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The AAPG/Datapages Combined Publications Database

Pacific Section of AAPG


Field Guide to the Monterey Formation between Santa Barbara and Gaviota, California, 1994
Pages 1-15

Overview of the Stratigraphy of the Monterey Formation Along the Coastline between Santa Barbara and Gaviota, California

J. Scott Hornafius


Several stratigraphic sections of the Monterey Formation along the coastline west of Santa Barbara are well exposed and easily accessible. The stratigraphic sections between Naples Beach and Gaviota display a similar lithostratigraphic sequence over a twenty mile distance. The basal portion of the Monterey is represented by upper Saucesian and Relizian calcareous-siliceous shales that experienced large-scale soft-sediment deformation. The soft-sediment slumping suggests deposition on a slope in the late early Miocene. The overlying Luisian and lower Mohnian phosphatic shale interval is extremely organic-rich. Several phosphatic hardgrounds occur in the middle of this organic-rich interval (at the Luisian/Mohnian boundary). These hardgrounds correspond with a major depositional hiatus. The hardground interval suggests deposition on a banktop during the late middle Miocene. This hardground interval is absent in coastal exposures in the city of Santa Barbara. Instead, a 100-ft-thick porcelanite and chert interval is present at the Luisian-Mohnian boundary. This biogenic silica-rich interval resembles the upper Luisian and lower Mohnian fractured chert reservoirs in oil fields in the Santa Barbara Channel. The upper Monterey is dominated by siliceous shales and porcelanites. An increase in detrital content at the top of the Monterey Formation corresponds with the initiation of uplift of the Santa Ynez Range. Submarine canyon deposits that incise the Monterey Formation and are filled with Monterey chert cobble conglomerates or large blocks of Monterey shale in a matrix of Sisquoc shale suggest that the Santa Barbara coastline was on a shelf edge at the end of the Miocene.

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