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