DEPOSITIONAL CONTROLS ON FACIES DISTRIBUTION AND CYCLICITY OF THE UPPER CRETACEOUS COLLADES DE BASTURS RUDIST PLATFORM, SOUTH-CENTRAL PYRENEES, SPAIN

The Upper Cretaceous Collades de Basturs carbonate platform (south-central Pyrenees, Spain) displays several orders of cyclicity that can be attributed to the biological, tectonic, and environmental factors in operation at the time. The platform contains 3 distinct facies associations: (1) shallow shelf strata composed of dense hippuritid rudist biostomes and rudist skeletal grainstone, (2) slope strata composed of rudist-coral-sponge floatstone, and (3) deep shelf strata composed of marl and nodular skeletal wackestone. Shallow shelf strata extend down-dip for at least 3 kilometers with little change in thickness or facies, indicating a very flat, low-angle platform. Shallow strata eventually transition into slope strata, then deep shelf strata over a lateral distance of about a kilometer.

The platform is composed of three large-scale (c. >100m thick), shallowing upward, progradational cycles, that pass vertically upwards from deep shelf through slope, and into shallow shelf facies associations. Relative seal level changes on the order of 10’s of meters were required to account for facies shifts of this magnitude. Within each of the three cycles, the shallow shelf facies displays higher frequency cyclicity (typically 2-8 m thick). These high-frequency cycles start as a thin colonizing layer composed of coral-rudist rudstone that grades upwards into biostromes composed of densely-packed in situ hippuritid rudists. Skeletal grainstone and packstone cap these biostromes, and fill any topography that would have been present above, within, or downdip of the biostromes. These facies suggest that once the hippuritid biostromes were established, they grew to fill available accommodation. Wave activity reworked the biostromes and redistributed skeletal debris resulting in planar-topped skeletal grainstone cycle caps. A small-scale relative sea level rise permitted deposition of the next colonizing layer.

In addition to sea level, tectonic factors influenced platform growth on several scales. The platform is exposed on the southern flank of the actively growing Sant Corneli antcline, which created the topographic relief upon which the rudists grew. In addition, small syndepositional faults (slumps) occur at the transition from shallow shelf to slope, and may have localized the edge of platform.