Mesozoic geology of the Jackson Mountains, northwestern Nevada BRANCH J. RUSSELL* Department of Geological Sciences, Northwestern University, Evanston, Illinois 60201 ABSTRACT morphism and the development of only modest generally regions of oceanic and/or magmatic foliation, except near thrust faults, suggest that arc volcanism and sedimentation prior to mid- The Jackson Mountains Unit, a tectono- thrusting in the Jackson Mountains was thin Cretaceous time. The Paleozoic continental stratigraphic unit in the Jackson Mountains, skinned. margin of North America, recognized by Burch- northwestern Nevada, includes the Upper Trias- fiel and Davis (1972), apparently strikes north- sic pelagic, carbonate, and volcanogenic sedi- INTRODUCTION east through west-central and north-central mentary rocks of the Boulder Creek beds, the Nevada. Until now, regional tectonic models Upper Triassic to Middle Jurassic volcanic and The Black Rock Desert region in north- have considered the Black Rock Desert region volcanogenic sedimentary rocks of the Happy western Nevada is one of the least understood to be underlain by a late Paleozoic magmatic arc Creek complex, and the Lower Cretaceous provinces of pre-Tertiary rocks in the western (Moores, 1970; Silberling, 1973; Burchfiel and sedimentary rocks of the King Lear Formation. Cordillera. Considerably more is known of the Davis, 1972; Speed, 1977, 1979; and Stevens, The lower Mesozoic rocks of the Jackson pre-Tertiary paleogeography and tectonic his- 1977). Speed (1978a) proposed that cooling and Mountains Unit record a transition from sedi- tory of adjacent provinces (Fig. 1). The northern subsidence of a late Paleozoic magmatic arc in mentation on a starved lower slope-basinal Sierra (D'Allura and others, 1977; Schweikert, northwestern Nevada during early Mesozoic margin, locally interrupted by prograding depo- 1978), eastern Klamath (Irwin, 1977; Davis and time promoted the formation of a marine sition from shoaled Mesozoic and possibly Pa- others, 1978; Hamilton, 1978), and eastern basinal province. leozoic carbonate terranes, to the eruption of a Oregon-western Idaho (Vallier and others, My investigations of the Jackson Mountains, proximal intraoceanic magmatic arc. This mag- 1977; Brooks and Vallier, 1978) provinces were the largest and most continuous exposure of pre- matic arc, represented by the Happy Creek ig- neous complex, erupted at the margin of North 121 118 America that lay in northwestern Nevada during early Mesozoic and possibly later times. Between Middle Jurassic and mid-Cretaceous times, the Jackson Mountains Unit was sub- jected to multiple phases of deformation. Broad, open folding, probably resulting from shortening along a southwest-northeast axis (modern coor- dinates), affected the region between Middle Figure 1. Map shows Jurassic and Early Cretaceous times. location of study area and During Early Cretaceous times, southeast- distribution of selected northwest shortening produced cleavage, thrusts, provinces of pre-Tertiary and folds. The second phase of shortening, or- rocks in northeastern thogonal to the first, produced local intermoun- California, northwestern tain basins in which the fluvial sediments of the Nevada, eastern Oregon, Lower Cretaceous King Lear Formation were and western Idaho; BT simultaneously deposited, on erosional uncon- = basinal terrane of Speed formities above lower Mesozoic rocks of the (1978b). Jackson Mountains Unit, and deformed. Also during the second phase of deformation, late Paleozoic sedimentary rocks were thrust over the Jackson Mountains Unit. A later phase of thrusting affected second phase structures between Early and mid-Creta- ceous times. The absence of significant meta- *Present address: Gulf Research and Develop- ment Company, P.O. Box 37048, Houston, Texas 77236. Geological Society of America Bulletin, v. 95, p. 313-323, 8 figs., 1 table, March 1984. 313 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/3/313/3444819/i0016-7606-95-3-313.pdf by guest on 02 October 2021 314 B. J. RUSSELL Tertiary rocks in the Black Rock Desert region, of the margin of North America during early termediate igneous rocks in the Jackson Moun- have found that the para-autochthonous rocks in Mesozoic and perhaps later times. At least three tains, and named them the Happy Creek the Jackson Mountains include Upper Triassic, discrete phases of thin-skinned folding and/or volcanic series. Concluding that the Happy lower slope-basinal margin deposits that are in- thrusting deformed the Jackson Mountains Creek volcanic series is gradationally overlain terpreted to be part of the early Mesozoic basin- between Middle Jurassic and mid-Cretaceous by Permian and Triassic sedimentary and ig- al terrane of Speed (1978a, 1978b); however, times. Late Paleozoic rocks, allochthonous to neous rocks, he assigned it an age of Pe rmian or the basement of the early Mesozoic basinal ter- the Mesozoic rocks in the Jackson Mountains, older. However, the base of the Happy Creek rane in the Jackson Mountains is unclear. Be- are not discussed in detail here. volcanic series and its internal structure as well tween Late Triassic and Middle Jurassic times, a The work of Willden (1958, 1963), the only as the structure of the other pre-mid-Cretaceous magmatic arc erupted in and shed debris onto previous investigation of the pre-Tertiary rocks batholith rocks in the Jackson Mountains re- the basinal terrane. This magmatic arc, called in the Jackson Mountains, provided the frame- mained undefined. Willden also described the here the Happy Creek igneous complex, is inter- work for this study. He was the first to discuss Lower Cretaceous King Lear Formation. preted to have erupted through what was part extensive undifferentiated mafic rocks and in- Correlations of the Happy Creek volcanic se- EXPLANATION KJi Jurassic and Cretaceous igneous rocks JACKSON MOUNTAINS Unit King Lear Formation o M vo> H v I J"fih I Happy Creek igneous 4*225. complex fj-RbV///7///A i Boulder Creek beds o McGILL CANYON Unit Kl O J & depositional contact high angle fault thrust fault contact of uncertain type ->• axis of major fold —•— drainage Alaska Canyon Black Rock Desert Figure 2. Generalized geologic map of the central Jackson Mountains, northwestern Nevada; numbers identify selected subunits. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/95/3/313/3444819/i0016-7606-95-3-313.pdf by guest on 02 October 2021 MESOZOIC GEOLOGY, JACKSON MOUNTAINS, NEVADA 315 Figure 3. Selected generalized cross sections through the central Jackson Mountains. ries and associated rocks in the Jackson Moun- tive stratigraphic sequence or rock assemblage carbonate rocks. Within the study area, the tains with rocks elsewhere in the Black Rock that is bounded by faults and that differs structure of only the southernmost nappes of the Desert (Willden, 1964; Smith, 1966; Bonham, markedly from those of nearby partly or entirely McGill Canyon Unit has been studied in detail; 1969) have been the principal data base for coeval neighbors. A tectonostratigraphic unit however, reconnaissance structural examination models of the pre-Tertiary paleogeographic and may include subunits defined by structural of more northern nappes has revealed similar tectonic history of northwestern Nevada. criteria; the structurally defined subunits share fabrics and structures. The nappes of the McGill part or all of the stratigraphy of the tectono- Canyon Unit are allochthonous with respect to Geological Setting stratigraphic unit. all other prebatholith rocks in the Jackson Depositional contacts separate major internal Mountains (Figs. 3 and 4). Pre-mid-Cretaceous batholith rocks are ex- stratigraphic subdivisions in the tectonostrati- The Jackson Mountains Unit, the subject of posed throughout the Jackson Mountains and graphic units. In the Jackson Mountains, two this paper, consists of Mesozoic rocks grouped record a complex history of Paleozoic and tectonostratigraphic units are the McGill into three divisions of subunits. The subunits of a Mesozoic sedimentation, magmatic arc vol- Canyon Unit and the Jackson Mountains Unit. particular division share part or all of a specific canism, and deformation. With the exception of The McGill Canyon Unit, not discussed in stratigraphy. Subunits are bounded by faults of certain Jurassic and/or Cretaceous diorites and detail in this paper, consists of nappes (subunits) significant displacement and/or depositional granodiorites, the prebatholith rocks can be composed of upper Paleozoic volcanogenic and contacts with rocks of other subunits. The included in one of two tectonostratigraphic siliceous turbidites and sediment gravity flows, divisions of the Jackson Mountains Unit are units (Fig. 2). and/or hemipelagic and pelagic rocks that are (1) the Boulder Creek beds, composed of Upper A tectonostratigraphic unit, derived from the interpreted to be slope, base-of-slope, and/or Triassic carbonate and pelitic rock, volcano- definition of a tectonostratigraphic terrane inner-fan deposits (Russell, 1981). One or genic sedimentary rock, and minor volcanic (Jones and others, 1981), is defined as a distinc- more nappes include upper Paleozoic shelf-slope rock; (2) the Happy Creek igneous complex (the renamed and redefined Happy Creek volcanic series), consisting of Upper Triassic to Jurassic volcanic rock and minor volcano- genic sedimentary rock; and (3) the King Lear Formation, consisting of Lower Cretaceous fluvial sediments. JACKSON MOUNTAINS UNIT Mz Boulder