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

Abstract

Abstract In the Lower Saxony Basin of Northern Germany, the Earliest Cretaceous (Berriasian, ‘German Wealden’) is represented by organic-rich shales assigned to the Isterberg Formation, 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 has been studied with a combined approach including bulk rock organic geochemistry and palynofacies analysis. The lower part (W 1 to W 3) is interpreted to reflect very shallow-water conditions and episodic emergence, indicated by multiple occurrences of palaeosol 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 (W 4), indicating more distal conditions characterized by clay-dominated sedimentation, abundant lumachelle interbeds and a predominance of aquatic-derived organic matter. The newly acquired data fill a gap in the record between the proximal and distal strata of the Berriasian Lower Saxony Basin and provide new insights into the accumulation and preservation of organic matter within this exceptional depositional setting.