The Toarcian–Bajocian deltaic system in the North German Basin: Subsurface mapping of ancient deltas‐morphology, evolution and controls

Abstract

Research focussing on the morphology and morphodynamics of modern river deltas has contributed much to the understanding of reservoir geometry and prediction of hydrocarbon splays and deep geothermal aquifers. The interplay of allogenic and autogenic controls results in complex histories of modern river deltas comprising subsequent regressive and transgressive stages of delta formation. The application to ancient examples is still ambiguous and in particular the detailed morphological description of ancient deltas is often hampered by limited subsurface data. This basin‐scale study employs a data set of 15 cored wells and more than 450 logged wells in combination with lithofacies, biofacies and architectural analysis to reconstruct high‐resolution subsurface facies and sand thickness maps of the Toarcian–Bajocian deltaic system in the North German Basin. The results of this study show the evolution from smaller elongate river‐dominated delta types in the Lower and Upper Toarcian (15 000 to 20 000 km²), and a larger lobate river‐dominated delta in the Upper Aalenian (40 000 km²) to a cuspate mixed river‐dominated and wave‐modified delta in the Upper Bajocian (25 000 km²). River‐dominated deltas are formed of distributary deltaic channel belts and associated sheetsands of the delta plain and distributary‐mouth bar complexes with bar‐finger sand architectures of the delta front. Increasing marine processes in the Bajocian resulted in modification of delta complexes forming a shoreface setting with foreshoals, arc‐like chains of ooid‐bearing barrier shoals and back barrier lagoons. The complex history of the Toarcian–Bajocian deltaic system was mainly controlled by allogenic sea‐level fluctuations that triggered the delta progradation and subordinately by allogenic basin reorganization. Autogenic process change of channel avulsion led to locally reduced sediment flux and culminated in lobe abandonment and transgressive shorelines. Differential compaction of underlying deltaic deposits result in strong differences in accommodation space and contributed greatly to large‐scale delta shifts (>300 km).

Geochemical fingerprinting of key lithologies and depositional processes across the upper boundary of the Opalinus Clay (Aalenian, Middle Jurassic, northern Switzerland)

“…An overall regressive trend during the early Aalenian is nonetheless suggested from other locations across the epicontinental Central European Basin (Callomon, 2003; de Graciansky and Jacquin, 2003; Sandoval et al ., 2002; Teyssen, 1989; Zimmermann et al ., 2015; 2018), supporting the interpretation of Burkhalter (1996). Zimmermann et al .…”

Shoreline migration paths and depositional architecture of early–mid Miocene deltaic clinoforms in response to sea‐level changes in the north‐eastern shelf margin, South China Sea

“…The structureless or tabular cross‐strata and the lack of burrows (Fig. 11G and H) suggest a fresh or brackish, high‐energy environment with the absence of any marine influences (Zimmermann et al ., 2018). Basal scour surfaces and the upward fining trends of the single storey of FA1 (Fig.…”

3rd International Congress on Stratigraphy - Strati 2019

“…Barth G.* 1 , Obst K. 1 , Pieńkowski G. 2 , Hesselbo S.P. 3 & Leng M.J. 4 The Toarcian (upper part of the Lower Jurassic) starts in the eastern part of the Central European Basin (CEBS, including the North German Basin -NGB, and the Polish Basin -PB) with a widespread marine transgression from the west (Zimmermann et al, 2018). Thus, the facies changes from W to E from marine mudstones (named Posidonia and Dörnten shales) to brackish-marine silty and sandy deposits (so-called Green Series and Ciechocinek Fm).…”

“…The biostratigraphic framework of the NGB enabled the reconstruction of the sea-level record and correlation of epicontinental sequences with Boreal and Tethyan cycles (Zimmermann et al, 2015) and the analyses of depositional environments (Zimmermann et al, 2018) and palaeogeographic settings (Barth et al, 2018). Employing stable isotopes (δ 13 C and δ 18 O) and organic geochemistry, ongoing work is focused on environmental perturbations, such as the Late Pliensbachian event and Toarcian OAE.…”