| Five informal stratigraphic units can be recognized on well logs through the Marble Falls interval in the northern Fort Worth basin. Four of the units---an upper limestone, upper shale, lower limestone, and lower shale---are present in the eastern half of Wise County. These units interfinger to the west in Jack County with a heterolithic unit comprised of siltstones, mudstones and claystones. Facies recognized in core through the heterolithic unit and log curve shapes in the limestones and shales reveal shallowing-upward sequences formed as the basin filled. The basin fill contains authochthonous carbonate sediment produced in shallow epeiric seas and siliciciclastic (and perhaps carbonate) debris eroded off rising positive structures such as the Bend arch, Red River arch, Muenster arch, and the Ouachita thrust belt during the Ouachita orogeny. Revised correlations based on additional well control suggest the Comyn Limestone and the Forestburg limestone are the same lithostratigraphic unit. |
