Terrace scarp deflation as a renewable source for eolian sediments in an arctic periglacial setting K E E N E S W E l T A N D KEITH M A N N Swett K. & Mann K., 1986: Terrace scarp deflation as a renewable source for eolian sediments in an arctic periglacial setting. Polar Research 5 n . s . . 45-52. Glacial, glaciofluvial, and glaciolacustrine sediments deposited by the retreating Vibekes Glacier are being actively reworked into sand dunes and loess sheets o n the tops of glaciofluvial terraces down- valley from Vibekes Glacier and Vihekes S 0 . Active permafrost precludes trenching below 0.5 to I .0 meters. However, sedimentary structures, deflationary and abrasion features in shallow and surface deposits are visible. Although an armored pavement inhibits sediment deflation on the horizontal terrace surfaces, a combination of fluvial erosion, mass-wasting, and eolian processes o n terrace scarps provides a continuing source of sand- and silt-size materials for redeposition. Keene Swert and Keith Mann. Department of Geology, The Universiry of Iowa. Iowa Cit-v. Iowa, U.S.A. Februar), I986 (revised September 1986). Introduction Periglacial environments have frequently been cited as contributory or pivotal to the generation of Pleistocene and older eolian deposits (Tuck 1938; Cailleux 1942; Fristrop 1952; Pewe 1955; Smalley 1966; Selby, Rains & Palmer 1974; Nies- sen et al. 1984). However, few studies have docu- mented modem eolian processes and their sedi- ments actively forming in periglacial environ- ments. Although small in areal extent ( < 10 km2), the accumulating sand dunes and loess deposits on the terraces of Vibekes Elv (river) provide a modem example of eolian processes and their deposits. The sand and loess deposits described here are accumulating 10-20 km southeast of the present front of Vibekes Glacier in northern Hudson Land in central East Green- land at 74'5" and 23"W (Fig. 1). Vibekes S0 (lake) is adjacent to the terminus of Vibekes Glacier. Vibekes Elv flows from the lake and courses down valley through glacial, glacio- fluvial, and glaciolacustrine deposits. Numerous terraces occur at various heights on both sides of the river with the uppermost terrace being 150-200 m above the present river level. The terraces are marked by their unusually flat upper surfaces (Fig. 2). Cowie & Adams (1957) briefly described these terraces. Approximately 25 km downstream from the outlet of Vibekes S0, Vibe- kes Elv converges with a valley occupied by Wordies Glacier. Northwesterly winds coming down-valley off the Greenland ice sheet pre- dominate, although we occasionally observed up-valley southeasterly winds during our two week stay in the valley. Description of the terraces In this relatively short valley (25 km), Vibekes Elv has entrenched 150 to 200 m through glacial, glaciofluvial, and glaciolacustrine deposits. The glacial, glaciofluvial, and glaciolacustrine sedi- ments that form the terraces consist of sorted and poorly sorted clay- to boulder-size materials. These sediments are mostly unconsolidated, al- though there is slight competency to the glacio- lacustrine silt beds. The cobble- to boulder-sized material in these terraces is well to very well rounded. Approximately 35 km2 of unconsoli- dated sediments at the angle of repose ( = 30") are exposed along the terrace scarps. (Calcula- ted, assuming: (1) an approximate average relief of 175 m between the valley floor and the top of the upper prominent terrace, and (2) approxima- tely 100 km of terrace scarp surfaces along the meandering path of Vibekes Elv and its tributari- es.) Description of the eolian deposits Two genetically distinct types of eolian deposits 46 -----, - / HUDSON LAND 24" 23" Fig. /. Map of a portion of central East Greenland showing geographic relations of Hudson Land, the Vibekes Glacier. Vibekes So (lake), Vibekes Elv (river) a n d Wordies Glacier. Insert shows locality of area in East Green- land. exist within the Vibekes Elv valley: wind- shadow, 'cliff dunes and vegetative-baffled loess sheets. The cliff dunes are visually more prominent, but the loess sheets are probably more significant volumetrically and areally. Hobbs (1931) also observed these two types of deposits in southeast Greenland. The cliff dunes range i n size from a single large longitudinal dune nearly 0.75 km in length, 100 m wide and 20 m high (Fig. 3) to minor sand 'drifts' behind vegetated tufts (Fig. 4), or behind large boulders and cobbles. The larger dunes, restricted to terrace margins adjacent to the angle of repose scarps. appear to result from vertical eddies formed where the wind currents transport sand up the scarps shear at the horizontal terrace tops. Sedimentary structures of the sand dunes in- clude large-scale cross-bedding (larger than the access of o u r permafrost-limited excavations), ripples (Figs. 5 and 6) and surface scours (Fig. 7 ) . Two surprises emerged from the excavations made to observe internal sedimentary structures in the dunes: ( 1 ) cross-bedding on the up-wind, erosional side of the dunes (except for a thin veneer < 1 cm) was inclined at an angle consis- tent with the slip face on the lee side of the dune (Fig. 8), and (2) the existence of a - 25 cm thick, tabular bed of snow lying 18 cm beneath the surface, but above the permafrost surface on the lee side of the largest dune (Fig. 9). The volume and distribution of snow and ice layers deeper within the dune is an interesting subject for spe- culation, but further investigation would require equipment more persuasive than our folding shovels to excavate through the permafrost. Slump structures on the surface of many dunes may result from melting or sublimation of such snow or ice layers within the dunes. This process should be considered as a potential cause of erra- tic internal structures within periglacial and per- haps northern temperate zone dune deposits. Similar snow layers a n d associated slumping structures have been observed elsewhere in 47 Fig. 2. Photo looking northwest up the Vibekes Elv valley showing the prominent and flat upper terrace level and the angle of repose scarps to present river level. Here the terrace is approximately 150 meters above river level. Some bedding within the fluvial terrace gravels can also be seen. fig. 4. View of small sand ‘drifts’ to the lee of vegetated tufts. Entrenching tool handle is 50 cm long. Arrow indicates principal wind direction. modern sand dunes (Selby, Rains & Palmer 1974; Ahlbrandt & Andrews 1978; Ahlbrandt & Fryberger 1982), but perhaps have been over- looked as the cause of contorted or slumped bedding structures in ancient eolian deposits. Size-frequency distributions over the surface of the sand dunes are so variable that their analy- ses are judged to be non-significant. Of signifi- cance, however, are observations of the grain shapes on the dune surface. The sands were sepa- rated into 1/2 Phi class intervals, and the only grains on the stoss or lee sides of the dunes to show any degree of rounding were those larger Fig. 3. View across the upper terrace in the Vibekes Elv valley looking to the southwest. In the distance i s the largest of the observed dunes, approximately 20 m high, that sits on the edge of the erosional bank of the terrace. Closer to the photographer is a smaller longitu- dinal dune 2-3 rn high. than 0.125 mm; these were subangular. All o f t h e grains smaller than 0.125 mm were angular to very angular. This may reflect the relatively short distance of fluvial and eolian transport and hence opportunity for abrasion, or may merely reflect the lesser susceptibility of smaller grains t o abrasion (Kuenen 1960). The loess sheets, comprised principally of silt-size detritus, are mainly deposits trapped by tufted vegetation (species of Dlyas and Silene constitute the major vegetative tuft builders o r ‘bunch plants’). Tufted vegetative surfaces (Fig. 10) occupy a consider- able portion of the upper terraces and thus must trap volumes of loess far exceeding the sediment volume of the cliff dunes, albeit in a less con- spicuous fashion. At one locality where a tribu- tary stream dissected a loess deposit we observed 1 to 1.5 m of structureless loess, although some mottling and irregular fermginous o r organic staining boundaries were apparent within the loess (Fig. 1 I ) . We also observed lemming bur- rows on the surface, but did not see evidence of burrowing exposed in the cuts. Surficial tracks of musk oxen, fox, and birds abound, but preser- vation of these structures would be an unlikely event. Wind blown sand often fills the polygonal desiccation cracks that commonly exist between vegetative tufts. Surficial mud cracks are also present in several non-vegetated areas on the terrace surface, particularly adjacent t o the small ponds. 48 Figs. 5 and 6. Views of dune surface showing ripple marks of two different sizes. Entrenching tool handle is SO cm in each photo. Fig. 7. A granitic cobble o n the edge of the dune with a wind scoured excavation on the stoss (right) side. Scale beneath the scour structure in the photo is 3.5 cm wide. The granitic cobble is polished and incipiently fluted by sand abrasion. I t also exhibits minor exfolia- tion on its upper surface, perhaps due to feldspar expansion during weathering. Arrow indicates princip- al wind direction, Less significant are deposits of silt-sized mate- rials trapped in the small ponds that have formed either as kettle lakes or as abandoned channels on the terrace surfaces. That this sediment is actively accumulating was confirmed by the ease with which the fine sediment was stirred into suspension during a brief excursion into one of the ponds t o bathe. Deflation and abrasion features A deflated and abraided armored pavement (Fig. 12) veneers virtually all of the terrace surfaces that are not covered by low vegetation (comm- only clumped as 'bunch plants') or covered by water (the small ponds in abandoned channels or kettles). In a similar environment in southeast Greenland, Hobbs (1942) reported the deflation of dust, sand and smaller pebbles from outwash plains to form an armor of pebble pavement that protected the materials below from further defla- tion. In areas of active eolian sand transport, the pebbles, cobbles, and boulders exhibit highly polished surfaces and often show preferential removal of weathered rinds on the windward (generally up-valley) side (Fig. 7). Many of the rounded cobbles and boulders on the terrace surface show in situ frost shattering (Fig. 13) that produces modifications of clast shapes on the surface. Examination of cobbles and boulders buried within the terraces indicates that the frost shattering occurs only on the upper surface of the terraces and not within the buried gravels. Facet- ing, fluting, and polishing of the pebbles, cobb- les, and boulders on the armored pavement are common and well developed. Limestone clasts exhibit the highest degree of faceting and fluting, but even granite (Fig. 14) and quartzitic cobbles show discernible fluting. We observed multi- faceted wind-polished and fluted cobbles. It is unclear whether eolian abrasion generated the facets, or if merely eolian abrasion of fracture- formed facets produced them. An interesting type of eolian faceting, though of dubious geological significance, occurs where clumped vegetation (bunch plants) has trapped eolian sediments. Erosion of these clumps has produced 'facets' on their stoss sides much like the cobbles and boulders elsewhere (Fig. 15). Geomorphic processes and hypotheses of dune formation in the Vibekes Elv valley Although Vibekes Elv presently flows along the southwest flank of Wordies Glacier to the fjord 49 Fig. 8. Cross-bedding in the slip-face of a dune that is parallel or sub-parallel to the surface of the dune. Painted dark and white intervals on the monopod are 10 cm each. Fig. 10. View of irregular tufted vegetation on the upper terrace. The ‘bunch plants’ serve as an effective baffle to trap silt-sized sediment over large areas of the terrace. Individual tufts are approximately 10 cm high. Fig. 9. Excavation into the slip face of the largest dune that exposes a tabular snow layer 25 cm thick beneath I8 cm of sand. Bedding structures within the snow as well as the sand indicate that both the snow and the sand were prograded by eolian deposition on the lee side of the dune. Thin sand laminae within the snow further suggest that at the time the snow was being added to the dune, the snow cover was probably relati- vely thin so that both snow and sand surfaces were being deflated. Fig. 11. Scraping of an erosional bank in the loess reveals faint irregular and cross-cutting ?Liesegang color mottling. Some of the organic or ferruginous stains cross-cut faint bedding planes. at Wordies Bugt, the terraces and lacustrine sedi- ments observed in the lower Vibekes Elv valley offer convincing evidence that Vibekes Elv was, at one time, dammed by the Wordies Glacier to form a lake. This lake not only caused deposition of the glaciolacustrine sediments seen in the lower portions of the valley, but persistent lake levels, determined by the Wordies glacial dam, were also apparently the base level control for the prominent glaciofluvial terraces throughout the valley. Ephemeral standstills and periodic lowering of the Wordies glacial dam appear to have produced the prominent upper terrace and several minor terrace levels (Fig. 16). Thick glacial, glaciofluvial, glaciolacustrine and tributary fan deposits composing the broad terraces in Vibekes Elv constitute the immediate provenance for the modern eolian deposits. The ultimate bedrock source of the sediments is glacial and fluvial erosion of a diverse suite of igneous, 50 metamorphic and sedimentary rocks of Archean, Proterozoic, and Phanerozoic age. Schists, slates, phyllites, granites, gabbros, dolerites, sand- stones, conglomerates, cherts, limestones, and dolostones are all prominent lithoclast types in the cobble- to boulder-sized detritus of the terraces. Katabatic winds that cascade southeastward down from the Greenland ice sheet (Fig. 17) are reworking the finer-grained terrace sediments into cliff dunes and loess sheets. The cliff dunes appear to result from both vertical and horizontal wind eddies with steady progradation and pro- gressive thickening of the eolian sand deposits through time. The derivation of eolian sediments from poorly sorted glacial and periglacial precursor sedi- ments poses an enigma. The eolian deposits or a densely packed lag deposit of pebbles, cobbles and boulders (i.e. an armored surface) veneer the upper terrace surfaces. Therefore, except for reworking, the upper surfaces of the terraces cannot now be providing a sediment source for the eolian sand and loess. I n this setting, modern deflation processes acting on the terrace gravels would seem t o be self-inhibiting because of the armored pavement. Periodic advance and retreat of the glaciers might expose fresh surfaces of unwinnowed materials, but would readily erode the unconsoli- dated materials in the valley and, almost certain- ly, would rework and modify the eolian deposits beyond recognition. The outlet of Vibekes SB appears to be morainally dammed with no discernible outwash that might serve as a source of sand and/or loess. Direct observations of the processes currently operating in the valley of Vibekes Elv, however, offer fresh insight into a renewable source of deflatable sand- and silt- sized sediments. During our stay in the valley, the suspended air-borne sediment was often so thick as to obscure visibility, and near the terrace scarps the saltating sand was intolerable. Of tangential interest during these dust storms was that, if the day was sunny, one could observe the reflections of light from the cleavage planes of fine micas (mainly muscovite) that were suspended in the air. Fluvial erosion of the thick gravel deposits by the present course of the Vibekes Elv has produced vast exposures of angle of repose gravel slopes adjacent to the river. As wind deflates the finer particles (sand, silt, and clay) from these angle of repose surfaces, the pebbles, cobbles, and boulders roll downslope and accumulate at the base of the slope rather than remaining as a lag on the steeply inclined surfaces. Cowie & Adams ( l 9 5 7 ) , referring to material cascading down these slopes, stated that it includes ‘large boul- ders which, in the spring, can bowl at speed across the frozen river to the diversion of those who use it for sledging’. To an observer standing on the terrace cliff edge during the strong kata- batic wind storms there is little doubt that sand-, granule-, and pebble-sized materials are being transported up the scarp faces and onto the upper terrace. Meandering and lateral cutting by the river at its erosional level remove the coarse clasts from the bases of the slopes, and the pro- cess continues to slowly broaden the valley floor. Thus, the combined processes of lateral cutting by the river and mass wasting of the coarse detri- tus down the angle of repose slopes generate renewable a n d abundant sources of deflatable detritus to form the eolian deposits of Vibekes Elv. Summary and conclusions I n the periglacial valley of Vibekes Elv i n northern Hudson Land, central East Greenland, retreat of the Vibekes Glacier, probably during the last 6000 years, coupled with damming of the valley by a convergent glacier has left thick depo- sits of glaciofluvial and glaciolacustrine sedi- ments as prominent terraces on either side of the valley. Deposition and subsequent incision of these terraces by the river have produced vast areas of angle of repose slopes that are subject to deflation by present day katabatic winds. The deflated material derived from these scarps is being redeposited both as sand dunes and as vegetative-bound loess deposits on the tops of the terraces. The observed complimentary mechanisms of fluvial erosion, mass wasting, and deflationary processes acting on these scarps provide a constantly renewable source of fine sediment. The horizontal surfaces of the terraces are inhibited from deflation by the armored pavements. 5 1 Fig. 12. Surface of the terrace in a deflated area show- ing the coarse desert pavement of mixed lithologies. Fluting and polishing of the pebbles and cobbles are apparent. Scale at lower right is 3.5 cm wide. Fig. 15. Eolian faceting of a vegetative tuft. The consis- tent orientation of these ‘facets’ indicates that they are the product of eolian abrasion. Height of tuft is approximately 15 cm. Arrow indicates principal wind direction. Fig. 13. Photo of a partly buried dolomitic siltstone on Fig. 16. View across Vibekes Elv showing the multiple the upper terrace surface showing in situ frost shatter- erosional terrace levels formed during ephemeral ing of the cobble into flakes parallel to the bedding. ‘stand-stills’ in the lowering of the glacial dam at the Scale at upper left is divided into cm. confluence of the Vibekes Elv and the Wordies Glacier. Fig. 14. A partly buried granitic cobble on the Fig. 17. Photo across the Vibekes Elv valley on a mildly deflation surface of the upper terrace showing fluting windy day showing the wind-born sand and silt moving of the stoss side by eolian abrasion. Scale is 17 cm long. from right to left down-valley (southeastward). At Arrow indicates principal wind direction. times the sediment cloud reached heights of several hundred meters. Arrow indicates principal wind direc- tion. 5 2 The integration of c o m p l e m e n t a r y erosional and mass-wasting processes t o p r o v i d e a mechanism for g e n e r a t i n g continually d e f l a t a b l e s e d i m e n t s merits c o n s i d e r a t i o n as a n e x p l a n a t i - on for o t h e r modem, Pleistocene, or o l d e r peri- glacial eolian deposits. Acknowledgements. - The observations recorded here were made during an expedition to East Greenland supported by N.S.F. Grant DPP-84-41584. We appre- ciate the careful editorial reviews of the paper by Timothy Kemmis. Sherwood Tuttle, and Ian Smalley. References Ahlhrandt, T. S. 8( A n d r e w . S . 1978: Distinctive sedi- mentary features of cold climate eolian deposits, North Park, Colorado. Paleogeographr. Paleoclima- ro1og.r. Palt=oecologjZS.327--35 1. Ahlbrandt, T. S. & Fryberger. S. G. 1982: Introduction to eolian deposits. Pp. I 1 --47 in Scholle. P. A. & Spearing, D. (eds.): Sandstone Depositional Environ- nienfs. .4merican .4ssociation of Petroleum Geologists. Mrmoir 31. Cailleux, A. 1942: Les actions eolians periglaciaires en Europe. Societe Geologique France, Memoire 46. 176 PP. Cowie, J . W. & Adams, P. J. 1957: The geology of Cambro-Ordovician rocks of central East Green- land. Meddr. Grenland 153. Fristrop, B. 1952: Wind erosion within the arctic deserts. Geogr. Tidsskr. SZ. 5 1 -65. Hohbs, W. H. 193 1 : Loess, peddle bands and boulders from glacial outwash of the Greenland continental glacier. JoumalqfCeology 39.381 -385. Hobbs. W. H. 1942: Wind-the dominant transporta- tion agent within extramarginal zones to continental glaciers. JournaloJ'Geologp SO. 556-559. Kuenen, P. H. 1960: Experimental abrasion 4: Eolian Action. Journalof Geology 68,427-449. Niessen, A. C. H. M.. Koster, E. A. & Galloway, J. P. 1984: Penglacial sand dunes and eolian sand sheets: an annotated bibliography. Department of the Inferior. U . S . Geological Survqr openlfile report. 84- 167. Pewe. T. L. 1955: Origin of upland silt near Fairbanks, Alaska. Geol. Soc. ofAmerica Bull. 66,699-724. Selby. M., Rains, R. B. & Palmer, W. P. 1974: Eolian deposits of ice-free Victoria Valley, Antarctica. New Zealand Journal of Geology and GeophJric.7 17. 543-562. Smalley, I. J. 1966: The properties of glacial loess and the formation of loess deposits. Journal oJSediment- a n . Petrology 36,669-676. Tuck, R. 1938: The loess of the Matanuska Valley, Alaska. Journalof Geology 46.641 -653.