Lower Cretaceous Shelf-Margin Carbonate Facies, Northwestern Gulf of Mexico: ABSTRACT

Abstract

Lower Cretaceous carbonate sediments accumulated on a broad shallow-water shelf which completely encircled the Gulf of Mexico. Along the edge of this shelf, biogenic growth climaxed with a complex of rudist reefs, banks, bars, and islands. The shelf-margin trend has been recognized offshore north of Yucatan; along the east coast of Mexico in the Golden Lane and Poza Rica trend in the subsurface, and outcrops in the Sierra Madre Oriental; through Texas and Louisiana parallel with and approximately 100 mi inland from the present coastline; and offshore of Florida along the west Florida Escarpment. These shelf-margin deposits are best known from the subsurface Golden Lane and Poza Rica trend, the outcrop in the Sierra Madre Oriental, and the subsurface of south Texas, where t is commonly referred to as the Stuart City trend. Five major depositional environments have been recognized from the carbonate rocks of the south Texas Stuart City trend: shelf lagoon, shelf margin, upper shelf slope, lower shelf slope, and open marine. The shelf-lagoon facies include miliolid wackestone, mollusk wackestone, toucasid wackestone, and mollusk-miliolid grainstone. These facies accumulated under generally low-energy conditions in water depths from 0 to 20 ft (0 to 6 m). In contrast, the narrow band of shelf-margin carbonate beds is made up of algae-encrusted miliolid-coral-caprinid packstone, coral-caprinid boundstone, requienid boundstone, and rudist grainstone, all of which accumulated in moderate-energy to high-energy waters less than 15 ft (5 m) deep. Seaward of the shelf margin, the upper shelf-slope environment com rises the caprinid-coral wackestone and coral-stromatoporoid boundstone facies, and the lower shelf slope comprises the intraclast grainstone, echinoid packstone, and End_Page 416------------------------------ echinoid-mollusk wackestone facies. Farther seaward, in water depths greater than 60 ft (18 m), the open-marine environment is represented by the planktonic foram wackestone. Intraparticle, interparticle, and fracture porosity are present in the thick limestone section along the Stuart City shelf margin. Intraparticle porosity, in places reaching 20%, is common, although permeability in facies with intraparticle porosity is low. Facies with interparticle porosity greater than 5% have good permeability up to 10 md. Permeability in any facies may be enhanced by the presence of thin fractures which were common in several cores. Only four facies, however, have greater than 5% porosity and 5 md permeability--the algae-encrusted miliolid-coral-caprinid packstone, mollusk grainstone, rudist grainstone, and coral-stromatoporoid boundstone. Rudist grainstone is potentially the most consistent in terms of porosity and permeability, thickness, and lateral extent. End_of_Article - Last_Page 417------------