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

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). The river-dominated parts are composed of subsequently 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 reorganisation. 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 to much to large-scale delta shifts (>300 km). This article is protected by copyright. All rights reserved.