Holocene geologic history of a transform margin estuary: Elkhorn Slough, central California

Abstract

Elkhorn Slough is California's second largest estuary and the United States' first estuarine sanctuary. It occupies the western reaches of Elkhorn Valley, a relic river valley eroded by drainage pouring out of the Santa Clara and/or Great Valley of California into Monterey Bay during the early Pleistocene. During the mid-late Pleistocene Elkhorn Valley was tectonically truncated from its headwaters by right-lateral movement along the San Andreas Fault. During the last glacial maximum, 16–18 × 10 3 years before present ( b.p), local drainage in Elkhorn Valley incised a stream channel at least 29 m below present day sea-level, as its base level was progressively lowered. With the ensuing Holocene sea-level rise, marine waters invaded this incised channel, floored with non-marine sandy gravels, creating a high-energy tidal inlet at the mouth of Elkhorn Slough approximately 8000 years b.p. As sea-level continued to rise, the main channel of Elkhorn Slough became filled by an estuarine, fining-upward sequence characterized by progressively shallower, lower-energy deposits. A quiet water estuary, considerably larger than the present-day Elkhorn Slough, existed approximately 3000 years b.p. As the Slough was slowly filling, salt marshes developed along its landward margins and have progressively advanced toward the center of the Slough during the past 5000 years. Historical records indicate numerous natural changes in the Elkhorn Slough vicinity. In the mid-1850s a.d. Elkhorn Slough was a minor tributary to the much larger Pajaro-Salinas River system which shared a common entrance to the Pacific Ocean north of Moss Landing. In 1909 winter storms modified the course of the Salinas River to its present location south of Moss Landing, while Elkhorn Slough persisted as a tributary to the Old Salinas River channel. Construction of jetties at the Moss Landing Harbor in 1946 provided a direct link between the Pacific Ocean and Elkhorn Slough. At this time, salt marshes began to retreat from the axis of Elkhorn Slough as it evolved into its present form as a relatively stable estuarine embayment. Had the jetties not been installed, Elkhorn Slough would have likely evolved into a freshwater wetland and eventually into a dry alluvial valley within 2000 years. The future fate of Elkhorn Slough will undoubtedly be controlled by relative sea-level changes, sediment supply, and human activities.