Abstract: Age of Sedimentation/Diagenesis for Bedford Canyon Formation and Santa Monico Slate in Southern California; Rb/Sr Evaluation

Joseph J. Criscione, Terry E. Davis, Perry Ehlig

Previous isotopic studies of fine-grained sedimentary rocks suggest that Rb/Sr isochron ages, in many cases, approximate very closely the time of either sedimentation or diagenesis. In essence, the process of erosion, transportation, sedimentation and, in particular, diagenesis can produce such a high degree of mixing of detrital and authigenic components that a given bed of fine-grained sediment contains a practically homogeneous initial ⁸⁷Sr/⁸⁶Sr ratio (Ri), whose actual value is determined by properties of the source area. Furthermore, diagenesis also may lead to substantial variations in Rb/Sr ratios within a single bed, or within a closely related set of beds, at the same time as strontium isotope homogenization is approaching completion. If the e conditions are met, whatever the mechanism, a linear Rb/Sr whole-rock isochron may date the time when the ⁸⁷Sr/⁸⁶Sr ratio in the sediment was last uniform and when the variation in Rb/Sr ratios was introduced.

Rb/Sr isotopic dating methods have been applied to the Bedford Canyon Formation and the Santa Monica Slate, which are Mesozoic sedimentary formations exposed in basement complexes of the Santa Ana and Santa Monica Mountains, respectively. Unmetamorphosed, fine-grained sedimentary rocks (mainly shale and argillite) from these formations were analyzed to determine: (1) time of sedimentation or diagenesis; (2) location, geochemical, and petrologic characteristics of the source terrane; and (3) possible regional stratigraphic correlation between these rock units.

The Bedford Canyon Formation is a tectonically disturbed, flysch sequence containing sparse faunas of Bajocian (171 m.y.) and Callovian (156 m.y.) age. Rb/Sr isochron ages for fine-grained sediments indicate two separate sedimentation events: (1) Permian-Triassic (228 ± 4.3 m.y.--Ri 0.70965) and (2) Middle Jurassic (175.8 ± 3.2 m.y.--Ri 0.71267). The fossil ages correlate well with the youngest event identified by Rb/Sr methods but there is no fossil evidence to support the oldest sedimentation age. The Rb/Sr data define high-precision isochrons and show no indication of two-component mixing.

The Santa Monica Slate contains late Oxfordian (149 m.y.) to early Kimmeridgian (141 m.y.) fossils. Rb/Sr isotopic data indicate a possible sedimentation age of 163 ± 14 m.y. (Ri 0.7057). However, the isochron age may not be valid because: (1) the data (on a plot of ⁸⁷Sr/⁸⁶Sr versus strontium concentration), fits a well-defined, two-component, hyperbolic mixing curve; and (2) the scatter of points on the isochron exceeds experimental error. Thus, a common ⁸⁷Sr/⁸⁶Sr ratio apparently did not exist during sedimentation or diagenesis.

The Ri and Rb/Sr ratio for the Bedford Canyon Formation and Santa Monica Slate are distinctly different and presumably related to average geochemical characteristics of the source terrane. Bedford Canyon sediments probably were derived from an old, igneous-metamorphic terrane (high Ri and Rb/Sr) whereas Santa Monica Slate sediments may have been derived from a young, mafic volcanic source (low Ri and Rb/Sr). In the event that seawater strontium may have affected the ⁸⁷Sr/⁸⁶Sr ratio during sedimentation/diagenesis, the Ri for the Bedford Canyon Formation only could have been lowered from its original value and the Ri for the Santa Monica Slate been increased.

AAPG Search and Discovery Article #90963©1978 AAPG/SEG/SEPM Pacific Section Meeting, Sacramento, California