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Abstract

Beach cliffs north of San Diego, California, provide superb three-dimensional exposures of an exhumed Eocene submarine-canyon complex. Photomosaics aid in analyzing bounding surfaces and lithologic patterns.

An irregular sequence boundary defines the canyon floor. Two stratigraphic sequences comprise the canyon fill; two sequences lie beneath. The canyon base unconformably separates lagoonal and tidal deposits of the underlying Delmar and Torrey sequences from bathyal units of the Ardath Sequence. A second submarine sequence boundary occurs within the canyon succession, eroding into the top of the Ardath Sequence and dividing it from the overlying Scripps Sequence. A fifth (shallow-marine) sequence, which is not described here, truncates the Scripps Sequence to the north and inland. Pleistocene wave-cut terraces plane off the Eocene interval at the top of the cliffs. Internally the canyon complex displays multiple cross-cutting channels on a multitude of scales and with 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 canyon fill (Ardath Sequence) comprises amalgamated, pebbly and diffusely laminated sandstones, which fine upward to convolute-bedded finegrained 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; channel fill predominantly occurred during abandonment. Subsequent flows evacuated a multitude of cross-cutting successions. Coarser grained units often punctuate the mudstone channels. Some contain interbedded T(b)c-e turbidites. Others display a mantle of mudstone along their base overlain by a massive sandstone plug

Large slump blocks define the base of the Scripps Sequence. They line the sequence boundary and represent canyon rejuvenation. The overlying section displays laterally interconnected coarse-grained channels 100’s 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 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.

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