Depositional sequences in deep-shelf environments: a response to sea-levelchanges and shallow-platform carbonate productivity (Oxfordian, Germany and Spain)

Oxfordian deep-shelf deposits of southern Germany and southern Spain are ch aracterized by mart-limestone alternations that are stacked into small-scal e, medium-scale, and large-scale depositional sequences. The German section s contain autochthonous sponge reefs and associated fragments of microbiali tes, whereas the sections studied in Spain display tempestites composed of autochthonous deeper-water and allochthonous shallow-water particles. The f acies of the German limestones and marl layers has been analyzed in detail: condensed intervals (implied by glauconitization of particles, abundant ce phalopods, intense bioturbation, and generally more marls) are also enriche d in sponge reefs and associated particles, and in nannofossils. Limestone- rich intervals, however, contain fewer sponges and fewer nannofossils. Neit her bioerosion of sponge reefs nor nannofossil blooms can thus explain the abundance of carbonate mud that forms the limestones. Consequently, it is s uggested that most of the carbonate mud is exported from shallow platform a reas where carbonate productivity is high. The clay fraction was derived fr om weathering of massifs in the hinterland. The observed depositional sequences can be correlated between the studied s ections in Germany and Spain (situated in different paleotectonic and paleo climatic domains), and also between deeper-water sections and platform sect ions. This suggests that they formed through allocyclical processes. Compar ision with published time scales implies that the small-scale and medium-sc ale sequences formed in tune with the 100 kyr and 400 kyr orbital eccentric ity cycles, respectively. However, the number of mart-limestone alternation s is not always consistent with the expected number of 20 kyr precessional cycles (5 per 100 kyr cycle). The large-scale sequences reflect long-term ( "third order") sea-level changes. The observed marl-limestone alternations are interpreted to have formed thr ough cyclically varying export of carbonate mud from the platform towards t he deep shelf, the variations being controlled by climatically induced high -frequency sea-level fluctuations. Enhanced marl deposition on the deep she lf can be related to sea-level fall causing exposure of the shallow platfor m and reducing the area of carbonate production, or by rapid sea-level rise (maximum flooding) leading to partial or total drowning of the platform, a nd/or to retrogradation of facies belts. Enhanced carbonate deposition occu rs during transgression, when large production areas are created on the pla tform, or during late highstands, when progradation is forced and carbonate -mud export enhanced. Depending on the long-term trend of sea-level change on which the high-frequency fluctuations are superimposed, one high-frequen cy (20 kyr) sea-level cycle can thus create one or two marl-limestone alter nations, or only marry deposits. Consequently, one 100 kyr eccentricity cyc le may be formed of a variable number of marl-limestone alternations (commo nly 2 to 8). The studied deeper-water depositional sequences thus are stron gly linked to the history of the adjacent shallow carbonate platforms.