Abstract: Oscillating Estuarine Conditions Recorded in Holocene Deposits Near Present-Day San Francisco International Airport

Mary McGann

Holocene muds, with numerous shell layers and scattered shell fragments, comprise a 3.54 m core recovered in the southern San Francisco Bay in the vicinity of San Francisco International Airport (DJ6-93-SF, Core 6; 37°37^prime49.57^PrimeN, 122°21^prime59.20^PrimeW; 3.5 m water depth). The muds contain a subtidal estuarine benthic foraminiferal fauna found today at intermediate water depths (2-8 m) in the bay. The assemblage is characterized by a low-diversity (= 8 species), calcareous benthic fauna overwhelmingly dominated by Elphidiella hannai, E. excavatum selseyensis. Also abundant are Ammonia beccarii, Elphidiella hannai, E. excavatum clavatum and E. gunteri. Arenaceous species (Trochammina inflate and Textularia earlandi) are gene ally rare and planktic species non-existent.

The presence of E. excavatum selseyensis, abundant throughout the entire length of this core, previously was found to correlate with high organic content in the sediments. Therefore, it appears that the muds deposited at this locality have remained organic-rich throughout most of the Holocene. Fluctuations in the abundances of A. beccarii and E. hannai within this core, however, suggest possible changes in water temperature and salinity through time. Below 310 cm, and again from 225-90 cm, A. beccarii is abundant while E. hannai is rare or absent entirely, indicative of warm estuarine conditions with high or fluctuating salinities. From 310-225 cm and again near the top of the core (90-28 cm), E. hannai flourishes at the expense of A. beccarii, implying a decrease in water temperature and the re-establishment of less brackish conditions. The uppermost sample investigated in this core (10-12 cm) is unique in that E. excavatum selseyensis drops to its lowest abundance (49.8%), both A. beccarii and E. hannai are at, or near, their maximum occurrences, and the euryhaline species Trochammina hadai comprises a surprising 8.8% of the fauna. The latter species is widely distributed in the brackish waters of Japan and its only previous occurrence in this core is as a rare constituent of the benthic fauna from 20-22 cm.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California