Paleogeographic Implications of Disconformities in Monterey Formation, California

J. Scott Hornafius, Ronald J. Echols, Bernie E. Felber

A disconformity resulting from nondeposition between 10 and 14 Ma is represented by a series of phosphatic hardgrounds in the Monterey Formation along the coastline between Santa Barbara and Point Conception, California. The disconformity can be recognized in nearshore wells by the high gamma-ray activity of the hardgrounds. The interval represented by the hardgrounds is the age equivalent of the prolific lower chert member of the Monterey Formation in the South Elwood, Hondo, and Point Arguello oil fields in the Santa Barbara Channel. Paleobathymetric indicators and isopach maps suggest that the condensed sections represented by the hardgrounds formed in a banktop environment, whereas the expanded sections represented by the producing intervals in the Santa Barbara Chann l were deposited in a basinal environment. Gamma-ray correlations in the offshore Santa Maria basin suggest that banktop environments also occurred in parts of the Point Pedernales and San Miguel oil fields. X-ray diffraction data suggest that in hemipelagic settings, Monterey stratigraphic sections deposited in a banktop environment are more clay rich (and biogenic-silica poor) than age-equivalent sections deposited in a basinal environment. Because Monterey initial production rates are proportional to the biogenic-silica content of the tested interval, the depositional environment strongly influences the producibility of the Monterey.

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