科研之友
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摘要
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 Luzice (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 Ulicny (2001) in other parts of the basin, with the exception of very large-scale dunes that locally occurred in the present study area. %26lt;br%26gt;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 %26 Martinsen 1996). The genetic sequence-stratigraphic framework was then applied in construction of a time-slice reconstruction of regional palaeogeography from latest Turonian to middle Coniacian times. %26lt;br%26gt;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. The transgressive regressive history of the study area during latest Turonian and Early Coniacian reveals three major transgressions in the study area: (1) approximately at, or immediately prior to, the Turonian/Coniacian boundary, (2) at the base of CON 2 sequence (base of Cremnoceramus crassus crassus Zone), and (3) at the base of CON 3 sequence, within the Volviceramus koeneni Zone. Of the abovementioned maximum flooding episodes, (1) and (2) have their counterparts in a different depocentre in the same basin, but also in Western Canada (Walaszczyk et al. 2014), suggesting a significant role of eustasy in their formation. Further conclusions on eustasy require caution, however, because of the clearly significant role of accelerating subsidence as well as supply from an actively uplifted source area during the early and middle Coniacian. Local stacking patterns of deltaic bodies in the study area differ from coeval ones in the separate depocentre of Cesky Raj developed further east along the Luzice Fault Zone. It is likely that the increased tectonic activity between the latest Turonian and middle Coniacian was a precursor to the onset of shortening that eventually led to the well-known inversion of basins in the northern Alpine foreland. The exact kinematic role of individual tectonic structures remain to be understood in the near future, using data on evolution of basin-fill geometries through time, such as provided by this study.
- 出版日期2014-12
