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It is now recognized that depositional environments of shales can be expanded beyond the phrase "quiet-water" deposition. One of the difficulties in delineating different depositional environments in fine-grained sediments is that descriptions of sedimentary structures in shales are not common. If the structures are preserved by some diagenetic process, then they can be more readily used in the interpretation of depositional environments. Diagenetic carbonate concretions that formed by the localized precipitation of cement in pore spaces of sediment would be expected to preserve original fabrics. Because of good exposure and greater resistance to weathering, concretions may exhibit features that are difficult to see in the less resistant surrounding shale.
The Greenhorn Formation (Upper Cretaceous) north and west of the Black Hills is predominantly a shale with abundant limestone concretions. The concretions are generally restricted to certain zones. Shale exposures are generally unspectacular. The concretions, which are more resistant to weathering, contain a variety of physical and biogenic sedimentary structures. Commonly the structures are subtle. Polished slabs enhance or reveal structures not apparent in the field. Concretions from a given layer exhibit similar sedimentary structures. Parallel lamination is the most abundant physical sedimentary structure. Laminae range from 0.1 mm (.003 in.) to 1 cm (.4 in.) in thickness. They can be continuous with a constant thickness, continuous with a variable thickness, or discontinuous. Low angle ripple cross-stratification is present in some laminae. Graded bedding, cut-and-fill structures, and flame structures are also present. Biogenic structures include distinct burrows, burrow-disrupted layering, and thoroughly bioturbated textures.
Depositional textures of the Greenhorn concretions include mudstone, wackestone, and packstone. Major grain types are inoceramid prisms and fragments, foraminifera, quartz, fish debris, and pellets. In some concretions, pellets are deformed indicating compaction occurred prior to cementation.
The range of sedimentary structures, depositional textures, and abundance of quartz in Greenhorn concretions suggests variations in depositional conditions. Some laminae must have been deposited in quiet water with a current velocity close to zero while other laminae must have been deposited by currents of higher velocities. Cross-stratification is evidence that tractive currents were active in depositing and/or reworking the bottom sediments. The biogenic structures provide evidence of infaunal life and bioturbation.
Where shale exposure is good, the same structures are observed in the shale as in the concretions. Therefore, structures in concretions are representative of the surrounding shale at that level. Concretions should not be ignored as a source of information regarding sedimentary structures. They can be valuable in the interpretation of paleoenvironments of shale.
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