untitled 1 GEOLOGICAL SURVEY OF DENMARK AND GREENLAND BULLETIN 8 · 2005 Structural analysis of the Rubjerg Knude Glaciotectonic Complex, Vendsyssel, northern Denmark Stig A. Schack Pedersen GEOLOGICAL SURVEY OF DENMARK AND GREENLAND DANISH MINISTRY OF THE ENVIRONMENT 2 Geological Survey of Denmark and Greenland Bulletin 8 Keywords Northern Jylland, Denmark, Weichselian, glacial geology, glaciotectonics, thin-skinned thrust faulting, balanced cross-section, thrust- fault dynamics, imbricate duplexes, mud diapirs, piggyback basins. Cover The coastal clif f (99 m high at its highest point) at Rubjerg Knude on the west coast of Vendsyssel, northern Denmark. The lower two-thirds of the cliff, beneath the prominent dark sub-horizontal surface, forms part of the cross-section through the Rubjerg Knude Glaciotectonic Complex displaying imbricated thrust sheets composed of the Lønstrup Klint Formation (bluish-grey colour) and the overlying Rubjerg Knude Formation (yellow colour), both of Late Weichselian age. The thrust sheets are truncated by a glaciotectonic unconformity (the prominent surface), upon which the Kattegat Till Formation is only preserved as a boulder bed due to subsequent aeolian erosion of the till matrix. The upper third of the clif f comprises recent aeolian dune sands that have accreted over the last 100 years and now encroach on the Rubjerg Knude lighthouse, the top of which is just visible above the clif ftop. Photo: Stig A. Schack Pedersen (August 1984). Chief editor of this series: Adam A. Garde Editorial board of this series: John A. Korstgård, Geological Institute, University of Aarhus; Minik Rosing, Geological Museum, University of Copenhagen; Finn Surlyk, Geological Institute, University of Copenhagen Scientific editor of this volume: Jon R. Ineson Editorial secretaries: Esben W. Glendal and Birgit Eriksen Illustrations: Benny M. Schark and Alice Rosenstand Digital photographic work: Benny M. Schark Graphic production: Knud Gr@phic Consult, Odense, Denmark Printers: Schultz Grafisk, Albertslund, Denmark Manuscript submitted: 8 August 2003 Final version approved: 11 February 2005 Printed: 15 December 2005 This monograph has been accepted by the Faculty of Natural Sciences, University of Copenhagen, for public defence of the degree of Doctor of Science. ISSN 1604-8156 ISBN 87-7871-168-1 Geological Survey of Denmark and Greenland Bulletin The series Geological Survey of Denmark and Greenland Bulletin replaces Geology of Denmark Survey Bulletin and Geology of Greenland Survey Bulletin. Citation of the name of this series It is recommended that the name of this series is cited in full, viz. Geological Survey of Denmark and Greenland Bulletin. If abbreviation of this volume is necessary, the following form is suggested: Geol. Surv. Den. Green. Bull. 8, 192 pp. Available from Geological Survey of Denmark and Greenland (GEUS) Øster Voldgade 10, DK-1350 Copenhagen K, Denmark Phone: +45 38 14 20 00, fax: +45 38 14 20 50, e-mail: geus@geus.dk or Geografforlaget ApS Rugårdsvej 55, DK-5000 Odense C, Denmark Phone: +45 63 44 16 83, fax: +45 63 44 16 97, e-mail: go@geografforlaget.dk © Danmarks og Grønlands Geologiske Undersøgelse (GEUS), 2005 3 Contents Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 History of the present investigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Glacial tectonics – concepts and models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Previous conceptual models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Thin-skinned thrust faulting: the concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Thrust-fault modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Test model 1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Test model 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Test model 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Test model 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Test models: concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Concept of balanced cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Location and construction of the Rubjerg Knude cross-section . . . . . . . . . . . . . . . . . 23 Location. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Photogrammetric work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Digital editing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Construction of the balanced cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Geological setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Lithostratigraphy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Skærumhede Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Stortorn Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Lønstrup Klint Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Upper Weichselian lithostratigraphic units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Rubjerg Knude Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Kattegat Till Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Ribjerg Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Mid Danish Till Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Vendsyssel Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Structural description of sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Ulstrup Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Tectonic architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Sedimentary units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Lønstrup Klint Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Rubjerg Knude Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Structures and breccias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Thrust-zone breccias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Foreland-dipping hanging-wall flat faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Collapse structure in the Ulstrup Rende . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 Interpretation of structural development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 Stensnæs Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Tectonic architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Sedimentary units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 Lønstrup Klint Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 Rubjerg Knude Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 4 Imbricate duplex folding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Extensional faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Interpretation of structural development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 Martørv Bakker Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 Tectonic architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 Sedimentary units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 Lønstrup Klint Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Rubjerg Knude Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Imbricate duplexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Normal fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Hydrodynamic brecciation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Interpretation of structural development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Kramrende Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Tectonic architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Sedimentary units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 Lønstrup Klint Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Rubjerg Knude Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Thrust faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Kramrende diapir . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 Reverse faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 Interpretation of structural development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 Brede Rende Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 Tectonic architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 Sedimentary units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Lønstrup Klint Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Rubjerg Knude Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Diapir structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Brede Rende normal fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Frost wedges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Interpretation of structural development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 Sandrende Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Tectonic architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Sedimentary units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Lønstrup Klint Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Rubjerg Knude Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Structures and breccias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Normal faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 Diapir structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Frost wedge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Interpretation of structural development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Stenstue Rende Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Tectonic architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 Sedimentary units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Lønstrup Klint Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Rubjerg Knude Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 Thrust-fault structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 Hanging-wall anticlines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 5 Normal faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Slump folding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Interpretation of structural development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Grønne Rende Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Tectonic architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 Sedimentary units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 Lønstrup Klint Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 Rubjerg Knude Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112 Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 GR01 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 GR02 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 GR03 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 GR04 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 GR05 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 GR06 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 GR07 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 GR08 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 GR09 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 GR10 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 GR11 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 GR12 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 GR13 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Interpretation of structural development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Rubjerg Knude Fyr Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 Tectonic architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 Sedimentary units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 Anastomosing thrust-fault brecciation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 Interpretation of structural development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 Stortorn Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 Tectonic architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 ST01 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 ST02 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 ST03 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 ST04 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123 ST05 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 ST06 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 ST07 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 ST08 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124 ST09 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 ST10 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 Sedimentary units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 Interpretation of structural development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 Moserende Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 Tectonic architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 MR01 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 MR02 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 MR03 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 MR04 and MR05 thrust sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 MR06–MR08 thrust sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 6 MR09 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 MR10 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 MR11 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 MR12 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 MR13 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Sedimentary units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 Lønstrup Klint Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 Rubjerg Knude Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 Diapir structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 Thrust faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 Footwall synclines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 Interpretation of structural development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 Mårup Kirke Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 Tectonic architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 MK01 thrust sheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 MK02–MK04 thrust sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 MK05–MK07 thrust sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139 MK08–MK10 thrust sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 MK11–MK20 thrust sheets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 Sedimentary units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 Interpretation of structural development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 Fault-bend-fold model for duplex units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 Characterisation of thrust duplex MK11–MK20. . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Discussion of structural development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Ribjerg Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 ‘Store Blå’ and ‘Lille Blå’. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 Tectonic architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 Sedimentary units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 Skærumhede Group . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 Blå-unconformity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 Ribjerg Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146 Mid Danish Till Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Vendsyssel Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Interpretation of glacial geology and stratigraphic development . . . . . . . . . . . . . . . . 147 Dynamic development of the thin-skinned thrust faulting . . . . . . . . . . . . . . . . . . . . . . 148 Moserende Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 Moserende Section: summary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Stortorn Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 Stortorn Section: summary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 Rubjerg Knude Fyr Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 Rubjerg Knude Fyr Section: summary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 Grønne Rende Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 Grønne Rende Section: summary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Stenstue Rende Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Stenstue Rende Section: summary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 Sandrende Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 Sandrende Section: summary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 Brede Rende Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 7 Brede Rende Section: summary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 Kramrende Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 Kramrende Section: summary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 Martørv Bakker Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 Martørv Bakker Section: summary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 Stensnæs Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 Stensnæs Section: summary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 Ulstrup Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 Ulstrup Section: summary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 Summary of dynamic development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 Thrust-fault architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178 Balanced cross-section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179 Thrust brecciation and diapirism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 Thrust-fault dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Syntectonic deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Proglacial and subglacial deformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182 Glacial geological conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186 Appendix 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 Thrust-fault terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190 Appendix 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 Specification of photogrammetric work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 8 9 Abstract Pedersen, S.A.S. 2005: Structural analysis of the Rubjerg Knude Glaciotectonic Com- plex, Vendsyssel, northern Denmark. Geological Survey of Denmark and Greenland Bulletin 8, 192 pp. The Rubjerg Knude Glaciotectonic Complex is a thin-skinned thrust-fault complex that was formed during the advance of the Scandinavian Ice Sheet (30 000 – 26 000 B.P.); it is well exposed in a 6 km long coastal profile bordering the North Sea in northern Denmark. The glaciotectonic thrust-fault deformation revealed by this cliff section has been subjected to detailed structural analysis based on photogrammetric measurement and construction of a balanced cross-section. Thirteen sections are differentiated, characterising the distal to proxi- mal structural development of the complex. The deformation affected three stratigraphic units: the Middle Weichselian arctic marine Stortorn Formation, the mainly glaciolacustrine Lønstrup Klint Formation and the dominantly fluvial Rubjerg Knude Formation; these three formations are formally defined herein, together with the Skærumhede Group which includes the Stor- torn and Lønstrup Klint Formations. The Rubjerg Knude Formation was deposited on a regional unconformity that caps the Lønstrup Klint Formation and separates pre-tectonic deposits below from syntectonic deposits above. In the distal part of the complex, the thrust-fault architecture is characterised by thin flat- lying thrust sheets displaced over the footwall flat of the foreland for a distance of more than 500 m. Towards the proximal part of the complex, the dip of the thrust faults increases, and over long stretches they are over-steepened to an upright position. The lowest décollement zone is about 40 m below sea level in the proximal part of the system, and shows a systematic step-wise change to higher levels in a distal (southwards) direction. The structural elements are ramps and flats related to hanging-wall and footwall positions. Above upper ramp-hinges, hanging-wall anticlines developed; footwall synclines are typically related to growth-fault sedimentation in syntectonic piggyback basins, represented by the Rubjerg Knude Formation. Blocks and slump-sheets constituting parts of the Lønstrup Klint Formation were derived from the tips of up-thrusted thrust sheets and slumped into the basins. Mud diapirs are a prominent element in the thrust-fault complex, resulting from mud mobilisation mainly at hanging-wall flats and ramps. Shortening during thrust-fault deformation has been calculated as 50%. Only about 11% of the initial stratigraphic units subjected to thrust faulting has been lost due to erosion. The thrust-fault deformation was caused by gravity spreading of an advancing ice sheet. Over- pressured mud-fluid played an important role in stress transmission. The average velocity of thrust-fault displacement is estimated at 2 m per year, which led to compression of a 12 km stretch of flat-lying sediments, c. 40 m in thickness, into a thrust-fault complex 6 km in length. The thrust-fault complex is truncated by a glaciotectonic unconformity, formed when the advancing ice sheet finally overrode the complex. When this ice sheet melted away, a hill- and-hole pair was formed, and meltwater deposits derived from a new ice-advance (NE-Ice) filled the depression. The NE-Ice overran the complex during its advance to the main station- ary line situated in the North Sea. When this ice in turn melted away (c. 19 000 – 15 000 B.P.), the glacial landscape was draped by arctic marine deposits of the Vendsyssel Formation (new formation defined herein). _________________________________________________________________________________________ Author’s address Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: sasp@geus.dk 10 55N 5E 10E 15N 60N SCANDINAVIAN ICE SHEET NORWAY DENMARK Copenhagen Göteborg Møns Klint Bovbjerg Fur Knudeklint Hanklint Mols Hoved Rügen Ristinge Klint Lønstrup Klint SWEDEN Germany Baltic ice advance Swedish ice advance Norwegian ice advance 28 ka BP 30 ka BP 20 ka BP 17 ka BP 17 ka BP 17 ka BP 28 ka BP Fig. 1. Map of the Danish Basin indicating the distribution of the Scandinavian Ice Sheet during the three main ice advance events, with source areas in southern Norway, central Sweden and the Baltic, in the Middle–Late Weichselian. The approximate timing of the stationary lines are given; the early progressive ice advance is indicated in black, the subsequent late ice border lines in red. The locations of major glaciotectonic complexes formed during the ice advances are indicated by asterisks. 11 Introduction Glaciotectonic studies in Denmark have a long tradi- tion, and an important part of structural geology stud- ies in Denmark concern glacial tectonic deformation resulting from the southward advance of the Scandi- navian Ice Sheet in the Pleistocene (Fig. 1). The descrip- tion of the geological structures dates back to Pug- gaard (1851), who made one of the first extensive and detailed Danish structural analyses of a tectonic com- plex and provided a classic cross-section of Møns Klint. Johnstrup (1874) established the concept of glacial deformation. The next milestone in glacial tectonic studies in Denmark was by Jessen (1918, 1931), whose detailed survey of Lønstrup Klint (Fig. 1) included a structural analysis and an attempt at a glaciodynamic interpretation of the deformation structures observed. The Lønstrup Klint coastal section includes the Rubjerg Knude Glaciotectonic Complex, which is the subject of this study (Fig. 2). A Danish school of glaciotecto- nic studies subsequently developed (Madsen 1916; Jes- sen 1931; Gry 1940, 1941; Rosenkrantz 1944; Berthelsen 1973, 1975, 1978, 1979; Sjørring 1974, 1977, 1981, 1983; Rasmussen 1975; Petersen 1978; Houmark-Nielsen 1987, 1988; Pedersen 1987, 1993, 1996, 2000; Peder- sen & Petersen 1988, 1995, 1997; Pedersen et al.1988; Klint & Pedersen 1995; Jakobsen 1996), which has naturally been stimulated by geologists working with glaciotectonic structural geology internationally (Ban- ham 1977, 1988; Stephan 1980; Aber 1982, 1993; Ehlers 1983; van der Wateren 1985, 1992; Boulton 1986; Boul- ton & Hindmarsh 1987; Croot 1987, 1988; Meer 1987; Goldthwait & Matsch 1988; Aber et al. 1989; Hart 1990; Hart & Watts 1997; Bennett 2001). The similarity in structural geometry between gla- ciotectonic terrains and orogenic belts has led to pro- longed debate. Are glaciotectonic terrains scale mod- els for orogenic deformation? Or does the soft and synsedimentary nature of glaciotectonics differ in prin- ciple from that of fold belt deformation? Arguments for deformational similarity have been put forward by Berthelsen (1978, 1979), Banham (1988), Aber et al. (1989), van der Wateren (1992) and Pedersen (1987, 2000). These structural geologists share the opinion that the terminology of structural geology related to oro- genic belts is applicable in the description and dis- cussion of glaciotectonic complexes. The main differ- ences between deformation in metamorphically altered rocks and glaciotectonic deformation of soft sediments are: (1) the presence of ‘free’ water, which enables liquefaction and fluidisation, (2) the velocity of the deformation, and (3) the shallowness of penetrative deformation. In contrast, deformation of metamorphic rocks commonly involves alteration and recrystallisa- tion of minerals, processes that never apply to glacio- tectonics. The advantage of a study of glaciotectonic complexes is that the structures are at a scale that allows them to be studied in a single exposure, in contrast to fold belts where extensive field mapping and expensive geophysical investigations are typically required for adequate documentation of the structures. Further- more, many glaciotectonic complexes are geological- ly young, which means that the upper structural levels are still preserved and interpretation of the full dyna- mic development of structural complexes is possible. The structural architecture of glaciotectonic complexes may therefore serve as inspiration for the interpreta- tion of thin-skinned structural relationships in fold belts and thrust-fault deformation terrains. The structural analysis of the Rubjerg Knude Glaciotectonic Com- plex is presented as a mesoscopic model of a thin- skinned thrust-fault complex (Plates 1, 2). History of the present investigation This study focuses on the structural framework and dynamic development of the glacial tectonic thrust- fault complex at Rubjerg Knude, Lønstrup Klint. It is based on twenty years of investigations of the Løn- strup Klint cliff section. The author took up the study of glacial tectonic thrust-fault structures after having concluded a Ph.D. thesis on thin-skinned thrust fault- ing in the North Greenland fold belt (Pedersen 1979, 1981, 1982, 1986a, 1987). A large part of the study of the fold belt structures in Peary Land, North Green- land, was photogrammetric mapping (Pedersen 1979, 1981), undertaken at a time when geological map- ping by computer-assisted photogrammetry was under development in Copenhagen. This project was an inte- grated collaboration between the Geological Survey of Greenland, the Institute of Surveying and Photogram- metry of the Technical University of Denmark (DTU), the Geological Museum (GM) and the Geological Insti- tute (GI) of the University of Copenhagen. In the years 12 l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l Diamictite Glaciolacustrine and glaciofluvial sand and gravel Mobilised mud Non-marine sand Non-marine clay and silt Marine clay Rubjerg Knude Formation Lønstrup Klint Formation Stortorn Formation 0 500 1000 1500 2000 2500 3000 0 100 200 300 400 5 150014001300120011001000 2000 2100 2200 2300 2400 2500 350034003300320031003000 4100 4200 4300 4400 4500 560055005400530052005100 steps Ribjerg Moserende Kramrende Stenstue Rende Stortorn Grønne Rende Brede Rende Stensnæs Fig. 2. Geological cross-section of the Rubjerg Knude Glaciotectonic Complex. For details and legend, see Plate 1. 13 l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l Aeolian dunes Holocene peat Marine clay and sand Sandy till Glaciofluvial sand { Mid Danish Till Formation & Kattegat Till Formation Vendsyssel Formation Ribjerg Formation Top of dunes Clifftop – glacial abrasion surface Thrust, fault Unconformity Intraformational bedding 3500 4000 4500 5000 5500 6000 m 500 600 700 800 900 1000 m 2000 m1900180017001600 2600 2700 2800 2900 3000 m 4100 m40003900380037003600 4600 4700 4800 4900 5000 5100 m 6000 m590058005700 steps Rubjerg Knude Fyr Sandrende Martørv BakkerOddervej Ulstrup Ulstrup Rende Tvonnet Rende Mårup Kirke 14 up to 1990, techniques of geological mapping and construction of geological cross-sections based on multi- model photogrammetric analysis were developed and made available at DTU (Dueholm 1992). Initial inves- tigations in co-operation with K. Dueholm (DTU) and A.K. Pedersen (GM) proved the applicability of multi- model photogrammetry in the study of glaciotectonic cross-sections in Denmark by an examination of the Møns Klint clif f section (Pedersen 2000). Subsequent- ly, the photogrammetric investigation of the Rubjerg Knude clif f section was initiated, and forms the basis of the present work. Objectives The objectives of the study of the Rubjerg Knude Gla- ciotectonic Complex can be summarised as follows. 1. A description of an exceptionally well-exposed gla- ciotectonic complex, which can be taken as an ex- ample of a very low friction thrust-fault wedge, pre- sented as a detailed cross-section based on multi- model photogrammetric measurements of the Ru- bjerg Knude clif f section. 2. A demonstration of the techniques of balanced cross-section construction that permit interpretation of the unexposed parts of the thrust-fault complex. 3. The construction of a model for the dynamic de- velopment of the proglacial thrust system that dem- onstrates the sequential evolution of increasing de- formation intensity and the interplay with syntec- tonic depositional processes. 4. An interpretation of deformation processes within the framework of Danish glacial stratigraphy in the late Pleistocene (late Middle to Late Weichselian c. 30 000 – 20 000 years B.P.). Previous conceptual models The basic concept of glacial processes acting as the deformation agent was formulated by Johnstrup (1874). His concept was primarily focused on the formation of the spectacular cliffs at Møns Klint in south-eastern Denmark and on Rügen in north-eastern Germany. However, subsequently Johnstrup (1882) also includ- ed the formation of the steeply inclined floes exposed in the Lønstrup Klint clif f section in the classic exam- ples of glacial deformation in Denmark. (The term floes is frequently used in the old glacial geology lite- rature inspired by the idea that the dislocated sheets were ground- or permafrozen; in a structural geologi- cal context, floes are identical to thrust sheets or thrust- sheet segments.) Johnstrup’s main conclusions con- cerning the glaciotectonic origin of the deformation at Lønstrup Klint were: (1) the dislocations are super- ficial without extending down to a deep root zone, and are restricted to surface phenomena, (2) the di- rection of movement indicated from the dip of the dislocated floes corresponds to a uniform direction of ice advance, and (3) the dislocated floes formerly con- stituted one undisturbed area. The detailed mapping and construction of the cross-section was presented by Jessen (1918) in his geological description of the Vendsyssel map sheet. However, the final detailed de- scription of the dislocations at Lønstrup Klint was pub- lished later (Jessen 1931). In 1927, George Slater included a study of the Løn- strup Klint section as part of his thesis for a D.Sc. degree at the University of London, which also in- cluded a study of glacial deformation at Møns Klint. The most striking conclusion was that the glacial defor- mation at Lønstrup Klint was caused by englacial defor- mation. Slater (1927, p. 312) summarised thus: “… 2. The deposits represent the final positions of englacial material after the melting of the interstitial ice. 3. The type of structure is analogous to that seen in decaying Arctic glaciers, and is due to the arresting of move- ment of the frontal part of an overloaded ice-sheet. 4. The structure has been built up in the reverse direc- tion to the line of movement.” Slater (1927) interpre- ted the Lønstrup Klint section as a variety of glacial tectonics he termed ‘the stagnant-glacier type’. Subsequently, Axel Jessen and Karl Gripp exchang- ed ideas about proglacially formed glaciotectonic struc- tures, and concluded that the structures Jessen had observed at Lønstrup Klint were similar to those that Gripp (1929) described from the foreland of the ad- Glacial tectonics – concepts and models 15 vancing Holmströms Gletscher on Spitsbergen. In his detailed and comprehensive description of his inves- tigations, Jessen (1931) concluded that the disloctions cannot have formed englacially, but must be the result of pressure building up due to loading at the margin of the advancing ice. This pressure spreads out later- ally into the clayey units, which in the foreland react by splitting up into fractured dislocation sheets com- pressed in front of the advancing ice masses. Jessen (1931) also discussed the dif ficulty related to the displacement of the sheets without fracturing of the lithological units resulting in a complete col- lapse during deformation, and he pointed out that Johnstrup (1882) had suggested that the deformed layers could have been ground-frozen. Jessen’s (1931) more subjective arguments against Slater’s work con- cern the fact that Slater (1927) did not refer to Jessen’s (1918) substantial work on Vendsyssel and in particu- lar his published cross-section of Lønstrup Klint. Jes- sen pointed out that major anticlines in Slater’s cross- section between Mårup Kirke and Rubjerg Knude Fyr do not exist, and that Slater’s (1927) misinterpretation must be ascribed to his superficial investigations which did not allow him to check the way-up relationship of each limb in the fold structure (Jessen 1931). In his work on the glaciotectonic deformation of Palaeogene diatomites with ash layers in the Limfjor- den region, Gry (1940) compared these with the de- formation at Lønstrup Klint and supported the progla- cial deformation concept of Gripp (1929) and Jessen (1931). Furthermore, Gry proposed a gravity-spread- ing model for the deformation and attempted a very early balanced cross-section in the consideration of restoration of the dislocated thrust sheets (Fig. 3). How- ever, Gry (1940) proposed a cylindrical model for the thrust surfaces, and in his ‘back-stripping’ cross-sec- tion the floes were displaced along circular fault lines. Thus, in his dynamic consideration the floes were as- signed a standing position with their frontal parts ‘up in the air’ (Fig. 3), and he consequently concluded that more than 80% of the upper sand-series at Løn- strup had been eroded away by the advancing ice. In contrast to this point of view, Pedersen (1987) suggested that a large proportion of the upper sand- series was deposited syntectonically; this removed the requirement that a large part of the floes or thrust sheets had been eroded away. Pedersen (1987) interpreted the glaciotectonic thrust-fault complex as an example of gravity-spreading deformation, viewed in the light of the gravity-spreading experimental model presen- ted by Bucher (1956) and with reference to compa- rable gravity-spreading deformation in soft sedimen- tary rocks exemplified by the mudlumps in the Missis- sippi Delta (Morgan et al. 1968). Furthermore, the mudlumps or mud diapirs in the Lønstrup Klint imbri- cate fan were described, and interpreted as an integral part of a conceptual dynamic model for thrust-fault related mud diapirism and syntectonic sedimentation (Fig. 4). Sadolin et al. (1997) elaborated on the model of syntectonic sedimentation in the Lønstrup Klint sec- tion. Based on detailed sedimentological studies, they pointed out the importance of the unconformity that separates the lower muddy units (their unit A), from Diluvial sand Yoldia clay Fig. 3. A model for structural balancing of the dislocated floes in the Lønstrup Klint section suggested by Gry (1941). In his model, the displacement surfaces were regarded as cylindrical sections and due to the suggested amount of displacement about 80% of the dislocat- ed floes was subsequently eroded away. 16 the upper sandy units (their units B–D). The lower unit A was interpreted to have been deposited in a lake isolated from the former marine Kattegat–Ska- gerrak basin by either a damming of the advancing ice, in accordance with ideas also presented by Jes- sen (1918, 1931), or simply by isolation of the lake basin due to lowering of sea level in the late Pleis- tocene (Sadolin et al. 1997). The unconformity was interpreted to reflect a major drainage event of the lake basin before a shallow lacustrine basin was es- tablished, characterised by incursions of glaciofluvial deposition (units B–D of Sadolin et al.1997). During the deposition of units C and D, glaciotectonic thrust- ing commenced contemporaneously with the rise of mud diapirs and the formation of normal faults due to mass adjustments in the mobilised mud in the subsur- face (Sadolin et al. 1997; Fig. 5). The conceptual model presented here aims at an interpretation based on the concepts of thin-skinned thrust-fault tectonics. Although the scale is an order of magnitude smaller than in typical orogenic belts, it has not been found appropriate to introduce special terminology for the deformation structures in the Ru- bjerg Knude Glaciotectonic Complex. The concept of thrust-fault deformation and related structures is sum- marised in the following chapter. Thin-skinned thrust faulting: the concept It is difficult to judge exactly when the concept of thin-skinned thrust faulting nucleated, as it represents a gradual evolution of ideas over the last 25 years or more. However, Boyer & Elliot (1982) appear to have been the first to give a conceptual introduction to the basic principle of thin-skinned thrust faulting. Suppe (1983, 1985) improved the concept by defining and describing the geometry and kinematics of fault-bend folding. Jamison (1987) and Schirmer (1988) contri- buted with further improvements of geometric analy- sis of fold development in overthrust terranes and thrust-fault hanging-wall successions. McClay (1992) presented a glossary of thrust tectonic terms, and Erick- son & Jamison (1995) demonstrated viscous-plastic finite-element models of fault-bend folds. In 1997, an entire volume of the Journal of Structural Geology was devoted to thrust-fault tectonics. Among the pa- pers that particularly inspired and supported this study of glaciotectonic thrust faulting were those of Contre- ras & Sutter (1997), Medwedeff & Suppe (1997) and Mitra & Sussman (1997). Thrust-fault modelling To better understand the range of possible configura- tions of different structural frameworks of thrust-fault complexes, a series of computer models were tested with the aid of the program AUTOFAULT, a ‘Balanced Cross Section Program’ within the AutoCAD system frame (Ozkaya 1994). Four of these test models are demonstrated here to illustrate the thin-skinned thrust- fault concept (Figs 6–9). The basic function of the model is to define and con- struct a layer package onto which a thrust fault is add- ed and given a certain displacement. The program then calculates the configuration of the thrust sheet Fig. 4. A four-stage model for the development of mud diapirs related to thrust faulting in Lønstrup Klint suggested by Peder- sen (1987). Note that in the model the thrust zone of the hang- ing-wall ramp constitutes mobilised mud and that syntectonic deposits accumulate ‘piggyback’ between the thrust sheets. 17 100 m 100 m 100 m 100 m Sandrende thrust fault C C C B B B B A A A A A B C A Mud diapir x1 A x2 A C C Mobilized mud y1 y2 Extensional faults Mobilized mud x1 x2 y1 y2 x1 x2 y1 y2 Position of thrust-fault fractures UnconformityErosionally removed 2 1 D D m. s. l. Older Yoldia Clay 4 3 Fig. 5. The structural and depositional development of the Sandrende Section suggested by Sadolin et al. (1997). The model summarises four stages of development initiating with the formation of the regional erosional unconformity (1). Unit B was deposited in topographic lows above the unconformity, and thrust faulting initiated contemporaneously with the deposition of unit C (x 1 –y 1 and x 2 –y 2 denote same reference points separated by the thrusts, where x = footwall syncline and y = hanging-wall anticline) (2). Propagation along the thrust faults continued and unit C was deposited during increasing tilting of the thrust sheet. Normal-fault fractures formed in connection with the incipient diapirism (3). The Sandrende diapir rose during deposition of unit D and normal faulting propagated. In the proximal part of the thrust sheet, a network of conjugate extensional faults developed and interference between a new-formed satellite thrust and the normal faults af fected the complex. The tip of the thrust sheet was bent due to drag along the side of the rising diapir (4). Star symbol provides a reference point through the development stages. 18 Step 1 Step 2 Step 3 Step 4 Step 5 Step 6 Hanging-wall block Ramp Footwall block 50 m displacement 100 m displacement 150 m displacement 200 m displacement 300 m displacement 400 m displacement Lower flat Axial surface Upper flat = top surface Lower ramp hinge Upper ramp hinge Hanging-wall anticline Hinterland-dipping limb Foreland-dipping limb Hanging-wall flat Hanging-wall flat Hanging-wall ramp Upper footwall flat Footwall ra mp Décollement or lower footwall flat 0 100 200 300 400 m N S Fig. 6. Test model 1 of thrust-fault deformation constructed with the computer program AUTOFAULT (Ozkaya 1994). The model demonstrates the development of simple ramp propagation given increasing displacements. In the first four steps, the displacement is sequentially increased by 50 m, whereas a displacement of 100 m is added to steps 5 and 6. Note that a ‘typical upright anticline’ develops when the displacement is about twice the thickness of the layer package displaced. Moreover, the model illustrates the terminology applied in the text and defined in Appendix 1. 19 for the specific model constructed. Thus the program gives the ‘differential’ calculation model to an induced ‘integration’ solution configuration. Further thrust faults can be added, and be given new displacements, such that rather complex models can be constructed. How- ever, a few limitations of the program hamper realistic comparisons with nature. Thus the program cannot handle inclinations exceeding 60°. In general this is not a problem as ramp angles typically range between 10° and 35° and for rock mechanical reasons never exceed 45° (Ozkaya 1994). However, the problem of steep inclinations becomes important in complexes including superimposed deformation. A second limi- tation is that testing with superimposed displacements requires a construction with an upper flat located with- in the model. This results in an unrealistically high number of shallow upper flats in the models, as illus- trated below in test model 4 (see Fig. 9). Thirdly, the program cannot accommodate cross-cutting thrust-fault relationships, which limits the spacing and dip of ramps. Nevertheless, the test models give a good in- troduction to the thrust-fault concept, and demonstra- tion of models with basic layer package dimensions approaching the scale of thrust sheets involved in the Rubjerg Knude Glaciotectonic Complex can be achie- ved. A glossary of the thrust-fault terms used here is given in Appendix 1; note that only contractional thrust-fault structures are considered. Test model 1 The first AUTOFAULT model displays a simple thrust fault with one ramp connecting a lower and an upper flat (Fig. 6). The development of thrust-fault structures, in particular the fault-bend folding of the hanging- wall anticline, is demonstrated in six steps with increas- ing displacement. The ramp angle is 25°, and the layer package constitutes a lower unit 25 m thick where the lower flat (or the décollement zone) is located. Above this, one 25 m and two 20 m thick layers have been constructed, with a 30 m thick uppermost layer (Fig. 6). The model approaches the assumptions of parallel behaviour with preservation of layer thickness, no net distortion where layers are horizontal, and conserva- tion of bed length (Suppe 1983). Step 1 shows the gentle hanging-wall anticlinal fold- ing after 50 m displacement. Note the flat-topped na- ture of the hanging-wall anticline, which makes it al- most insignificant. The backlimb of the anticline dips toward the left, parallel to the ramp, and the axial surfaces defined by the bend above the lower ramp hinge and the bend of the hanging-wall anticline de- fine two kink bands dipping steeply to the right. By comparing steps 1 and 2 it can be seen that the spac- ing between the kink bands increases with increasing displacement. Step 2 gives the configuration after 100 m displace- ment. Here the forelimb dipping towards the foreland to the right starts to be a significant part of the struc- ture. Note the increase in spacing between the kink bands in the backlimb structure. The kink bands de- fine minor zones of weakness, which could develop into small reverse faults as in the thrust model dem- onstrated by Wiltschko (1979). These are referred to as back thrusts. Step 3 shows the structural development after 150 m displacement. Note that the flat-topped hanging- wall anticline now has a more angular upright form, where the kink bands fanning up from the positions near the upper ramp hinge approach each other. How- ever, in the model the anticline maintains its flat-topped structure and retains two axial surfaces (kink bands). Step 4 demonstrates the formation of the upright, angular hanging-wall anticline, where the amount of displacement is close to the length of the thrust-fault ramp. Due to the geometric adjustments the hanging- wall ramp is shorter than the footwall ramp. The dis- placement is 200 m corresponding to about two times the thickness of the thrust sheet. Step 5 shows the structural development after 300 m displacement. The hanging-wall anticline becomes even more flat-topped and the space between its axial surface kink bands increases. Note that the foreland- dipping forelimb is linked to the hanging-wall ramp displaced along the footwall flat, and the hinterland- dipping backlimb corresponds to the hanging-wall flat bent up along the footwall ramp. Step 6, with a displacement of 400 m demonstrates that the main structural configuration is maintained, except for the increase in spacing between the back- limb and the forelimb. Test model 2 The second AUTOFAULT model demonstrates the propagation along a thrust fault dif ferentiated into a décollement zone, a lower ramp, an intermediate flat, an upper ramp and an upper flat bringing the thrust fault up to the top surface (Fig. 7). The model is con- 20 structed with two lower units, 40 m in thickness; the décollement zone is located in the second layer. The lower layers mimic the lower clay units of the Løn- strup Klint stratigraphy, and two c. 25 m thick layers overlie them. The top layer is 50 m thick, but while not comparable to any part of the stratigraphy in the Lønstrup Klint section, its construction yields a better demonstration of the development envisaged. The lower ramp is given a dip of 25° and the upper ramp a dip of only 15° to reflect the principle of increasing angle of fracturing with increasing depth (Hobbs et al. 1976; Pedersen 1996). The distance between lower and upper ramps along the intermediate flat is c. 250 m, and three steps are presented in Fig. 7. Step 1 is given 50 m displacement and two hang- ing-wall anticlines immediately appear. The steep ramp clearly initiates the formation of an upright anticline with steeply dipping limbs. Between the two hang- ing-wall anticlines, an intervening syncline forms above the intermediate flat. The involute surface of the syn- cline provides the location for a broad, shallow basin. Step 2 shows the structural development after 100 m displacement. This demonstrates clearly that the intervening syncline is an obvious site for a piggy- back basin to develop. Note that the steeply dipping forelimb of the hanging-wall anticline above the lower ramp would be the obvious site for erosion and the source of material feeding into the piggyback basin. Step 3 demonstrates that with a displacement of 200 m, the piggyback basin becomes narrow and is ele- vated to a higher position as a consequence of the displacement up along the upper ramp; it is eventual- ly lifted out of the position for being a centre of depo- sition. With increasing displacement, the frontal part of the thrust sheet develops into a wedge-shape structure. Hanging-wall block Lower hanging-wall ramp Lower ramp Upper ramp Upper flat Upper hanging-wall ramp Upper hanging-wall ramp Footwall block 50 m displacement 100 m displacement 200 m displacement Piggyback basin Piggyback basin Intermediate flat Lower flat Fault-bend folding Step 1 Step 2 Step 3 0 100 200 m N S Fig. 7. Test model 2 of thrust-fault deformation constructed with the computer program AUTOFAULT. The model demonstrates the development of thrust-fault propagation along a lower and an upper ramp and the connecting flats. Note in this model the formation of two anticlines divided by a syncline, the depression of which is the obvious location of a piggyback basin. 21 Test model 3 The third AUTOFAULT model aims at constructing an imbricate complex by branching faults fanning up from the same décollement level (Fig. 8). The model is con- structed with a lower 20 m thick unit in the top of which the décollement zone is located. Above the dé- collement zone, three units with a combined thick- ness of 50 m form the lower part of the thrust sheets, and the succession is capped by an upper 20 m thick unit. In three sequential steps, the principle of piggy- back thrusting is demonstrated (Fig. 8). Step 1 shows 100 m displacement along a deep- rooted ramp dipping 30°. Note the normal architec- ture of the hanging-wall anticline results from the ramping (compare with Fig. 6, step 3). Step 2 demonstrates the re-orientation of the piggy- back thrust sheet by the introduction of 100 m displace- ment along a 18° dipping ramp in front of and below the first thrust fault. Note that the accumulated dis- placement of the first thrust sheet amounts to c. 200 m. Step 3 shows an additional 100 m displacement along a low-angle 12° dipping ramp. Although the model demonstrates the main architecture of the imbricate fan illustrated by Pedersen (1987), it is a fairly simple model which may have only little relevance to natural conditions. Test model 4 The final AUTOFAULT model demonstrates the more likely formation of a steeply dipping imbricate fan or duplex (Fig. 9). The model is given the same strati- graphic units as in Test Model 3 (Fig. 8). A longer dé- collement zone is located in the middle of the lower- most unit, in addition to an intermediate flat in the third layer, while the upper flats are located within the uppermost unit. The initial steps in the construc- 1 1 2 3 1 2 100 m displacement 100 + 100 m displacement 300 m accumulated displacement Simple ramp Piggyback thrust sheet Branching thrust fault Branching thrust-fault imbricate fan Step 1 Step 2 Step 3 0 100 200 m N S Fig. 8. Test model 3 of thrust-fault deformation constructed with the computer program AUTOFAULT. The model demonstrates the formation of an imbricate fan by successive thrust-fault splays branching up from the main décollement zone. The encircled numbers refer to the sequential phase of thrust imbrication. The model is probably not comparable to structures formed in nature, but can be regarded as an introduction to test model 4 (Fig. 9). 22 tion of this model are similar to the examples demon- strated above, and hence only the final two steps are illustrated (Fig. 9). However, these give a convincing illustration of the increase of dips in an imbricate thrust- fault complex. Step 1 illustrates the final structural architecture af- ter 140 m displacement of thrust sheet 1 along the décollement zone, the lower ramp, the intermediate flat, an upper ramp and onto the upper flat (dips of ramps c. 25°). Thrust sheets 2–5 were formed by branch- ing ramps (dip of ramps c. 15°) with a displacement of c. 80 m added to each thrust fault. Finally, the lead- ing thrust sheet (6) is displaced 90 m along the lower décollement zone and a deep-rooted 30° dipping ramp. Note that the branching ramp imbricates are carried piggyback on thrust sheet 6. Furthermore, it should be noted that a long trailing segment of thrust sheet 6 occurs between the décollement zone and the inter- mediate flat. If this trailing segment becomes chopped up into duplexes between the two deep-rooted ramps, it will affect the overlying imbricates by vertical eleva- tion and the formation of antiformal stacks. Step 2 illustrates the over-steepening of the imbri- cates stacked onto the backlimb of the hanging-wall anticline of thrust sheet 6 arising from the addition of 100 m displacement to step 1 along the leading thrust rooting down to the lower décollement zone. Test models: concluding remarks A set of principles may be derived from the test models. 1. The level of elevation of the reference surface is directly related to the number and sizes of ramps the thrust sheet has passed. A ramp rooting down to a deep flat level corresponds to a high elevation of the topmost reference surface. In contrast, if a top reference surface is positioned at the same level as in the foreland, the thrusting corresponds to a trans- lation along a flat. 2. The steeper the ramp, the earlier its time of forma- tion. Gently dipping ramps are initiated at a late stage of deformation in areas proximal to the foreland. 3. The thickness of a piggyback basin reflects its du- ration as depocentre. Thus a small thickness of pig- gyback basin fill indicates an early trapping of the basin by overthrusting of a hanging-wall block. 4. A thick succession in the piggyback basin reflects a long period of translation of the thrust sheet along a long flat. Step 1 Step 2 Fig. 9. Test model 4 of thrust- fault deformation constructed with the computer program AUTOFAULT. The model demonstrates an imbricate fan (see Fig. 8) subjected to fault- bend folding during piggyback translation of an underlying hanging-wall flat propagation along a footwall ramp. The footwall ramp propagation will consequently result in increasing dips of the thrust sheets in the imbricate fan. Encircled numbers indicate successive thrust sheets. Concept of balanced cross-section The principle of the balanced cross-section in struc- tural analysis of thrust-fault systems was elegantly outlined by Dahlström (1969) and further improved by Suppe (1985). The application of balanced cross- sections in glaciotectonics has been demonstrated by Croot (1987), Klint & Pedersen (1995) and Pedersen (1996). In the construction of the balanced section, two different functions are applied: (1) the line balance, and (2) the volume balance, which in a 2-D cross- section corresponds to area balance. The first func- tion concerns the length of displacement, whereas the second function concerns the preservation of volume in the deformed cross-section compared with the re- stored undeformed cross-section (for demonstration see Plate 2). The basic method of balancing a cross- section (Dahlström 1969) is restoration by defining a pinpoint to be fixed to the foreland and then restor- ing the thrust sheets back to their initial pre-deforma- tional position. Thus one begins at the foreland and then by line balancing the thrust sheets are pulled back sequentially to their position prior to displace- ment. This requires a measure of displacement, which is the essential, but often difficult figure to achieve without some range of uncertainty. Details concerning the construction of the balanced cross-section of the Rubjerg Knude Glaciotectonic Com- plex (Plate 2) are given below. 23 Location The Rubjerg Knude cross-section is 6124 m long and extends from the coastal cliff immediately south of Lønstrup, Ribjerg, to about 300 m north of the ramp leading down to the beach at Nørre Lyngby (Fig. 2, Plate 1). The strike of the section is 17°, which is nearly parallel to the direction of the coastline. This is also approximately perpendicular to the main concentra- tion of structural strikes (bedding, thrust faults and fold axes; Fig. 10). The cross-section was consequently constructed to fit a general plane of orthographic pro- jection with a projection axis striking 107°. The Rubjerg Knude cross-section covers only the Rubjerg Knude Glaciotectonic Complex. Thus it is not as extensive as the cross-section of Lønstrup Klint con- structed by Jessen (1918, 1931), which extends from the cliff at the northern fringe of Lønstrup to the north- ern part of the beach at Løkken (see Fig. 12). The UTM co-ordinates (zone 32, ED50) of the end points of the Rubjerg Knude cross-section are 547512, 6370243 (N-end point) and 545251, 6364783 (S-end point). Photogrammetric work The cross-section of Rubjerg Knude Glaciotectonic Complex (Plate 1) is based on a multi-model photo- grammetric investigation of the cliff section using the method described by Dueholm (1992). Oblique pho- tographs were taken from a Cessna fixed-wing air- craft with a Minolta XG2 camera with known optical specifications, calibrated at the laboratory of photo- grammetry at the Technical University of Denmark. Standard 24 × 36 mm diapositive colour film was used, and the photographs were taken with 66% overlap from a distance of 200–300 m with an inclination an- gle of c. 35°, which provided the basis for setting up 67 stereoscopic models. In the laboratory, the orien- tation of the stereo-models was carried out based on ground control points adapted from two sets of verti- cal aerial photographs at a scale of 1:25 000, namely D9202 G 1365–66 and KMS 9203 A509–10 taken in May 1992. The stereoscopic instrument used was a Kern DSR 15 analytic plotter with a DEC VMS operating system and the special attached GEOPROGRAM developed N S 0 100 200 m 100 m displacement on youngest thrust fault 200 m displacement on youngest thrust fault 1 1 2 3 5 2 3 4 5 4 6 6 Location and construction of the Rubjerg Knude cross-section 24 1 2 3 N n = 52 1 2 3 n = 60 N 1 2 3 n = 83 N 1 2 3 n = 83 N A B C D Fig. 10. Stereographic projection diagrams of the orientation of structural elements in the Rubjerg Knude cross-section. The stereo- grams, lower hemisphere, equal area (Schmidt) net, display the concentration of the poles to bedding planes (black dots) or thrust planes (black triangles). A and B are measurements taken from Jessen (1931), and C and D are data produced in this study. Contour intervals are 1, 2.5, 5, 7.5, 10, 12.5, and 15%. The density point in all four diagrams is close to 197°/35°. Comparing the two sets of diagrams demonstrates that the structural orientation has been maintained despite c. 100 years erosion corresponding to c. 125 m retreat of the coastal cliff section. Black squares (D) indicate normal fault planes. Blue lines/numbers indicate principal compres- sion axes. 25 by Dueholm (1992). In the stereoscopic models, the geological features were outlined by the floating mark and digitised by the attached computer. The digitised data were stored for the later construction of the cross- section and the transformation for other programs applied for the management of the cross-section dis- play. The scale of the Rubjerg Knude cross-section in the analytic plotter version is 1:500, and the accuracy of the plotted data is about 25 cm (for further details, see Appendix 2). Digital editing In order to represent the cross-section in a publish- able display, the digitised data were transferred to ARC- INFO at the GIS-laboratory at the Geological Survey. Here it was transformed into an ARC-VIEW project, which served as the computer tool for editing the cross- section. Thus all areas were converted to closed poly- gons, which were annotated to fit the legend of litho- logies. During this editing, interpretations were made to finish the display of the cross-section, in particular interpretations of the scree-covered parts of the sec- tion. This was carried out contemporaneously with the construction of the balanced cross-section (see be- low), and a few additional corrections were added to the Rubjerg Knude cross-section. Some new exposures along the cliff section appeared in 1997–1999, which added to a better understanding of the structures in the transition from the frontal part of the glaciotec- tonic complex to its foreland. These have been incor- porated into the ARC-VIEW project. The final editing of the cross-section concerned the balanced cross-section. The construction of the bal- anced section was digitised and transformed into an ARC-VIEW project, and the subsequent interpretation of the extension of the thrust-fault ramps below sea level was added. Thus the Rubjerg Knude cross-sec- tion comprises a display of the exposed part of the cliff section with lithological and structural identity added as themes. Furthermore, the cross-section includes an interpretation of the thrust-fault structures in the sub- surface. Finally, a balanced construction was added l l 2900 3000 m Dc Dm Ds α α L/R-u L/R-u T T T T L/R-u L/R-u Fig. 11. Illustration of the method used for estimation of the displacement for the balanced cross-section. Above the main erosional unconformity at the top of the cliff, the extension of the thrust sheet tip is constructed by the intersection between the thrust fault (T) and the L/R-unconformity (L/R-u) based on the angle (±) between the bedding of the thrust sheet and the hanging-wall ramp. Dm, displacement measured; Dc, displacement constructed from tip-extension; Ds , displacement estimated from the interpretation of thrust-fault trace under the scree cover. The section illustrated is part of the Rubjerg Knude Fyr Section (Plate 1). 26 Table 1. The distribution of areas in the balanced cross-section (Plate 2) Balance (Plate 2A) Ramps (Plate 2B) Section* Number of areas Area (m2) Section* Number of areas Area (m2) 01UL 5 23 048 01UL 13 24 302 02SN 13 8965 02SN 18 8536 03MB 15 28 443 03MB 21 30 944 04KR 10 24 158 04KR 22 18 390 05BR 28 34 143 05BR 40 33 548 06SR 28 33 218 06SR 49 31 588 07SS 32 26 421 07SS 31 23 973 08GR 55 49 842 08GR 47 45 118 09RF 30 22 827 09RF 26 21 458 10ST 54 43 674 10ST 41 36 656 11MR 69 51 902 11MR 55 45 342 12MK 95 82 226 12MK 87 62 763 13BL 8 17 922 13BL 2 14 313 NrLy 2 5437 13RI 1 4405 PTR 3 2538 MD 1 472 Ve 4 9818 * The annotated numbers of sections (05BR) correspond to the sequential location of each section in a distal–proximal order, and the capitalised letters refer to the general abbreviation of the section names (see Plate 2). to the cross-section project, such that each thrust sheet is annotated in a balanced restored cross-section as well as in the structural cross-section displaying the geometry of the ramps and flats (Plate 2). Construction of the balanced cross-section The construction of the balanced cross-section for the Rubjerg Knude Glaciotectonic Complex was based on the geological cross-section, which displays the geo- metry of the thrust sheets in sufficient detail to allow calculations of their displacements and cross-section- al areas (Plates 1, 2). The method of balancing neces- sitates that the thrust sheet closest to the foreland is the first to be restored to its pre-deformational posi- tion. Therefore, the balancing works backwards from the distal to the proximal deformation area, and con- sequently the annotation of the thrust sheets begins with the first thrust sheet restored. In the balanced cross-section of the Rubjerg Knude Glaciotectonic Complex, the thrust sheets are additionally annotated according to that part of the cliff in which they occur: two capital letters refer to the name of the section and a number refers to its position from leading edge to trailing end of the section. Thus, KR01 is the thrust sheet nearest to the foreland in the Kramrende Sec- tion. A thrust fault is referred to according to the thrust sheet it displaces. However, the trailing footwall ramp is referred to the annotation of the footwall block, which underlies the hanging-wall ramp/flat of the thrust sheet displaced over it. Thus the KR02 hang- ing-wall ramp is displaced up along the KR01 foot- wall ramp. Although one of the basic conditions in construct- ing balanced sections is the preservation of volumes, which in the areas strongly affected by mud remobili- 27 sation and diapirism is difficult to maintain, the exer- cise has been carried out to match a balanced section to the mapped and interpreted thrust-fault framework. So despite the uncertainties and the demand for inter- pretation of the geometry and magnitude of eroded thrust sheet tapers, the construction of the balanced section added significantly to the understanding of the duplex framework (Plate 2B). In the Rubjerg Knude cross-section (Plate 1), the displacement is measured and estimated mainly from the distance between the intersection of the L/R-un- conformity (the unconformity between the Lønstrup Klint and Rubjerg Knude Formations, see below) and the footwall ramp, and the intersection of the L/R- unconformity and the hanging-wall ramp (Fig. 11). How- ever, the tips of the thrust sheets are generally eroded away, so the first approximation is from the L/R-un- conformity footwall point to the point where the hang- ing-wall ramp is truncated by the glaciotectonic un- conformity at the top of the cliff. The second approx- imation is the addition of the distance estimated from the size of the tip eroded away. This estimate is based on a simple geometric construction of the tip-triangle from the dips of the hanging-wall ramp and the L/R- unconformity, respectively (Fig. 11). This line balance is subsequently controlled by the width of the piggy- back basin more or less corresponding to the upper footwall flat. All the measured displacements are strictly restricted to the minimum distance to avoid unrealis- tic exaggerations. Therefore the actual displacements might be slightly greater. The area balance is based on a calculation of all the areas annotated in Plate 2. The computer-supported calculation was carried out with the ARC-INFO pro- gram, and the calculations of the areas in the bal- anced cross-section and the ramp cross-section devi- ate by less than 10% (Plate 2A, B). This is regarded as a reasonable correspondence considering the various sources of error (Table 1). In general, the sections have a smaller area in the ramp cross-section (Plate 2B) due to the erosion of areas above the main head- of-cliff unconformity, and in most sections the number of areas is higher due to the increased complexity of the geometry in the reconstructed structural cross-sec- tion (Plate 2B). Geological setting The Rubjerg Knude Glaciotectonic Complex incorpo- rates deformed sedimentary deposits that belong to the upper part of the mainly marine succession known previously as the Skærumhede series (Jessen et al. 1910). This succession was deposited in the northern part of the Danish Basin in the late Pleistocene, after the late Saalian terrestrial glaciation retreated from Den- mark (Houmark-Nielsen 1987, 1999; Knudsen 1994). The major source area for deposits in this part of the Danish Basin is the Scandinavian basement in south- ern Norway and central Sweden, that comprises Pre- cambrian Fennoscandian granites and gneisses over- lain by Palaeozoic metasediments, including Permian volcanics and their related intrusive magmatic rocks of the Oslo province (Oftedahl 1981). The extrabasi- nal indicator boulders reflect these source areas, which were situated between the centres of ice-cap nucleation and the depositional basin (Milthers 1909; Smed 1995). The boundary between the northern part of the Danish Basin and the south-western part of the ele- vated Scandinavian basement is covered by the Ska- gerrak, the sea covering a deep depression (about 500 m deep) known as the Norwegian Channel (Sejrup et al. 1987, 1994, 1998). One of the important discussions concerning the glaciation of Denmark during the last stadial focuses on how the ice from Norway advanced across the Skagerrak about 30 000 years ago. The prob- lem involves the dynamics of the ice stream along the southern coast of Norway, the so-called Norwegian Channel Ice Stream, and the interaction between the marine and terrestrial parts of the ice cap in south- west Norway (Larsen et al. 2000). Associated prob- lems include the filling of the deep trench in Skager- rak, and the termination of marine conditions in Ska- gerrak, Vendsyssel, and the northern North Sea as well as the Kattegat (for locations, see Fig. 12). The marine environment referred to as the Older Yoldia Sea, which extended into the Vendsyssel re- gion, formed in the Late Saalian, and the climatic change from a mild climate in the Eemian to a glacial 28 AB 58˚N 58˚N 56˚N 56˚N Skagen Hirtshals Frederikshavn Mols Djursland Samsø Hven Hundested Glumsløv Ristinge Klint Bovbjerg Jylland Jylland Læsø Anholt Langeland 0 100 km50 NORWAY SWEDEN Limfjorden Skagerrak North Sea Skagerrak Kattegat Kattegat Vendsyssel Mors GERMANYFig. 13 Lodbjerg Vendsyssel SD KT MSL M SL Lim fj or d en I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I DENMARK 12˚E 12˚E Lønstrup Lim fjorden Jæren N orwegia n Tren ch Fyn Sjælland Skåne Lønstrup Klint 29 climate in the Weichselian is recorded in a series of wells drilled in north Jylland and the Kattegat region (Knudsen & Lykke-Andersen 1982; Lykke-Andersen 1987; Lykke-Andersen & Knudsen 1991; Knudsen 1994). Towards the end of the Middle Weichselian the Scandinavian Ice Sheet over southern Norway built up. The ice streams were drained from a main spillway in Oslo Fjord moving out through the Norwegian Chan- nel along the coastline of southern Norway (Larsen et al. 2000). A change in the dynamics of the Scandina- vian Ice Sheet over southern Norway forced the gla- ciers to progress south-westward across the Norwe- gian Channel. The ice advanced into the northern North Sea, where a glacial cover was established about 29 000 years B.P. and lasted until 22 000 years B.P., when the first recurrence of marine conditions (the ‘Young Yoldia Sea’) was recorded (Sejrup et al. 1994, 2000). This glacial coverage was probably closely con- nected with the fall in sea level, amounting to 120 m below present sea level (Fairbanks 1989; Bard et al . 1993), which could have hampered the active drain- age of the Norwegian Channel Ice Stream. The ice spread southward over the Skagerrak causing the Kat- tegat basin to be dammed by the ice margin and ter- restrial areas to be established in the central part of the North Sea (Sadolin et al. 1997; Houmark-Nielsen 1999). As a consequence, the Kattegat–Skagerrak re- gion began to dry up due to the general sea-level fall; this is reflected in the progression from arctic marine conditions in the Skærumhede series to brackish and glaciolacustrine environments. This change took place at about 32 000 years B.P. (Table 2), and may have been accentuated by the addition of meltwater from the advancing Norwegian Ice (Jessen 1918; Sadolin et al. 1997). The dramatic drainage of the lake basin in the Kat- tegat towards the North Sea is recorded by a signifi- cant erosional unconformity in the sedimentary suc- cession at Lønstrup Klint (the L/R-unconformity), dat- ed as close to 29 000 years B.P. (Sadolin et al. 1997). Shortly afterwards, the basin was once again dammed and shallow lacustrine and fluvial environments were established while proglacial thrust faulting was initi- ated reflecting the relatively fast advance of the ice margin (Sadolin et al. 1997). The thin-skinned thrust faulting in the Rubjerg Knude Glaciotectonic Com- plex involved an accretionary wedge extending more than 12 km to the south in front of the advancing ice margin. The lowermost décollement level was situat- ed in the marine clays of the Older Yoldia Sea. After a compression of about 50%, the glaciotectonic complex was formed (Pedersen 1987) leaving a large part of the area between Lønstrup and Hirtshals as a depres- sion corresponding to the ‘hole’ and the Rubjerg Knu- de Glaciotectonic Complex to the ‘hill’, in a ‘hill-and- hole’ pair in the sense of Aber et al. (1989). Subse- quently the Norwegian Ice truncated the glaciotec- tonic complex and the deposition of the Kattegat Till Formation concealed its structures. The Norwegian Ice advanced down to a stationary line (Figs 1, 12) cross- ing central Denmark from west to east, whose posi- tion is inferred from the distribution of the Kattegat Till Formation (Houmark-Nielsen 1987, 1999, 2003; Pedersen & Petersen 1997). After its termination at the stationary line (Figs 1, 12), the Norwegian Ice melted back. It was succeeded by the main south-west ice advance of the Scandina- vian Ice Sheet, which extended out to the Main Sta- tionary Line (Ussing 1903; Houmark-Nielsen 1987, 2003; Pedersen et al. 1988). In northern Jylland, the isostatic depression due to the loading of the ice sheet was substantial. The termination of the glaciation in Denmark thus resulted in interference between eus- tatic sea-level rise and isostatic rebound with a com- plex depositional development during the re-estab- lishment of the Younger Yoldia Sea in the Skagerrak– Vendsyssel–Kattegat region about 17 000 years ago. This may be summarised as a forced regression under progressively falling sea level due to the isostatic rise of the Vendsyssel region (Richard 1996). The Venne- bjerg and Rubjerg Knude hilly islands probably formed part of a larger island archipelago extending out into the North Sea. Terrestrial conditions were established at the end of the Weichselian. At Nørre Lyngby (Fig. 13), a de- pression was formed above a neotectonic fault zone that predated Older Dryas time (Lykke-Andersen 1992). In this depression, lacustrine gyttja and fluvial sand of Older Dryas and Allerød age were deposited; a large number of mammalian remains have been found in these deposits indicating an arctic to sub-arctic rein- Facing page: Fig. 12. Location map. Map (A) shows the main part of the Danish Basin with the surrounding land areas. SDKT is the position of the stationary line for the Norwegian Ice Advance (SDKT is an abbreviation of southern distribution of Kattegat Till Fm). MSL is the Main Stationary Line for the Scandinavian Ice Sheet at the glacial maximum in the Late Weichselian. Map (B) gives the position of relevant geographical localities in Denmark as well as the location of Fig. 13, the geological map of Vendsyssel. 30 deer steppe also populated by hunters (Jessen & Nor- dmann 1915; Aaris-Sørensen 1995). During Holocene time, the Vendsyssel region was affected by isostatic rebound (Mertz 1924). At Løn- strup Klint, this resulted in a 25 m elevation of the heterolithic sediments of the Younger Yoldia Sea. Bogs developed in the depressions on the glacial peneplain at the end of the Stone Age and the beginning of the Bronze Age (Jessen 1918). Up to 1.5 m of peat accu- mulated; when this is exposed in the clif f surface and blocks of peat fall down onto the beach, the peat is locally called martørv (sea-peat). The locality names Martørv Bakker (sea-peat hill) and Moserende (bog- gully) refer to these deposits. The geomorphology of the cliff is strongly influ- enced by the thrust-fault structures. The clayey parts of the thrust sheets form ridges that form projections along the coast between gullies that are eroded out in the sandy parts (Schou 1949). Springs typically well out at the surface between the clayey and sandy units and more incised gullies (render in Danish) are formed where the drainage is concentrated. Although the lo- cation of gullies and the clif f line have retreated about 100 m since A. Jessen constructed the first cross-sec- tion of Lønstrup Klint, it has been possible to retain his names in the present cross-section (Plate 1). The general erosion rate of the cliff is about 1.3–1.5 m per year (Jessen 1918; Pedersen 1986b). Landslides occur very frequently, particularly at sites where mud dia- pirs are located in the cliff section. Where glaciofluvi- al deposits dominate the cliff section, there is a marked tendency for aeolian dunes to accumulate on top of the cliff (Pedersen 1986b). Wind action on the mo- raine plateau on top of the cliff has eroded the fine- grained material away from the till deposits, leaving a stone pavement as the residual trace of the glacially truncated surface. Aeolian sand migration intensified about 300–400 years ago (Jessen 1918), one of the consequences being the burial and abandonment of the Old Rubjerg Church. The high aeolian dunes on top of Rubjerg Knude have accumulated during the last 100 years. The Rubjerg Knude lighthouse was built in 1900 (Bendsen 1981) when dunes were less than 10 m high. Today the tops of the dunes are close to 100 m above sea level corre- sponding to a vertical dune accumulation of nearly 50 m. The present-day steep nature of the dunes was probably stimulated by the artificial dune protection fences. However, the steady erosion of the cliff indi- cates that the lighthouse will fall into the sea about ten years from now. Table 2. Radiocarbon dates, Rubjerg Knude and Lønstrup Klint, Vendsyssel, northern Denmark Stratigraphic unit Vendsyssel Fm Vendsyssel Fm Vendsyssel Fm Rubjerg Knude Fm Rubjerg Knude Fm Lønstrup Klint Fm Stortorn Fm Stortorn Fm Stortorn Fm Locality Lønstrup Klint Lønstrup Klint Lønstrup Klint Sandrende Lønstrup Klint Sandrende Ribjerg Mårup Kirke Stortorn Lab. ID no. K-858 K-2670 AAR-2134 AAR-2265 AAR-4066 Ua-4454 AAR-4067 AAR-4068 AAR-4069 Material Mollusc Mollusc Mollusc Plant Mollusc Moss Mollusc Mollusc Mollusc 14C age ka B.P. 13.9 ± 0.2 14.7 ± 0.2 14.5 ± 0.2 30.9 ± 0.5 43.0 ± 1.3 29.2 ± 1.4 29.6 ± 0.4 30.9 ± 0.4 31.3 ± 0.4 Calib. age ka B.P.* 16 ± 1 17 ± 1 17 ± 1 33 ± 1 46 ± 3 32 ± 1 33 ± 1 34 ± 1 34 ± 1 13C‰ PDB+ 0.6 –27.3 3.3 –29.1 1.5 1.7 1.3 Ref.‡ (1) (2) (3) (4) (5) (4) (5) (5) (5) * Calibrated ages are calculated according to Bard et al. 1993 and Kitagawa & van der Plicht (1998). + Relative to PDB standard.‡ References: 1: Krog & Tauber (1974); 2: Knudsen (1978); 3: Richardt (1996); 4: Houmark-Nielsen et al. (1996); 5: this study. 31 Holocene Vendsyssel Formation Glacigene formations Upper Cretaceous chalk 20 km5 10 150 10˚30’10˚00’ Nørre Lyngby Mårup Løkken Rubjerg Knude Skærumhede Skagen Frederikshavn Hirtshals Kattegat Skagerrak Location of well Lø ns tr up K lin t Lønstrup Sandrende Vennebjerg DGU No. 1.287 DGU No. 10.4 DGU No. 8.137 Stortorn 57˚30’ DGU No. 8.137 Fig. 13. Geological map of Vendsyssel showing the location of three wells referred to in the text. 32 H ol oc en e M id dl e W ei ch se lia n La te W ei ch se lia n Sk æ ru m he de G ro up 11.5 19–15 27–23 30–28 32–30 35–32 Fo rm at io n U nc on fo rm ity Li th ol og y C la y Si lt Pe bb le Sa nd C ob bl e Fo ss ils St ru ct ur es an d gr ai n si ze s D ir ec tio na l el em en ts T hi ck ne ss A ge k a B. P. (c al en da r ye ar s) 30 20 10 0 m Recent aeolian dunes Main cliff-section unconformity Nr. Lyngby Beds Vendsyssel Formation Mid Danish Till Formation Ribjerg Formation Blå-unconformityeu Kattegat Till Formation Rubjerg Knude Formation eu L/R-unconformity Lønstrup Klint Formation Stortorn Formation gu gu eu eu Clay Silty mud Dropstones in clay and silty mud Sandy mud Sand Till Gravel Plant fossil dated Shell dated Palaeocurrent Direction of ice movement Erosional unconformity Glaciotectonic unconformity f. m. c. eu gu Peat and gyttja