Composition and distribution of sedimentary organic matter in nearshore Berriasian strata (“German Wealden”) of the eastern Lower Saxony Basin, NW Germany

Abstract

In the Lower Saxony Basin (LSB) of Northern Germany, the earliest Cretaceous (Berriasian, “German Wealden”) is represented by organic-rich shales assigned to the Isterberg Fm., which have been deposited under brackish-lacustrine conditions in a partially restricted basin. Towards the southern basin margin, these shales interfinger with nearshore sandstones sourced from the Deister-Hils delta. In order to better understand the environmental conditions and depositional dynamics of the Isterberg Fm., the sedimentary organic matter (OM) has been studied with a combined approach including bulk rock organic geochemistry and palynofacies analysis. The lower part (W1–W3) is interpreted to reflect very shallow water conditions and episodic emergence, indicated by multiple occurrences of palaesol horizons, rhizoliths and intercalated coal beds formed within a mud-dominated and protected domain of the delta plain. The overlying sand-rich interval represents a nearshore deltaic setting. A distinct change is observed in the upper part (W4), indicating more distal conditions characterised by clay dominated sedimentation, abundant lumachelle interbeds and a predominance of aquatic-derived OM. The newly acquired data fill a gap in the record between proximal and distal strata of the Berriasian LSB and provide new insights into the accumulation and preservation of OM within this exceptional depositional setting. Supplementary material at https://doi.org/10.6084/m9.figshare.c.7093491