Mesozoic Geology of the Jackson Mountains, Northwestern Nevada

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 igneous 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

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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.

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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 volcanogenic sedimentary rock; and (3) the King Lear Formation, consisting of Lower Cretaceous fluvial sediments.

JACKSON MOUNTAINS UNIT

Mz Boulder Creek Beds Jackson Mountains Unit Characteristics. The Boulder Creek beds, a division of the Jackson Mountains Unit, are exposed in mildly to intensely deformed subunits. The subunits of the Boulder Creek beds referred to in this paper are numbered in Figure 2. The Boulder Creek beds include (1) Upper Triassic interbedded thin pelitic rocks, chert, and calcarenite, (2) Upper Triassic clastic Figure 4. Diagrammatic representation of tectonostratigraphic units in the Jackson carbonate rocks, (3) blocks of late Paleozoic Mountains. All thrust faults are Mesozoic in age. and lower Mesozoic platform and slope carbon-

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 E co ©E CO C O © Lithology Character of Rocks >. d> Unit CO Unit CO co CO

thrust fault thrust fault carbonate olistostrome conglomerate and volcanogenic • O* Ao , sandstone

co calcirudite and minor calcarenite (0 Boulder Boulder à. D , polymict breccia CO Creek Creek

sub unit calcarenite and minor calcirudite; CO subunit conglomerate and volcanogenic minor interbeds of calcareous co sandstone peli tic rock co 1 • K> - ÍV o © 3 c -.c ;¿>. ci a "j L'i u: calcarenite and micrite; pelitic -> rocks include Norian Halobia Q. amygdular porphyritic feldspar lava 3 © of intermediate composition T3 pelitic rocks and chert; cut locally MIMMI interbedded thin chert, calcarenite and calcareous pelitic rocks •o by diatremes and porphyritic dikes a * 1 thrust fault thrust fault volcanic flow breccia o O H <91 W '55 Happy co V < ; CO co Creek v basaltic andes i te lava and flow Boulder .S L- breccia k_ CO volcanogenic breccia 3 mm I- o -3 complex ^ N/ CO Creek w. - CO © © i » y /? a. subunit • a •. • a volcanogenic sedimentary breccia & a ' £7 3 and sandstone coarse-grained volcanogenic 4 « O O sandstone gradational contact between vol- <1) a. Boulder canogenic sandstone and nonvol- .»'ft. canogenic rocks which include a co Creek chert, limestone, and pelitic z> medium-grained volcanogenic co rocks; succession cut locally sandstone co subunit by diatremes and porphyritic rocks fine-grained volcanogenic sandstone and pelitic rocks interbedded thin chert, pelitic thrust fault rocks, and limestone (some beds Boulder © contain upper Triassic fossils); block of massive carbonate Creek fine-grained volcanogenic sandstone, a pelitic rocks, chert, and limestone a _z - ? - i - subunit ss D 5

Figure 5. Columnar sections of selected subunits of the Boulder Creek beds.

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ate deposits, and (4) Upper Triassic to Jurassic round chert grains in laminated (centimetre canic rocks. The volcanogenic sedimentary volcanogenic sedimentary rocks. Locally, vol- scale) pelitic rocks suggests that microfossils rocks include breccias, conglomerates, and sand- canic rocks are intercalated with volcanogenic (probably radiolaria) became silicified follow- stones. The volcanogenic sedimentary breccias sedimentary rocks. Generalized columnar sec- ing deposition. are composed of angular to subrounded tions of selected Boulder Creek subunits are The pelitic rocks are in places interbedded boulders, cobbles, and pebbles of volcanogenic shown in Figure 5. with thin-bedded calcarenite and dark, grainy sedimentary breccia; volcanogenic sandstone; Along the northeastern range front, the basal chert. The calcarenites are generally fine grained volcanic rocks; diorite; and lesser amounts of Boulder Creek contacts are thrust faults and/or and dolomitized, and some contain Late Triassic limestone, pelitic rocks, chert, and soft green igneous contacts. The basal subunits are over- conodonts. The calcarenite beds are 2 to 5 cm mudstone, all in a sandy volcanogenic matrix. thrust by other subunits and/or covered by thick and display centimetre-scale laminations. Volcanic rocks in these breccias include porphy- Quaternary alluvium (Fig. 2). In the southern The depositional mechanism of the calcarenites ritic pyroxene feldspar rocks, porphyritic pyrox- and western regions of the study area, subunits is unclear. ene rocks, and porphyritic to cryptocrystalline of the Boulder Creek beds are succeeded con- Several Boulder Creek subunits contain felsic rocks. The breccia beds range from 0.5 to formably by those of the Happy Creek complex calcwackes, calcarenite, and/or calcirudite 10 m thick, are discontinuous (lensoidal or fill or are overthrust by other Boulder Creek rocks composed of angular to rounded grains of silici- channels), and are massive except for occasional and by the McGill Canyon Unit. fied limestone, crinoid fragments, and chert in a very weak grading or preferred clast orientation; Age controls in the Boulder Creek beds (Rus- calcareous mud matrix. Most of these carbonates they are probably mass-flow deposits. sell and others, in prep.) are as follows: occur in continuous beds that range from 0.1 to Volcanogenic conglomerates in the Boulder 1. Subunit 1, to the west of and allochthonous 2 m thick and exhibit TBE, TBCE, TABE, or TAB Creek beds are composed of subangular to to Subunit 2, consists of thinbedded calcareous sequences. Some calcarenites are coarse- to rounded cobbles and pebbles of black and gray pelitic rocks bearing fossils of Late Triassic (Late fine-grained, thick (0.5-10 m) beds that are chert; fine-grained laminated sandstone; tan, Karnian to Middle Norian) age that are grada- massive or grade from calcirudites and occupy gray-brown, and buff limestone; pyroxene tionally overlain and cut by channels filled with channels > 10 m deep. The clastic carbonate feldspar porphyry; feldspar porphyry; tuffaceous calcarenite and calcirudite that contain blocks of rocks are interpreted to be mass-flow and and felsic rocks; and volcanogenic sedimentary Mesozoic coral. Pebbles in conglomerates in turbidity-current deposits. rocks with a groundmass of calcareous sand. Subunit 1 contain late Paleozoic fauna, and the Olistostromes in Boulder Creek beds include The conglomerates are massive except for weak matrix of these conglomerates includes Late Tri- (1) calcarenite and calcwacke, ranging from imbrication, vary from < 1 m to 5 m thick, and assic conodonts. carbonate rocks similar to those described above occur either as lensoidal beds that display 2. Subunit 2 (Fig. 2) includes interbedded to thick-bedded rocks (2-10 m), to massive calc- scoured contacts with subjacent beds or as thin pelitic rocks, limestone, and chert that in- arenite with no internal structure except occa- channel fillings, and are interpreted to be some cludes radiolaria of Late Triassic (Norian) age, sional subplanar concentrations of silicified type of mass-flow deposit. which grade conformably to volcanogenic sedi- limestone pebbles; and/or (2) beds and blocks Volcanogenic microbreccias, arenites, and mentary rocks. Subunit 2 is succeeded con- of gray silicified limestone-pebble conglomerates wackes in the Boulder Creek beds consist formably by a subunit of the Happy Creek composed of late Paleozoic limestone pebbles in of angular to subangular, granule to fine-grained complex. a matrix of Late Triassic micrite that display TA plagioclase feldspar; volcanic (porphyritic and 3. Limestone clasts of Late Triassic (Norian) sequences and grade into calcarenites and tuffaceous) fragments; and minor chert, pelitic age are present in volcanogenic sedimentary calcwackes similar to those described above; rocks, limestone, and quartz in a calcite-chlorite- rocks in Subunit 4. and/or (3) blocks of massive, dark gray lime- illite groundmass. The volcanogenic microbrec- 4. Interbedded thin pelitic rocks, limestone, stone with an irregular honeycomb-like net- cias and sandstones also include angular to and chert in Subunit 6 contain blocks of Per- work of silicified limestone, which float in a rounded cobbles and pebbles of limestone and mian and latest Middle to earliest Late Triassic pebbly micrite with their internal bedding soft green mudstone; limestone clasts in vol- carbonate. and/or long dimension subparallel to the lower canogenic sandstones in a Boulder Creek sub- Lithology. Some subunits of the Boulder contact of the olistostrome. unit along the northeastern range front bear Creek beds include thin interstratified pelitic Some limestones in Boulder Creek Subunit 6 Norian conodonts. Commonly, the sandstones rocks, calcarenite, and chert. The pelitic rocks are composed of massive 0.05- to 1-m-thick calc- contain impressions, leaves, and associated range in color from dark brown to dark gray to arenites and calcwackes, interbedded with thin branch structures that measure up to 10 cm in black and include variably calcareous fine- beds and stringers of silicified pelite, chert, and width. The coarse- and finer-grained volcano- grained and finer sandstone and variably micrite, and include ammonites and conodonts genic sandstones generally range from 0.05 to calcareous mudstone. The sandstones are prin- of latest Ladinian to earliest Karnian age; the 3 m thick, are laterally continuous (> 100 m), cipally wackes composed of angular and round stringers of chert and silicified pelite are spectac- and often exhibit TA, TAB, TABE, and TBE chert, angular quartz, carbonate and pyrite, and ular spiculites (N. J. Silberling, 1980, personal sequences, which strongly suggests that they are very rare feldspar grains in a calcareous mud commun.). Other limestones in Subunit 6 yield turbidity-current deposits. The microbreccias matrix. Beds are thin (2 to 10 cm thick), laterally Permian fossils (Willden, 1963). Some of the and some coarser volcanogenic sandstones are continuous, and graded or laminated. Some fossil-bearing limestones in Subunit 6 include massive or laminate on the centimetre scale, pelite beds exhibit flame structures, truncated numerous asymmetric, chaotic, sporadically lo- range from 0.05 to 5 m thick, and are laterally cross laminations, and TCDE and TDE sequences; cated minor folds, and others are cut by mafic discontinuous, in part filling channels. They are consequently, these beds are interpreted to be dikes. The absence of similar deformation and probably mass-flow deposits. turbidity-current deposits. Other pelite beds are igneous dikes in thin-bedded pelitic rocks and Fine-grained, thin-bedded volcanogenic sand-

laminated on a < 1- to 2-cm scale and show no chert adjacent to the limestones indicates that stones, exhibiting TDE and T^DE sequences, in other internal structure; the laminations mark these limestones are exotic blocks. places with considerable cross lamination and changing concentrations of chert, carbonate, or Most of the Boulder Creek subunits contain convolution, are considered to be turbidity- pyrite grains. The concentration of the distinctly volcanogenic sedimentary rocks and minor vol- current deposits. Fine-grained feldspar laminates

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occur either in thin-bedded sequences or in asso- isochron is interpreted to date later eruptions of homogeneous, massive, and difficult to dis- ciation with the volcanogenic turbidites; they are the Happy Creek complex. tinguish from extrusive rocks, except in thin sec- hemipelagic deposits and/or bottom-current Lithology and Physical Character of Litho- tion. Most of the known intrusive rocks reworkings. somes. The following is a brief petrographic associated with the Happy Creek occur in dikes The volcanic rocks in the Boulder Creek beds description of the igneous rocks in the Happy <20 m across. include medium- to fine-grained pyroxene Creek complex; a more detailed analysis of the Some of the diorites and quartz diorites, as- feldspar porphyries, similar to the basaltic petrography, chemistry, and tectonic setting of sumed to be intrusions, may have crystallized in andesites and andesites in the Happy Creek the Happy Creek complex is given in Russell the interior of well-insulated flows; the contacts complex discussed below, and amygdular por- (1981). of these bodies are generally obscured. The in- phyritic lava composed of medium- to fine- The Happy Creek complex contains basaltic trusive rocks are concentrated in two areas grained feldspar phenocrysts in a red aphanitic andesite and andesite that are cryptocrystalline (Trout Creek Spur and the area east of Alaska matrix. The amygdular porphyritic lavas exhibit to porphyritic (hypocrystalline), with pheno- Canyon) (Fig. 2). The diatremes in the Boulder pillow structures and contain angular fragments crysts of labradorite and/or andesine, augitic Creek Subunit of Alaska Canyon are within 250 of pelite arid carbonate. Deposits of jasperite and pyroxene, and magnetite. The groundmass of m of extrusive-intrusive rocks in the Happy carbonate fill some of the voids between the the basaltic andesites and andesites includes mi- Creek complex. The extrusive-intrusive-diatreme pillows and fractures in the amygdular lava. The croliths of plagioclase; clinopyroxene; opaques; neighbors may indicate proximal magma con- amygdules, < 0.5 cm in diameter, are composed altered glass; and amygdules (Q%-30% modal duits. Dikes commonly cut volcanic rock and of calcite and/or silica. volume) < 2 mm in diameter filled with chlorite, volcanogenic sedimentary rock throughout the In places, it is not possible to determine if calcite, and epidote. These volcanic rocks are range. some of the volcanogenic breccias are of igneous locally albitized and/or sericitized near younger The volcanogenic sedimentary rocks include or sedimentary origin. These breccias lack intrusions. Textures of the volcanic rocks range breccia, microbreccias, and sandstones that are internal structure, are rich in tuffaceous rocks, from hyalophitic to intersertal. indistinguishable from volcanogenic sedimen- and range from 1 to > 20 m thick. Intrusive rocks in the Happy Creek complex tary rocks of the Boulder Creek beds. Along the southeast wall of Alaska Canyon, range from diorites to quartz diorites composed Boulder Creek Subunit 2 is cut locally by poly- of labradorite and/or andesine, clinopyroxene, The King Lear Formation mict breccia composed of angular clasts of vol- magnetite, and 0% to 7% modal volume intersti- canogenic sandstone, pelitic rocks, calcarenite, tial quartz. Local albitization and sericitization is Characteristics. The Lower Cretaceous King and chert in a porphyritic feldspar to aphanitic common near younger intrusions. Textures of Lear Formation, described in part by Willden matrix. The polymict breccia is interpreted to be the intrusive rocks range from hypidiomorphic (1958), overlies an erosional unccnforrnity a diatreme because it cuts bedding at high and granular to subophitic. above the Happy Creek complex or the Boulder low angles, lacks internal structure, and is com- The Happy Creek complex includes lavas, Creek beds wherever a basal contact is exposed posed of a homogeneous clast population similar autobreccias, volcanic-flow breccias, intrusive (Fig. 2). Locally, the King Lear Formation in composition to the wall rock set in a volcanic rocks, and minor volcanogenic sedimentary varies in composition from gravel to mudstone matrix. rocks. Lavas and autobreccias of basaltic composed almost entirely of volcanogenic rock, andesite and andesite occur as 5 to 30+-m-thick, to conglomerate to mudstone compcsed pre- Happy Creek Igneous Complex gray, red, or green beds that are laterally contin- dominantly of terrigenous rock (Fig. 6). Several uous on the scale of the subunits where their subunits also include sporadic thin-bedded la- Characteristics. Exposed throughout the contacts can be traced. The lavas are generally custrine limestones, some of which contain study area, the Happy Creek complex is an massive but contain occasional pillow structures freshwater mollusks of Early Cretaceous age extrusive-intrusive complex of basaltic andesite, and/or angular pebbles and smaller fragments of (Willden, 1958, 1963). andesite, diorite, and quartz diorite, with minor metacarbonate and metapelite. The angular Thrust faults that cut or bound subunits of the sedimentary rocks. The igneous rocks occur as boulders and cobbles in the autobreccias are vir- King Lear Formation have brought Happy well-exposed sequences of lava, volcanic-flow tually indistinguishable in composition from the Creek and King Lear rocks over other K ing Lear breccia, and intrusions. matrix. rocks. High-angle faults also cut or bound King The Happy Creek complex consists of several Volcanic-flow breccias of basaltic andesite Lear subunits. subunits that are separated from neighboring and andesite are gray, red, and/or green and are Lithology. The King Lear Formation in- rocks by thrust faults, high-angle faults, ero- exposed in flows or in massive exposures with cludes volcanogenic gravel, conglomerate, sand- sional unconformities, intrusive contacts, and no apparent contacts. Clast types range from stone, and mudstone. The lowest King Lear contacts of uncertain type. One Happy Creek 99% to 75% volcanic rocks, which include angu- strata above the Happy Creek complex are prin- subunit in the vicinity of Alaska Canyon is lar boulders to pebbles of porphyritic pyroxene cipally volcanogenic gravels and conglomerates everywhere bounded by thrust and high-angle rocks, porphyritic feldspar rocks, porphyritic composed of angular to round clasts of Happy faults, erosional unconformities, and igneous pyroxene-feldspar rocks, diorite, and minor fel- Creek complex rocks, pink and/or green coarse- contacts, except along its southwestern margin, sic and tuffaceous rocks. Angular cobbles and grained diorite, porphyritic felsic rock, and por- where it conformably overlies the Upper Trias- pebbles of pelite, sandstone, and carbonate phyritic pyroxene rock in a volci.nogenic sic Boulder Creek Subunit 2 discussed above make up the nonvolcanogenic fraction in the sandstone matrix. These gravels as well as most (Fig. 2 and 5). Thus, stratigraphic evidence indi- volcanic-flow breccias. The matrix of the of the volcanogenic sedimentary rocks in the cates that the Happy Creek may have begun volcanic-flow breccia ranges from coarse- to King Lear Formation are devoid of organic mat- erupting as early as the Late Triassic. A whole- fine-grained pyroxene feldspar porphyry to ter or graphitic impressions and are stained dark rock Rb/Sr isochron age of 160 Ma, la = aphyric. red by hematite. In some localities, there is a 35 m.y., was determined from four rock samples Intrusive rocks, diorite, and quartz diorite as- terrigenous component (described below) in the near King Lear Peak (Russell, 1981). This sociated with the Happy Creek complex are basal King Lear rocks.

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phyry lavas. The pyroxene phenocrysts are Rock equant, euhedral to subhedral pseudomorphs Lithology Character of Rocks composed of amphibole, chlorite, and opaques. Unit Serie s The groundmass consists of highly altered mi- Syste m croliths of columnar feldspar and pyroxene, equant pyroxene, and altered glass. The feldspar mixed terrigenous and volcanogenic microliths are generally sericitic, calcitic, and "a o o û o o conglomerate, sandstone, and albitic. Pyroxene microliths are variably altered sandy mudstone and replaced by chlorite, calcite, sericite, and o a o e ö 9 King » O O o O 0 brown opaque dust. Amygdules consist of chlorite and/or calcite. o e o o OO00O Lear O o o O OOOÖ r.TT?". 7 volcanogenic conglomerate, sandstone Where unfaulted sections of the King Lear # «#• Formation rocks are exposed, the volcanogenic Lowe r and sandy mudstone; minor terrigenous Formation 9 O & O ¿> O 4 ' 4 ö O <3 Û 0 OOOOOO®0 sandstone, and mudstone imentary rocks (Fig. 6). ¿f ç Ö ej û a O & * erosional unconformity Depositional Environment of the V v " Jackson Mountains Unit V V basaltic andesite, andesite, diorite V V V

t o V v and quartz diorite Boulder Creek Beds and the Happy Creek Complex. In some Boulder Creek subunits, in- U.Triassi c MJurassi c terstratified thin beds of Upper Triassic pelitic turbidites and laminates, chert, and calcarenite Figure 6. Diagrammatic columnar section of a King Lear subunit in the Happy Creek show continuous planar contacts, except where area. locally disrupted by small slump structures (<1 m thick) or channels (<7 m deep and <100 m Most of the volcanogenic gravels and con- ally interbedded with sandstones. Some sandy wide) filled with coarse-grained clastic carbonate glomerates have very low mud fractions; how- mudstones are massive and lensoidal. The sandy or volcanogenic sedimentary rocks interpreted ever, some are notably mud rich. The mudstones are interpreted to be principally to be turbidite and mass-flow deposits. Locally, volcanogenic gravels and conglomerates range water-laid deposits. they contain blocks and olistostromes of plat- from 0.5 m to 15 m thick and are lensoidal or Mixed terrigenous and volcanogenic conglo- form and slope carbonate deposits. The inter- truncated by channels over lateral distances of merates were deposited on erosional unconfor- bedded thin turbidites, pelagic deposits, and 25 to 200 m; some channels are 10 to 25 m mities above the Boulder Creek beds and the associated carbonate slumps, similar to rocks deep. The volcanogenic gravels and conglomer- Happy Creek complex and on the volcanogenic described by Cook (1979), are interpreted to be ates occur in several different types of deposits: sedimentary rocks of the King Lear Formation. lower slope-basinal margin deposits. (1) tabular beds, interpreted to be water-laid They are composed of rounded to subrounded The mildly deformed Boulder Creek Subunit deposits, that are associated with sheet-like beds cobbles and pebbles of pelitic rocks, chert, 2 contains important clues to the relationship of sandstone and mudstone and that fill both quartz arenite, feldspathic sandstone, limestone, between the Boulder Creek beds and those of channels, and are themselves channeled; (2) len- and volcanic rock in a sandy matrix similar in the Happy Creek complex. The lowest exposed soidal beds that exhibit poorly defined inverse composition to the clast population. These con- strata in Boulder Creek Subunit 2 are lower grading characteristic of sieve deposits (Bull, glomerates are moderately to well sorted, 0.5 to slope-basinal margin pelagic and thin turbidite 1972); and (3) loaf-like deposits, interpreted to 10 m thick, tabular, and continue laterally from deposits that include blocks of massive calcaren- be debris flows, that contain high fractions of 100 to 500 m before pinching out or being trun- ite and calcirudite. Above the carbonate blocks, mud and are <10 m thick x 25 to 50 m long. cated by channels. thin volcanogenic wackes are interbedded with Associated with the volcanogenic gravels and Sandy mudstones contain varying amounts of thin interstratified lower slope-basinal margin conglomerates are volcanogenic arenite, wackes, medium- to very fine-grained sand composed of deposits of Late Triassic age. The thin volcano- and sandy mudstones. The volcanogenic sand- feldspar, chert, and quartz in a green to red mud genic wackes also indicate deposition on a lower stones are composed of angular to subangular, matrix. Mudstone beds range from 0.01 to 0.5 m slope-basinal margin for the following reasons: very coarse- to very fine-grained feldspar, vol- thick and are rarely continuous over distances (1) the lack of debris flows and channel deposits canic rocks, magnetite, and quartz with a red >25 m. and the rarity of slumps indicate deposition in a mud matrix. The sandstone beds exhibit lamina- Gray to pink muddy limestone exposed in low topographic gradient; (2) sand coarser than tions and/or cross-bedding, range from 0.2 to King Lear subunits throughout the study area as medium grained is absent; and (3) beds exhibit 1 m thick, and are truncated by channels or fill thin beds are either pinched out or truncated by other characteristics of lower-fan to fan-fringe channels. They are interpreted to be water-laid channels over distances of >50 m. Some of these deposits, according to the criteria of Nelson and deposits. limestones (Fig. 2, Sites 4a and 4b) contain Nilsen (1974). These characteristics include The volcanogenic sandy mudstones include Early Cretaceous mollusks. TBCDE and TCDE sequences, medium- to fine- angular fine to very fine-grained feldspar and Present in the King Lear Formation in the grained channel sandstone, thin (<10 cm) quartz in red mudstone and occur as beds <0.25 Happy Creek drainage area are roughly 5-m- medium- and fine-grained beds with great lateral m thick, exhibiting fine laminations, and gener- thick, fine-grained, amygdular pyroxene por- continuity, and a volume of hemipelagic deposi-

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 320 B. J. RUSSELL

TABLE I. DATA ON FOLDS IN PREBATHOLITHIC ROCKS

Phase of Structure Dimensions Orientation Remarks Rocks deformation

po Minor folds of beds 1/2 - < 5 m Randomly oriented F.A. of adjacent Occur sporadically in fine-grained rocks; some McGtll Canyon A.A. - < «20° folds in the same outcrop are overturn»! and contain thrusts and/or Boulder Creek beds nappe-like structures; not shown here

pl Major folds of beds 1/2 - > 1 km F.A.. SE-trending, shallow plunge Interpreted from data; no axial traces located; McGill Canyon Unit A.A. - > 90° Figure 8 A Boulder Creek beds

P2 Minor folds of beds 1/2 - < 50 m F.A. lie in NE-striking, steeply dipping Type IB and IC; not shown here McGill Canyon Unit A.A. - < 120° girdle Boulder Creek beds

P2 Major folds of beds 1/2 - > 0.5 km F.A. and A.P. same as P2 minor folds Observed in fidd and interpreted from fabric McGill Canyon Unit A.A. - > 90° of beds data in Figu'e 8B; some are shown in Boulder Creek beds, Happy Creek Figure 2 complex King Lear Formation

(Subunit Kkls)

P2 Major folds of beds 1/2 -> 0.5 km F.A.. NE-trending, shallow plunge Observed in field and interpreted from fabric King Lear Formation A.A. - > 90° data; Figure 8A; some are shown in Figure 2

P2 Minor folds of cleavage 1/2 - < 5 m F.A. and A.P. same as ?2 minor folds Observed in field; not shown here McGill Canyon Unit A.A. - < 90° of beds Boulder Creek beds

Noie: I /2 = nalf-wavelenglh of ihe fold; A.A. = fold apical angle: F.A. = fold axis: A P axial surface of fold.