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

摘要

The Upper Turonian Gro?berg 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 Gro?berg 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 Gro?berg Ramp. Carbon stable isotope?geochemistry cannot significantly contribute to the stratigraphic calibration of the Gro?berg Formation due to the?depleted and trendless bulk-rock δ~(13)C values, probably resulting from a shallow-water aquafacies with depleted δ~(13)C?DIC?values and low δ~(13)C values of syndepositional and early diagenetic carbonate phases. However, strongly enriched skeletal?calcite δ~(13)C values support a correlation of the Gro?berg 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 Gro?berg Formation are characterized by unconformities. Sequence boundary?SB Tu 4 at the base is a major regional erosion surface (erosional truncation of the underlying Kagerh?h Formation?in the Regensburg area, fluvial incision at the base of the Seugast Member of the Roding Formation in the Bodenw?hr?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 Gro?berg Formation is concomitant to the deposition of the fluvial Seugast Member and the onlap of the?marginal-marine “Veldensteiner Sandstein” onto the Fr?nkische Alb. The unconformity at the top of the Gro?berg Formation?(late Late Turonian SB Tu 5) is indicated by a ferruginous firm-/ hardground and an underlying zone of strongly?depleted δ~(13)C 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.