Late Mississippian girvanella-bryozoan mud mounds in southern West Virginia

Girvanella-bryozoan mud mounds of the Upper Mississippian Bluefield Formation in southeastern West Virginia formed in a very shallow marine environment in the presence of terrigenous sedimentation. Of the several mud mounds present, three mounds, a larger one and two smaller ones, were studied petrographically. The influence of both terrigenous clastic and carbonate deposition produced a characteristic sequence of mound development: 1) initial establishment by fenestrates, Girvanella, brachiopods, crinoids, and other organisms; 2) deposition of quartz, clay and ooids; 3) re-establishment by fenestrates, Girvanella, brachiopods, and crinoids; 4) repetition of steps two and three; 5) cessation of mound development by progradation of terrigenous clastic sediment. Fenestrate bryozoans played a more dominant role in the development of these low diversity, terrigenous influenced carbonate mud mounds than in carbonate-dominated, high diversity bioherms and mud mounds. Fenestrate bryozoans inhibited sediment movement, trapped sediment, and stabilized the mounds by providing a non-binding framework. Allogenic controls, not autogenic, dominated the succession of the mud mounds. The environmental stress caused by turbidity resulted in a low diversity fauna and prevented faunal succession that typically occurs in other mud mounds. Interpretation of the principal components analysis (PCA) of the point-count data suggests that the constituents of the Girvanella-bryozoan layers are autochthonous, whereas the constituents of the interbedded siliciclastic layers are allochthonous. More importantly, PCA indicated that there were two different sources providing allochthonous material. The mud mounds were affected by both terrigenous progradation from the north and east and carbonate sediment from the south-southwest. Fine quartz sand and clay were the dominant terrigenous sediments from the north and east, whereas ooids were derived from carbonate shoals to the south-southwest. Deposition of autochthonous and allochthonous sediment produced an alternating pattern of Girvanella-bryozoan layers and siliciclastic layers. Ordination of the principal components shows that the smaller mounds (B and C) are distinguishable from the larger mound (A) based on constituent grains. The larger mound was more stable in respect to erosion by the influx of the terrigenous clastics than the smaller mounds.