Sea gullies are localized dissections of California borderland upper basin slopes; the gullies form groups, covering distances of 10 to 16 km off points and narrow shelves. Gullies have youthful "V" cross-sectional profiles and relief of less than 100 m; they may extend to basin floors, creating small, coalescing submarine cones.

Gully development is attributed to subaerial stream erosion during eustatic sea-level lowering and outer-shelf exposure. Also, during rapid regional uplift, shelves become narrower and short, steep streams characteristic of immature drainage patterns prograde across shelves, depositing unstable coarse debris near the shelf break. Differential relief, overburden instability, and earthquakes trigger subaqueous mass flows, creating gullies by headward sapping; and at the base of slope, linear aprons of coarse conglomeratic debris accumulate.

Four stages of nearshore basin deposition are recognized in the lower Capistrano Formation (upper Miocene, Mohnian) at Dana Point, California. Basal deposits of fine siltstones, sandstones, and diatomite represent abyssal deposits. These are overlain abruptly by conglomerates interbedded with structureless coarse sandstones, superseded by graded sandstones and siltstones interpreted as mid-submarine-fan deposits.

Basin subsidence, tectonic activity, and increased erosion climaxed during the late Miocene. Conglomeratic debris flows are also associated with sinistral-coiled Globigerina pachyderma, indicating the late Miocene N-17 period of glaciation and possible sea-level lowering. Coarse debris accumulated near the shelf break, and gullied submarine slopes developed. Undermining through liquefaction caused debris flow down these gullies; the flows "froze" at the abrupt slope/basin junction, forming linear, composite conglomerate units. Overlying fan sequences reflect basin maturity dominated by canyon point sources.

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