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Mid- to Late-Paleozoic Deformation On Buck Mountain, Nevada
AuthorWhitmore, Ross James
AdvisorCashman, Patricia H.
Geological Sciences and Engineering
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Abstract: New detailed structural mapping of the strata capping Buck Mountain in White Pine Co., Nevada, has documented two distinct episodes of folding. The structures and angular unconformities at Buck Mountain are consistent with late-Paleozoic deformation and regional unconformities documented previously elsewhere in northern and central Nevada.Folds in the Morrowan-Atokan Ely Limestone are erosionally truncated and depositionally overlain by the mid-Desmoinesian Hogan Formation. The lacuna developed at this stratigraphic level is temporally consistent with a regional unconformity ("C5" of Snyder et al., 2000). This unconformity is an upper boundary on regional west-vergent folding. In addition, two disconformities are found on Buck Peak. Both disconformities correlate with known regional unconformities, one Desmoinesian-Missourian ("C6") and the other Missourian to Sakmarian (mid-Wolfcampian) ("P1"). F1 folds are pre-Desmoinesian in age and are the oldest structures discussed. These folds range from close west-vergent to open, upright, and symmetric folds. The range of fold geometries is driven by localization of strain during shortening. F1 folds are erosionally truncated and refolded around a younger northwest fold axis. When F1 folds are restored, their original plunge is ~10° to the northeast.The map-scale syncline fold at Buck Mountain is a syncline developed in the Ely Limestone, Hogan Formation, and both Lower and Upper Strathearn formations and is therefore part of a second fold set (F2). It is a gentle to open, symmetric, upright, cylindrical fold plunging ~15° to the northwest (323°) with a wavelength of about 1 km and unknown amplitude. The age of this fold is poorly constrained at Buck Mountain (post-Sakmarian to Oligocene), but it appears to correlate with Mesozoic folds in adjacent ranges. No evidence was found for low-angle faulting on Buck Mountain. Previous work identified a single attenuation fault at the base of the mountain (Nutt, 2000). In addition, reconnaissance work described intense folding at the top of the range, suggesting the presence of a different low-angle fault there. However, this study found that the folds are not localized around structurally defined surfaces. Rather, they occur at all structural levels. Thick massive carbonate layers form gentle long wavelength folds and local minor faults with 2-3 meters of offset. Thinner-bedded carbonate layers form open folds with shorter wavelengths. The contrast in fold style and apparent intensity is interpreted as a competency response to shortening between thick- and thin-bedded carbonate layers. The presence of mid-Pennsylvanian folds at Buck Mountain is important because it: 1) correlates with other sub-C5 folds and faults in eastern Nevada and extends the known geographic extent of these structures, 2) helps constrain the orientation of the regional Pennsylvanian deformation, and 3) documents a regional eastward decrease in intensity of mid-Pennsylvanian folds and faults.