Gravity Tectonics in Submarine Canyon Fill on a Subducting Margin: Paleogene Strata at Point Lobos

Clifton, H. Edward
[email protected]

The Carmelo Formation at Point Lobos State Natural Reserve is a remarkable accumulation of deep-water sandstone and conglomerate that generations of geologists have examined. Its geometry, fauna, and physical structures indicate deposition in the upper reaches of a submarine canyon. The exposures, which are continuous for nearly 1.5 km along the Reserve’s South Shore, are superb. The rocks, however, display some odd features. Intervals exist where the stratigraphy and structure seem to lack coherency; elsewhere they are locally shattered by faults of diverse sizes that display lateral offset. Evidence exists of large-scale substratal deformation and disaggregation of semi-consolidated sediment. Although horizontal Carmelo stratification elsewhere in the Reserve suggests that there has been little, if any, regional tilting, the Carmelo strata adjacent to the contact with underlying Cretaceous granodiorite in the southern part of the Reserve are vertical, concordant with the lithologic boundary. Near the basal contact in the northeast (up-stream) part of the Reserve, large conglomeratic slides from the canyon wall suggest a sudden evacuation of previously deposited fill in the axis of the canyon. These observations, in combination, suggest that a giant slide of previously deposited sediment within the canyon occurred, possibly due to over-steepening of the subducting margin. Remarkable oblique aerial photographs, taken as part of the California Coastal Records Project, allow the delineation of slip surfaces within the rock that provide an explanation of many of the anomalous features in the Carmelo. It is questionable whether any of the rocks exposed on the South Shore are in their original place.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013