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Microfacies Analysis of Recent Carbonate Environments in the Southern Gulf of California, Mexico — A Model for Warm-Temperate to Subtropical Carbonate Formation

¹ Department of Geological and Environmental Sciences, Stanford University, Stanford, CA, 94305
² Centro Interdisciplinario de Ciencias Marinas, Playa el Conchalito, Apdo. Postal 476, La Paz, B.C.S. 23000, Mexico
³ Department of Geological and Environmental Sciences, Stanford University, Stanford, CA, 94305

The La Paz area in the southwestern Gulf of California, Mexico, provides an ideal site for studying Recent warm-temperate to subtropical carbonate environments. Carbonate factories include small pocket bays, a rhodolith dominated carbonate shelf, and a mixed-carbonate siliciclastic high-energy beach. Underwater mapping and constituent analysis have revealed free-living coralline red algae in the form of rhodoliths to be the main carbonate producers, contributing 33% of the biogenic constituents to the sediment. Other significant contributions come from corals (20%), molluscs (18%), echinoderms (5%), and benthic foraminifera (4%). The benthic foraminiferal community includes mixtures of tropical and temperate species. This compositional pattern stands in marked contrast to (1) better-studied coral and green algae dominated tropical carbonate systems and (2) foraminifer, bryozoan, and mollusc dominated cool-water carbonates. Cluster analyses of biogenic constituents and benthic foraminifera revealed distinctive coral, coralline red algal, and molluscan microfacies. However, subdivisions characterized by benthic foraminifera more closely reflect the bottom facies observed during underwater mapping as opposed to patterns defined by cluster analysis of biogenic components. Diagnostic features for interpreting similar environments found in the fossil record include the (1) co-occurrence of coralline red algae and corals together with the absence of calcareous green algae, (2) presence of 5–10 genera of larger benthic foraminifera, (3) laterally and vertically not extensive character of environments, and (4) changes of microfacies over short distances.

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