Abstract

This study interprets the controls on cyclic and secular changes in a shallow-water hemipelagic system of the Bohemian Cretaceous Basin using (i) spatial and temporal changes in facies and geometries of the hemipelagic succession and (ii) correlation of the hemipelagic cycles with the coeval nearshore siliciclastic sequences.

Three orders of hemipelagic rhythms, characterized by changes in lithology between foram-dominated limestones (> 70% CaCO3) and mudstones (< 30% CaCO3), are recognized: couplets, first-order bundles, and second-order bundles. The hemipelagic bundles are interpreted to reflect changes in siliciclastic flux that followed transgressive-regressive movements of the adjacent shoreline. Upward-muddying intervals correspond to regressive trends whereas downward-muddying intervals are interpreted as due to transgressions. The upward-muddying and downward-muddying intervals of the second-order hemipelagic bundles display offlapping and onlapping internal geometries, respectively, suggesting that the transgressive-regressive cycles were coupled with changes in potential accommodation in the hemipelagic system. These relationships may point to sea-level forcing of the second-order hemipelagic bundles. Finally, spectral analyses of gamma-ray signatures of the hemipelagic strata suggest that the second-order hemipelagic bundles were driven primarily by the Milankovitch cycles of "long" eccentricity (c. 400 kyr), whereas the first-order hemipelagic bundles record an interplay of the cycles of "short" eccentricity (c. 100 kyr) and obliquity (c. 40 kyr). Thus, the transgressive-regressive, and possibly sea-level, cycles acted as mediators of Milankovitch-driven changes in insolation.

The secular onset of carbonate-dominated conditions in the distal part of the basin does not follow any secular transgressive trend but coincides with a major change in differential subsidence/uplift pattern of the study area. It is interpreted to reflect an increase in the background carbonate production in the basin due to a tectonically and/or climatically induced change in circulation and associated acceleration of water-mass exchange with the pelagic-carbonate factory of northwestern Europe.

In summary, processes related to relative sea-level changes and transgressive-regressive shoreline movements are capable of producing hemipelagic rhythms, but it is the interplay of these processes with "background" oceanographic conditions (regional circulation patterns, open-marine trophic resource levels, etc.) that controls the actual character of the hemipelagic rhythms (e.g., whether carbonate or clastic dominated). Thus, changes in hemipelagic conditions as expressed in the rock record are rarely proportional to either changes in sea level and terrigenous input alone or changes in the "background" oceanography alone. Sequence stratigraphic analysis of hemipelagic strata must consider the spatial and temporal variability in the local oceanographic conditions that result from a combination of the above processes.

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