Molluscan Assemblages of Bahia Concepcion: Holocene Analogs to Pliocene Marine Rift Basin Faunas of Baja California, Mexico

Degree Year

1995

Document Type

Thesis - Open Access

Degree Name

Bachelor of Arts

Department

Geology

Abstract

What is the applicability of present day biofacies and faunal assemblages for paleoenvironmental interpretation? This study considers the molluscan fauna of two analogous marine rift basins of Baja California, Mexico: the Holocene Bahia Concepcion and the Pliocene Loreto Basin. Molluscan faunal data were collected from 49 sample sites throughout Bahia Concepcion and from 4 partial stratagraphic sections from the Loreto Basin. The Holocene data were subjected to Q - mode and R - mode cluster analysis to delineate molluscan biofacies (sites with a similar faunal composition) and species assemblages (groups of species that commonly co - occur). Variation in faunal composition, ecological variables and depth produce distinct environmental zonation in Bahia Concepcion. This variation primarily tracks variation in substrate, water depth, salinity, and environmental energy. The applicability of the Holocene data for paleoenvironmental interpretation was tested in the Loreto Basin. Q - mode cluster analysis combining species presence - absence data for both the Holocene and Pliocene demonstrates that marginal marine to shallow marine conditions prevailed during much of the history of the Loreto Basin.

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