The Santa Margarita Formation in Ventura County, California, represents a transgressive-regressive sequence of rock deposited in a late Miocene shallow marine and paralic environment. A relatively steady low rate of subsidence with an initially low rate of sedimentation produced the transgressive phase. Increased rate of sedimentation produced a seaward progradational sequence or regressive phase.

Rocks that represent the transgressive sequence are herein divided into two facies: (1) facies A, a basal conglomerate zone and thick beds of bioturbated, locally fossiliferous, fine-grained sandstone with some large-scale cross bedding; and (2) facies B, laminated mudstone, phosphatic mudstone, and pelletic phosphate. Rocks that represent the regressive sequence conformably overlie facies B and are herein divided into three facies: (1) facies C, medium to very coarse-grained fossiliferous sandstone characterized by small to large-scale cross bedding; (2) facies D, thin to thick beds of laminated, nodular, and honeycomb gypsum and intercalated claystone, and lenticular-bedded and interlaminated mudstone and very fine-grained sandstone; (3) facies E, medium to thick-bedded mudstone and medium to very coarse-grained sandstone characterized by cross bedding.

Facies A represents marine deposition in a high-wave-energy, shoreface environment. Facies B represents deposition in a low-wave-energy, embayed, inner-shelf environment. Facies C was deposited in a high-wave-energy, east-west-trending, shoreface environment consisting of barriers, bars,

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tidal channels, and shoals. Facies D was deposited behind the barrier complex in hypersaline lagoon, evaporitic intertidal-flat, and supratidal-falt environments. Facies E represents deposition on a prograding fluvial plain. A modern analog for the late Miocene barrier-lagoon-tidal flat complex exposed in Sespe Creek area is the coastal zone between San Felipe, Baja California, and the present mouth of the Colorado River.

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