Detailed mapping and reevaluation of biostratigraphic data provide new insights into the regional stratigraphic significance of the Ordovician Comus Formation at its type locality at Iron Point, Edna Mountain, Humboldt County, Nevada. Mapping of the internal stratigraphy of the Comus Formation yielded six new subunits and a previously unrecognized formation that is potentially correlative to the Middle Ordovician Eureka Quartzite. The age designation of the Comus Formation was reexamined, using the most current understanding of Ordovician graptolite biostratigraphy. The species of graptolites found in the Comus strata at Iron Point are Late Ordovician, in contrast to the Middle Ordovician age assignment in previous studies. Structural analyses using the new detailed mapping revealed six deformational events at Iron Point. The first fold set, F 1 , is west-vergent and likely correlative to mid-Pennsylvanian folds observed nearby at Edna Mountain. The second fold set, F 2 , records north–south contraction and is likely correlative to Early Permian folds observed at Edna Mountain. The King fault is a normal fault that strikes north and dips east. It truncates the F 1 and F 2 fold sets and has not been active since the Early Permian. The Silver Coin thrust strikes east, places the Ordovician Vinini Formation over the Comus Formation, truncates the King fault, and is not affected by the F 1 and F 2 fold sets. Timing of the Silver Coin thrust is unknown, but it is likely post-Early Permian based on crosscutting relationships. The West fault strikes southeast and dips southwest. It truncates the Silver Coin thrust on the west, and the fault surface records several phases of motion. Finally, Iron Point is bounded on the east side by the Pumpernickel fault, a normal fault that strikes north and dips east. The movement on this structure is likely related to Miocene to Recent Basin and Range faulting. Several key findings resulted from this detailed study of the Ordovician rocks at Iron Point. (1) Based on detailed mapping of the internal stratigraphy of the Comus Formation at Iron Point, it is here interpreted to be correlative with the autochthonous Late Ordovician Hanson Creek Formation rather than the well-known “Comus Formation” that hosts Carlin-style gold mineralization in the Osgood Mountains to the north. (2) The Comus Formation at Iron Point is autochthonous, and the Roberts Mountains thrust is not present at Iron Point, either at the surface or in the subsurface. (3) The stratigraphic mismatch between Iron Point and Edna Mountain requires a fault with significant lateral offset between the two areas; its current expression could be the West fault. (4) West- and southwest-vergent structures at Iron Point and Edna Mountain are rotated counterclockwise relative to northwest-vergent structures at Carlin Canyon and elsewhere in northern Nevada. This relationship is consistent with large-scale sinistral slip along the continental margin to the west.

Previous structural interpretations of the Sonoma Range in north-central Nevada have concluded that the Tobin thrust fault—regarded as the equivalent of the Golconda thrust fault—is younger than other thrust faults of post-Triassic age in the range. However, thrust emplacement of the distinctive oceanic upper Paleozoic rocks of the Golconda allochthon over a large region in western and north-central Nevada, and perhaps even beyond, seems to have taken place prior to deposition of Triassic strata in the region. Hence, the structural relationships in the Sonoma Range that bear on the age of the Golconda thrust fault have been questioned. Restudy of the critical part of the Sonoma Range in the vicinity of Clear Creek shows that the oldest faults in the area that bound rocks of the Golconda allochthon and therefore may represent the Golconda thrust fault are, in fact, segments of a single fault that has been displaced by several successive slices of the Clear Creek thrust fault, the north end of which cuts Triassic rocks exposed in the northwestern Sonoma Range. Furthermore, the geometry of rocks displaced since Triassic time on the Clear Creek system of thrust faults suggests that the faults regarded as parts of the Golconda thrust fault in the Sonoma Range are offset segments of the type Golconda thrust fault as exposed about 15 km to the northeast. Consequently, the Golconda thrust fault in its type locality, as well as in the Sonoma Range, is evidently older than faults that cut Triassic rocks, and its age relationships do not conflict with the generally accepted Late Permian or Early Triassic time of emplacement of the Golconda allochthon. Integrated into this structural reinterpretation of the Sonoma Range are several other conclusions and findings of more than local significance, including the following: (1) Prior to emplacement of the Golconda allochthon, lower Paleozoic rocks in the Sonoma Range area, such as the Harmony and Valmy Formations and perhaps the Preble Formation, were intricately deformed and faulted together, presumably during the middle Paleozoic Antler orogeny. (2) Coarse clastic detritus derived from the Harmony and Valmy Formations occurs in the Golconda allochthon of the Sonoma Range, which suggests that it was originally deposited along the North American continental margin. (3) Radiometric ages of plutonic rocks in the Sonoma Range suggest that post-Triassic displacement, perhaps as gravity slides, of parts of the Golconda allochthon on the Clear Creek system of thrust faults took place between about 170 and 100 m.y. ago.

Paleozoic and Mesozoic rocks exposed near Quinn River Crossing, Humboldt County, Nevada, constitute two terranes that have played important roles in the tectonic history of this region. Data collected from these terranes provide insight into the tectonic events that shaped the margin of western North America during the late Paleozoic and the early Mesozoic. The Jackson terrane at Quinn River is composed of a chert-quartzite arenite containing Late Devonian and pre–Late Devonian chert pebbles. This unit sits below a Jurassic(?) ash-flow tuff that has embedded limestone blocks, mafic and felsic volcanic clasts, and hornblende porphyry blocks K-Ar dated at 191 ± 9 Ma. Thrust over the Jurassic(?) ash-flow tuff is the Black Rock terrane, which includes the Lower Permian Bilk Creek limestone, Permian clastic and arc-derived volcaniclastic rocks, and the Upper Permian to Middle Triassic Quinn River Formation, composed of chert, siltstone, shale, sandstone, and tuffaceous and volcaniclastic rocks. The Jackson terrane has an enigmatic paleogeographic relation with the Black Rock terrane. Lithologic, age, structural, and biogeographic data indicate that the Black Rock terrane at Quinn River is correlative with the eastern Klamath terrane in northern California. In contrast, the Black Rock terrane at Quinn River is lithologically, biogeographically, and structurally distinct from coeval rocks of the Golconda allochthon in the nearby Osgood Mountains and Hot Springs Range. The lack of Sonoman deformation in the Black Rock terrane, the lack of Permian arc-derived volcaniclastic rocks in the Golconda allochthon in the Osgood Mountains and Hot Springs Range, the absence of any linking facies between these two areas, and the presence of post–Early Jurassic deformation in rocks between these areas all do not support models that imply relative proximity of the Black Rock arc terrane to the northwesternmost part of the Golconda allochthon during Permian-Triassic time. The evidence implies instead that the present relative positions of the Black Rock terrane and the northwesternmost exposures of the Golconda allochthon are the result of younger Mesozoic tectonic events unrelated to the original paleogeographic settings of these rocks.

D espite their remarkable diversity, the Paleozoic and Mesozoic rocks of northwestern Nevada are related in both their depositional and structural history. In the northern part of the region, lower and middle Paleozoic rocks were greatly deformed during the Antler orogeny, after which shallow-water limestone and clastic rocks of the Antler sequence of Pennsylvanian and Permian age were deposited upon them. The Havallah sequence, an entirely different facies of upper Paleozoic rocks including several thousand feet of fine-grained clastics, chert, and volcanic rocks, is thrust eastward over the Antler sequence and older rocks. Overlying the Havallah are Permian and Triassic volcanic rocks of the Koipato sequence and two different facies of lower Mesozoic sedimentary rocks, the Winnemucca and Augusta sequences. Part of the Augusta sequence of latest Early to Late Triassic age is interpreted as an eastern and more shoreward facies of the Winnemucca sequence, which ranges from Middle Triassic to Early Jurassic in age. The rocks in the southern part of the region, south of a belt in which there are few pre-Tertiary outcrops, represent the same time span as those farther north, but rock units and structural features differ. Greatly deformed lower Paleozoic rocks along the southeastern and southern margin of the region are unconformably overlain by the Diablo sequence, the lower part of which resembles the Antler sequence and the upper dominantly volcanic part, the Koipato sequence. Overlying the Diablo is the Luning sequence, mainly of calcareous sedimentary rocks ranging from latest Middle Triassic through Early Jurassic in age. The upper part of the Luning sequence was laid down during the initial folding and thrusting of Mesozoic orogeny. Volcanic rocks of the Gillis sequence, tentatively dated as Middle or Late Triassic, are thrust over the Luning sequence from the west. Three periods of orogeny deformed the Paleozoic and Mesozoic rocks of the area. The Antler orogeny took place during late middle Paleozoic time; this was followed by the Sonoma orogeny during the middle or Late Permian and by folding and thrusting that commenced in the late Early Jurassic and persisted into Cretaceous time. Thrust faulting, represented in some places by faults with great horizontal displacement, characterized all three orogenies. The sequence of thrusts in the northern part of the area, and therefore the paleogeographic interpretation of the upper Paleozoic and Mesozoic rocks, has not been completely resolved. Two interpretations can be made: that the Golconda thrust, bringing the Havallah over the Antler sequence from the west, is one of the youngest of the Mesozoic thrusts and controls the distribution of the Augusta sequence of Triassic rocks; or that the Golconda thrust is older, developed during the Sonoma orogeny, and did not affect the geographic relationships of the Augusta to the Winnemucca sequence.

Abstract The Golconda Group was deposited during the early to middle Chesterian, a time of transition in the Mississippian from when conditions favored ooid formation (Valmeyeran) to when conditions were unfavorable (late Chesterian). As a result, the Golconda is one of the youngest Chesterian carbonate units in the Illinois basin containing abundant ooids. Oolitic limestone is common along the western, southern, and eastern margins of the basin. Elsewhere, oolitic deposits are more sporadic and thin. The presence of a positive area (i.e., the Ozark dome) in Missouri induced conditions appropriate to ooid formation in the southwestern part of the basin. Antecedent topography formed by bioaccumulation also favored oolite deposition. Terrigenous influx and storm activity tended to discourage ooid formation. Although there are similarities between Valmeyeran and Chesterian oolite characteristics, notable differences exist.