--> Distribution of Mesozoic Basement Rocks in Santa Maria Basin, California, by Hugh Mclean; #91024 (1989)

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Distribution of Mesozoic Basement Rocks in Santa Maria Basin, California

Hugh Mclean

Published drilling records from the Santa Maria basin (SMB) report that approximately 200 wells reached bottom in basement variously called Franciscan, Knoxville, Jurassic, Cretaceous, serpentine, and granite. All of these units except granite have been mapped in areas adjacent to the basin. Samples from approximately 50 SMB wells indicate that the northern part of the basin between the Pezzoni-Casmalia and Santa Maria basin faults is floored mainly by potassium feldspar-bearing arkosic strata that resemble the Great Valley Sequence (GVS). The arkosic rocks are inferred to be Campanian or younger, and structurally overlie rocks of the Franciscan complex. Basement in the middle of the basin consists of ophiolitic rocks that extend southeastward from Point Sal. The ophiolit is overlain by Upper Jurassic and Lower Cretaceous GVS strata, and by locally derived heterolithic sandstone and conglomerate of the middle Tertiary Lospe Formation. The southern area of the SMB that lies north of the Lompoc-Santa Ynez fault is underlain by Upper Jurassic(?), Lower Cretaceous and pre-Campanian Upper Cretaceous strata that also structurally overlie Franciscan melange.

Great Valley Sequence units in the SMB were identified using sandstone petrofacies that previous workers applied to Upper Jurassic and Cretaceous sandstone sequences west of the Sur-Nacimiento fault. Graywacke phacoids in Franciscan melange tend to mimic GVS quartz, feldspar, and lithic fragment (QFL) ratios, but lack potassium feldspar, suggesting that several GVS petrofacies were tectonically incorporated into the Franciscan and subsequently altered to varying degrees. Relative abundance of smectite, illite, and chlorite in pelitic interbeds, using x-ray diffractograms are a useful adjunct in distinguishing basement rock types because clay minerals tend to reflect provenance and burial history.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.