MODERN WARM-TEMPERATE AND SUBTROPICAL SHALLOW-WATER BENTHIC FORAMINIFERA OF THE SOUTHERN GULF OF CALIFORNIA, MEXICO

Abstract

The La Paz area of the southwestern Gulf of California, Mexico, provides an ideal setting for studying the distribution of Recent warm-temperate to subtropical littoral to inner neritic foraminiferal assemblages in a carbonate-dominated setting. Four representative subenvironments including two pocket bays, a rhodolith-dominated carbonate bioherm, and a mixed carbonate-siliciclastic high-energy beach were selected for detailed foraminiferal analysis. Assemblages of benthic foraminifera were compared to bottom observations, time-series temperature recordings, and occurrences of other carbonate-producing biota. Foraminifera are of intermediate importance among carbonate producers in the La Paz environments, making up 4% of the total calcareous biota. However, they reach abundances of up to 27% in middle shelf localities where species numbers are also highest. Miliolid foraminifera, dominated by the genus Quinqueloculina, are the most abundant group and frequently comprise more than 60% of foraminiferal assemblages. Larger foraminifera include Peneroplis pertusus pertusus, P. pertusus arietina, Amphisorus hemprichii, Gypsina vesicularis, and Sphaerogypsina globulus. The distribution of shallow-water assemblages in the La Paz environments is largely controlled by substrate type. Q- and R-mode cluster analyses revealed eight distinct foraminiferal biofacies that appear to be related to microlithofacies defined on the basis of biogenic components. Assemblages include mixtures of tropical and temperate species as demonstrated by the abundance of Peneroplis pertusus, Amphisorus hemprichii , and Quinqueloculina spp., which are representative of tropical communities. In contrast, the most abundant species with temperate affinities include Buccella parkerae, Gavelinopsis sp., Haplophragmoides sp., and Oolina melo. Application of the Murray model for distinguishing cool-temperate, warm-temperate, and subtropical faunas clearly points to the warm-temperate to subtropical character of the foraminiferal assemblages associated with the carbonate factories and environments of the southern Gulf of California.