ABSTRACT: Sequence Architecture of Lower Cretaceous Carbonate Shelf, Gulf Coast
R. W. Scott
Cretaceous carbonate shelves in the Gulf Coast and the Chihuahua trough in Arizona record both eustatic and tectonic events. The Lower Cretaceous Comanche shelf in Texas and Louisiana consists of five major depositional/seismic sequences. The regionally extensive bounding surfaces of each sequence are either drowning unconformities or exposure unconformities. Drowning is inferred where shallow shelf facies are overlain by deeper water pelagic facies. Exposure is inferred where coastal terrigenous facies overlie marine shelf carbonates or paralic clastics. The interval of downlap and maximum flooding may be either a sharp contact that corresponds with the sequence boundary, or may be a gradational interval where deeper facies change to shoaling-upward facies.
Medial Albian terrigenous clastics terminated the lower Albian carbonate shelf in the northern part of the Chihuahua trough. Farther south, the carbonate shelf persisted into the middle Albian. In Texas and Louisiana, lower and middle Albian shelf margins prograded southward up to 15 km. Low-angle simple sigmoidal sequence geometry (type 1) suggests low-energy areas of the shelf. Steep, complex-oblique sequences (type 2) suggested high-energy areas. During the late Albian, the shelf margin was drowned in many places, and pelagic facies overlie the shelf facies.
Rates of sediment accumulation within the low-energy prograding Fredericksburg sequence shelf margin are nearly two times those in the forereef basin and seven times greater than in the updip shoreward portion (8.98 cm/k.y.; 4.85 cm/k.y.; 1.28 cm/k.y.). numerous hiatuses probably punctuate the interbedded sand and carbonate section in the updip, nearshore part of the shelf. Even rates in the pelagic shelf of the Washita sequence are greater than in the updip intrashelf East Texas embayment (1.69 cm/k.y. compared to 1.28 cm/k.y.).
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990