--> ABSTRACT: Controls of Eustasy, Tectonics, and Environmental Factors on the Deposition of Mid-Cretaceous Carbonate Platforms, South-Central Pyrenees, Spain, by Peter Drzewiecki, J. A. Toni Simo; #91020 (1995).

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Controls of Eustasy, Tectonics, and Environmental Factors on the Deposition of Mid-Cretaceous Carbonate Platforms, South-Central Pyrenees, Spain

Peter Drzewiecki, J. A. Toni Simo

Eustasy, tectonics, and environmental factors were important controls on the carbonate deposition of the Cenomanian-Turonian sequences of the south-central Pyrenees, Spain. These controls have contributed in varying degrees to the following features which are not easily explained with conventional sequence stratigraphy: 1) sediment shedding and the emplacement of carbonate megabreccias during sea level highstand, 2) the formation of a steep, rimmed platform in the absence of reef-constructing organisms, 3) platform progradation during a marine transgression, and 4) the drowning of a healthy carbonate platform in response to changes in physicochemical conditions of the oceans.

A lower sequence contains a 250 m thick slope succession of hemipelagic sediments interbedded with clast-supported, margin-derived and matrix-supported, slope-derived breccias which is interpreted to have been deposited during a sea level highstand, and attests to the large volume of sediment produced during this time. Tectonic activity is most likely responsible for the megabreccias, and not erosion during sea level lowstands as would be predicted by contemporary sequence stratigraphic models. This sequence also contains a steep margin composed of coarse rudist packstones and grainstones. Despite the steep profile, in place reef-constructing fauna are rare. The occurrence of a syndepositional fault beneath the margin, and the rapid cementation of carbonates are interpreted to be responsible for the margin geometry.

An upper sequence contains evidence of progradtion during the marine transgression. Carbonate production rates exceeded the rate of creation of accommodation space, leading to the formation of stratal relationships characteristic of sea level highstand and seaward-stepping platforms (downlap). This sequence was drown at the Cenomanian-Turonian boundary in response to changes in oceanic conditions related to a globally recognized Oceanic Anoxia Event. Physicochemical changes of the ocean superseded any eustatic, tectonic, or biological control that may have operated on the system. Carbonate production rates are extremely sensitive to physicochemical marine conditions, a fact that is often overlooked in studies of carbonate sequence stratigraphy. Only by assessing the relationships amon eustatic, tectonic, and oceanic factors can a full understanding of the controls on carbonate depositional processes be approached.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995