The Turonian/Coniacian (T/C) boundary in the Upper Cretaceous of the Elbe Valley/Saxony (Germany)
Inoceramid Workshop (2 ; 1996 ; Freiberg, Niemcy)
Three lithofacies characterize the Turonian/Coniacian sequences in the Upper Cretaceous of the Elbe Valley. The marly lithofacies, between Weinbohla - Dresden - Heidenau, consists of marls and calcareous clays (Strehlen Formation). A transitional lithofacies, with alternations of marls, clays and blocky sandstones, follows to the southeast (area between Rosenthal - Pirna - Lohmen). A sandy lithofacies, with blocky sandstones, is developed even farther to the southeast (Elbsandsteingebirge). The T/C boundary is situated in the lower part of the Strehlen Formation in the marly lithofacies, known from several boreholes. Coniacian index ammonites are absent across the T/C boundary in the three investigated boreholes apart from a deformed and incomplete specimen of Placenticeras cf. orbignyanum (GEINITZ). Therefore, bivalves must be used to place the T/C boundary. The Didymotis event II (Dresden-Marienhof Borehole), with Cremnoceramus waltersdorfensis (ANDERT), lies at the top of the Upper Turonian M. scupini Zone. The rarity or absence of the bivalve Didymotis in the Dresden-Blasewitz and Graupa boreholes is caused by facies changes. The FAD of Cremnoceramus rotundatus (TROGER non FIEGE), especially the C. rotundatus Event (Graupa Borehole, Hinterjessen Marl), can be used to place the T/C boundary. A small interval below the FAD of C. rotundatus and above the Didymotis II event, which may belong to the basal Coniacian, yields Cremnoceramus waltersdorfensis (ANDERT), Cremnoceramus waltersdorfensis hannovrensis (HEINZ), Mytiloides carpathicus (SIMONESCU) and Placenticeras cf. orbignyanum (GEINITZ). In the sandy lithofacies fossils are rare. It is not possible to place the boundary in either the Herrenleite Sandstone or in sandstone d. In the marly lithofacies of the Upper Cretaceous of Saxony bracketing of the T/C boundary is possible by means of planktonic foraminifera. It is possible to identify the evolution of planoconvex double-keeled globotruncanids of the Dicarinella hagni - Dicarinella primitiva - Dicarinella concavata group from the Upper Turonian. In the Dresden-Blasewitz and Dresden-Marienhof boreholes this group is represented in the T/C boundary interval by Marginotruncana paraconcavata PORTHAULT and Dicarinella cf concavata (BROTZEN). They are of rare occurrence because they are facies controlled. It is not possible to recognize the boundary between the scheegansi and concavata zones because of the restricted occurrence and rarity of the index forms in the investigated sections. In the Dresden-Blasewitz and Dresden-Marienhof boreholes the benthonic species Stensioeina granulata (OLBERTZ) was identified.
Bibliogr. 16 poz.