SEQUENCE STRATIGRAPHY OF A MIDDLE EOCENE SUBMARINE-CANYON COMPLEX AND RELATED STRATA, SAN DIEGO, CALIFORNIA

Beach cliffs north of San Diego, California, provide superb three-dimensional exposures of a Middle Eocene submarine-canyon complex. A submarine sequence boundary defines the canyon base. Two sequences comprise the fill; two sequences are truncated beneath. Workers previously interpreted the strata as a succession of time-equivalent, interfingering formations. We instead interpret multiple sequence boundaries between and, in some cases, within formations.

The canyon floor is a "sand-on-sand" sequence boundary that lies within a single (lithostratigraphically defined) formation. This surface unconformably separates lagoonal and tidal deposits of the underlying Delmar and Torrey sequences from bathyal units of the Ardath Sequence. The erosional surface is plucked and stepped, and displays injection features, pry-ups, intraclast-filled scours, and erosional remnants protruding up into the overlying fill. Basinward, the sequence boundary cuts out the Delmar and Torrey sequences, placing deep-water deposits onto a kaolinitic paleosol which caps the Mount Soledad Sequence.

Internally the canyon complex displays multiple cross-cutting channels on a multitude of scales and with widely diverse lithologies. The lowermost section (Ardath Sequence) comprises amalgamated, pebbly and diffusely laminated sandstones, which fine upward to convolute-bedded fine-grained sandstones, which then grade into laminated to bioturbated silty mudstones. The mudstones fill channels that exhibit a sinuous morphology. Multiple erosional episodes scoured each channel; deposition predominantly occurred during abandonment.

A second submarine sequence boundary occurs within the canyon succession, eroding into the Ardath Sequence and dividing it from the overlying Scripps Sequence. This sequence boundary is a "mud-on-mud" contact. Large slump blocks line the base of the sequence boundary and represent canyon rejuvenation. The overlying section displays laterally interconnected coarse-grained channels arranged in a braided architecture. Complete channels contain basal conglomerates overlain by pebbly sandstones that grade upward to interbedded sandstone and siltstone, and finally mudstone.

Lithologic predictability within ancient submarine canyons is problematic. Variable channel fills in this Eocene system produce complex vertical and lateral patterns. However, the canyon succession exhibits overall large-scale fining-upward trends above each sequence boundary, yielding a general facies model.