Bounding surfaces, Lithologic Variability, and Sandstone Connectivity within Submarine-Canyon Outcrops, Eocene of San Diego, California

May, J.A.,, DDD Energy, Denver, CO, and J.E. Warme, Colorado School of Mines, Golden, CO

Abstract

Beach cliffs provide superb three-dimensional exposures of an exhumed submarine-canyon complex in the Eocene strata of Black's Beach, San Diego, CA. Photomosaics aid in analyzing bounding surfaces and lithologic patterns. The basal surface is an irregular sequence boundary that unconformably separates lagoonal and tidal deposits from bathyal units. Internally the canyon complex displays multiple cross-cutting channels on a multitude of scales and widely diverse lithofacies. Individual channels range from subtly scoured and only a few meters deep to over 1 km wide and up to 100 m deep. The lowermost fill (Torrey Sandstone) comprises amalgamated, pebbly and diffusely laminated sandstones. These fine upward to convolute-bedded fine-grained sandstones, which grade into laminated to bioturbated silty mudstones (Ardath Shale).

Mud-dominated channels of the Ardath have a meandering morphology. Multiple erosional episodes scoured each channel; fill occurred during abandonment. Subsequent flows evacuated cross-cutting successions. Coarser grained units often pun ctuate the mudstone channels. Some contain interbedded "T(b)c-e" turbidites. Others display a mantle of mudstone along their base overlain by a sandstone "plug."

A second sequence boundary occurs within the canyon. Large slump blocks line this surface, representing canyon rejuvenation. The overlying section (Scripps Formation) displays laterally interconnected channels hundreds of meters wide but less than 30 m thick, arranged in a braided architecture. Complete successions in these channels contain basal conglomerates overlain by pebbly sandstones that grade to interbedded sandstone and siltstone, and finally mudstone.

Lithologic predictablity within ancient submarine canyons is problematic. Variegated channel fills in this complex produce variable vertical and lateral patterns. However, the canyon system does exhibit large-scale fining-upward trends above each sequence boundary, yielding a general facies model.