The Capistrano Embayment comprises a distinct geologic unit of the southern California area. Paleobathymetry, sediments, and microfaunas within the embayment indicate that it has had a different structural history, different from the Los Angeles Basin on the north. Marine invasion of the trough began in the Paleocene and ceased in the Pleistocene. This report develops a detailed history of the embayment from middle Miocene to late Pliocene time using data from two well exposed sections within the boundaries of the embayment.

Analogy between ecologic niches of living benthonic Foraminifera and fossil forms encountered indicates that middle bathyal depths were attained by the middle Miocene. By late Pliocene the trough was filled to shelf depths. Repetitive changes in morphology of benthonic species provides additional evidence of varying paleobathymetry.

Variation in abundance of cool and warm-water planktonic Foraminifera indicates three periods of distinctly cool surface temperature between late Miocene and late Pliocene time. Increase in radiolarian diameter provides evidence for surface temperatures in sediments barren of Foraminifera.

Peak radiolarian abundance suggests that the deepest point in basinal evolution existed in the early Pliocene at which time water depth neared 1,750 meters.

Correlation of the two sections is based on: (a) a Mohnian horizon of Globigerina pachyderma which coil sinistrally, (b) the uppermost point of abundant radiolarian tests, (c) the uppermost point of the radiolarian Prunopyle titan, and (d) the horizon of peak radiolarian number. These mutually corroborative planktonic criteria demonstrate the time transgressive relationship of existing stages based on benthonic Foraminifera. Repettian faunas, for example, appear much earlier in the rapidly filling southern end of the embayment than in the northern deeper area.

A restricted, oxygen-deficient, closed-basin system, characterized by laminated diatomaceous sediments, originated during a period of early Miocene diastrophism. Closed-basin conditions allowed only a marginal benthonic foraminiferal fauna to exist, analogous to the existing fauna of the oxygen deficient Santa Barbara basin. Restricted basin plain conditions prevailed until the end of the Miocene.

Instantaneously deposited coarse sediments (turbidites) emphasize pulses of structural activity in the middle Miocene, late Miocene, and middle Pliocene. Sediments provide evidence of a landmass to the west of the embayment. Turbidites are recognized on the basis of displaced Foraminifera, plant material, and sedimentary structures.

Paleoecologic and sedimentary analyses delineate a characteristic basin-filling sequence in the Capistrano Embayment, similar to events taking place today in the Gulf of California and off southern California.

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