Paleosols of the Upper Devonian Foreknobs Formation of Western Virginia and Eastern West Virginia

Terry, Dennis O.; McClung, Wilson; Eriksson, Kenneth A.

Twenty paleosol profiles were measured from four separate exposures of the Upper Devonian Foreknobs Formation of northeastern West Virginia. Paleosols were identified by the presence of root traces, ped structure, slickensided curvilinear fracture planes, and differences in color suggestive of horizonation. We have identified four main types of paleosols within the Foreknobs Formation which represent both differences in soil orders (as per USDA Taxonomy) and gradations in the degree of pedogenic features within individual profiles (on a scale of 1-4 with 4 = greatest), as well as pedologic features critical to paleoenvironmental and paleoclimatic interpretations. Profiles range in thickness from 0.2 to 1.0 m and represent pedogenically modified fining-upward, fluvial packages. The most distinctive macroscopic pedogenic feature within the paleosols of the Foreknobs Formation is horizonation, which is expressed as varying degrees of relict vs. pedogenic sedimentary structures and manifests as differences in erosional relief and the concentration of rooted horizons near the top of individual profiles. The most common root types within the Foreknobs Formation are light greenish gray to pale yellow drab-haloed root traces. Wedge-shaped aggregates and angular to subangular blocky peds are most common within these paleosols and range from 0.3 to 2 cm in size and become smaller toward the tops of individual profiles. Curvilinear fracture planes with slickensided surfaces are common, but not ubiquitous, throughout these profiles. The most common microscopic features within paleosols of the Foreknobs Formation include distinctive pedogenic fabrics within the soil matrix and zones of reduction associated with drab haloed root traces. Glaebules are absent from these profiles. Soil fabrics include skel-lattisepic, vosepic, and clinobimasepic varieties. Paleosols of the Foreknobs Formation formed in response to relative changes in sea level. Periods of greater soil formation are likely the result of relatively large drops in sea level associated with 4th-order sequence boundaries and likely represent the interfluves of the incised valley fills throughout this sequence, whereas the groupings of multiple, weaker soils likely represent subaerial exposure of individual 5th-order sequences. The facies overlying individual paleosols are variable, and reflect both the magnitude of sea level rise and the position along the parasequence for that particular profile.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013