Re-Evaluating the Age of Quaternary Conglomerate in the Ojai Valley of the Ventura Basin: Implications for the Tectonic Evolution of the Western Transverse Ranges

R. V. Heermance
California State University, Northridge, CA, [email protected]

Along the northern edge of the Ventura basin, the Ojai Valley trends northeast-southwest and is sandwiched between the Topatopa Mountains to the north and Sulpher Mountain to the south. Stratigraphy within the Ojai Valley consists of syntectonic Miocene and Pliocene strata truncated along an angular unconformity and overlain by a distinctive Quaternary “Ojai” conglomerate. The conglomerate contains primarily Eocene sandstone clasts derived from the Topatopa Mountains to the north, with clasts derived from the south confined to only the uppermost part. The angular unconformity at the conglomerate’s base implies an episode of uplift, deformation, and erosion within the late Pliocene and/or early Quaternary, but the precise timing of this event is unconstrained due to contradictory age-estimates for the conglomerate. Furthermore, the age of the north-to-south switch in the clast provenance is unconstrained. To date, the conglomerate has been interpreted as either 1) upper Quaternary terrace deposits (15,000-100,000 y.b.p.) or 2) middle Quaternary Saugus Formation equivalent (200,000-800,000 y.b.p). Both age estimates are problematic and imprecise. The conglomerate must be older than 100,000 years due to its greater dip beneath the more flat-lying terrace surfaces. A “Saugus” age, however, is inherently imprecise; moreover a direct correlation with other parts of the Saugus Formation has not been shown. New mapping suggests the conglomerate is syntectonic and occupies structurally controlled depressions formed due to uplift of the adjacent ranges. Ongoing research includes 1) mapping the three-dimensional geometry of the Ojai conglomerate, 2) describing any facies or provenance changes within the conglomerate that may imply local provenance changes due to uplift of the surrounding ranges, 3) using in-situ cosmogenic burial ages to date the deposits, and 4) using magnetostratigraphy to locate the Brunhes-Matayama (~780,000 y.b.p.) boundary. Together, this new work will allow the interpretation of uplift of Sulpher Mountain and constrain slip rates across faults that cut the Ojai conglomerate.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009