Genetic stratigraphy of Coniacian deltaic deposits of the northwestern part of the Bohemian Cretaceous Basin
Abstrakt
Based on a combination of outcrop and subsurface data, this paper presents a description and interpretation of principal lithofacies and regional-scale depositional geometries in sandstones of early to middle Coniacian age in the northwestern part of the Bohemian Cretaceous Basin. A depositional model of nearshore deposition presented here interprets the depositional system as dominated by coarse-grained deltas that prograded from the faulted northern basin margin, the present-day Lužice (Lausitz) Fault Zone.
The deltaic systems were the primary loci of clastic sediment deposition upon entering the basin, but the deposits of the delta fronts as well as part of the prodeltaic facies were further reworked by vigorous tidal currents, in particular in shallow-water settings. Steep, Gilbert-type foresets formed mainly in delta fronts that prograded into deeper water, reaching up to c. 100 m in some units.
This depositional model is largely analogous to that developed by Uličný (2001) in other parts of the basin, with the exception of very large-scale dunes that locally occurred in the present study area. Regional correlations utilising well-log and core data made it possible to correlate nearshore sandstone units to their fine-grained time-equivalents in the mud-dominated offshore realm, and subdivide the depositional record into genetic sequences, bounded by surfaces of maximum transgression (sensu Helland-Hansen & Martinsen 1996).
The genetic sequencestratigraphic framework was then applied in construction of a time-slice reconstruction of regional palaeogeography from latest Turonian to middle Coniacian times. In comparison to the upper Turonian (TUR 7) sequence, dominated by vertical stacking of sandstone bodies deposited in shallow water, sequences CON 1 to CON 3 were deposited in a setting of generally increasing depth through time, most probably due to increasing subsidence rate.