Forgotten in a sinkhole: Stratigraphy, depositional setting and biofacies of the Cenomanian to Lower Turonian Wellheim Formation (Bavaria, southern Germany)

During early Late Cretaceous times, a stepwise transgression turned vast areas of the former northern Tethyan margin into peri-continental shelf seas. In Bavaria (Germany), this process is recorded in the predominantly marine strata of the Danubian Cretaceous Group (Niebuhr et al. 2009), which document the transgression of the Late Cretaceous sea onto the margins of the Mid-European Island, consisting of the Rhenish and Bohemian massifs. The entire succession documents an almost symmetrical transgressive-regressive cycle with a maximum flooding interval during the upper Middle Turonian (Niebuhr et al. 2014).

In the type region of the Danubian Cretaceous Group, the Regensburg-Kelheim area, the basal marine unit that unconformably overlies the palaeo-land surface is the Lower to lower Upper Cenomanian Regensburg Formation (greensands). It is followed by the uppermost Cenomanian-lowermost Turonian silty marls of the Eibrunn Formation and the spiculitic-calcareous siltstones (Reinhausen Member) and sandstones (Knollensand Member) of the Lower Turonian Winzerberg Formation. Towards the west, the Wellheim Formation replaces these strata, being restricted to up to 130-m-deep post-depositional karst depressions in Upper Jurassic carbonates spread over an area of approximately 200 km² between Solnhofen and Neuburg an der Donau. The Upper Jurassic carbonates are overlain by a basal, several decimetres to a few metres thick residual clay layer (Neuburg Clay), sharply overlain by coarse-grained arkosic sands. Both units constitute the Lower Cretaceous terrigenous Schutzfels Formation. The onset of the Early Cenomanian transgression is documented by shallow marine, fine-grained sandy basis beds with foraminifers, sponge spicules and bivalve shells (predominantly oysters), belonging to the basal Wellheim Formation. At the boundary to the overlying Neuburg Kieselerde Member (a siliceous earth), common opaline nodules with the Early Cenomanian inoceramid bivalve index fossil Inoceramus crippsi occur, representing the crippsi Event associated with the first intra-Cenomanian sequence boundary SB Ce 1. The spiculitic silts of the Neuburg Kieselerde Member are completely decalcified and consist of c. 80 % quartz, c. 15 % kaolinite and c. 5 % biogenic opal-CT, with traces of illite as well as manganese and iron oxides. Up-section, the input of detrital quartz sand increases, causing a gradual transition from the Neuburg Kieselerde Member to the overlying sandy beds, which are capped by an unconformity surface at the base of a silicified conglomerate (sequence boundary SB Tu 1). The four marine lithounits described above, i.e. sandy basis beds, Neuburg Kieselerde Member, overlying sandy beds and the silicified conglomerate, form the typical succession of the Wellheim Formation. The Neuburg Kieselerde Member of the Wellheim Formation yielded five inoceramid species that allow for precise biostratigraphic dating: Inoceramus crippsi crippsi Mantell, 1822 indicates an Early Cenomanian age for the lowermost part, while Inoceramus hoppenstedtensis Tröger, 1967 persists into the early Middle Cenomanian. Inoceramus pictus pictus J. Sowerby, 1829 and I. p. cf. concentricoundulatus Tröger, 1967 indicate a Late, but not latest, Cenomanian age for the middle part. Mytiloides mytiloides (Mantell, 1822) occurs in the upper, i.e. Lower Turonian part of the Neuburg Kieselerde Member.

With approximately 100 species, the invertebrate macrofauna of the Neuburg Kieselerde Member of the Wellheim Formation is probably the most diverse fossil assemblage of the Danubian Cretaceous Group (Schneider et al. 2013). It preserves a soft-bottom community dominated by epifaunal and semi-infaunal bivalves and siliceous sponges, which, however, is biased due to near-complete early diagenetic loss of aragonitic shells. The fauna provides evidence of a shallow to moderately deep, calm, fully marine environment, which is interpreted as a large-scale embayment. Although so far most people may never have heard about the Neuburg Kieselerde Member, its unique composition of crypto- to nano-crystalline silica and clay minerals made the material an essential component of synthetic colours, rubber, plastic, varnish and (car) polish, and the respective sediment has thus entered a majority of our households straight through the front door