Establishing Foraminifera Based Biofacies Within Shallow Marine Deposits, Carpinteria Slough, CA: Implications for Southern California Sea-Level Studies

Abstract

Foraminiferal assemblages are useful for interpreting ancient marine and coastal sediments and strata through the defining of biofacies zones and reconstructing past sea levels and climates. However, their use for these purposes is dependent on knowing the environmental controls on their distribution, which varies regionally. In this study, we investigate the environmental variables controlling the distribution of foraminifera in Carpinteria Slough in southern California. The foraminiferal assemblages were determined at 29 sample locations within the marsh. Assemblages were complimented by measurements of elevation, pore-water salinity, pore-water pH, and grain size. Total organic carbon was also measured in 16 of these samples. Three distinct biofacies were identified using Q-mode cluster analysis. Four species of foraminifera were shown to have statistically different mean abundances between the three biofacies zones, through the implementation of one-way analysis of variance tests (ANOVA). Zone 1 is defined by mean abundances of Miliammina fusca comprising between 30 and 60% of the entire sample. Zone 2 is defined by mean abundances of Elphidium excavatum comprising between 0 and 2% of any sample, and Ammonia parkinsonian comprising between 2 and 4% of any sample. Zone 3 is defined by a mean abundance of Balticammina pseudomacscerens comprising between 45 and 50% of any sample. Correlation analysis, principle component analysis, and ordinary least squares linear regressions suggest that the distributions of foraminiferal species in Carpinteria Slough are linked to the environmental variables of elevation and salinity. Principle component analysis demonstrates that the percent of total variance in assemblage data is explained by elevation (33.68%), salinity (16.52%), pH (12.49%), and median grain size (12.42%). Although, the link between environmental variables and foraminiferal assemblages lacks definitive correlations, a final ANOVA, based upon elevation, is able to separate the foraminiferal data into two elevation zones. Zone A consists of all elevations below 0.83m and Zone B consists of all elevations above 0.83m. Therefore, foraminifera from southern California marshes provide limits to the refining of past relative sea-levels at the sub-meter scale. They also provide valuable constraints on the depositional interpretation of older coastal sediments and strata.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017