Facies and integrated stratigraphy of the Upper Turonian (Upper Cretaceous) Großberg Formation south of Regensburg (Bavaria, southern Germany)

Abstract

The Upper Turonian Großberg Formation of the Regensburg area (Danubian Cretaceous Group, Bavaria, southern Germany) has a mean thickness of 20–25 m and consists of sandy bioclastic calcarenites and calcareous sandstones which are rich in bryozoans, serpulids and bivalves (oysters, rudists, inoceramids). Eight facies types have been recognized that characterize deposition on a southward dipping homoclinal ramp: the inner ramp sub-environment was characterized by high-energy sandwave deposits (sandy bioclastic rud- and grainstones, bioclastic sandstones) with sheltered inter-shoal areas. In mid-ramp settings, bioturbated, glauconitic, calcareous sand- and siltstones as well as bioturbated, bioclastic wacke- and packstones predominate. The carbonate grain association of the Großberg Formation describes a temperate bryomol facies with indicators of warm-water influences. An inferred surplus of land-derived nutrients resulted in eutrophic conditions and favoured the heterozoan communities of the Großberg Ramp. Carbon stable isotope geochemistry cannot significantly contribute to the stratigraphic calibration of the Großberg Formation due to the depleted and trendless bulk-rock δ¹³C values, probably resulting from a shallow-water aquafacies with depleted δ¹³C DIC values and low δ¹³C values of syndepositional and early diagenetic carbonate phases. However, strongly enriched skeletal calcite δ¹³C values support a correlation of the Großberg Formation with the mid-Late Turonian positive Hitch Wood isotope event (Hyphantoceras Event of northern Germany). This interpretation is supported by biostratigraphic data and a range from the Mytiloides striatoconcentricus Zone into the lower My. scupini Zone is indicated by inoceramid bivalves. Both the base and top of the Großberg Formation are characterized by unconformities. Sequence boundary SB Tu 4 at the base is a major regional erosion surface (erosional truncation of the underlying Kagerhöh Formation in the Regensburg area, fluvial incision at the base of the Seugast Member of the Roding Formation in the Bodenwöhr area towards the north and northeast). It is suggested that this unconformity corresponds to a major sea-level drop recognized in many other Cretaceous basins below the Hitch Wood or Hyphantoceras Event. The transgression and highstand of the Großberg Formation is concomitant to the deposition of the fluvial Seugast Member and the onlap of the marginal-marine “Veldensteiner Sandstein” onto the Fränkische Alb. The unconformity at the top of the Großberg Formation (late Late Turonian SB Tu 5) is indicated by a ferruginous firm-/ hardground and an underlying zone of strongly depleted δ¹³C values. The abrupt superposition by deeper marine marls of the lower Hellkofen Formation (uppermost Turonian–Lower Coniacian) may be connected with inversion tectonics at the southwestern margin of the Bohemian Massif.