SLOAN FORMATION: APPLICATION AND IMPLICATIONS OF A FORGOTTEN CARBONIFEROUS STRATIGRAPHIC UNIT IN THE LLANO UPLIFT, TEXAS

The Barnett Shale and Marble Falls Limestone have become synonymous with the Mississippian and Pennsylvanian systems in the Llano Uplift Region. Biostratigraphy demonstrates a time transgressive facies shift between the shale and limestone facies these formations represent. Because this facies shift straddled the Mid-Carboniferous boundary, and because there exists an ingrained interpretation of an unconformable boundary interval, the contact between the Barnett and Marble Falls has been treated as juxtaposed biostratigraphic zones separated by unconformity. Where limestone and shale interbeds mark the transition, current practice shifts the formation boundary to the lowest limestone or highest shale in the interval according to local “age appropriate” fossils. A clear and consistent boundary must be recognized for reliable research.

F.B. Plummer (1947) proposed a classification for these strata in which the Marble Falls Group comprises the Big Saline Fm and underlying Sloan Fm. The top of the Sloan was denoted by the Gibbons Conglomerate (basal Big Saline) above, and its base denoted by a lithologically monotonous Barnett Shale below. He defined the Sloan as a distinctively thin bedded, black, subcrystalline, fossiliferous limestone with common shale partings 2 to 30 in thick near the base.

Sequence stratigraphic principles illustrate genetic linkage of the Chappel-Barnett-Sloan package. The bounding unconformities are marked by the basal Chappel and the Gibbons, which overlies increasingly older strata where removed by this exposure event. The Barnett and Sloan represent the maximum flooding and high-stand systems tracts. Reinstating the Sloan Fm resolves the time transgressive facies change from shale to limestone, and obviates use of lithostratigraphic units for chronostratigraphic subdivision. It enables the Barnett to be defined strictly as shale, and the upper Marble Falls (Big Saline) to be recognized as genetically distinct, marked by the basal Gibbons Conglomerate. The shale and limestone interbeds between comprise the distinct Sloan Fm. Placing the Chappel-Barnett-Sloan within the context of a third order sequence, bounded above and below by marked unconformities, reinforces the possible occurrence of a conformable Mid-Carboniferous boundary in the eastern Llano Uplift.