SEDIMENTOLOGY AND DEPOSITIONAL SETTING AS PHYSICAL EVIDENCE FOR A CONFORMABLE MISSISSIPPIAN-PENNSYLVANIAN BOUNDARY, LLANO UPLIFT, TEXAS

The Mississippian Barnet and the Pennsylvanian Marble Falls formations straddle the Mid-Carboniferous (Cm/Cp) boundary in the northern Llano Uplift of central Texas. Sedimentation at this time was controlled by a structural foredeep extension of the Fort Worth Basin along the Ouachita orogenic front. In the Mid-continent of North America the Cm/Cp boundary is marked by a paleosoil separating the uppermost preserved Mississippian strata from the lowest Pennsylvanian strata. The unconformity is further marked by an irregular lower contact and a marked shift in lithology. The amount of record lost in this hiatus decreases southwesterly, suggesting a general connection to the ocean in that direction. Nevertheless, the standard interpretation places an unconformity between the Barnet and Marble Falls.

The depositional setting, petrology, character of the contact, and lack of physical evidence of exposure indicate Cm/Cp conformability within the Barnet/Marble Falls contact. The depositional environment, as indicated by the petrology of both units and their tectonic setting, indicates distal shelf accumulation. Interbedding of these units produces a gradational contact over a 1.5-5meter interval in several locations. This gradational interval was termed the Sloan formation of the Marble Falls group (Cheney, 1940). Usage of this nomenclature would alleviate the dispute concerning where the Barnet ends and the Marble Falls begins, since these terms have become synonymous with Mississippian and Pennsylvanian respectively. The contact between these two units indicates a gradual change in depositional environment, recording a shallowing upward but still distal shelf environment. There is no evidence of an irregular, bed cutting lower contact or paleosoil within this interval. The Barnet shale weathers to soil in the modern semiarid climate at exposures no older than sixty years. Exposure during the Cm/Cp interval would have formed a considerable paleosoil. The shale interval is laminated throughout, lacking evidence of paleosoil formation. The physical evidence suggests conformability. Therefore, an unconformity should not be forced between these formations based solely on the fact that the Cm/Cp boundary interval is a marked unconformity in the northern Mid-continent.