Snyder, Walter S.¹, Davydov, Vladimir I.¹, Trexler, James, H., Jr.², Cashman, Patricia H.², and Schiappa, Tamra, A.³, (1) Department of Geosciences, Boise State University, MS-1535, Boise, ID 83725-1535, (2) Department of Geological Sciences and Engineering, MS 172, University of Nevada, Reno, Reno, NV 89557, (3) Department of Geography, Geology, & the Environment, 319 Advanced Technology and Science Hall, Slippery Rock University of Pennsylvania, Slippery Rock, PA 16057-1326
The late Paleozoic continental margin of western North America is characterized by a succession of tectonically-controlled, unconformity-bounded sedimentary basins. This was first recognized for Permian basins (e.g., Snyder et al., 1989), but has also been extended for the entire upper Paleozoic (see Trexler et al., this volume). The Permian part of the tectonostratigraphic story starts in the latest Pennsylvanian with the C6-P1 or Lower Strathearn interval which is best manifested in Carlin Canyon, where Lower Strathearn strata overlap folded and faulted older units (see Trexler et al., 2003, 2004). Another episode of at least regional tilting and/or folding is marked by the P1 unconformity which places the Upper Strathearn units unconformably across subjacent strata. The Upper Strathearn is not everywhere preserved beneath the P2 unconformity, but it can be traced discontinuously from NE Nevada to SE California. The P2-P4 interval is marked by the abrupt appearance of the basinal facies of the Dry Mountain Trough and includes related basins from SE California to Idaho and Montana. P2 is typically a subtle angular unconformity, and to date no major sub-unconformity folding or faulting have been documented. Nevertheless the marked change to a basinal facies (with black shales, ammonoids and conodonts) documents a sudden and rapid subsidence that requires a tectonic drive. P3 is locally observed in central Nevada where it post-dates what appears to be a minor folding event, but regionally it has not yet been fully documented. P4-Tr1 encompasses the Phosphoria and Park City basins, with P5 reflecting a long recognized internal event within the Phosphoria. P4 is typically described as a disconformity to a slight angular unconformity, but in Central Nevada, near Eureka it is marked by the appearance of the conglomeratic Garden Valley Formation, which in the Diamond Range truncates subjacent folds. The Sonoma orogeny encompasses the Tr1 lacuna - a protracted interval of missing strata within western North America. Deformation associated with the Sonoma orogeny has only been documented internally within the Golconda allochthon and Triassic strata do not overlap the Golconda thrust. Thus the final emplacement of the Golconda allochthon, which is normally viewed as the Sonoma orogeny, is more likely a Jurassic event (Dunston et al., 2001).
The extent of pre-Sonoma, Permian phase(s) of deformation has yet to be fully assessed. Importantly, many of the tectonic events noted above can be traced westward into the Antler highlands and perhaps beyond. For example, the P2 event may have expression not only within the foreland and Antler highlands, but also in the Golconda allochthon - thus complicating our understanding of the tectonics of that terrane and the Sonoma orogeny. Finally, we have just initiated studies to determine if the conglomerates of the Garden Valley Formation and their apparent equivalents in the Edna Mountain Formation, record a significant Middle-early Late Permian event; the relationship of these conglomerates to the Phosphoria-Park City event and possible expression in the Golconda allochthon is at present unknown.
Dunston, Jacob F., Northrup, C.J., and Snyder, W.S., 2001, Post-latest Triassic Thrust Emplacement of the Golconda Allochthon, Sonoma Range, Nevada: Geological Society of America, Abstracts with Programs, v. 33, p. 327.
Snyder, W.S., Spinosa, C., and Gallegos, D.M., 1989, Late Pennsylvanian to Permian tectonism along the western United States Continental margin: Geological Society of America, Rocky Mountain-Cordilleran Section Meeting, Abstracts with Programs, v. 21, no. 5, p. 147.
Trexler,, James H., Cashman, Patricia H., Snyder, Walter S., Davydov, Vladimir I., 2004, Late Paleozoic tectonism in Nevada: Timing, kinematics, and tectonic significance: Geological Society of America Bulletin, v.116, no. 5, p. 525-538.
Trexler,, James H., Cashman, Patricia H., Cole, James C., Snyder, Walter S., Tosdal, Richard M., Davydov, Vladimir I., 2003, Widespread effects of middle Mississippian deformation in the Great Basin of western North America: Geological Society of America Bulletin, v. 115, no. 10, p. 1278-1288.