Abstract: Contrasting Facies Successions in Cyclic Pennsylvanian and Cretaceous Mixed Carbonate-siliciclastic Shelves

J. A. Simo, G. L. Gianniny, D. Rodrigues De Miranda

Shallow-water, mixed carbonate-siliciclastic stratal geometries are commonly a repetitive succession of transgressive onlapping wedges and highstand prograding shingles. Although this geometric framework is observed on many Phanerozoic low-angle platforms, rock types and facies successions can vary, especially if geologic times with contrasting amplitudes of relative sea level changes such as in the Pennsylvanian icehouse and Cretaceous greenhouse are compared. Two case-studies, the Pennsylvanian of the Paradox basin and the Upper Cretaceous of the Pyrenean basin, illustrate these two end members.

Desmoinesian sequences on the western shelf of the Paradox basin are characterized by a lower onlapping wedge of siliciclastic sandstone, an up-dip transgressive open-marine carbonate, an early highstand prograding mixed carbonate-siliciclastic sandstone and siltstone, and a late highstand succession of carbonate buildups, oolitic shoals and stromatolites. In contrast, Santonian shelf parasequences in the south-central Pyrenees are characterized by an older onlapping wedge with rudist and corals on the seaward side of the platform margin, and a younger prograding mixed carbonate-siliciclastic succession with diminishing quartz abundance basinward. Both the Pennsylvanian sequences and Cretaceous parasequences have an early onlapping wedge and later prograding, shingled wedge. The progr ding wedges consist of older clastic-rich and younger carbonate-dominated facies belts. However, Pennsylvanian buildups occur during late highstand, while Cretaceous buildups occur during transgression. Pennsylvanian sequences also contain a great diversity of rock types and multiple exposure surfaces. These contrasts may reflect the differences in the relative sea-levels changes during high-amplitude icehouse and low-amplitude green-house climate.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994