Geological Survey of Denmark and Greenland Bulletin 20, 2010, 23–26 23 The distribution of sand in deltas depends on the delta re- gime: wave, fluvial or tidal-dominated delta (Orton & Read- ing 1993; Bhattacharya & Giosan 2003). During the Early Miocene, three delta complexes built out from the Fenno- scandian Shield into the eastern North Sea Basin (Rasmus- sen 2004). The oldest delta complex, which is informally named the Billund delta, is located in Jylland (Fig. 1). This delta complex was mainly wave-dominated (Rasmussen & Dybkjær 2005; Hansen & Rasmussen 2008; Rasmussen 2009a). Recently, it has been demonstrated that in modern wave-dominated delta environments sand mostly accumu- lates on the updrift portion of the delta (Fig. 2) whereas alter- nating mud and sand, e.g. barrier-lagoon complexes, occupy the downdrift portion of the delta system (Bhattacharya & Giosan 2003). The current study shows that most of the sand in the submarine part of the Miocene wave-dominated Bil- lund delta (mainly lower shoreface and delta slope sand) was deposited downdrift to the delta front and thus differs from the foreshore and uppermost shoreface accumulation found in recent delta complexes. The aim of this study is to map the distribution of subma- rine delta sand in the Billund delta complex. A detailed un- derstanding of the distribution of delta sand, such as found in this delta complex, is crucial for developing predictive tools in sequence stratigraphy and for seismic interpretation, especially in the application of seismic attribute analysis of the subsurface. Geological setting The eastern North Sea Basin was subject to inversion in the Early Miocene (Rasmussen 2009b). The inversion tectonism resulted in high sediment input into the Norwegian–Danish Basin and progradation of delta complexes. During the Early Miocene, the deltas built far out into the basin (Rasmussen 2004) and were predominantly wave-dominated (Rasmus- sen & Dybkjær 2005; Fig. 1). The Middle – Late Miocene was characterised by deposition of marine, clayey sediments at water depths of more than 100 m (Rasmussen 2009a). At the end of the Late Miocene and during the Pliocene the shoreline prograded several times across Denmark (Rasmus- sen et al. 2008) and reached the central part of the North Sea both during the latest Late Miocene and Late Pliocene. In the Early Miocene, a warm temperate to subtropi- cal, humid climate prevailed (Larsson-Lindgreen 2009). The subtropical climate continued into the early Middle Miocene, but was succeeded by a marked climatic cooling in the middle Middle Miocene. Apart from an interval in the Early Pliocene the climate was relatively cool during the remaining part of the Neogene. During the Miocene and Pliocene, the region was located in the northern part of the zone of prevailing westerly winds, which led to a high wave energy regime in the eastern part of the North Sea Basin due to the long fetch across the North Sea (Galloway 2002; Ras- mussen et al. 2008). Distribution and grain size of sand in the Miocene wave-dominated Billund delta, Denmark Erik S. Rasmussen and Jens Bruun-Petersen Fig. 1. Palaeogeographical reconstruction of the Billund delta. The Bil- lund delta is located in the central part of Jylland. 50 km 56°N 8°E 10°E Sacalin island Upd rif t Downdrift Lo ng sh ore cu rr en t Black Sea Sãrãturile strandplain Danube: Sf. Gheorghe 10 km N Fig. 2. Example of a modern wave-dominated delta. Note the amalgama- tion of beach ridges on the updrift portion of the delta, and spits and barri- ers that enclose lagoons on the downdrift flank. Yellow: sand. Green-grey: mud. Modified from Bhattacharya & Giosan (2003). © GEUS, 2010. Geological Survey of Denmark and Greenland Bulletin 20, 23–26. Open Access: www.geus.dk/publications/bull 2424 Sand distribution in the Billund delta The development of the Billund delta complex was studied from high-resolution seismic data and borehole data. The delta complex was deposited during a period of a high rate of sediment supply (Rasmussen 2009a). Based on a number of boreholes drilled in central Jylland it is possible to reconstruct the Billund delta system in the area. The N–S-orientated correlation panel shows a series of sand-rich lobes prograd- ing across clay-rich successions (Fig. 3A). The gamma-ray log of the sand-rich units shows a serrated pattern, with gener- ally decreasing values upwards. The grain size is dominated by fine- to medium-grained sand with the latter dominating in the upper part of the succession. Coarse-grained sand and gravel occur in the uppermost part (Fig. 3A). At the north- ernmost borehole, the upper part is characterised by coarse- grained sand overlain by a succession of alternating fine- and medium-grained sand. The log pattern at this site is char- acterised by generally upward increasing gamma-ray values (Fig. 3A). The sand was deposited in clinoforms, with a dip of 7–10° according to seismic data (Hansen & Rasmussen 2008; Rasmussen 2009a). Locally, at the top of the clinofor- mal package, erosive features are seen with concave-upward structures that are filled with a succession showing transpar- ent or subparallel reflection patterns (e.g. Rasmussen 2009a). The updrift part of the delta complex is represented by the Løvlund and Grindsted boreholes (Fig. 3B). At these sites alternating sand- and mud-rich successions dominate. The 10–20 m thick sand-rich part is dominated by grey, fin- grained sand. Medium- to coarse-grained sand occurs in the upper part. In the westernmost borehole, at Grindsted, the lower part is dominated by 1–2 m thick sand-rich deposits intercalated in a mud-dominated succession. Large cuttings show that a substantial part of the succession at this site con- sists of alternating thin sand beds and clay layers. The sand beds are normally graded. The downdrift flank of the delta complex is characterised by grey, medium- to coarse-grained sand. Pebbles are com- mon and clasts with diameters up to 2 cm have been found. The sand-rich succession is 20–50 m thick (Fig. 3B), but seis- mic data indicate a thickness up to 75 m immediately north of the Billund well (Hansen & Rasmussen 2008). Thin mud layers have been found in a few samples, but mud is not a common lithology. Depositional environment A prograding depositional system is indicated by the sedi- mentary succession characterised by S–SW- dipping clino- forms and the general coarsening-upward trend seen in the boreholes (Fig. 3). The mud-dominated part of this prograd- ing system is dominated by marine palynomorphs (Dybkjær 2004). Their concentration decreases upward, indicating a shallowing-upward succession with increasing terrestrial in- fluence. The fining-upward succession found in the upper part of the Hammerum well is interpreted as fluvial chan- nel deposits (Rasmussen et al. 2006; Rasmussen 2009a). Fig. 3. Two correlation panels of the Billund Formation. A: N–S correlation panel showing a strike section of the Billund Formation. The length of the profile is c. 45 km. B: E–W correlation panel showing a cross section of the Billund Formation. The length of the profile is c. 20 km. GR: gamma-ray log. LOD: last occurrence datum. m b.s.: m below surface. A B Clay and clayey silt Fine-grained sand Medium-grained sand Coarse-grained sand Gravel LOD of Homotryblium spp. abundant Lithology in borehole Main boundaries Depositional environments LOD of D. phosphoritica common Sand (marine) Sand/gravel (continental) Clay (marine) Clay (lagoonal) Vejle Fjord Fm Vejle Fjord Fm Billund Fm 250 240 230 220 210 200 190 180 170 230 220 210 200 190 180 170 160 230 220 210 200 190 180 170 160 240 230 220 210 200 190 180 170 160 VandelBillundGrindsted Løvlund m b.s. GRm b.s. GR m b.s. GR m b.s. GR Billund Fm Billund Fm Vejle Fjord Fm Vejle Fjord Fm 240 230 220 210 200 190 180 170 160 150 220 210 200 190 180 170 160 150 140 130 120 180 170 160 150 140 130 120 210 200 190 180 170 160 150 140 130 Billund m b.s. GR Store Vorslunde m b.s. GR Fasterholt, Klyngholt m b.s. GR Hammerum m b.s. GR Addit Mb A B West East South North 25 A prograding system overlain by fluvial channels (Hansen & Rasmussen 2008; Rasmussen 2009a) indicates a deltaic depositional environment. The development of spits and barrier complexes south-east of the main area of prograda- tion implies a depositional system characterised by longshore transport of sediment (Rasmussen & Dybkjær 2005). The predominance of storm deposits with hummocky and swa- ley cross-stratification and other types of tempestites (Ras- mussen & Dybkjær 2005) indicates a wave-dominated delta front. In the modern wave-dominated Rhône Delta, sediment (sand) transport to the delta platform and prodelta slope oc- curs in two ways (Maillet et al. 2006). (1) During storms, erosion of the foreshore and upper shoreface leads to trans- port of sand partly to the outer delta platform and partly to the delta slope and (2) during floods, sand is transported in migrating dunes from the fluvial system towards the mouth bar. Sand accumulations at the slope break of the delta plat- form may destabilise the area by increasing the angle of the delta slope (the equilibrium profile). Slope failure may re- sult from the steepened slope or from changes in pore-water pressure due to wave action, resulting in deposits being shed directly down the delta front as mass-flow deposits. High sediment supply to the Billund delta occurred from the riv- ers (Rasmussen 2009a). The supply was probably dominated by bedload transport as indicated by the braided channels that dominated the fluvial system feeding the delta (Rasmus- sen et al. 2006). Therefore, migration of dunes towards the delta platform was more effective than in the modern Rhône Delta, where man-made constructions have significantly reduced the sediment influx to the delta. Direct sediment supply from the fluvial system to the Billund delta complex is indicated by the occurrence of large clasts, up 2 cm, in the lower part of the delta slope. Such large clasts have never been reported from Miocene shoreface deposits (e.g. Rasmussen & Dybkjær 2005) indicating that the hydro-dynamic condi- tions in the shoreface zone was unfavourable for transport of clasts of that size. The central and the downdrift parts of the Billund delta slope were thus dominated by sedimentation of medium- to coarse-grained sand with its source in the main river system. Failure at the mouth bar, spit and downdrift beach thus sourced sediments for deposition on the delta slope (Fig. 4). The deposition of graded, predominantly fine- grained, sand beds in the updrift part of the delta complex indicates sedimentation from suspended sand clouds gener- ated by storms or from diluted turbidity currents (Fig. 4). The finer-grained character here reflects that the source was the stacked beach ridges from the updrift flank of the delta. The sand in this part of the delta has been effectively sorted during transport along the shoreline before deposition and is therefore finer grained. For example, the barrier and spit systems of the Billund delta found 25 km south-east of the main delta consists of fine- to medium-grained sand with few intercalations of gravel (Rasmsussen & Dybkjær 2005). The clasts of the intercalated gravel layers do not exceed 5 mm. Discussion The distribution of sand in recent wave-dominated deltas is characterised by coherent sand accumulation, e.g. amalga- mated beach ridges in the updrift portion of the delta com- plex (Fig. 2; Bhattacharya & Giosan 2003). The downdrift Erosion of coastline during storms (mainly fine sand) Erosion of slope during storms (mainly coarse sand). Clay is resuspended and exported towards the shelf Failure due to instability of slope after a flood Direct fluvial influx of sand and gravel during floods Fluvial accretion of mouthbar during floods Fig. 4. Depositional model for a wave-domi- nated delta showing typical areas with erosion and deposition. At the river mouth fluvial sand accretes to the mouth bar, with high potential for slope failure. Direct influx of sediment onto the delta slope may occur during floods. In the downdrift portion of the delta front wave- reworked sand (relatively coarse-grained) forms spits and barriers that may be eroded during storms. On the updrift flank fine- to medium- grained sand is deposited as beach ridges which are a source of sediment supply to the delta slope. Consequently, finer-grained sand is deposited in this area. 2626 flank is commonly dominated by river-borne clay and sand deposited in lagoons protected by spits and barriers. A diffe- rent pattern of sand distribution is seen in the Billund delta where most of the sand was deposited in a downdrift position of the main delta (Fig. 4). This different depositional pat- tern can be explained by both the depositional environment and the geological setting. The Billund delta prograded into a basin with relatively deep water (c. 100 m) and the delta front was relatively steep (c. 7–10°; Hansen & Rasmussen 2008). Fluvially transported, coarse-grained sediments were at times shed directly down the delta slope as mass-flow de- posits (Fig. 4). The high-wave energy regime in the region and the high frequency of storms also enhanced sand trans- port downdrift of the main delta lobe and some of this was directed offshore beyond the delta slope break and deposited in deeper water (Fig. 4). The distribution of submarine sand in the downdrift setting is important, because foreshore and uppermost shoreface sediments are rarely preserved in the geological record. Therefore, in deltaic systems of the same character as the Billund sand, with steeply dipping, 50–100 m high and asymmetric clinoforms, the reservoir sand is most likely found in the downdrift portion of the wave-dominated delta. This type of delta is best developed in a ramp setting that has undergone a tectonic phase, which resulted in a sudden increase in accommodation space, and is characterised by a high sediment supply due to the formation of a high relief in the hinterland. In such a setting, the fluvial system is dominated by bed-load transport and migration of dunes to the delta front is a common phenomenon. The pro- portion of river-borne sediment is also important. Longshore currents and wave processes can move the fine-grained frac- tion downdrift and offshore and thereby lead to concentra- tion of sand on the main delta platform. Acknowledgements Environment Centres Ribe, Ringkøbing and Århus are thanked for finan- cial support. References Bhattacharya, J. P. & Giosan, L. 2003: Wave-influenced deltas: geomor- phological implications for facies reconstruction. Sedimentology 50, 187–210. Dybkjær, K. 2004: Dinocyst stratigraphy and palynofacies studies used for refining a sequence stratigraphic model – uppermost Oligocene to lower Miocene, Jylland, Denmark. Review of Palaeobotany and Paly- nology 131, 201–249. Galloway, W.E. 2002: Paleogeographic setting and depositional architec- ture of a sand-dominated shelf depositional system, Miocene Utsira Formation, North Sea. Journal of Sedimentary Research 72, 447–490. Hansen, J.P.V. & Rasmussen, E.S. 2008: Structural, sedimentologic, and sea-level controls on sand distribution in a steep-clinoform asymmetric wave-influenced delta: Miocene Billund Sand, eastern Danish North Sea and Jylland. Journal of Sedimentary Research 78, 130–146. Larsson-Lindgren, L. 2009: Climate and vegetation during the Miocene – evidence from Danish palynological assemblages. Litholund theses 19, 20 pp. + 3 appendices. Maillet, G.M., Vella, C., Berné, S., Friend, P.L., Amos, C.L., Fleury, T.J. & Normand, A. 2006: Morphological changes and sedimentary processes induced by the December 2003 flood event at the present mouth of the Grand Rhône River (southern France). Marine Geology 234, 159–177. Orton, G.J. & Reading, H.G. 1993: Variability of deltaic processes in terms of sediment supply, with particular emphasis on grain size. Sedi- mentology, 40, 475–512 Rasmussen, E.S. 2004: Stratigraphy and depositional evolution of the up- permost Oligocene – Miocene succession in western Denmark. Bulle- tin of the Geological Society of Denmark 51, 89–109. Rasmussen, E.S., 2009a: Detailed mapping of marine erosional surfaces and the geometry of clinoforms on seismic data: a tool to identify the thickest reservoir sand. Basin Research 21, 721–737. Rasmussen, E.S. 2009b: Neogene inversion of the Central Graben and Ringkøbing–Fyn High, Denmark. Tectonophysics 465, 84–97. Rasmussen, E.S. & Dybkjær, K. 2005: Sequence stratigraphy of the Up- per Oligocene – Lower Miocene of eastern Jylland, Denmark: role of structural relief and variable sediment supply in controlling sequence development. Sedimentology 52, 25–63. Rasmussen, E.S., Dybkjær, K. & Piasecki, S. 2006: Neogene fluvial and nearshore marine deposits of the Salten section, central Jylland, Den- mark. Bulletin of the Geological Society of Denmark 53, 23–37. Rasmussen, E.S., Heilmann-Clausen, C., Waagstein, R. & Eidvin, T. 2008: The Tertiary of Norden. Episodes 31, 66–72. Authors’ addresses E.S.R., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: esr@geus.dk J.B.-P., Environment Centre Ribe, Sorsigvej 35, DK-6760 Ribe, Denmark.