Depositional Patterns of Miocene Facies, Middle Texas Coastal Plain

Miocene strata along the Middle Texas Coast form an offlapping sequence of fluvial and deltaic sandstones and shales overlying shelf shales of the Oligocene(?) Anahuac Formation. Subdivision of Lliocene strata into thin timecontemporaneous units reveals that the axis of Miocene sand deposition migrated gulfward with time until it reached a position 2 to 12 miles (7.5 km) seaward of the present coast. There, strike-fed Miocene sands were deposited in a vertical stack gulfward of the axis of maximum Oligocene Frio sand deposition. A minor transgression occurred in late Miocene followed by further gulfward progradation of Miocene depositional systems.Mapping of Miocene sandstone distribution, along with examination of electrical-log characteristics, reveals the existence of three depositional systems within each time-contemporaneous unit: (1) an updip fluvial system with dip-oriented sands, (2) a high-destructive deltaic-strandplain system with dominantly strike-oriented sands, and (3) a downdip, distal deltaic-shelf system composed of thin sands separating prodelta and shelf muds.Structural features greatly influenced Miocene deposition. Vertical stacking of fluvial channel-fill facies suggests a deep-seated structural control of channel positions. During much of the Miocene, large growth faults and basinal subsidence stabilized delta locations by accommodating all incoming sediment, thereby preventing progradation.