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.

Three brachiopod faunas discussed herein record different depositional and tectonic settings along the southwestern margin of Laurentia (North America) during Devonian time. Depositional settings include inner continental shelf (Cerros de Los Murciélagos), medial continental shelf (Rancho Placeritos), and offshelf continental rise (Rancho Los Chinos). Ages of Devonian brachiopod faunas include middle Early (Pragian) at Rancho Placeritos in west-central Sonora, late Middle (Givetian) at Cerros de Los Murciélagos in northwestern Sonora, and late Late (Famennian) at Rancho Los Chinos in central Sonora. The brachiopods of these three faunas, as well as the gastropod Orecopia , are easily recognized in outcrop and thus are useful for local and regional correlations. Pragian brachiopods dominated by Acrospirifer and Meristella in the “San Miguel Formation” at Rancho Placeritos represent the widespread Appohimchi Subprovince of eastern and southern Laurentia. Conodonts of the early to middle Pragian sulcatus to kindlei Zones associated with the brachiopods confirm the ages indicated by the brachiopod fauna and provide additional information on the depositional setting of the Devonian strata. Biostratigraphic distribution of the Appohimchi brachiopod fauna indicates continuous Early Devonian shelf deposition along the entire southern margin of Laurentia. The largely emergent southwest-trending Transcontinental arch apparently formed a barrier preventing migration and mixing of many genera and species of brachiopods from the southern shelf of Laurentia in northern Mexico to the western shelf (Cordilleran miogeocline) in the western United States. Middle Devonian Stringocephalus brachiopods and Late Devonian Orecopia gastropods in the “Los Murciélagos Formation” in northwest Sonora represent the southwesternmost occurrence of these genera in North America and date the host rocks as Givetian and Frasnian, respectively. Rhynchonelloid brachiopods ( Dzieduszyckia sonora ) and associated worm tubes in the Los Pozos Formation of the Sonora allochthon in central Sonora are also found in strati-form-barite facies in the upper Upper Devonian (Famennian) part of the Slaven Chert in the Roberts Mountains allochthon (upper plate) of central and western Nevada. Although these brachiopods and worm tubes occur in similar depositional settings along the margin of Laurentia in Mexico, they occur in allochthons that exhibit different tectonic styles and times of emplacement. Thus, the allochthons containing the brachiopods and worm tubes in Sonora and Nevada are parts of separate orogenic belts and have different geographic settings and tectonic histories. Devonian facies belts and faunas in northern Mexico indicate a continuous continental shelf along the entire southern margin of Laurentia. These data, in addition to the continuity of the late Paleozoic Ouachita-Marathon-Sonora orogen across northern Mexico, contradict the early Late Jurassic Mojave-Sonora megashear as a viable hypothesis for large-magnitude offset (600–1100 km) of Proterozoic through Middle Jurassic rocks from California to Sonora.

U-Pb isotopic dating of detrital zircons from a conglomeratic barite sandstone in the Sonora allochthon and a calciclastic sandstone in the Mina México foredeep of the Minas de Barita area reveals two main age groups in the Upper Devonian part of the Los Pozos Formation, 1.73–1.65 Ga and 1.44–1.42 Ga; and three main age groups in the Lower Permian part of the Mina México Formation, 1.93–1.91 Ga, 1.45–1.42 Ga, and 1.1–1.0 Ga. Small numbers of zircons with ages of 2.72–2.65 Ga, 1.30–1.24 Ga, ca. 2.46 Ga, ca. 1.83 Ga, and ca. 0.53 Ga are also present in the Los Pozos sandstone. Detrital zircons ranging in age from 1.73 to 1.65 Ga are considered to have been derived from the Yavapai, Mojave, and Mazatzal Provinces and their transition zones of the southwestern United States and northwestern Mexico. The 1.45–1.30 Ga detrital zircons were probably derived from scattered granite bodies within the Mojave and Mazatzal basement rocks in the southwestern United States and northwestern Mexico, and possibly from the Southern and Eastern Granite-Rhyolite Provinces of the southern United States. The 1.24–1.0 Ga detrital zircons are believed to have been derived from the Grenville (Llano) Province to the east and northeast or from Grenville-age intrusions or anatectites to the north. Several detrital zircon ages ranging from 2.72 to 1.91 Ga were probably derived originally from the Archean Wyoming Province and Early Paleoproterozoic rocks of the Lake Superior region. These older detrital zircons most likely have been recycled one or more times into the Paleozoic sandstones of central Sonora. The 0.53 Ga zircon is believed to have been derived from a Lower Cambrian granitoid or metamorphic rock northeast of central Sonora, possibly in New Mexico and Colorado, or Oklahoma. Detrital zircon geochronology suggests that most of the detritus in both samples was derived from Laurentia to the north, whereas some detritus in the Permian synorogenic foredeep sequence was derived from the evolving accretionary wedge to the south. Compositional and sedimentological differences between the continental-rise Los Pozos conglomeratic barite sandstone and the foredeep Mina México calciclastic sandstone imply different depositional and tectonic settings.

The Ouachita-Marathon-Sonora orogen is a 3000-km-long belt of deformed Paleozoic rocks bordering the southern margin of the Laurentian (North American) craton. Extending from Mississippi and Arkansas (Ouachita) southwestward through Texas (Marathon) and westward through Chihuahua and Sonora (Sonora), the orogenic system formed during a late Paleozoic collisional-subductional event. This event resulted in closure of the Rheic ocean and the development of the orogen as the southern edge of the Laurentia plate was subducted beneath a northward-advancing Gondwanan (South American) continental-margin arc. Foredeeps and foreland basins and uplifts were created on the Laurentian plate continentward of the orogen. Regional stratigraphic and structural relations indicate original physical continuity of the Ouachita-Marathon-Sonora orogenic belt along the entire southern margin of Laurentia. In the Neoproterozoic and Early Cambrian, the supercontinent Rodinia rifted along trends later followed by the Ouachita-Marathon-Sonora orogenic belt. During the breakup of Rodinia, the promontories and embayments that developed along the south margin of Laurentia are related to a northeast-striking rift system (oceanward of the continental edge) deformed by northwest-striking synrift transform faults that offset the rift-parallel cratonal margin. Initial deposits of the rifted margin were sediments deposited in tectonic sags and basins, which opened oceanward, and in depressions adjacent to transform faults. Shelf deposition began with Middle Cambrian clastic and carbonate sediments and continued with deposition of mostly shallow-marine carbonate sediments. Offshelf deposition began with Upper Cambrian (oldest rocks recognized in thrust sheets) clastic sediments and continued into the Early Mississippian with deposition of deep-marine clastic and subordinate carbonate sediments in continental-rise and ocean-basin settings. Deformation of the southern margin of Laurentia resulted from its late Paleozoic diachronous oblique collision with the South American part of Gondwana and development of volcanic-plutonic arc and associated fore-arc and back-arc assemblages along the northern margin of Gondwana. Westward younging of foredeep and foreland-basin depocenters and decreasing age of basin-fill sediments are in accord with the westward migratory closure of the Laurentia–Gondwana suture. Cambrian to Lower Mississippian preorogenic sediments were deposited in offshelf settings along the margins of the two continents, and Upper Mississippian to Permian synorogenic sediments were deposited in the deep-water ocean basin between the two converging continents and associated arcs, in troughs within the evolving allochthons, and in foredeeps and foreland basins on the craton margins. Ocean-basin sediments were scraped off the ocean floor and transported as allochthons formed during continental collision, in advance of synchronous Pennsylvanian and Permian foredeeps and foreland basins and uplifts. These allochthons were part of a large accretionary wedge formed above a south-dipping subduction zone and thrust northwestward 50–200 km above Laurentian continental-shelf and foredeep rocks. The late Paleozoic synorogenic foreland basins and uplifts developed cratonward as far north as the Transcontinental arch and Ancestral Rocky Mountains. The timing and sense of movement of these intracratonic structures have complex relationships to the collisional margin. Deformation in all three segments of the Ouachita-Marathon-Sonora orogenic belt began in mid-Mississippian time and ended diachronously in the Late Pennsylvanian in the Ouachita Mountains, Early Permian in the Marathon region, and Late Permian in Sonora. This represents predominantly north to northwest contraction. The westward migratory termination of orogenesis and the related development of foredeeps along the Ouachita-Marathon-Sonora belt are consistent with oblique convergence of Gondwana (Africa and South America) with Laurentia and require some clockwise rotation of South America following initial collision in the Ouachita Mountains region. The Sonora segment of the Ouachita-Marathon-Sonora orogenic system shows that northwestern Gondwana and its associated volcanic-arc terrane once lay south of western Laurentia (south of central Sonora) and was not restricted to areas to the east of Sonora as often shown in Pangaean reconstructions. Data south of the Laurentia continental margin indicate that the Gondwanan crust was extremely variable. Ashfall tuffs in Upper Mississippian through lowermost Pennsylvanian hemipelagites and turbidites in the Ouachita Mountains and Marathon region, and rhyolite flows and ashfall tuffs (bentonites) in Permian flysch in the southern Pedregosa basin of north-central Mexico indicate explosive volcanism along the northern margin of Gondwana. Geochemistry of the upper Paleozoic extrusive Sabine Rhyolite in the flysch sequence of the Sabine uplift area indicates a continental-arc origin. Several inferred remnants of Gondwana crust and volcanic-arc rocks in the southern United States and northwestern Gulf of Mexico are collectively referred to as the Sabine block. Other inferred remnants of Gondwana crust and volcanic-arc rocks in northern Mexico include the Coahuila block in Coahuila and southeast Chihuahua, and the El Fuerte block in Sinaloa, Sonora, and adjacent Chihuahua. Several lines of critical evidence contradict a late Paleozoic or Mesozoic megashear through Sonora. Both the Rodinia rift system and the Ouachita-Marathon-Sonora orogen represent a continuous southern continental margin westward to Baja and Alta California, across the supposed trace of the hypothetical Mojave-Sonora megashear. Although structures are present that offset the orogen and its associated rift system, their sense of offset is right-stepping rather than left-stepping as required by the megashear. Minor displacements by postorogenic faulting together with right-stepping transform faulting cumulatively represent ∼300 km of displacement in contrast to 600–1100 km of left-lateral offset postulated for the megashear. Stratigraphic facies changes around the southwest end of Laurentia do not support offset by such a megashear. Paleozoic biostratigraphic faunal groups persist throughout the length of the orogen to Baja California and are distinct from those in the Cordilleran margin. These, together with later Mesozoic faunal groups, continue across the hypothetical trace of the megashear with no offset. Finally, paleomagnetic data by other workers are not supportive of the megashear. The megashear concept is not compatible with the information presented in this chapter.

Abstract Nearly continuous successions of late Proterozoic through Upper Devonian rocks are widespread in the western United States between the structural fronts of the Sevier and Sonoma orogens (Plates 2-1 to 2-6, 3-1 to 3-4). West of the Sonoma orogenic front in Washington, Oregon, northe Rn and southe Rn California, western Idaho, and western Nevada, rocks of this age are limited to many small areas of exposures, most of which are shown on Plate 3-5. East of the Sevier orogenic front, some large areas on the cratonic platform contain only partial stratigraphic records because of eithe R nondeposition or erosion. The western limit of mapped carbonate-shelf rocks can be obtained from only a few tectonic windows because those rocks disappear beneath thrust plates of western facies rocks of the same age or of younger and older rocks that moved eastward as much as 200 km or more during the Antler and later orogenies. Figure 1 shows Devonian and older paleotectonic features, major post-Devonian faults, and locations of the 12 generalized stratigraphic columns shown in Figure 2.

Abstract The Ordovician Whiterock Series of western Utah, Nevada, and southern California constitutes a vast carbonate platform that grew during a single offlap-onlap cycle, 12 m.y. in duration, in late Arenig through Llandeilo time, when most of the North American continent was exposed to subaerial weathering. Evolution of the platform took place in four phases during which carbonate sedimentation initially kept pace with, then generally exceeded, the rate of relative subsidence. The margin of the platform was initially occupied by a carbonate-shoal complex rich in Nuia and calathid algal reefs, but at its climax it became composed mostly of Girvanella-rich oncolites and receptaculitid algae. Seaward of the margin, slope deposits consist of fine-grained carbonate and terrigenous clastic sediments, largely as parted to ribbon limestones with hardgrounds and slumped horizons, punctuated by limestone conglomerates. Rapid accretion of massive prograding oncolite-sand shoals during the second phase impeded water circulation to the open platform shoreward, or east, of them. The result was lagoonal accumulation of euxinic black shales and intercalated storm deposits as tongues of coquina, oncolite, and fossiliferous rudstone in an intra-shelf basin. Slowing of relative sea-level rise during phase 3 is indicated, especially in the south, by extensive peritidal deposits, which prograded westward precisely over the most substantial parts of the previous oncolite-rimming facies and assumed their function as the shelf margin. Shoreward and northward, the shallow shelf was covered by burrowed carbonate mud. Quartz sands prograded from the northeast toward the southwest and west, eventually overwhelming most of the carbonate platform. Dolomitization and karstification in a seemingly anomalous southwest-northeast, narrowly elongate area from the Talc City Hills to the Sheep Range appear to have taken place prior to deposition of quartz sands. The vast scale of this carbonate platform is such that all its elements may not be seen at a single location, in contrast, for example, to the Permian Basin sediments of the Guadalupe Mountains of New Mexico. Rather, the three-dimensional geometry of the White-rockian platform is evident only through careful measurement of stratigraphic sections and interpretation of depositional facies patterns, which are then tied together through detailed biostratigraphic correlation. The unique nature of the sediments that compose this platform is a function of its equatorial setting on a leeward continental margin and the lack of any large skeletal metazoans during this interval of Ordovician time.

At several localities in southwestern Nevada and adjacent California the Mississippian-Pennsylvanian boundary coincides with the contact between the Eleana Formation and the overlying Tippipah Limestone, or their equivalent strata. Near Red Canyon southeast of the Eleana Range, southern Nye County, Nevada, a thin limestone in the Eleana containing the ammonoids Cravenoceras hesperium Miller and Furnish and C. merriami Youngquist of late Chester age is separated by approximately 100 ft of shale and minor quartzite from platy limestone of the Tippipah, containing Diaboloceras aff. D. neumeieri Quinn and Carr of Morrow age. Three inches of conglomeratic limestone marks the base of the Tippipah. In the hills northwest of Frenchman Flat, southern Nye County, 14 ft of shale separates a limestone in the Eleana containing late Chester brachiopods from the basal Tippipah containing the ammonoids Bisatoceras, Diaboloceras, and Stenopronorites. In the hills southwest of Indian Springs, northwestern Clark County, Nevada, we refer a similar sequence (the Indian Springs Member of Longwell and Dunbar, 1936, of the Bird Spring Formation) to rocks equivalent to part of the Chainman Shale and Tippipah Limestone. The Chainman equivalent is about 85 ft thick and contains Mississippian (Chester) brachiopods 25 ft below the top. The basal Tippipah equivalent contains Bisatoceras, Diaboloceras, Stenopronorites, and Syngastrioceras. One foot of conglomeratic limestone marks the base of the Pennsylvanian. The same lithologic sequence is recognized on the northeast flank of the Nopah Range, southeastern Inyo County, California. The Tippipah ammonoids appear closest to late Morrow forms, indicating a hiatus at the base of the Pennsylvanian in this region. A Mississippian-Pennsylvanian disconformity is further suggested by the abrupt lithologic change and the conglomeratic limestones.