Monitoring tectonically controlled marine to lacustrine transitions using organic facies — Ridge Basin, California, U.S.A.

Abstract

Ridge Basin is an elongate, intermontane basin filled with Upper Miocene/Pliocene clastics, located ∼80 km north-northwest of downtown Los Angeles. Over 13 km of marine, lacustrine and fluvio-deltaic sandstones and shales are present. Because of southeastward depocenter migration throughout basin development, no more than 4–5 km of vertical section are present in any one location. This has resulted in an ideal situation to study organic facies variability, namely a thick, thermally immature stratigraphic succession with only minor variations in maturity level. In this paper, we use molecular, isotopic and petrographic data to help identify systematic variability in organic facies in the Castaic, Ridge Route and Peace Valley Fms. (all Late Miocene), and relate these variations to the paleolimnology, tectonic history and oil source rock potential of the basin. As strike-slip movement continued along the San Gabriel fault during the Late Miocene, the sedimentation regime changed from a marine dominance to one controlled by lacustrine and fluvio-deltaic processes. This transition results in changes in the organic facies within the basin, as reflected in: (a) a systematic decrease in organic carbon isotope ratios; (b) an increase in relative yields of phenolic and n-aliphatic structures in the kerogen pyrolysate; (c) an increasing occurrence of perhydro-β-carotene (indicating episodic development of a halocline in the water column); and (d) decreasing pristane/phytane and C 29 C 28 sterane ratios. Unusual patterns of kerogen maceral distribution and A-ring sterane methylation are also observed. These geochemical data support the existence of a marine embayment in the early Late Miocene, yielding to a lacustrine and fluvio-deltaic setting which experienced considerable variation in bathymetry, salinity and oxicity. Low levels of thermal maturity preclude the occurrence of hydrocarbon source rocks, although source rock potential is present in sporadic localities. Optical and molecular maturity indicators show no systematic variation with stratigraphic succession. This lack of variation is consistent with the known tectonic development in the basin and the “shingled” sedimentation pattern resulting from a migrating depocenter. Internal variability in the sterane and hopane epimer ratios may eventually be useful in estimating the uneven extent of overburden in the Peace Valley and Ridge Route Fms., leading to applications elsewhere in intermontane lacustrine basins with tectonically migrating depocenters.