Depositional and Structural Framework of the Distal Frio Formation, Texas Coastal Zone and Shelf
The basinward limit of the progradational continental platform of the Oligocene Frio depositional episode is generally defined by the zone of lower Miocene growth faults and expansion. The Frio offlap paleocontinental margin consisted of two broadly arcuate deltaic headlands, centered in the Rio Grande and Houston Embayments. Along each headland, delta lobes locally built to the shelf edge and onto the continental slope. Between the deltaic depocenters lay a narrow muddy shelf that graded landward into shoreline deposits of an extensive barrier/lagoon and strandplain coast. The Frio depositional sequence can be conveniently subdivided into three operational map units. During deposition of the lower unit, the Frio continental margin prograded rapidly gulfward, constructing a new continental slope. Large submarine canyons were cut along the south flank of the South Texas Norias delta system. The middle Frio unit records margin offlap; however, the Houston delta system did not prograde much beyond the platform established by the earlier surge of deposition. Local submarine erosion and subsequent deposition of onlapping sandstone and mudstone created the deep-water Hackberry Embayment. In contrast to the lower and middle Frio, the upper Frio interval was characterized by coastal stability interrupted by episodes of retreat and transgression. Sandy, distal facies of the delta and shoreline systems include frontal-splay, slump, andstorm-deposited inner-shelf sandstones. Such distal facies are commonly thinlyinterbedded with prodelta and shelf silt and mudstone and may be quite discontinuous.Offlap deposits of the upper slope of the Frio depositional episode are dominantlymudstone; local ponding of sandy, distal delta sandstone within intraslope basins and cut and fill of minor slope channels created deep-water reservoir facies that have proven to be productive of gas and condensate. Onlap slope deposits, which infill major erosional canyons and shelf-edge reentrants, have proven particularly productive.
Galloway, W. E., 1986, Depositional and Structural Framework of the Distal Frio Formation, Texas Coastal Zone and Shelf: The University of Texas at Austin, Bureau of Economic Geology, Geological Circular 86-8, 16 p. doi.org/10.23867/gc8608.
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The University of Texas at Austin, Bureau of Economic Geology