Facies Analysis and Shelfbreak Evolution of Two Mid-Cretaceous Carbonate Sequences (South-central Pyrenees, Spain)

Peter A. Drzewiecki and J. A. (Toni) Simo

Two Cenomanian-Turonian sequences of the south-central Pyrenees (Spain) record three distinct evolutionary phases of the shelfbreak that are best revealed through detailed analysis of facies distribution patterns. The sequences were deposited in a rift basin, and the original location of the margin was controlled by inherited Albian rift topography.

The lower sequence records the first evolutionary phase. It has a ramp facies succession at the shelfbreak that grades landward into a broad, flat, semi-restricted lagoon facies. Evidence of a well-developed margin facies is lacking, and the slope grades basinward from coarse packstones into hemi-pelagic sediments. The upper sequence records the evolution to a flat-topped platform with a steep slope and abrupt facies changes. Shallow, open marine facies were deposited over a broad region of outer shelf and a distinct high-energy margin facies was established. As the platform evolved, carbonate megabreccias derived from the margin and upper slope bypassed the middle slope and were deposited in the lower slope. These characteristics are attributed to increasing slope angle, that originaled by syndepositional tectonism and erosion/cannibalization of the margin and upper slope. The final evolutionary phase occurs at the top of the upper sequence, when the platform was drowned in response to environmental changes. Pelagic sediments draped the entire platform, and a distally-steepened ramp morphology was established.

These sequences provide excellent examples of the use of detailed facies analysis to interpret platform evolution in response to tectonically-induced changes in shelf-to-basin relief. Whereas the shallow shelf changed little, the shelfbreak and slope experienced dramatic changes in both lithofacies and facies distribution (both lateral and vertical), through a combination of erosive and depositional processes. These observations have implications for seismic interpretation, where platform slopes are often critical to deciphering regional sequence stratigraphy.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California