The Kirchrode I and II boreholes: technical details and evidence on tectonics, and the palaeoceanographic development during the Albian

The technical details of drilling and coring at the Kirchrode I and II site s are presented. At these sites, a sequence of claystones and marlstones fr om an Albian shelf basin was recovered. Constraints on the ages of the sedi ments in the two boreholes are provided by the occurrence of the inoceramid bivalve Actinoceramus sulcatus, the first appearance of which is used to d efine the Middle/Upper Albian boundary and by observed facies changes that can be correlated to the established lithostratigraphy. The cores from the two boreholes provide a rather complete, 285-m-long sequ ence of the Upper Albian, with a 155.5-m-long overlap. Analysis of the tect onic structures showed considerable shortening in the Middle and Lower Albi an part of the sequence due to normal faulting. Of the Upper Albian, only t he lowermost part is affected by faults. The increase in sedimentation rates of terrigenous detritus and of marine b iogenic carbonate, which occurs in the basal part of the C. auritus Subzone , is interpreted to reflect a regional change to a more humid climate and r egional tectonic movements (uplift of the Rhenish Bohemian massif, subsiden ce of the Lower Saxony basin intensified locally by halokinetic movements). The further increase in marine productivity in the latest Albian may be re lated to upwelling of more nutrient-rich deep water along submarine relief in this shelf sea. Identification of Milankovitch cyclicity documented by the fluctuating CaCO 3 contents of the sediments is used (i) to constrain the minimum time repre sented by the Upper Albian deposits, and (ii) to determine the duration of the sea level cycles (Cycle V: greater than or equal to1.6 Ma, Cycle VI: gr eater than or equal to2 Ma), and (iii) to establish the duration of the Lat e Albian ammonite subzones (e.g. Callihoplites auritus Subzone: 2.1 Ma). Av erage sedimentation rates determined from the identified 100-ka eccentricit y cycles show a stepwise increase in sedimentation rates from 1-2 cm/1000 a in the Lower Albian dark claystones to 7-13 cm/1000 a in the late Late Alb ian. In addition to the general deepening trend through the Late Albian, two, ne arly completely documented 3rd-order sea-level cycles in the Upper Albian o f Kirchrode I were recognised, plus another one, cut short by faulting, at the base of the Upper Albian (documented in Kirchrode II). These global sea -level cycles were identified on the basis (a) of the sequence of the abund ance maxima of selected benthos and plankton groups, (b) of trends in the f luctuations of the CaCO3 content, and (c) of the abundance of glauconite. T he transgression periods in this Upper Albian deep shelf-basin are characte rised by intensified circulation. This intensified circulation is found to have affected first the surface-near waters, resulting e.g. in an increase in the abundance of immigrant plankton and nekton species from the Tethys. At a later stage the deep water was affected, supporting then an increased population of suspension-feeding benthos, and causing condensation and eros ion in the sediment at the sea floor. (C) 2001 Elsevier Science B.V. All ri ghts reserved.