Abstract: Evidence for a Tectonic Control of the Rincon-Monterey Transition, South Elwood Field, Santa Barbara-Ventura Basin

Nathan R. Miller

In Santa Barbara-Ventura Basin, deposition of the petroliferous Monterey Formation and subjacent Rincon formation is generally interpreted as a transgressive response to basin deepening, within a Neogene cycle of basin formation and fill. However, the relative interplay of tectonic, eustatic, and productivity influences on deposition is poorly constrained, largely because of poor preservation and limited regional exposures of the Rincon-Monterey contact. Although a gradational lithologic change is recognized (from massive bedded clay shales to less detrital carbonaceous, phosphatic, and siliceous mudstones), onset of Monterey deposition is often considered to occur above a widely reported bentonite/tuff,

traditionally correlated to the rhyolitic Tranquillon volcanic center. Thus, the depositional context of the Rincon-Monterey transition is uncertain in terms of fundamental influences.

Detailed analysis (core description, chemostratigraphy, and ⁸⁷Sr/⁸⁶Sr stratigraphy) of a continuous 300 m core of the Monterey and uppermost Rincon Formations from the offshore South Elwood field offers a fresh perspective on this issue. The bentonite marker bed is thin or missing in the South Elwood core, and the Rincon-Monterey contact is instead picked from electric log response corresponding to an upward transition to diminishing detrital input and increasing organic matter content. Up-section decreases in detrital proxy indicators (Al₂O3, K₂O, TiO₂) and concomitant increase in total organic carbon confirm this change. Although the gross lithologic transition is visually subtle, a dramatic increase in the stratigraphic frequen y of volcanoclastic components (largely as subcentimeter bentonites of probable rhyolitic affinity) occurs at this level. The base of this volcanostratigraphic succession is conspicuous as an 8-m-thick interval of interbedded calcareous mudstones and thin turbidite sands. Microprobe analysis reveals that the sands are an admixture of plutonic/metamorphic detritus and intermediate-grade volcanic phenocrysts (silicic andesite to dacite). The Santa Cruz Island volcanics or eastern equivalents are the closest source of intermediate volcanism. Based on ⁸⁷Sr/⁸⁶Sr stratigraphy, the transition occurs at ~20 ± 1 Ma, predating the Monterey base at Naples Beach immediately above the "Tranquillon" bentonite by 1 to 2 m.y. The stratigraphic frequency of bentonite occurrence declines episodically, and is essentially dormant by ~15 ± 1 Ma.

Coincidence of the Rincon-Monterey lithologic transition with the increase in volcanostratigraphic frequency suggests that the onset of Monterey deposition at South Elwood was tectonically controlled. The record of silicic-to-intermediate volcanism may correspond to an episode of transtensional rifting and active volcanism along the northwest (Tranquillon) and southeast (Santa Cruz Island) margins of Santa Barbara-Ventura Basin, associated with rotation and northwesterly translation of the western Transverse Ranges. Wrench down-dropping of the basin floor may have fostered restricted detrital influx and basinal circulation, necessary for the subsequent accumulation and preservation of laminated carbonaceous and biosiliceous lithofacies.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994