ABSTRACT

Along the south Texas shelf margin, a vast carbonate shelf complex with an extensive barrier reef system and abundant shallow lagoon and skeletal shoal deposits existed during Aptian-Albian time. The Sligo Formation represents over 609.6 m (2,000 ft) of deposition along this margin.

Facies types along the shelf edge were quantitatively delineated by cluster analysis of detailed point-count data from 90 thin sections of whole cores from five wells. In addition, studies of 42.6 m (140 ft) of core slabs and thin sections of well cuttings from four other wells were used to establish a regional depositional model.

Along the Sligo shelf edge, three major facies occur: reef or reef rubble (two subfacies), back reef (three subfacies), and lagoonal (two subfacies). Reef facies are dominated by caprinids and also contain solenoporid algae, stromatoporoids, and an assortment of corals. Behind the reef, a spectrum of extensive back-reef deposits interfinger with shallow (5 m or 16.4 ft), lagoonal sediments. Farther behind the shelf-margin reef complex, along the outer shelf, benthic foraminifera, peloids, and ooids were deposited in high-energy shoals, and are interbedded with low-energy lagoonal sediments.

Two types of buildups probably existed along the Sligo shelf margin and the equivalent Cupido shelf margin to the south: wave-resistant coral-caprinid-stromatoporoid barrier reefs (adjacent to restricted lagoonal facies), and low-lying rudist banks (adjacent to diverse, washed lagoonal facies).

On a regional scale, the extensive barrier reefs of Australia and Belize are modern analogs for the vast Sligo reef trend. On a local depositional scale, the Permian Capitan reef complex in southeastern New Mexico is an ancient analog for the low-lying rudist banks of the Sligo shelf margin and the associated shoal deposits of the outer shelf.