Bluefield Formation

The Bluefield Formation can be subdivided into four lithological units or "packages" at its thickest extent. The oldest two packages are primarily grey shale and fossiliferous limestone, while the younger two packages are primarily siltstone, sandstone, and red mudstone. Four distinct types of sediment deposition and flooding cycles are preserved, labelled types A-D. Type A and B cycles are most common in the younger packages and type C and D cycles are most common in the older packages. These cycles and sequences of the Bluefield Formation are most well preserved at the depocenter of the Bluefield Formation, at the eastern edge of Mercer County, West Virginia. Here, at least 28 cycles have been tracked. Elsewhere they may be incomplete due to erosion within the basin as it was experiencing deposition.[1]

Type A cycles involve fine black shale grading upwards to brown laminated mudstone, then thickly-bedded red mudstone and mudcracked siltstone layers, before a flood event returns the area to carbonate-rich mudstone. Ostracods, root casts, and intense bioturbation were common in many layers. Coal fragments are found in the basal black shale layers. Carbonate nodules and slickensides were present in red mudstone, indicating that they were vertisols. Type A cycles represent brackish lagoons which were able to dry out into mudflats in a hot, semi-arid climate. The mudflats accumulated soil, as well as streams that formed silty crevasse splay deposits.[1]

Type B cycles were similar to type A cycles but were thicker and differed in other ways. The basal black shale is richer in coal and grades into siltier brown mudstone and then interbedded layers of siltstone, sandstone, and silty mudstone. The top of a cycle is characterized by a thick layer of rippling cross-bedded sandstone filled with rootcasts. Type B cycles represent a delta influenced by tides which alternate the supply of fine and coarse sediments. However, bioturbation makes it difficult to precisely track tidal forces. The top of a cycle preserves a sandbar in the process of being colonized by plants. Once seawater re-submerges the area and starts a new cycle, plants contribute to the richness of coal within the basal shale.[1]

Type C cycles begin with bioturbated mudstone which abruptly transitions into a thick sequence of limestone. The limestone can be characterized as thin layers of fossil-rich grainstone interbedding with broader layers of packstone. Rarely, brown laminated mudstone may be present above the limestone. The upper part of the cycle involves conglomerate covered by sandstone and siltstone filled with plant fragments, ripple marks, and occasionally tetrapod footprints. In type C cycles, a transgression floods a mudflat and allows it to be colonized by marine organisms. Repeated storm event lead to alternating limestone grain size, and eventually tidally-influenced sandbars manifest as the shoreline shifts back.[1]

Type D cycles primarily involve a thick sequence of laminated mudstone grading from shale to siltstone. The sequence is occasionally interrupted by fossil-rich packstone which grades upwards into mudstone. The thickest packstone layer is often located at the top of the sequence, though dolomite-rich mudstone may lie in its place. Type D cycles are entirely marine deposits found on the continental shelf of the ancient marine basin. Though this is a mostly stable environment, it is regularly disrupted by storms (forming siltstone) and turbidity flows (forming packstone). The gradual increase in sediment size is tied to a regressing coastline, allowing shallow-water sediments to influence the environment more.[1]

Collectively, the cycles and "packages" of the Bluefield Formation represent a series of large-scale sea level changes, each lasting several hundred thousand years. This data is consistent with identifying each package as a fourth-order sequence. This likely means that they are tied to glacial periods as the earth was transitioning from a greenhouse to icehouse climate. The individual sediment cycles represent smaller-scale (50-100 thousand year) glacial sea level fluctuations, also known fifth-order sequences or parasequences. The upper two packages of the Bluefield Formation have more parasequences and more coal near the depocenter, but areas further away (towards the basin margin) had fewer parasequences and less coal. This indicates that the basin margins had less subsidence compared to the depocenter. The relatively larger impact of sea level changes leads to more erosion along the basin margin, allowing many parasequences and coal beds to be eroded away completely.[1]