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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)

Abstract


Journal of Sedimentary Research, Section B: Stratigraphy and Global Studies
Vol. 70 (2000), No. 2. (March), Pages 392-407

Depositional Previous HitSequencesNext Hit in Deep-Shelf Environments: A Response to Previous HitSeaNext Hit-Previous HitLevelNext Hit Changes and Shallow-Platform Previous HitCarbonateNext Hit Productivity (Oxfordian, Germany and Spain)

Bernard Pittet (1), Andre Strasser (2), Emanuela Mattioli (1)

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 Previous HitsequencesNext Hit. 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 Previous HitcarbonateNext Hit mud that forms the limestones. Consequently, it is suggested that most of the Previous HitcarbonateNext Hit mud is exported from shallow platform areas where Previous HitcarbonateNext Hit productivity is high. The clay fraction was derived from weathering of massifs in the hinterland.

The observed depositional Previous HitsequencesNext Hit 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 Previous HitsequencesNext Hit formed in tune with the 100 kyr and 400 kyr orbital eccentricity Previous HitcyclesNext Hit, respectively. However, the number of marl-limestone alternations is not always consistent with the expected number of 20 kyr precessional Previous HitcyclesNext Hit (5 per 100 kyr cycle). The large-scale Previous HitsequencesNext Hit reflect long-term ("third order") Previous HitseaNext Hit-Previous HitlevelNext Hit changes.

The observed marl-limestone alternations are interpreted to have formed through cyclically varying export of Previous HitcarbonateNext Hit mud from the platform towards the deep shelf, the variations being controlled by climatically induced high-frequency Previous HitseaNext Hit-Previous HitlevelNext Hit fluctuations. Enhanced marl deposition on the deep shelf can be related to Previous HitseaNext Hit-Previous HitlevelNext Hit fall causing exposure of the shallow platform and reducing the area of Previous HitcarbonateNext Hit production, or by rapid Previous HitseaNext Hit-Previous HitlevelNext Hit rise (maximum flooding) leading to partial or total drowning of the platform, and/or to retrogradation of facies belts. Enhanced Previous HitcarbonateNext Hit deposition occurs during transgression, when large production areas are created on the platform, or during late highstands, when progradation is forced and Previous HitcarbonateNext Hit-mud export enhanced. Depending on the long-term trend of Previous HitseaNext Hit-Previous HitlevelNext Hit change on which the high-frequency fluctuations are superimposed, one high-frequency (20 kyr) Previous HitseaNext Hit-Previous HitlevelNext Hit 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 Previous HitsequencesNext Hit thus are strongly linked to the history of the adjacent shallow Previous HitcarbonateTop platforms.


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