Abstract

Limestone and shale in the Marble Falls interval in eastern Wise County interfinger to the west in Jack County with a heterolithic unit of siltstone, mudstone, and claystone. Six facies are present in the heterolithic unit. Facies A is light gray, laminated spiculitic siltstone with thin interbeds of dark gray to black laminated mudstone. Facies B consists of nearly equal amounts of highly bioturbated crinoidal siltstone and black claystone. Facies C is characterized by discontinuous siltstone laminae. Facies D has notable amounts of quartz in the silt fraction and fewer sponge spicules, but otherwise resembles facies C. Facies E is black claystone with thin interbeds of moderately bioturbated, spiculitic siltstone. Facies F consists of black claystone with thin interbeds of non-bioturbated, spiculitic siltstone. The two facies (E, F) comprised mainly of black claystone are present only in the lower third of the core, and the one (F) in which the interbedded siltstones are not bioturbated is found only at the base of the core. Facies C comprises most of the cored interval. It is interbedded with facies E in the lower third of the core and passes upward in to facies A and B at the top of the core. The stacking of facies reveals a change from a low-energy anoxic setting to a high-energy marine environment with a thriving infauna. This change in depositional conditions represents a shift from basinal to slope and shelf settings during the filling of the Fort Worth Basin in the late Pennsylvanian.