Carboniferous stratigraphy of eastern San Saba County and western Lampasas County, Texas
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Carboniferous sedimentary rocks in eastern San Saba County and western Lampasas County are composed of an assemblage of lithosomes now assigned to the Houy, Chappel, Barnett, Marble Falls, Smithwick, and Strawn formations. The limestone, shale, and sandstone comprising these formations are genetically interrelated and are the result of two major episodes of sedimentation. Chert pebbles, carbonate sand, and mud accumulated as lag gravel, strand-line sand, and marine mud facies of the Houy, Chappel, and Barnett formations during an early Carboniferous marine transgression across exposed Ordovician rocks. Circulation in the marine basin must have been restricted as evidenced by the abundance of black organic material and general lack of benthonic fauna in the Barnett shale. With continued transgression, circulation improved and shelf carbonate facies of the Marble Falls Formation were deposited. The Marble Falls is divisible into three members informally termed Lower, Middle, and Upper Members. Detailed analyses of the characteristics and distribution of the facies in the Lower Member show that initiation of carbonate deposition coincided with establishment of a slope-break in the shelf at the western side of the Fort Worth Basin, and that the facies accumulated as oolitic shoals, interior sand, channel fills, and several types of algal build-up on a Bahaman-type platform. Sedimentation more than kept pace with platform subsidence; facies depocenters remained approximately fixed on the platform, but higher energy facies expanded at the expense of lower energy facies. Ultimately the platform either built to sea level and/or sea level dropped, subaerially exposing the rocks of the Lower Member, During the second Carboniferous marine transgression, shale and limestone of the Middle and Upper Members of the Marble Falls Formation accumulated on an open marine shelf as algal build-ups, calcarenite shoals, and restricted lows in which marine mud and spiculitic carbonate were deposited. Vertical, as well as lateral, facies changes are numerous, and the facies pattern is distinctly different from the facies pattern in the Lower Member, suggesting that facies in the Middle and Upper Members were subject to different sedimentologic controls. Terrigenous sediment of the Smithwick and Strawn formations, derived from the rising Ouachita fold-belt to the east, accumulated in a fluvial-deltaic depositional system that filled the Fort Worth Basin and prograded westward across the Marble Falls carbonate shelf. It is postulated that this terrigenous influx, occurring with carbonate deposition, is principally responsible for the facies pattern displayed in the Middle and Upper Members of the Marble Falls Formation. Locally superimposed on the delta environments was a fanlike system characterized by limestone conglomerate derived from small fault blocks of Marble Falls limestone exposed in the area. The picture was somewhat analogous to modern bajada deposits that are interbedded with tidal mud flats along the north end of Baja, California. On the basis of lithologic and time-stratigraphic relations, an integrated regional, fluvial-deltaic and carbonate shelf depositional model is proposed to explain the genetic relationships among facies of the Middle and Upper Members of the Marble Falls, Smithwick, and Strawn formations in Central Texas. This model also explains outcrop patterns previously interpreted to represent separate episodes of sedimentation and erosion.