Abstract: Recent Peritidal Carbonate Environments of the La Paz Area, Baja California Sur, Mexico: A Model for Subtropical Carbonate Deposition

The La Paz area in the southwestern Gulf of California, Mexico, provides an ideal site for studying Recent subtropical peritidal carbonate sediments and depositional environments. Intertidal carbonate sand flats and subtidal carbonate factories are well developed along the margins of the young active rift system. Siliciclastic admixtures only become important close to rocky shorelines and below a water depth of 30m, where they can account for up to 80% of the sediment. Underwater mapping and sampling efforts have revealed coralline red algae and corals as the main carbonate producers along with significant contributions from mollusks and benthic foraminifera. This compositional pattern stands in marked contrast to (1) the much better studied coral and green algae dominated fully tropical carbonate systems and (2) to foraminiferae and mollusk dominated temperate carbonate settings. Coralline red algae in the La Paz study site occur as both attached algal crusts and free living unattached red algal nodules (rhodoliths). Environments with similar biogenic constituents have been described from a variety of Cenozoic and modern carbonate systems. However, the significance of the co-occurrence of red algal nodules and corals in the characterization and identification of transitional subtropical carbonates has, to date, been neglected.

A detailed study of the benthic foraminiferal community  has revealed that assemblages include mixtures of tropical and temperate species clearly supporting the subtropical character of the La Paz carbonates. This is demonstrated by the abundance of Spirolina arietina, Peneroplis pertusus, Amphisorus hemprichi, and Quinqueloculina compta, which are representative of tropical communities. In contrast, the most abundant species with temperate affinities include Poreoponides cribrorepandus, Glabratella californiana, and Triloculina triaonula. 

AAPG Search and Discovery Article #90940©1997 AAPG Foundation Grants-in-Aid