Phosphatic Hardgrounds and Hiatus Concretions in Neogene Marine Sequences of California Coastal Ranges
R. E. Garrison, K. B. Follmi, K. M. Friede, M. Kastner, P. C. Ramirez
Distinctive phosphatic intervals mark hiatuses of varying durations at several localities along the central California coast. At Capitola in the La Honda basin, a complex series of carbonate-cemented hardgrounds containing authigenic phosphatic nodules and marine vertebrate bones occurs within 50 cm of the upper Purisima Formation (Pliocene). Paleomagnetic measurements and diatom stratigraphy indicate that this 50-cm interval represents a hiatus of about 1 m.y. (4.5-3.5 Ma). At Naples Beach in the Santa Barbara basin, a severely condensed section of about 7 m of interbedded nodular phosphatic hardgrounds, phosphatic marlstones, and dolomites appears to represent as much as 5 m.y. (14.5-9.5 Ma). In the Lompoc Quarry of the Santa Maria basin, a thin phosphatic hardground at the contact between the Monterey and the Sisquoc Formations represents about 0.5-0.7 m.y. (between 7.0 and 6.0 Ma). At Mussel Rock in the Santa Maria basin, a nodular phosphatic conglomerate occurs at the boundary between the Monterey and Sisquoc Formations; the age of the conglomerate determined from diatom biostratigraphy is approximately 6.2 Ma (Messinian), but no time gap can be recognized given the resolution of diatom age dating.
The phosphatic hardgrounds of Capitola, Lompoc Quarry, and Naples Beach appear to correspond to high sea level stands, whereas the conglomerate at Mussel Rock coincides with a sea level lowstand.
During high sea level stands, depocenters shifted landward, leading to clastic-sediment starvation possibly accompanied by current-induced erosion and winnowing, which in turn led to condensation and early diagenetic carbonate and phosphatic cementation. Persistent starvation, as at Capitola, Lompoc Quarry, and Naples Beach, led to the development of complex mature hardgrounds, whereas relatively short-lived starvation followed by reworking during lowered sea level stands promoted the development of phosphatic conglomerates, as at Mussel Rock.
AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.