The Bohemian Cretaceous Basin combines features of a shallow-water (mostly < 100 m) epicontinental seaway formed during a global transgression with those of a tectonically active, transtensional setting. The basin formed under a greenhouse climate and was affected by strong axial currents. Dense well-log coverage, combined with locally high-quality exposures and biostratigraphic control, make it possible to examine in three dimensions the geometries of genetic sequences and interpret their controlling variables. Sand-dominated deltas formed sequences at several spatial scales that reflect nested transgressive-regressive cycles with durations ranging from tens of thousands of years to millions of years. Progradation directions and distances, thicknesses and internal geometry of the individual sequences were controlled primarily by intrabasinal faulting, basin-scale changes in subsidence rate, eustatic fluctuations and localized bathymetric changes due to successive filling of the basin. Along-strike change in sediment input from different parts of the source area and a short-lived uplift of a secondary clastic source provided additional controls on the sequence geometry. Efficient hypopycnal transport combined with redeposition of fine clastics in shallow water promoted development of steep slopes of sand-dominated deltas while preventing downlap of muddy clinoforms; most of the suspended load became deposited downcurrent in subhorizontal or gently dipping bottomsets. Longterm accommodation rates were low during the Early to Middle Turonian, with minor intrabasinal faulting, but became accelerated in the Late Turonian and Early Coniacian. This acceleration was caused at least partly by increased subsidence rate accompanied by structural partitioning of the depocentre and partly compensated by increased sediment input indicating increased uplift rates in the Western Sudetic Island source area. This event probably reflected an increase in the regional strain rate in Central Europe. The succession of two major flooding events in the Early Turonian and late Early Coniacian, separated by a low-accommodation interval in the Middle Turonian, shows a close similarity to published estimates of long-term eustatic curves. However, the eustatic component of accommodation rate in the Bohemian Late Turonian and Coniacian is difficult to separate from accelerated subsidence. In several cases, evidence for short-term (100 kyr scale) forced regressions, independent of basinal structural activity, suggests small-scale eustatic falls at rates which, as presently understood, cannot be explained other than by a glacio-eustatic mechanism.
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palynological sea-level proxies in hemipelagic sediments: A critical assessment from the Upper ABSTRACT Geochemical and palynological records are presented for an expanded Turonian -Coniacian hemipelagic succession in the central Bohemian Cretaceous Basin. A high-resolution stratigraphic framework is provided by biostratigraphy and organic carbon stable-isotope (δ 13 C org ) chemostratigraphy. A short-term (100 kyr) sea-level curve has been derived from high-resolution transgressive/regressive maxima / shore-proximity data established from basin-wide sediment geometries. The viability of geochemical and palynological parameters as potential sea-level proxies is tested against this independently derived sea-level record. Elemental chemostratigraphy is demonstrated to offer a reliable means of identifying medium-to long-term (0.4 -2.4 Myr) sealevel trends. Manganese maxima are associated with periods of high sea level, and troughs with intervals of low sea level. Falling Mn contents accompany regression and rising values transgression. Major transgressive events associated with medium-term sea-level change are marked by sharp increases in Ti/Al ratios, but short-term (100 kyr) sea-level cycles are not consistently identified. Long-term δ 13 C org variation and dinoflagellate cyst species richness are positively correlated and show similarities to the sea-level curve. Baseline trends have a cycle duration close to the 2.4 Myr long-eccentricity cycle. Dinocyst species richness closely follows short-term changes in sea level, with marked increases in dinocyst diversity coincident with most short-term flooding events. Periods of rapid sea-level rise caused an influx of a more diverse "outer shelf" assemblage into the study area, together with the addition of shallower water species, some of which may have been transported into the central basin by hypopycnal flows. Changes in the proportion and abundance of peridinioid dinoflagellate cysts (principally Palaeohystrichophora infusorioides) were controlled principally by changing nutrient levels. Proximity proxies derived from geochemical and palynological data are not always consistent with the independent sea-level model. This exemplifies the need to understand all factors influencing elemental geochemical and palynological proxies before making simplistic sea level interpretations.
This study presents a synthesis of currently available data on the distribution of Cenomanian-age palaeodrainage systems in the Bohemian Cretaceous Basin, filled by fluvial and estuarine strata, and an interpretation of their relationships to the basement units and fault systems. Much of the progress, compared to previous studies, was made possible by a recent basin-scale evaluation of Cenomanian genetic sequence stratigraphy. Several local palaeodrainage systems developed in the basin, separated by drainage divides of local importance and one major divide -the Holice-Nové Město Palaeohigh -which separated the drainage basins of the Tethyan and Boreal palaeogeographic realms. The locations and directions of palaeovalleys were strongly controlled by the positions of inherited Variscan basement fault zones, whereas the bedrock lithology had the subordinate effect of narrowing or broadening valleys on more vs. less resistant substratum, respectively. The intrabasinal part of the palaeodrainage network followed the slopes toward the Labe (Elbe) System faults and was strongly dominated by the conjugate, NNE-trending, Jizera System faults and fractures. Outlet streams -ultimate trunk streams that drained the basin area -are interpreted to have followed the Lužice Fault Zone toward the Boreal province to the Northwest, and the Železné hory Fault Zone toward the Tethyan province to the Southeast. At both the northwestern and southeastern ends of the Bohemian Cretaceous Basin, shallow-marine or estuarine conditions are proven to have existed during the early Cenomanian. Direct evidence for syn-depositional subsidence during the early to mid-Cenomanian, fluvial to estuarine phase is very rare, and the onset of deposition by fluvial backfilling of the palaeodrainage systems was driven mainly by the long-term rise in global sea level. Subtle surface warping, mostly without detectable discrete faulting, is inferred to have been a response to the onset of the palaeostress regime that later, with further stress accumulation, led to subsidence in fault-bounded depocentres of the Bohemian Cretaceous Basin and uplift of new source areas. •
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