Depositional environments of the Lower Crooked Fork Group near Wartburg, Morgan County, Tennessee

The relatively unstudied lower Crooked Fork Group is described and discussed in terms of petrology and depositional environments from 14 exposures near Wartburg, Morgan County, Tennessee. These formations are Early Pennsylvanian in age and include (oldest to youngest), the Dorton Shale, Crossville Sandstone, Burnt Mill Shale, and the Coalfield Sandstone. The Crooked Fork Group is only exposed in the northern Cumberland Plateau of Tennessee.

The Dorton Shale contains seven distinct subfacies; five were traceable across the study area. In general, the Dorton represents an embayment at least partially open to the sea. Initial conditions leading to peat-formation (represented by the Rex Coal) were terminated by a local transgression. The lower one-third of the Dorton (subfacies 1 to 4) records anoxic conditions, slow sedimentation rates, and little current or wave/tidal activity. These conditions are interrupted by a period of oxygenation and intense bioturbation (subfacies 2) and a storm event (subfacies 3). The upper two-thirds of the Dorton (subfacies 5, 6 and 7) reflect increased current or tidal/wave activity and greater sedimentation rates; the latter evidenced by dewatering structures and discreet slump zones.

The Crossville is a complex sand body which represents delta front sands that prograded across the bay sediments of the Dorton. The Crossville in the study area displays characteristics of both marine and fluvial processes. Seven fades recognized within the Crossville are: (1) reworked distributary bar sands, (2) interbedded flaser sublitharenites and quartzarenites, (3) washovers, (4) thick-bedded channels, (5) multistory channels, (6) abandoned channel fill, and (7) interbedded sublitharenites and quartzarenites.

The Burnt Mill Shale overlies and in one place interfingers with the top of the Crossville, The Burnt Mill is dominated by a stagnant bay facies, although splay, minor mouth bar, and splay abandonment fades were also recognized.

The Coalfield Sandstone unconformably overlies the Burnt Mill and appears more fluvially dominated than the Crossville. The Coalfield contains "dirty sands," abundant fossil twig debris, and numerous channel forms, some of which are abandoned. The Coalfield displays 3 facies: small channels, abandoned channel fill, and large, low-sinuosity channels or chenier-like deposition.