Sand Bank and Dune Facies Architecture of a Wide Intracratonic Seaway

Abstract

Abstract The 200- to 300-m-thick Raukelv Formation was deposited in an intracratonic seaway between East Greenland and Norway during the Late Jurassic-earliest Cretaceous (Volgian-Valanginian), a period of considerable tectonic unrest in the region. The formation is three-dimensionally exposed over 900 km2. Four basic architectural elements are recognized: 1. The sandbank (SB) element is composed of single, tabular mega-cross-beds 15-50 m thick, which extend over the whole outcrop area (900 km2). 2. the sand-dune (SD) element consists of a large-scale tabular cross-bedded coset, 2-8 m thick, which extends over the whole outcrop area (900 km2). 3. the sandsheet (SS) element comprises a burrowed and glauconitized, mainly trough cross-bedded coset, 2-10 m thick, with sheet geometry, which extends over most or all of the outcrop area; and 4. the siltstone (SI) element is 2-20 m thick, has sheet geometry, and extends over at least 100 km2. The SB, SD and SS elements are topped by planar, glauconitized commonly pebble-sttewn omission surfaces with U-shaped burrows. Most sand banks of the SB element migrated eastward from the western coastline, whereas the linear dunes of the SD element migrated southward, parallel to the basin axis and to the coast. Sedimentation seems to have been governed mainly by coast-parallel or offshore-directed currents set up by storms and tides. The low-amplitude 3-D dunes of the SS element migrated toward the south or, rarely, toward the north. Rates of subsidence and sediment influx can be considered as relatively constant for the time interval in question. The succession consists of stacked SB, SD, SS and SI elements. A clear, systematic, sequential stacking order has not been observed, although there is a tendency for development of an SB, SS, SD cycle. A total of four SB, three SD, three SS and two SI elements has been recorded. Deposition is interpreted as controlled mainly by fluctuations in regional sea level during a period of lowstand. The individual elements reflect fairly rapid regional sea-level fluctuations on a background of the overall lowstand. The main transport of sand in the basin took place in extensive fields of linear sand dunes (SD element). Sand banks (SB element) were initiated at times of high rate of sea-level rise, and the main growth by eastward progradation took place during highstands. Some bank tops were eroded during sea-level fall, whereas others were drowned during rise. In both cases the tops became glauconitized and burrowed during the succeeding rise. The omission surface was slowly covered by bioturbated siltstone (SI element) during high rate of sea-level rise. In case of slower rate of rise a reworked, transgressive sheet sand (SS element) was formed. Sea level is estimated to have fluctuated between 30 and 50 m during deposition of the SB elements. These oscillations had a duration of about 1 m.y. and were superimposed on a lowstand following the Kimmeridgian highstand. The lowstand had a duration of about 10 m.y.