Depositional Sequences in Deep-Shelf Environments: A Response to Sea-Level Changes and Shallow-Platform Carbonate Productivity (Oxfordian, Germany and Spain)

Abstract

ABSTRACT Oxfordian deep-shelf deposits of southern Germany and southern Spain are characterized by marl-limestone alternations that are stacked into small-scale, medium-scale, and large-scale depositional sequences. The German sections contain autochthonous sponge reefs and associated fragments of microbialites, whereas the sections studied in Spain display tempestites composed of autochthonous deeper-water and allochthonous shallow-water particles. The facies of the German limestones and marl layers has been analyzed in detail: condensed intervals (implied by glauconitization of particles, abundant cephalopods, intense bioturbation, and generally more marls) are also enriched in sponge reefs and associated particles, and in nannofossils. Limestone-rich intervals, however, contain fewer sponges and fewer nannofossils. Neither bioerosion of sponge reefs nor nannofossil blooms can thus explain the abundance of carbonate mud that forms the limestones. Consequently, it is suggested that most of the carbonate mud is exported from shallow platform areas where carbonate productivity is high. The clay fraction was derived from weathering of massifs in the hinterland. The observed depositional sequences can be correlated between the studied sections in Germany and Spain (situated in different paleotectonic and paleoclimatic domains), and also between deeper-water sections and platform sections. This suggests that they formed through allocyclical processes. Comparision with published time scales implies that the small-scale and medium-scale sequences formed in tune with the 100 kyr and 400 kyr orbital eccentricity cycles, respectively. However, the number of marl-limestone alternations 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 through cyclically varying export of carbonate mud from the platform towards the deep shelf, the variations being controlled by climatically induced high-frequency sea-level fluctuations. Enhanced marl deposition on the deep shelf can be related to sea-level fall causing exposure of the shallow platform and reducing the area of carbonate production, or by rapid sea-level rise (maximum flooding) leading to partial or total drowning of the platform, and/or to retrogradation of facies belts. Enhanced carbonate deposition occurs during transgression, when large production areas are created on the platform, 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-frequency (20 kyr) sea-level cycle can thus create one or two marl-limestone alternations, or only marly deposits. Consequently, one 100 kyr eccentricity cycle may be formed of a variable number of marl-limestone alternations (commonly 2 to 8). The studied deeper-water depositional sequences thus are strongly linked to the history of the adjacent shallow carbonate platforms.