ABSTRACT

Modern shelf margins like Yucatan constitute models for predictable biofacies trends and sedimentary facies patterns in fossil limestone hydrocarbon reservoirs. Podocopid (benthic) ostracodes are very abundant in modern carbonate sediments of this area (Quintana Roo, Mexico). Host sediments are distributed in shallow, brackish-water lagoons and bays. Seaward, in higher energy environments, carbonate shelf sediments coarsen to oolitic sand and even gravel size. Five widely spaced quantitative (total--live + dead) benthic samples collected from each of these two markedly different depositional environments contain six characteristic ostracode species. Ternary diagrams of the first three dominant species from low energy lagoons indicate: (1) Hemicyprideis n. sp. dominates the ostracode fauna living in these micritic Mg-calcite-rich muds, (2) second-order dominants are usually Xestoleberis sp. aff. X. margaritea, with some Paranesidea bensoni and others, (3) third-order dominants consist of P. bensoni and P. gigacantha. These ostracode species characterize low-energy micrites and organic muds of the lagoons. The percentage spread among the five samples of the first-order dominants is about 52%; whereas, this spread is approximately 24% among the second and third-order dominants. Ternary plots of the first three dominants in marine strait grainstones reveal: (1) P. gigacantha dominates these oolitically coated sediments, (2) second-order dominants are almost equally divided between Paranesidea n. sp. and P. gigacantha, (3) third-order dominants consist of P. n. sp. and Neonesidea longisetosa. This suite of ostracodes distinguishes high-energy sand waves and unstable carbonate substrates of the marine strait. The percentage spread among the five samples of the first-order dominants is about 32%; whereas, this spread is approximately 16% among the second and third-order dominants.

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