Abstract

Goleta Slough (Santa Barbara County, California) is typical of several small estuaries and lagoons located along the semiarid southern California coast. The slough presently consists of dendritic, shallow subtidal channels surrounded by intertidal salt-marsh vegetation (mostly Salicornia virginica). It is the last remaining remnant of a large marine embayment that spread into the Goleta Valley basin as sea level rose toward the close of the last glacial period.

Stratigraphic data from numerous Goleta Valley water wells indicate that the late Pleistocene embayment was filled by peripheral fluvial sands which wedge out into marine silts and clays closer to the slough’s ocean entrance. Sequential air photographs (1928 to present) reveal alluvial fans advancing into the slough from the adjacent Santa Ynez Mountains during the late stages of infilling. Living and relic foraminiferal faunas suggest that these fans reduced an open, well-flushed coastal lagoon to a system of narrow subtidal channels leading from a restricted ocean inlet, frequently closed by longshore sedimentation.

Five paleoenvironments associated with lagoon and salt-marsh settings were recognized in shallow cores collected from the slough: (1) subtidal ponds and channels, (2) intertidal flat, (3) intertidal salt marsh, (4) marsh creeks, and (5) alluvial fans. The subtidal lagoon sediments contain a diverse stenohaline (fossil) foraminiferal assemblage that has been replaced in the present slough interior by a single living euryhaline species (Ammonia becarri). Open coast foraminifera are presently found only near the slough’s ocean entrance, where conditions approximate a lagoonal environment. The occurrence of marsh sediments in cores collected beyond the present slough boundary indicates a reduction in the extent of the fringing salt marsh. The present marsh vegetation lacks the zonation typical of areas subject to regular tidal flooding. These faunal and floral changes indicate that a steady decline in marine influence has accompanied infilling and closure of the Goleta Slough.

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