The late Paleozoic continental margin of western Pangea was in tectonic flux from at least the mid-Paleozoic Antler orogeny through the Late Permian–earliest Triassic Sonoma orogeny. This tectonism is registered by the periodic and apparent synchronous initiation and/or disruption of sedimentary basins and their associated paleogeographic highs along the entire length of the continental margin. The continental margin was not “passive” during the late Paleozoic, as is commonly believed. The possible tectonic drive(s) for this tectonism are problematic and include (1) terrane–continent collisions, (2) transpression and transtension along a long-lived translational margin, (3) far-field stresses related to continent–continent collision along the Appalachian–Ouachita–Marathon margins, and (4) shifts in mantle-plate interactions and resulting changes in global plate motions and intraplate stresses. Regardless of the specific tectonic driver, it must encompass the periodic and synchronous nature of these tectonic events and factor in the influence of preexisting crustal structures.

The Late Permian to earliest Triassic Sonoma orogeny has long been envisioned as the result of an arc-continent collision that closed the Havallah oceanic basin, creating the Golconda allochthon, which was emplaced eastward onto the western edge of the continental margin along the Golconda thrust. Critical reevaluation of available stratigraphic, biostratigraphic, and structural data raise some fundamental issues with this scenario, including: (1) The Golconda allochthon experienced multiple phases of deformation both older and younger than the Sonoma orogeny; (2) the tectonostratigraphic successions in the Golconda allochthon record a disrupted depositional history; (3) these punctuated events and unconformities are mirrored by simultaneous punctuated tectonic disruptions of the adjacent continental margin; (4) some of the lithotectonic units within the Golconda allochthon have clear ties to a magmatic arc. These observations indicated that the Havallah basin did not originate as a simple, post-Antler orogeny rift basin, nor is the Mediterranean model for opening of a basin a solution to the initiation of this basin. Instead they imply a more complex paleogeography for the Havallah basin. The Late Permian–earliest Triassic closure of the Havallah basin did result in the development of the Golconda allochthon sensu stricto , but final emplacement of the Golconda allochthon was likely an Early–Middle Jurassic event.

Abstract The Cortez district is in one of the four major Carlin-type gold deposit trends in the Great Basin province of Nevada and contains three giant (>10 Moz) gold orebodies: Pipeline, Cortez Hills, and Goldrush, including the recently discovered Fourmile extension of the Goldrush deposit. The district has produced >21 Moz (653 t) of gold and contains an additional 26 Moz (809 t) in reserves and resources. The Carlin-type deposits occur in two large structural windows (Gold Acres and Cortez) of Ordovician through Devonian shelf- and slope-facies carbonate rocks exposed through deformed, time-equivalent lower Paleozoic siliciclastic rocks of the overlying Roberts Mountains thrust plate. Juxtaposition of these contrasting Paleozoic strata occurred during the late Paleozoic Antler orogeny along the Roberts Mountains thrust. Both upper and lower plate sequences were further deformed by Mesozoic compressional events. Regional extension, commencing in the Eocene, opened high- and low-angle structural conduits for mineralizing solutions and resulted in gold deposition in reactive carbonate units in structural traps, including antiforms and fault-propagated folds. The Pipeline and Cortez Hills deposits are located adjacent to the Cretaceous Gold Acres and Jurassic Mill Canyon granodioritic stocks, respectively; although these stocks are genetically unrelated to the later Carlin-type mineralization event, their thermal metamorphic aureoles may have influenced ground preparation for later gold deposition. Widespread decarbonatization, argillization, and silicification of the carbonate host rocks accompanied gold mineralization, with gold precipitated within As-rich rims on fine-grained pyrite. Pipeline and Cortez Hills also display deep supergene oxidation of the hypogene sulfide mineralization. Carlin-type mineralization in the district is believed to have been initiated in the late Eocene (>35 Ma) based on the age of late- to postmineral rhyolite dikes at Cortez Hills. The Carlin-type gold deposits in the district share common structural, stratigraphic, alteration, and ore mineralogic characteristics that reflect common modes of orebody formation. Ore-forming fluids were channeled along both low-angle structures (Pipeline, Goldrush/Fourmile) and high-angle features (Cortez Hills), and gold mineralization was deposited in Late Ordovician through Devonian limestone, limy mudstone, and calcareous siltstone. The Carlin-type gold fluids are interpreted to be low-salinity (2–3 wt % NaCl equiv), low-temperature (220°–270°C), and weakly acidic, analogous to those in other Carlin-type gold deposits in the Great Basin. The observed characteristics of the Cortez Carlin-type gold deposits are consistent with the recently proposed deep magmatic genetic model. Although the deposits occur over a wide geographic area in the district, it is possible that they initially formed in greater proximity to each other and were then spatially separated during Miocene and post-Miocene regional extension.

Abstract The Williston Basin Bakken petroleum system is a giant continuous hydrocarbon accumulation. The petroleum system consists of source beds in the upper and lower Bakken shales and reservoirs in the middle and upper Three Forks, the Pronghorn member of the Bakken, and the middle Bakken. The petroleum system is characterized generally by low-porosity and permeability reservoirs, organic-rich source rocks, and regional hydrocarbon charge. The USGS (2013) mean technologically recoverable resource estimates for the Bakken Petroleum System is 7.375 billion barrels oil, 6.7 TCF gas, and 527 million barrels of natural gas liquids ( Gaswirth et al., 2013 ). In the western US, relative sea level changes may be a combination of glaciation in the southern hemisphere, regional flexural tectonics related to the Antler orogeny, epeirogenic uplift, and/or localized structural movement ( Cole et al., 2015 ). The controls are not fully or clearly differentiated in the rock record. The Three Forks is a silty dolostone throughout much of its stratigraphic interval. The Three Forks ranges in thickness from less than 25 ft to over 250 ft in the mapped area. Thickness patterns are controlled by paleostructural features such as the Poplar dome and the Nesson, Antelope, and Cedar Creek anticlines. Thinning and/or truncation occurs over the crest of the highs and thickening of strata occurs on the flanks of the highs. The Three Forks unconformably (?) overlies the Birdbear in the Williston Basin and in turn is unconformably overlain by the Bakken Formation. The Three Forks consists of one overall deepening upward third-order sequence consisting of continental sabkha dolostones and anhydrites at the base changing to supratidal dolostones in the middle part to intertidal dolostones and mudstones in the upper part. The unit is subdivided into six shallowing upward parasequences by various authors. For mapping purposes, a three subdivision scheme has been adopted for this paper (i.e., upper, middle, lower). Most of the development activity in the Three Forks targets the upper Three Forks. The Bakken Formation in the Williston Basin consists of four members: a basal member (dolostone, limestone, and siltstone) recently named the Pronghorn; a lower organic-rich black shale; a middle member (silty dolostone or limestone to sandstone lithology); and an upper organic-rich shale member. The Bakken Formation ranges in thickness from a wedge edge to over 140 ft with the thickest area in the Bakken located in northwest North Dakota, east of the Nesson anticline. The Bakken appears to be composed of one complete third order sequence and part of a second third order sequence. The basal Pronghorn to middle part of the Middle Bakken represents one complete sequence (lowstand to transgressive to highstand system tracts). The Pronghorn to Lower Bakken Shale represent lowstand to transgressive system tracts. The lower Middle Bakken (facies A-C) represents a highstand system tract (falling stage system tract in upper part). The upper Middle Bakken (facies D-F) through the Upper Bakken Shale represents part of another third order sequence (lowstand to transgressive system tract). The Middle Bakken has an oolitic, bioclastic, sandy middle facies (facies D) which represents a lowstand deposit. This is overlain by the upper Middle Bakken (facies E-F) and the Upper Bakken Shale which is a transgressive system tract. Part of the overlying Lodgepole represents the highstand part of the second sequence. Sharp downlap surfaces are noted at the base of the Middle Bakken and the base of the Lodgepole. The downlap surfaces represent the transition from transgressive system tracts to highstand system tracts. Maximum flooding surfaces are found in the middle and upper portions of the upper and lower Bakken shales.

The Bisoni-McKay area, a structurally isolated, fault-bounded horst, offset eastward at the south end of the Fish Creek Range, displays a geologic terrane that is previously unrecorded in Nevada, and perhaps elsewhere in North America. This unique terrane is an olistostrome that was shed eastward by listric faulting from the east side of the migrating Antler orogenic forebulge in Late Devonian (early Famennian, ca. 373 Ma) time. Stratigraphic identification of Devonian olistoliths and enclosing matrix that constitute the olistostrome, as well as overlying postemplacement units, is supported by correlation to formations in the main part of the Fish Creek Range and to the northwest in the northern Antelope Range. Precise zonal dating of map units and revised dating of Antler orogenic events are provided by 38 conodont collections recorded in the Devonian/Carboniferous (D/C) Conodont Database and by small collections of conodonts embedded in siltstone and mudstone. Our revision of regional geologic history uses Devonian conodont zones to measure “deep time” to circa millions of years before present. The upper Upper Devonian (Famennian) tongue of the Woodruff Formation was deposited directly on the olistostrome and is overlain by clastic Mississippian synorogenic deposits. These deposits were shed eastward from the evolving Antler highland and related Roberts Mountains allochthon into the Antler foredeep. We propose the following revised dates for important Devonian tectonic events in Nevada: initiation of Antler orogeny, ca. 385 Ma; downwarping of Pilot backbulge basin, ca. 382 Ma; initial uplift of the Antler highland, ca. 373 Ma; third, major pulse of highland uplift, ca. 364 Ma. A summation of regional geologic history indicates that the elapsed time from start of Antler orogeny to start of Roberts Mountains thrusting was ~30 m.y.

The 45 km 2 map area is situated at the south end of the Hot Creek Range in central Nevada, ~16 km east of the buried leading edge of the Mississippian Roberts Mountains thrust. Three eastward-trending left-slip faults divide the area into four structural blocks. The southernmost block is occupied solely by upper Oligocene volcanic rocks. The narrow northernmost block, now occupied surficially by valley fill and volcanic rocks, represents the south end of the main part of the Hot Creek Range, from which the study area is offset. The middle two blocks display different aspects of the eastward-traveled outer crater rim created by the ca. 382 Ma (early Late Devonian, middle Frasnian) Alamo impact. The Alamo impact was produced by a 5-km-diameter bolide, most likely a comet, which excavated a transient submarine crater 44–65 km in diameter. Comparison of thin (8–12 m) Alamo Breccia deposits in the northern of the middle two blocks with a more easterly, thick (35–42 m) Alamo deposit in the main Hot Creek Range, 4 km north of the map area, suggests that these blocks traveled many kilometers eastward. The northern of the middle two blocks contains a large olistolith capped by the thin breccia, whereas the southern block contains a larger olistolith lacking an Alamo Breccia cap. Three Devonian pulses of the Antler orogeny are better documented in the chapter on the Bisoni-McKay area. Here, the first Antler pulse in latest Middle Devonian time is obscured within an ~9 m.y. hiatus enlarged by excavation of the Alamo impact crater. The second Antler pulse is recorded by the ~4 m.y. hiatus produced by the regional unconformity between the lower and upper members of the Woodruff Formation. The third Antler pulse is documented by an ~8 m.y. regional hiatus between the Mississippian Webb Formation and Upper Devonian Woodruff Formation. In previous papers, we had interpreted this pulse to initiate the Antler orogeny.