Subaerial to submarine transitions in early Miocene pyroclastic flow deposits, southern San Joaquin basin, California
Dacitic pyroclastic flow deposits within the Tecuya Formation of the San Emigdio Mountains, California, exhibit a clear transition from subaerial to submarine deposition across the southeastern margin of the San Joaquin basin. The dacitic pyroclastic deposits are part of an informal volcanic member within the Tecuya Formation, consisting of a lower dacitic unit and an upper basaltic unit. Volcanogenic facies transitions within each of these units can be traced westward, obliquely across paleoshoreline, for 15 km. Depositional environments of the dacitic unit are well constrained by underlying nonvolcanic siliciclastic and overlying basaltic deposits.

The lowermost dacitic unit consists of pyroclastic surge and fallout deposits, which overlie nonmarine alluvial-fan deposits and were produced by subaerial, phreatomagmatic eruptions. A second, magmatic eruptive stage resulted in deposition of dacitic pyroclastic flows on top of the subaerial surge and fallout deposits, and within marine, nearshore to outer-shelf settings. These pyroclastic flows, generated from subaerial vents, were high-concentration, poorly fluidized flows which traveled down paleoslope into the marine part of the basin. Upon reaching water, the hot pyroclastic flows continued to flow subaqueously, partially mixed with water, and were quenched. Transitions to cool submarine pyroclastic debris flows may have occurred. Surface transformations of the submarine pyroclastic flows/pyroclastic debris flows generated dilute pyroclastic turbidity flows, which at times became detached and continued to flow independently downslope. Progradation of submarine pyroclastic flow deposits to the west into deeper marine settings likely accompanied the growth of subaerial volcanic vents to the east. A second, probably submarine, vent also produced submarine pyroclastic flow deposits in the western part of the basin. Deposition of reworked tuff and epiclastic debris flows within marine settings accompanied primary pyroclastic surge, fallout, and flow deposition.

The Tecuya volcanic member is an important ancient example in which the depositional environments of subaerial to submarine pyroclastic deposits are well constrained by bounding units. The characteristics of the subaerial and submarine pyroclastic flow deposits within the Tecuya volcanic member thus may be useful in other ancient volcaniclastic successions which lack bounding units of obvious paleoenvironmental origin.