Stratigraphy and Petrology of Middle Silurian McKenzie Formation in West Virginia

The Middle Silurian McKenzie Formation at the type section near McKenzie Station, Maryland, is composed of thinly interbedded shale and limestone with minor amounts of siltstone. These same rock types are predominant in cuttings from wells drilled farther west over a distance of 50 mi (80 km). Southwest from the eastern panhandle of West Virginia, however, the McKenzie contains coarser clastic material. In Pocahontas County, Travis subdivided the formation into three units: an upper shale and limestone, a middle sandstone, and a lower limestone and shale. Toward the west in the subsurface, the upper shale and limestone facies largely is replaced by dolomite, but thin shale beds still are present near the top of the formation. The middle sandstone persists as a distinct un t as far as Roane, Kanawha, and Boone Counties, a distance of approximately 100 mi (160 km). Farther west in the state, sandy dolomite is present at this level. The lower limestone and shale unit of outcrop areas in Pocahontas County becomes more calcareous and dolomitic westward and is still recognized as a sandy, dolomitic limestone as far west as Ohio and Kentucky. Correlations made by using gamma-ray logs and sample descriptions show that the Lockport Formation of Ohio is laterally equivalent to the lower two-thirds of the McKenzie. The upper one-third correlates with most of the three lower Salina units in Ohio. The McKenzie Formation in the eastern panhandle represents accumulation in a marine environment along a low coastal plain where rivers supplied clay and silt. Minor fluctuations of sea level and intermittent turbulent and calm conditions created alternating open-marine to intertidal environments. On the southwest in Pocahontas County, Travis interpreted the rocks of the lower unit as having been deposited under conditions ranging from normal marine to intertidal. The sandstone unit was formed as a beach deposit during a minor regression; it is overlain by a shallow-marine shale and limestone facies. An increasing quartz sand content in the top signaled another regression and the beach deposit of the overlying Williamsport Formation. In a complete McKenzie core in Wayne County, the paleoenvironment of the lowest unit was a bryozoan-stromato-poroidal bioherm, a middle sandy dolomite represents intertidal deposition, and an overlying oolitic facies is considered to be of a nearshore environment. These three units comprise the Lockport Member. Stromatolitic dolomite in the upper McKenzie member is interpreted as intertidal and supratidal algal mats. Small gas shows and commercial pays previously referred to as production from the Keefer Sandstone or Big Six sand are actually from the bioherm facies of the lower McKenzie. Gas has been encountered in this facies in approximately 75 percent of the wells drilled through the McKenzie in Wayne County. Gas shows also have been encountered in this zone in Cabell and Mingo Counties. Farther north, in Jackson, Wood, and Roane Counties, gas shows are recorded higher in the McKenzie at the top of the Lockport Member, in a porous zone referred to as the "Ohio Newburg."