The Thornton Reef has long been a model of reef sedimentology, but as thick tar occurs in the upper 22 m of the porous and deeply eroded buildup, Thornton is also a well-exposed fossil oil field.

Thornton Reef is about 2 km in diameter and bowl-shaped in cross section. The reef consists of radially and steeply dipping flank beds of dolomitized crinoidal wackestone and minor coral boundstone. Reef porosity (5 to 10%) is dominantly secondary, consisting of fossil molds, vugs, intercrystal voids, and fractures. At time of deposition porosity was probably high (50 to 70%), and consisted largely of intraparticle and interparticle pores of all sizes. Abundant hardgrounds and palisade-cemented grainstones suggest major reduction of depositional porosity by syndepositional submarine and marine phreatic carbonate cementation. Secondary dolomite preferentially replaced abundant fine-grained carbonate sediment but only partly replaced fossils. Leaching removed the remaining calcitic part of the fossils, slightly enhancing porosity. Extensive fracturing of the reef began at deposition and continued throughout the reef's geologic history, producing fractures that may extend hundreds of meters laterally. The fractures may be open or filled with syndepositional carbonate or younger terrigenous sediment. Thornton Reef's superb exposures and reservoir scale assure its importance to geologists studying reef facies and porosity.

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