UNIVERSITY OF NEVADA RENO
General Geology of Triassic Rocks at Alaska Canyon in the Jackson Mountains, Humboldt County, Nevada
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in geology
by
Lynn Roy Fuller U\
November 1986 MINES 1 LIBRARY T^i e St'S
9k li %
The thesis of Lynn Roy Fuller is approved:
UNIVERSITY OF NEVADA
RENO
November 1986 i i
ACKNOWLEDGMENTS
I wish to thank Joseph Lintz, my advisor, for his constant encouragement and support. He introduced me to this investigation, accompanied me in the field and made countless valuable suggestions during the course of the investigation.
During three summers of field work, many friends and family members offered their help and support. They gave up their vacation time to indure the alkali dust and heat of the Black Rock Desert in summer. To the following I wish to offer my sincere appreciation: Steve Hamilton, Dan Howe, James Wilkinson, Barclay Anderson and Mel, Diane, Monte and Steen Fuller.
I also wish to thank Dan Howe, Tom Lugaski, and James Firby for many hours of stimulating, informal discussion on the problems and implications of these rocks.
I am also appreciative of the financial support received for the project from the Sunmark Exploration Company of Denver, Colorado. i i i
ABSTRACT
Triassic mudrocks, ca1carenites, conglomerates and limestone of the Boulder Creek beds are exposed in a deformed stack of imbricate thrust nappes along the range front of Jackson Mountains in Humboldt County, Nevada. Deposition occurred in a terrane of basinal associations after the Sonoma Orogeny.
A section of these rocks are exposed along the northeast wall of Alaska Canyon. There, a quartz-rich
sequence of laminated and non- 1 aminated mudrocks with channelized debris flows and immature turbidites along with thick carbonates are interpreted as a middle to upper deep-sea fan regime.
Ammonites (Arcestes, Pi scotropites) and pelecypods (Ha 1 ob i a ) collected from sediment gravity flows and mudrocks respectively correlate with fossils from the Pine Forest Range in Humboldt County, Nevada and the so-called Hosselkus Limestone in Shasta County, California. Structural data collected at and near Alaska Canyon agrees with previous research and indicates a north-northwest to south-southeast compressiona 1 orientation. TABLE OF CONTENTS
Introduction......
Purpose and Scope of the Study...... 6
Geology of the Jackson Mountains...... 8 McGi 11 Canyon unit...... 10 Boulder Creek beds...... 11 Happy Creek igneous complex...... 13 King Lear Formation...... 14
Alaska Canyon...... 15
Results of the Study...... 21 L i tho 1 ogy...... 21 Environment of deposition...... 25 Paleocurrent analysis...... 34 Paleontology...... 37 Structure...... 44 Regional Setting...... 48 Golconda allochthon...... 48 Sonomi a...... 49 Western Mesozoic marine province...... 51 Rocks of the marine province...... 52 Section at Alaska Canyon...... 54 Structural implications...... 56 Summary......
References...... 62 Appendix 1 - Lithologic Descriptions...... 71 Appendix 2 - Systematic Paleontology...... 85 INTRODUCTION
The objective of this study was to determine the age, depositional nature and probable extent of limestone and related sedimentary rocks that occur along the western front of the Jackson Mountains in the vicinity of Alaska Canyon (figure 1).
The Jackson Mountains are located in central Humboldt County, Nevada. This range forms the eastern boundary of the Black Rock Desert, an extensive alluvial and playa flat occupying parts of three counties in northwestern Nevada. Alaska Canyon (figure 2 and 3) is a short, steep drainage along the western side of the range (sections 12, 13; T.39N., R.30E.). It can be located on the U.S.G.S. 7.5 minute Hobo Canyon Quadrangle.
Topography, climate and vegetation in this region is typical for the Great Basin. The relief is moderately high. The Jackson Mountains rise from an elevation of 1220 meters (4000 feet) on the Black Rock playa to 2716 meters (8910 feet) at King Lear Peak in about 4.8 linear kilometers (3 linear miles). The summers here are hot and the winters cold. Humboldt County is arid to semiarid with an annual precipitation of between about 12.7 and 25.4 centimeters (5 and 10 inches)(Wi1 den, 1 964). Desert shrubs
4
FIGURE 3: Mouth of Alaska Canyon (A) viewed from about one half mile south. Prominant thrust nappe (TN) at the top of the northeast wall terminates the section described in this paper. 5
such as the common sagebrush (Artemisia tridentata) and rabbit brush (Chrysothamnus nauseosus) along with the
juniper (Juniperus utahensis) characterize the vegetation. Wilden (1958, 1963, 1964) did most of the preliminary work in the Jackson Mountains. His geologic map (1964) of Humboldt County includes the Jackson Mountains at a scale of 1.250,000. Russell (1981) studied the pre-Tertiary
pa 1eogeography and tectonic history of the central Jackson Mountains. Russell also mapped the central portion of the range on a scale of 1:24000.
The middle Paleozoic to upper Mesozoic rocks of the Jackson Mountains represent a variety of depositional environments including: basinal, slope, subaqueous and subaerial magmatic, alluvial fan, and f1uviati1e-1acustrine (Russell, 1981). These rocks may provide important clues that could increase our understanding of the Paleozoic and Mesozoic tectonic development of this portion of the Cordillera. 6
PURPOSE AND SCOPE OF THE STUDY
This study began in the summer of 1981. Sunmark Exploration Company of Denver, Colorado, graciously offered financial assistance toward a thesis that would undertake a study of the Triassic-Jurassic(?) carbonates around Alaska Canyon in the Jackson Mountains and compare them with those in the Pine Forest Range across the Black Rock Desert. During that first summer measurements and descriptions were made of the sections in the two ranges. With the obligation to Sunmark fulfilled, the research was directed to the carbonates and related rocks in the vicinity of Alaska Canyon in the Jackson Mountains. Part of a second summer was spent mapping the area near Alaska Canyon, and a portion of a third summer was used to look at sedimentary features, collect paleocurrent data and measure structural orientations also in and around Alaska Canyon. Both the structural complexity and the rugged topography in and around Alaska Canyon provided an interesting challenge. Mesozoic folding and thrusting produced a deformed stack of thrust nappes. The nature of the rocks also added to the problem. The highly fractured mudrocks, thick-bedded sandstones and relatively unfossi1iferous , massive limestones when structurally 7
imbricated, deformed and exposed discontinuous1y on shear cliffs and steep talus slopes are difficult to map and correlate. Because of the structural complexity no realistic cross section could be generated to accompany the geologic map.
At the beginning of the second summer of study, a probable continuous section was located on the northeast wall of Alaska Canyon. The description of the lithological and depositional nature of the rocks of this section is a major objective of this paper. While it appears that the section is continuous, it is by no means complete. The section does not contain all the rock units present at this locality, but it is believed representative of depositional conditions for similar and depositionally related carbonate and clastic rocks found between Alaska Canyon and Bliss Canyon to the north. 8
Geology of the Jackson Mountains
The rocks of the Jackson Mountains have been divided into two tectonostratigraphic units by Russell (1984). These are the McGill Canyon unit and the Jackson Mountains unit. A tectonostratigraphic unit (Russell, 1984 after Jones and others, 1981) is a distinctive stratigraphic sequence or assemblage which is fault bounded and markedly differs from nearby partly or entirely coeval units.
The older tectonostratigraphic unit in the Jackson Mountains is the McGill Canyon unit. It contains Paleozoic volcanogenic and siliceous turbidites, sediment gravity flows, hemipelagic and pelagic rocks, and some carbonate units (Russell, 1984).
The Jackson Mountain tectonostratigraphic unit consists of Mesozoic rocks of Upper Triassic to Lower Cretaceous age. This unit is subdivided into three
subunits. They are: (1) the Upper Triassic Boulder Creek beds composed of carbonates, pelitic to conglomeratic
sedimentary rocks along with minor volcanic rock; (2 ) the Upper Triassic and Jurassic Happy Creek igneous complex containing basaltic andesite, andesite, diorite, quartz diorite with minor sedimentary rocks; and (3) the Lower Cretaceous King Lear Formation containing alluvial fan, 9
fluvial and lacustrine rocks.
Both the McGill Canyon and Jackson Mountains units share a similar deformationa1 history (Russell, 1984). Two episodes of folding and cleavage formation are recorded.
The first phase of folding affected all pre-King Lear rocks and is associated with large, cryptic, open folds with axes that trend northwest-southeast. The evidence is somewhat tenuous for this phase of folding and assumes insignificant rotation and displacement along thrusts. These folds are very large, of a scale approximating the width of the
Jackson Mountains and with the principal axis of shortening approximately southwest-northeast.
The second phase of deformation is better documented and is orthogonal to the first phase (Russell, 1984). It produced northwest-verging, shallowly plunging major folds and a northeast-striking foliation in the King Lear rocks. This phase of deformation also produced major and minor folds in pre-King Lear rocks with fold axes that parallel the above mentioned foliation of the King Lear rocks. The principal axis of shortening for this phase of deformation is northwest-southeast.
According to Russell (1984) there were at least two episodes of thrusting. One episode appears to be associated with the second phase of folding since cleavage is folded near the thrusts with resultant fold axes that 10
lie parallel to second phase cleavage. A later phase of thrusting with a north-south trace cuts rocks containing second phase features.
The Paleozoic and Mesozoic rocks of the Jackson Mountains acquired a very stable postfolding secondary magnetization, probably during the Late Cretaceous. This remagnetization probably accompanied the emplacement of granitic plutons in northwestern Nevada that occurred at or near the end of the Mesozoic. No appreciable tectonic rotation has occurred since that remagnetization (Russell and others, 1982).
The McGill Canyon Unit:
The McGill Canyon unit contains late Paleozoic rocks and is in thrust contact the with younger rocks of the Jackson Mountains. The McGill Canyon rocks are about 3 km thick with the base unexposed. The unit is a succession of deep water marine deposits capped by a massive shelf-margin limestone. Terrigenous, volcanic arc and shoaled terranes represent the source of detritus for the slope and base of slope basinal deposits (Russell, 1981).
According to Russell (1981), the McGill Canyon unit may represent a portion of the exotic arc containing microplate known as Sonomia (postulated by Speed, 1979), or it may represent some other exotic block or an arc related basin (forearc or backarc) proximal to the late Paleozoic western edge of North America. Similar late Paleozoic rocks occur in northwestern Nevada, northern California, eastern Oregon and western Idaho (Russell, 1981).
The Jackson Mountains Unit:
The Mesozoic Jackson Mountain tectonostratigraphic unit is divided into three lithologically distinct subunits by Russell (1984). They are the Upper Triassic basinal Boulder Creek beds; the Upper Triassic(?) to Lower
Cretaceous(?) magmatic arc related rocks of the Happy Creek igneous complex and the Lower Cretaceous alluvial
fan/fluvial/lacustrine King Lear formation. Each of these subunits will be discussed in more detail below.
The Boulder Creek Beds:
The rocks of the Boulder Creek beds, the subject of this study, are mildly to intensely deformed and include (1)Upper Triassic interbedded pelitic rocks, chert, and 12
calcaremte; (2)Upper Triassic clastic carbonate rocks; (3)b1ocks of late Paleozoic and lower Mesozoic carbonate rocks; (4)Upper Triassic to Jurassic volcaniclastic rocks and some intercalated volcanic rocks (Russell, 1984). Rocks of the Boulder Creek beds occupy thrust or igneous contacts except in places in the southern or western parts of the range where they are succeeded conformably by the Happy Creek igneous complex. The base of the subunit is not exposed (Russell, 1981).
The Boulder Creek subunit contains thin pelitic turbidites and laminates, chert and calcarenite which show continuous planar contacts, except where channelized or deformed by soft sediment deformation. They locally contain olistoliths of platform and slope carbonate deposits. The Boulder Creek beds are interpreted to be lower slope basinal deposits. Some Boulder Creek beds contain volcaniclastic rock and coarsen and thicken upsection. These rocks contain more sand and display mass flow character indicating a proximal volcanic source and increased gradient (Russell, 1984). 13
The Happy Creek Igneous Complex:
Succeeding conformably upon the Boulder Creek beds are the rocks of the Happy Creek igneous complex. This marks the transition from basinal slope to magmatic arc deposition. Some Happy Creek lavas contain pillow structures indicating subaqueous deposition. Large plant impressions and graphitic plant fossils in Happy Creek volcanogenic sediments indicate subaerial exposures as well (Russell, 1984).
Since the Happy Creek overlies the Upper Triassic Boulder Creek beds this constrains its maximum age. A whole rock Rb/Sr isochron of 160 Ma (1 standard deviation = 35m.y.) from four samples near King Lear Peak (Russell, 1981) are interpreted as dating later eruptions of the Happy Creek complex. The minimum age of the Happy Creek is constrained by the superjacent early Cretaceous King Lear formation (Russell, 1984).
The Happy Creek igneous complex is basaltic andesite and andesite varying from cryptocrysta11ine to porphyritic. Intrusives range from diorites to quartz diorites. The complex includes lavas, breccias, intrusives and some volcanogenic sedimentary rock (Russell, 1984).
Russell's (1981) petrochemical data indicates that the Happy Creek complex "represents a single, fractionated, 14
subalkaline to alk3line magma suits of island arc affinity."
The King Lear Formation:
The Early Cretaceous King Lear formation overlies an erosional unconformity above either the Happy Creek igneous complex or the Boulder Creek beds where basal exposures exist. Rock texture ranges from mud to conglomerate. Composition ranges from volcanogenic in the lower part to vo 1 canogenic and terrigenous above. The unit contains a few thin-bedded mollusk bearing lacustrine limestones (Russell, 1984). The mollusks are of Early Cretaceous age (Wilden, 1958, 1953).
Russell (1984) believes that the non-1acustrine King Lear rocks represent subaerial alluvial fan and various types of fluvial deposits coincident with deformation of Happy Creek and older rocks in the Lower Cretaceous. 15
ALASKA CANYON
The rocks of Alaska Canyon were originally described by Wi1 den ( 1964), who referred to them as the Triassic- Jurass i c(? ) carbonate unit. Russell (1981) in his dissertation on the Jackson Mountain subdivided these rocks into units Triassic x and Triassic y. Triassic x grades upward into volcanogenic sedimentary rocks of the Triassic Boulder Creek beds. Unit Triassic y is lithologically similar to unit Triassic x; but since no depositional contacts between the two units were found, Russell preferred to separate the two units. Unit Triassic y is present in the lower part of Alaska Canyon, the site of this study. Russell (1984) includes both units Triassic x and Triassic y in the Upper Triassic Boulder Creek beds. The Boulder Creek unit is a subdivision of the Jackson Mountain tectonostratigraphic unit (Russell, 1984). The unit contains carbonates, pelitic and volcanogenic sedimentary rocks along with minor volcanic material. Figure 4 shows a view down Alaska Canyon along the northeast wall from about 300 meters (330 yards) upcanyon. This report describes the section from from (A) to (B) on figure 4. Fossils pelecypods (Halobia) described in this report are present in mudrock approximately at (H). Beds 16
FIGURE 4: Northeast wall of Alaska Canyon. (A) to (B) is the traverse of the described section, (H) Halobia bearing mudrocks, (Ar) Arcestes bearing sediment gravity flows, (D)debris flows used for paleocurrent analysis. 17
containing ammonites described in this report are located approximately at (Ar). Pebbly debris flows sampled for paleocurrent data are approximately at (D). Due to the structural complexity, beds belonging to this large fold cannot definitely be traced either up, down or across the canyon. Sitting atop of the described section and shown more clearly in figure 5 is a thrust nappe of massive limestone. This nappe can be traced across the canyon to
the southwest side. Figure 6 shows the deformed nature of
the rocks on the southwest side of the canyon. Figure 7 is a photograph of the northern most extension of the unit on the northeast side of Bliss Canyon.
The rocks between Alaska and Bliss canyons are massive carbonates and various clastic units ranging from mudstones to cobble conglomerates. The various rock types are often in thrust contact, deformed and resting at high angle. FIGURE 5: Northeast wall of Alaska Canyon: Described section was through lower right hand corner to (B), (TN) is large prominent thrust nappe capping section, (SG) are sediment gravity flows in large fold showing probable lenticular nature, (Ar) is Arcestes bearing sediment gravity flows, (H) is Halobia bearing mudrocks. FIGURE 6: Southwest wall of Alaska Canyon showing the highly deformed nature of massive carbonates setting on mudrocks at base of wall. 20
FIGURE 7: Northern extension of unit in Bliss Canyon: massive carbonates (MC) in probable thrust contact with clastic rocks (CR) containing mudrocks to conglomerates. Carbonates and associated clastic rocks pinch out on the north side of canyon (at camera). 21
RESULTS OF THE STUDY
Lithology:
The Boulder Creek beds from Alaska Canyon north to Bliss Canyon contain: massive micritic, peloidal, and
crinoidal limestones; laminated and non- 1 aminated mudrocks;
vo1canic1 astic sandstones; ca1carenites; and both matrix and clast supported conglomerates. The relationship of these various rocks is not always clear because of the complicated structure and uncertain nature of the contacts. Many thrust contacts occur, and any contact between mudrocks and the more competent beds is suspect. Because of the deformation, contacts have accommodated some movement which makes it difficult to interpret the exact nature of the contacts. The canyon may also contain some limestone olistoliths (Russell, 1981) which may further complicate relationships. Olistoliths are exotic blocks that slid into place from upslope during the original deposition.
Figure 8 is a graphic representation of the section along the northeast wall of Alaska Canyon through the prominent fold shown in figure 4 and 5. Detailed descriptions and thin section photomicrographs are located 200 meters
AA: dark gray, massive pelmicrite
A: dark gray, weathering mottled tanish-gray, massive, crinoidal biopelmicrite containing rounded, redish-gray to black chert grains
B: dark gray, massive biopelmicrite C: dk. gray, bedded, calc, siltstone D: dark gray, biopelmicrite
E: dk. gray, quartzose intramicrite
F: dark gray, thin to thick bedded, bioclastic intramicrite; Arcestes, D i scotropites
G: dark gray, weathering light tan to brown, thin to thick bedded, quartzose micrite; Halobia
H: dk. gray, bioclastic intramicrite; debr'is flows
I: very dk. gray, weathering tan, thin to thick bedded, slightly fissile, laminated, quartz poor, calcareous mudstone
J: very dark gray, quartzose intra micrite; thin interbeds contain laminated, quartzose mudstone
K: gray to dark gray, massive, crinoidal biomicrite
FIGURE 8; Graphic representation of the described section on the northeast wall of Alaska Canyon. Detailed descriptions and thin section photomicrographs are contained in appendix 1. 23
in appendix 1.
While the section is lithologically variable, chert and detrital quartz are pervasive throughout most of the section. Many of the rocks contain considerable chert, but it is not all detrital. Much of the chert and some finely crystalline quartz appears to be replacing certain
carbonate intraclasts. These carbonate intraclasts show all stages of replacement. Quartz, although often a minor component, is pervasive throughout the section. It is usually medium-grained or finer, angular to subrounded and usually has normal extinction.
Feldspar and nonsedimentary lithic fragments are conspicuously absent from most of the section. The exception is one thin, granule to pebble-sized grainstone near thq bottom section (sample JM 004C in appendix 1). This bed contains feldspar and sedimentary, volcanic, Plutonic and metamorphic quartzite lithic clasts. The bed occupies a position between a massive, crinoidal biomicrite and a dark laminated mudrock. Besides its lithological content, this thin bed is also anomalous in other ways. The clasts are better rounded than in other rocks of the section, little or no mud is present and many of the clasts are agal in nature. The nature of this bed suggests a nearby, high energy, shallow water source of mixed provenance. 24
The source of the quartz in these rocks is unknown. Triassic sedimentary rocks in northwestern Nevada are characterized by their high quartz content, and the source of this quartz is debated. Silberling and Wallace (1969) envision a fluvial source originating east of Colorado Plateau. Speed (1973b) favors a contemporaneous volcanic terrane in westcentral Nevada. Considering the presence of Paleozoic fossils present in clasts of the Triassic Boulder Creek beds, a Paleozoic sequence like the McGill Canyon unit is also a very likely source of quartz. The McGill Canyon unit contains volcanogenic, lithic and quartzose sandstone along with volcanogenic breccia and conglomerate, chert and limestone (Russell, 1981). This suggests to this researcher that at least some of the quartz present in Triassic basinal sequences in northwestern Nevada came from subaerial portions of a Paleozoic terrane (Sonomia?) with rocks similiar to those now exposed at McGill Canyon, Black Rock Point (Howe, 1975) and the Quinn River Crossing (Ketner and Wardlaw, 1981). Probable equivalents of these rocks extend west to California (Schweickert and Snyder, 1981) and perhaps north to Oregon. 25
Environment of Deposition:
The rocks described in this paper from the northeast wall of Alaska Canyon exhibit characteristics of rocks deposited on the mid- to upper- reaches of a deep-sea fan. The upper fan facies association has been described by several investigators (Walker and Mutti, 1973; Nelson and Nilsen, 1974; Rupke, 1977). This association is
characterized by thick-bedded, coarse-grained, lenticular, turbidite sandstones with poorly developed Bouma sequences along with laminated and/or bioturbated mudstones exhibiting DE or CDE Bouma sequences. These represent channel and interchannel deposits, respectively, in the middle and lower reaches of the fan, medium and coarse sands with complete ABCDE or BCDE Bouma sequences predominant over interchannel muds and silts with CDE or DE sequences.
Proximal turbidite sequences include the following characteristies according to Walker (1967): thick beds, coarse grain sizes, individual sandstones often amalgamated to form tfrick beds, beds irregular in thickness, scours, washouts and channels, mudstone partings between sandstones poorly developed or absent, sand/mud ratio high, beds ungraded or crudely graded, beds with sharp bottoms and usually sharp tops, AE Bouma sequences common, laminations 26
and ripples infrequent, scour marks occur more frequently than tool marks.
The rocks in described section at Alaska Canyon vary from dark, carbonaceous, laminated mudrocks to cobbly
debris flows (f1oatstones) and thick to massive carbonate
sands (wackestone-packstones). They contain many of the characteristics of proximal turbidite sequences. Beds are often thick, coarse grain sizes (to cobble) are often
encountered and individual sandstones are often amalgamated or contain only thin mud partings. The sandstones display a high sand/mud ratio and are usually poorly graded with
sharp bottoms and usually sharp tops. Immature turbidites (AE Bouma sequences) and debris flows are common. Laminations and ripples in the sandstones are unusual. Basal scours are present but tool marks are rare.
The mudrocks in Alaska Canyon are quartz-rich and laminated except where laminations have been destroyed by bioturbation . Lamination is due to change in grain size and change in color (figure 9). Coarser grained laminae are usually graded and may show faint lamination (Bouma division D). These coarse-grained laminae are usually lighter in color due to their higher quartz content (and probably lower carbon content). The finer grained laminae (Bouma E) often show evidence of bioturbation. These rocks are interpreted to represent interchannel fines. The FIGURE 9: Thin section (lOx) of laminated mudrock from the southwest wall of Alaska Canyon probably representing sub-sea fan interchannel deposit. Light colored zones are quartz-rich, coarser grained, layers probably deposited from low-density turbidity currents. Darker, muddy layers show burrowing(B) and flame structures(F). coarser laminae probably represent deposition from low-density turbidity currents which can develope as a dilute tail to a high-density turbidity current (Moore, 1969; Rupke and Stanley, 1974). These rocks generally show some burrowing, and many are so bioturbated as to destroy all traces of laminations. They contain considerable pyrite and vary from gray to nearly black with gray to
white 1nter1 aminae. The mudrocks are nonfossi1 iferous excepting those which contain nektop1anktonic or pseudop1anktonic pelecypods (Halobia) which probably floated down from the photic zone after death.
Dispersed throughout the mudrocks are channelized sediment gravity flows from several centimeters to several meters thick with well-rounded siliceous, calcareous and bioclastic debris floating in a fine-grained matrix (figs.
10 & 11). Some of these beds represent debris flows while others are probably immature turbidites. The nature of the debris flows are indicated by their lithologically distinct bottoms and scoured soles; their massive, poorly graded nature; their random fabric and their longitudinal rather than transverse clast orientation.
The thick, massive limestone in the Alaska Canyon section may represent large blocks (ol istoliths) that have slid downslope to be incorporated into the section (Russell, 1981). Evidence for this is somewhat 29
FIGURE 10: Sediment gravity flow from the northeast wall of Alaska Canyon. Interpreted as a coarse, immature, graded turbidite rather than a debris flow. Clasts to pebble size show some degree of imbrication parallel long axis of clast orientation (to right). Lithologically distinct base lacks scours or definite tool marks. Larger, reddish mud clast (MC) floats above pebbly zone. FIGURE 11a: Polished slab of debris flow (actual size) from northeast wall Alaska Canyon showing poorly graded fabric, scoured base, distinct top. Silicified clast(F) contains several light colored fusilinids. Paleocurrent was parallel basal scours, into photograph.
FIGURE lib: Side view of same flow as figure 11a showing preferred orientation of pebbles and cobbles parallel trend of basal scours. 31
circumstantial. The contacts between these blocks and other rocks are sharp and nongradationa1. The sharp lithological disparity might be considered evidence of the olistostromic nature of the limestone. The coarse debris flows indicate some gradient, perhaps enough to accommodate the movement of these blocks. This massive limestone appears to lack lateral continuity suggesting that they are large blocks rather than beds. However it is also likely that the massive limestone has been implaced in the section tectonically. Indeed, with so much deformation and faulting nearly every lithologically distinct contact is suspect.
The thick, massive limestones in Alaska Canyon are crinoidal, peloidal and intraclastic wackestones- packstones. They might also represent proximal turbidite sands shed from a shelf or platform margin. Lateral discontinuity would be expected if these sands were deposited as channel fill or suprafan lobes. Mudrock, sand and conglomerate containing sequences are commonly encountered in laterally and longitudinally prograding deep-sea fans (Rupke, 1977).
Paleocurrent data from four successive debris flows (figure 13) strengthens the interpretation of a mid to upper fan regime. The orientations of the pebbles and cobbles in the soles of these different flows have the same 32
orientations. Generally upperslope flows vary the least in paleocurrent trends (except for over-levee splays),
becoming increasingly divergent downslope (Walker and Mutti, 1 973 ; Nelson and NiIson; 1 974, Mutti, 1 977 ).
Not encountered in this section but present between Alaska and 31i s s canyons is a thick unit of organized,
clast supported, well rounded, pebble/cobble conglomerate (figure 12). The best place to see this is the steep, north-south trending canyon between Alaska and Bliss canyons. Individual beds with color or clast size differences are welded together and some beds display channel filling. Some clasts in these conglomerates contain what appear to be Paleozoic corals. Organization of clasts indicate that these rocks probably represent deposition by density current processes in upper submarine fan channels (Walker, 1979; Collinson and Thompson, 1982). 33
FIGURE 12: Fractured, clast supported, massive conglomerate with thick beds amalgamated, in places cut and filled by channels. Clasts containing probable Paleozoic corals. Located along range front between Alaska Canyon and Bliss Canyons. 34
Paleocurrent Analysis:
The sediment gravity flows in Alaska Canyon contain few sole marks useable in paleocurrent analysis, but the bottom of many of the coarser debris flows contain gravel or cobble-sized clasts with preferred orientation. While many clasts are distributed throughout these flows, they are usually more abundant at the bottoms, and during transport these clasts have scoured the soft mud over which they passed. The soles of 4 flows are well exposed along the base of the northeastern wall of Alaska Canyon (figure 4). From these, 50 measurements of the long axis of elongated pebble and cobble clasts were sampled to determine their preferred orientations. The data are displayed in figure 13.
The clasts in sediment gravity flows are usually oriented parallel to the flow direction (Collinson & Thompson, 1982). Traction transport does however occur especially in low concentration turbidity currents (Rupke, 1977), in which case the orientations of elongated clasts can be transverse to flow. These beds however show erosive scours along their soles (figure 11) parallel the long axis of the clasts indicating that the long axis parallels the paleocurrent direction. Unfortunately definite imbrication and/or flute castes, etc. are lacking and therefore the 35
FIGURE 13: Rose diagram displaying paleocurrent data. Data is based on elongated pebble orientation from 4 debris flows at the base of the northeast wall in Alaska Canyon. 36
sense "to" of the pa 1eocurrents could not be determined. The 4 flows analyzed for paleocurrent direction are contained within the large plunging fold along the northeastern wall of Alaska canyon. To determine the original orientation of these clasts, the fold had to be opened and then rotated to horizonal on a stereonet (Ragan, 1968 after Ramsey, 1961).
Caution must be exercised in the interpretation of these paleocurrent data. First of all the data is limited to only a few related beds on one portion of the paleoslope. Secondly the deformed nature of these beds makes restoration risky. Lastly it is possible that there may of been large scale tectonic rotations since deposition of these rocks. Pa 1eomagnetic studies by Russell and others (1982) indicate their have been no rotations at least since Cretaceous.
Even if the reconstructed paleocurrent orientations from Alaska Canyon are correct and haven't been tectonically rotated they may not represent the paleoslope along the basin margin, since conduits and channels are abandoned with resulting changes in current direction during natural cycles of lateral progradation (Rupke, 1978 after Kruit and others, 1975). Much more data is required to understand the shape and extent of the depositional basin. There is also the question of whether these rocks 37
were sven deposited in their present pa 1eogeographic positions. It now seems that much of the growth of western North America is the result of accretion of terranes displaced thousands of kilometers (Jones and others, 1982). Despite the unlikely nature, it is possible that these rocks could have been deposited elsewhere and rode into their present location along with some exotic terrane.
Paleonto1ogy :
The Boulder Creek beds in and around Alaska Canyon contain few identifiable fossils. This probably relates more to the depositional environment than to any subsequent deformation.
Along the northeast wall and approximately 200 meters (220 yards) upstream from the mouth of Alaska Canyon, a calcareous, tan-weathering, dark gray mudrock occasionally contains pelecypod impressions on its weathered surfaces. At least two species of Ha I ob i a are present in these rocks. Because these rocks are highly fractured and the impressions are visible only on weathered surfaces, no complete specimens are available. Without complete specimens only tentative assignments can be made. The two forms are Halobia cf. H. superba and Halobia cf. H. 38
austriaca (plate 4 S 5).
Immediately upsection (down canyon) from the Halobia bearing mudrocks, several thick beds of gray, weathering
brown, quartzose wackestone/packstones contain ammonites which are visible on their weathered surfaces (figure 14). These beds are relatively unfractured and massive. It is possible, with difficulty, to fracture these beds with a sledge and occasionally the rock will fracture around rather than through these ammonites. In this way it was possible to obtain about a dozen partial and a few nearly complete specimens.
Most of the ammonites freed in the above manner are a smooth-shelled, globose form belonging to the genus Arcestes. From their consistent form, these all seem to belong to the same species, A. pacificus (plate 2). Several of the speci.mens collected were nearly complete and possessed clearly definable sutures.
These same rocks yielded one complete and one partial specimen of a more compressed, sculptured ammonite. These specimens were well enough preserved to allow assignment to the species Piscotropites mojsvarensis (plate 1). Although not found in Alaska Canyon but worth reporting is a second arcestid which was found during the early stages of this study while investigating the Triassic-Jurassic(?) (Wilden, 1964) limestone in the Pine
40
Forest Range. This second arcestid is a more compressed, but contemporaneous form, A. carpenteri (plate 3). These specimens were located in gray-weathering dark-gray limestone in an unnamed canyon immediately south of Dike Hot Spring (sect. 36 of T.43N., R.30E.) along the eastern front of the Pine Forest Range (figure 1). A single specimen of carpenteri found after those reported here was reported by McDaniel (1982).
All of the above fossils (including those from the Pine Forest Range) are Karnian (early Upper Triassic) in age. They have all been described from the so-called Hosselkus limestone in Shasta County, California (Smith, 1927). (The phrase "the so-called Hosselkus limestone" is used because of a probable incorrect and unresolved stratigraphic assignment. For a discussion of this, see Silberling, 1959, pg. 21.) Smith assigned them to the Juvavites subzone of the Tropites subbullatus zone. This zone is now the Tropites we 11eri zone in western North America (Tozer, 1980).
In the three summers spent in Alaska Canyon and its vicinity, the above represent the only identifiable fossil material. The rocks do however contain other fossils. Rocks with abundant broken bioclastic fragments or shelly impressions are common. Also collected was a small reptilian bone fragment probably from a marine reptile. 41
Corals of probable Paleozoic age and fusilinids are present in lithic clasts of conglomerates. Samples from the
northeast wall of Alaska Canyon were dissolved and examined for conodonts, but none were found.
Few other fossils have been reported from Alaska Canyon. Wilden (1964) assigned an age of Triassic-
Jurassic(?) to these rocks but mentions no fossils. His age assignment, one assumes, was based primarily on
lithological similarity with rocks of that age in the Pine Forest Range. Russell (1981, 1984) reports: a single
HaIobia specimen in calcareous pelitic rock in Alaska Canyon (found by R.C. Speed and identified by N.J.
Silberling as Halobia cf. H. cordi1lerana); boulders of early Mesozoic coral (identified by N.J. Silberling); a Permian fauna (diagnosed by D.L. Jones) in silicified limestone pebbles in a conglomerate olistolith; late
Triassic conodonts in the matrix of the above conglomerate; and early Mesozoic, probably Triassic, radiolaria in two chert beds along the southeast wall of Alaska Canyon (identified by D.L. Jones).
Most of the macrofossils present in the Boulder Creek beds in Alaska Canyon and its vicinity are fragments indicating transport and reworking. The ammonites and halobid pelecypods are the exception. No complete specimens of Halobia were collected, but this is the result 42
of later deformation and subsequent cleavage of these rocks. Ha 1obia impressions are observable only on weathered surfaces and in thin section. The highly
fractured nature of these rocks greatly preclude complete specimens. The fossils themselves are not reworked. Thin section of a Halobia bearing rock (sample JM 041 in appendix 1) reveals the extremely delicate valves to be complete. Almost any reworking of these fragile shells would probably destroy them.
los_itra_, a Jurassic genus closely allied to Halobia (and the younger but related Daonella), have a wide distribution and are often found in sediments without other benthonic organisms. It is likely that these pelecypods were either pseudop1anktonic (attached to floating plant material) or nektop 1 anktonic (a swimming member of the plankton). Either existence could explain wide distribution and abundance in certain sediments with limited benthonic organisms (Jefferies and Minton, 1965; Sell wood, 1978). It is likely that the Halobia present in the mudrocks of the Boulder Creek beds drifted down after death from photic zone rather than being a part of the benthos. If these halobids were benthonic, they were living on (not in) the substrate. Thin sections (sample JM 041 in appendix 1) of these rocks reveal that the delicate halobid valves are complete and parallel indicating that 43
they were probably deposited on the surface of the substrate and were not reworked.
The ammonite bearing beds in Alaska Canyon are
interesting. It is somewhat difficult to interpret the
abundance, completeness and low diversity of the ammonites in these beds. It is likely that the low diversity is
merely a function of selective preservation. However, the low diversity can also be due the harshness of the
environment. These rocks were deposited in the middle or distal reaches (Russell, 1981) or the middle and upper
portions (this paper) of a deep-sea fan. This is generally a region of low biological diversity, but were these ammonites living on or above (nektobenthonic) this substrate, or were they merely carried here from upslope by sediment gravity flows? The latter is probably most likely. Beds containing the ammonites are discrete, massive beds from several centimeters to several meters thick with muds intercalated between. The bottom of these beds are somewhat uneven probably due to soft sediment deformation, but they lack scour and tool marks. In thin section (sample JM 001 in appendix 1) these beds are poorly sorted calcareous mudrocks with abundant mud supported sand and gravel-sized lithic and biogenic clasts floating throughout. These beds fit the described characteristics of modern sediment gravity flows (Cook and Mullins, 1983). 44
The ammonites of these beds are often deformed or somewhat crushed. It wasn't possible to determine whether these specimens were crushed before or during transport or after deposition. These ammonites along with other bioclastic, intraclastic, and peloidal debris plus quartz and chert silt, sand and gravel were probably shed from carbonate producing shelf and were carried to the site of deposition by sediment gravity flows.
Structure:
The Boulder Creek beds in the vicinity of Alaska Canyon are highly deformed. They are intensely folded and thrust faulted. Between Alaska Canyon and Bliss Canyon to the north, the sequence is interpreted as a stack of steeply dipping, deformed and imbricated thrust nappes. At the thrust contacts the folds are small (with wavelengths of centimeters to several meters). Away from the thrust contacts the folds are broader (with wavelengths from several meters to several hundred meters). The pelitic rocks, clastic rocks and thinner bedded limestone are both deformed and shattered primarily by axial plane cleavage. Only the very thick, massive limestone remains relatively unfractured. 45
The structural data are displayed in figure 15, a plot of the poles to bedding and poles to cleavage for all rocks from Alaska Canyon north to Bliss Canyon. This plot
suggests a steeply dipping girdle striking approximately north-northwest to south-southeast.
The data presented in figure 15 displays considerable scattering. This is also true of Russell's (1981) data for the same rocks. Plots made by Russell although scattered, suggest a similiar girdling. Russell suggests two possible
explanations for the dispersion of structural orientations. First, the nature of the contacts between basinal rocks and clastic carbonate rocks are poorly understood and may be tectonic rather than depositional. Second, folding of
dissimilar and/or non-homogeneous materials can produce non-cylindrical folds.
Much of the problem in sampling the structural fabric lies in the difficulty of obtaining reliable attitudes.
The fine-grained clastic rocks are non-competent and are badly deformed and shattered. The competent, very thick and massive limestone while less deformed and shattered is
difficult to get reliable attitudes from because of massive nature. Also some of the very thick massive limestones present in the unit may be either lenticular, sub-sea fan depositional lobes or olistoliths, in which case a sequence of lithological non-homogeneity results which when deformed could produce non-cy1indrica1 folds.
47
Even though the data shows a considerable degree of scattering, the generally north-northwest to
south-southeast trend of compressive deformation supports the work of Russell (1981), who found similar trends (a
phase of northwest-southeast compression plus a later episode of east-west thrusting) in both the Paleozoic and Mesozoic rocks in the Jackson Mountains. This phase of compressive deformation may have been preceded by an
earlier, orthogonal phase; but the evidence for the first
phase is somewhat tenuous. The second, better documented
phase of compressive deformation deformed early Cretaceous (the King Lear formation) and older rocks (Russell, 1981).
Oldow (1983) suggests that this second phase of deformation may be related to left-oblique subduction during the late
Jurassic to.early Cretaceous which he believes responsible for the Luning-Fencemaker thrust of west-central Nevada and the Sevier fold and thrust belt of southwestern Nevada and western Utah.
The rocks between Alaska Canyon and Bliss Canyon to the north are cut by various igneous dikes and pipes. The steep, unnamed, north-south trending canyon between Alaska and Bliss Canyons follows a thick, soft dike of gabbro. The southeast wall of Alaska Canyon about 350 meters upstream is cut by a large diatreme containing angular sedimentary clasts in a porphyritic to aphanitic volcanic matrix. 48
REGIONAL SETTING
The Golconda Allochthon:
Any discussion of Triassic rocks in northwestern Nevada requires consideration of their relationship to the Golconda allochthon. This allochthon of central and northern Nevada contains an accretionary prism known as the Havallah sequence which is composed of oceanic rocks that were obducted onto continental North America during the Sonoma Orogeny sometime between late Permian and the early Mesozoic. Rocks of the Havallah sequence contain: ridge-type tholeiitic pillow lavas; ridge-type massive sulfide, siliceous iron and manganese deposits; pelagic to hemipelagic chert and argillite; and si 1 icic 1 astic , calcareous and vo 1 canic 1 astic turbidites. Fossils from the Havallah sequence indicate an age of Late Devonian to early Late Permian (Snyder and Brueckner, 1983). The Havallah sequence is an allochthonous stack of tectonically imbricated units which are-highly deformed and bounded on the lower surface by a single or multiple sole thrust, the Golconda thrust. Havallah rocks are unconformably overlain in the northern Tobin Range by the relatively undeformed Lower and Middle Triassic Koipato and 49
Star Peak groups (Silberling and Roberts, 1962; Stewart and others, 1977). This would seem to indicate that the emplacement of the Golconda allochthon was pre-Koipato. However, Dickinson (1977) pointed out another possibility; since the Koipato and Star Peak rocks are not found overlapping the Golconda thrust, it is possible that they merely rode in on top of the previously deformed Havallah rocks.
Sonomi a:
Speed (1979) proposed that a collision of a Paleozoic microplate (known as Sonomia) with North America in Triassic time was responsible for the Sonoma Orogeny. Sonomia, according to Speed, consists chiefly of island arc terranes which migrated from a "relatively distant site". Moving with left-lateral convergence above a west dipping subduction zone, Sonomia "propelled" before it an accretionary prism of oceanic sediments (the Havallah sequence of the Golconda allochthon). Overthrusting of the prism onto North America produced the Golconda allochthon. The present location of Sonomia would lie west of the Golconda allochthon and inboard of the younger disrupted oceanic rocks. Within these boundaries the presence of 50
Sonomia is indicated by scattered occurences of Permian or pre-late Middle Triassic rocks (Speed, 1979). others
(Schweickert and Snyder, 1981; Snyder and Brueckner, 1983) have suggested that the Klamath/Sierra arc terrane which is outboard of the Golconda allochthon may be the terrane that collided with North America during the Sonoma Orogeny. After the collision with North America, Sonomia according to Speed (1979) subsided to be later covered by Mesozoic and Cenozoic rocks. This subsidence may have been due to thermal contraction (Speed, 1979).
The paleogeographic site of origin for Sonomia is disputed primarily due to the occurrence of "unusual" Permian faunas from rocks located in the region of the proposed collided arc. For a discussion of this subject see Skinner and Wilde, 1965; Howe, 1975; Laule, 1978; Stevens, 1977; Ketner and Wardlaw, 1981; Ross and Ross, 1983.
The maximum age of the Sonoma orogeny is constrained by the rocks of early Late Permian (Guada1 upian) age contained within the Golconda allochthon, but the minimum age is still uncerta-in (Gabrielse and others, 1 983 ). The bulk of the evidence, however, seems to favor a Late Permian or Early Triassic age for the event. 51
Western Mesozoic Marine Province:
Extensive marine deposition occurred throughout much of the western Great Basin during the early Mesozoic. This deposition is both shelf and basinal as well as arc related (Speed, 1978A). The eastern Great Basin (eastern Nevada and western Utah), as well as parts of Arizona, Wyoming, Montana and Idaho, was also a marine province during the early Triassic (Carr and Pauli, 1983). The lack of early Mesozoic strata across central Nevada and at least one fluvial unit (Grass Valley formation) associated with western shelf rocks indicates that what is now central Nevada was likely an upland separating these two marine provinces during the early Mesozoic (Speed, 1978B). Historically these two marine troughs were once believed separated by a late Paleozoic to early Mesozoic geanticline (Nolan, 1943). Carr and Pauli (1983), however, see little evidence of the eastern marine trough shoaling against any western highlands. Rather they believe that marine deposition in the eastern trough (the Cordilleran Miogeocline) extended west to the continental margin until the Sonoma event (the docking of Speed's microcontinent, Sonomia, and the obduction of the Havallah sequence). This event they believe was responsible for cessation of marine deposition in the once persistent Cordilleran miogeocline. 52
This view is not shared by other researchers (Collinson and Hasenmuel1er, 1978) who see evidence for a positive, orogenic belt between the early Mesozoic marine province to the west and the Cordilleran miogeosyncline.
Rocks of the Western Mesozoic Marine Province:
Marine rocks of early Mesozoic age make up most of the pre-Tertiary strata in the Northwestern portion of the Great Basin (Speed, 1978a). These rocks were deposited east of the Klamath and Sierran arc(s) (Speed, 1978b; Snyder and Brueckner, 1983; Oldow, 1983) in a basin that might have been produced by thermal contraction after the Sonoma orogeny (Speed, 1978b). These marine rocks define a province that was subsident for parts of Triassic and Jurassic times. The rocks of this province can be grouped into three distinct lithological terranes, basinal, shelf and volcanic arc (Speed, 1978a, 1978b).
Rocks of the shelf terrane are carbonate platform and bank deposits which are also present to a much lesser degree in the basinal and volcanic arc terranes. Carbonates dominate the terrane with lesser quartzose clastic rocks. Mafic volcanics occur locally and may indicate "tensional differential subsidence" (Speed, 1978b). The carbonates in the northern parts of the
terrane (Star Peak group) are about 1 km thick while in the southern part of the terrane they are from 2-3 km thick (Speed, 1978b).
The rocks of the basinal terrane are mostly Triassic pelites and interstratified quartzose clastic rocks of known or suspected deep water origin. The age range and thickness of this basinal succession is uncertain. The known ages indicate that the northern portion of the terrane is Late Triassic while the southern part of terrane contains rocks of Late Triassic to Early Jurassic Age (Speed, 1978b). Definite correlation of similar rocks in extreme western Nevada, northern California and the northern Sierra has not been proved but is suspected
(Speed, 1978b). The volume of exposed basinal rocks has been estimated by Burke and Silberling (1973) at 105 cubic kilometers. Of this, 70% is mudstone which is nearly homogeneous in composition throughout the terrane, containing abundant quartz silt with illite and chlorite as the clay fraction (Speed, 1978b).
The volcanic arc terrane contains extrusive rocks of intermediate and siliceous composition with related volcanogenic sedimentary and interstratified carbonate rocks. These were generally deposited in a marine setting. 54
The age control for this terrane is meager, but the maximum age is a Rb-Sr age of 215 m.y. (Triassic) from the Singatse Range. The thickness of rocks of this terrane is also not well known, but values of about 3 km in the Singatse and Pine Nut Range are reported. In many areas of the volcanic arc terrane, carbonate and then basinal sedimentation succeeded volcanic and volcanogenic deposition in the late Triassic (Speed, 1978b).
The Section at Alaska Canyon
The Jackson Mountains are located in the basinal terrane of Speed (1978b). Russell's work (1981, 1984) indicates that the Boulder Creek beds represent deposition in a basinal setting, but then during or after late Triassic basinal sedimentation was overwhelmed by arc related volcanic and volcanogenic deposition (the Happy Creek igneous complex). Exactly when arc related deposition began and how long it persisted in the Jackson Mountains has not been determined. The superjacent lower Cretaceous King Lear formation constrains the minimum age of arc related deposition. From rocks exposed along the northeastern wall at 55
Alaska Canyon, Russell (1981) concluded that a carbonate source terrane and/or conduits from it must have prograded over fine-grained rocks of an abyssal plain, sub-sea fan fringe or starved slope. His reasoning is as follows: (1)The fine-grained rocks lack debris flows, channel deposits or slumps (except for large limestone
olistoliths); (2)course-grained rocks are absent and (3) the fine-grained rocks include turbidites with low
sand-to-shale ratios, thin beds (0.05-0.20m) and CDE and DE Bouma sequences with interbeds of pelagic and hemipelagic deposits (no evidence cited).
This researcher believes that the fine-grained rocks in Alaska Canyon are more likely interchannel, mid-to-upper deep-sea fan deposits for the following reasons:
(1)Interca1ated lithologies include channelized debris flows; thin to thick-bedded, immature, course-grained, graded wackestone-packstone turbidites and thick to very thick-bedded 1enticu1ar(?), amalgamated, quartzose calcarenites (probably representing fan-channel or fan-lobe deposit). (2)Bedded cherts are absent. (3)Laminated mudstones with DE Bouma sequences probably represent interchannel fan deposits. (4) Interca1ated debris flows with very consistent paleocurrent trends are typical of upper slope fan sequences. 56
Structural Implications
The broad open folding along a northeast-southwest axis of shortening between Middle Jurassic and early Cretaceous times (Russell, 1981, 1984) may be related to Nevadan deformation (Oldow, 1983). The Nevadan Orogeny occurred during a brief (2-3 m.y.) but intense interval of compressional deformation during the Upper Jurassic (Schweickert and others, 1983). It has been postulated (Schweickert and Cowan, 1975; Schweickert, 1978, 1981) that Nevadan deformation occurred as a result of a collision of an exotic oceanic island arc with the western edge of North America.
Deposition of the Lower Cretaceous King Lear rocks occurred during later compressional deformation. The axis of shortening for the second event was northwest-southeast. The timing and compressional orientation of this event indicates possible relationship to activity along the Luning-Fencemaker thrust of west-central Nevada and the Sevier fold and thrust belt of southeastern Nevada and western Utah (Oldow, 1983). The Luning-Fencemaker and the Sevier fold and thrust belts were active during Late Jurassic and Early Cretaceous time. They are apparently back-arc fold and thrust belts with their northeast- southwest structural orientations apparently resulting from acKsaccinwow .*.*
57
left-oblique subduction (Oldow, 1983). The relatively undeformed tract between these two active fold and thrust belts may indicate that this interposed zone may have been decoupled at depth (Speed, 1983). This suggests that the Luning-Fencemaker and Sevier belts may have shared a common deco 11 ement. The surface expression of shortening localized primarily in these two belts might indicate a crustal weakness at the sialic margin (Luning-Fencemaker thrust) and the Paleozoic miogeoclinal hingeline (Sevier thrust belt) (Oldow, 1983). 58
SUMMARY
Exposed from just south of Alaska Canyon north to Bliss Canyon along the range front of the Jackson Mountains is a lithologically diverse and highly deformed succession of marine Triassic rocks. These rocks were deposited in either a marginal back-arc basin of the Kalamath-Sierra arc (Schweickert and Synder, 1981; Snyder and Brueckner, 1983) or in basin formed on a thermally contracted and subsided, exotic, late Paleozoic, arc-containing terrane known popularly as Sonomia (Speed, 1979). The exact extent of this early Mesozoic basin is not known, but rocks believed deposited in it are now exposed in west-central and northwestern, Nevada, northern California (Speed, 1978b) and perhaps eastern Oregon. This basin (the western marine province of Speed, 1978b) contains three lithologically distinct terranes. An extensive carbonate and deltaic shelf forms the eastern margin of the province; the southern portion of the province is occupied by a volcanic arc terrane and the northern portion of province contains mostly basinal associations.
The Jackson Mountains lie within the basinal terrane of the Mesozoic marine province. Within the Jackson Mountains, Russell (1981) described a succession of 59
Triassic-Jurassic basinal and/or slope deposits succeeded by prograding volcanogenic deposits containing beach and terrigenous material. He named this succession the Boulder Creek beds. In a later paper (1984), Russell included the Triassic section at Alaska Canyon in the Boulder Creek beds.
At Alaska Canyon, Russell (1981) concluded that the fine-grained marine rocks exposed along the canyon's northeast wall were basinal or slope base deposits; and these were prograded over by a carbonate source terrane or its conduits. This researcher's interpretation of the same sequence in Alaska Canyon is that it is part of the mid or upper portion of a deep-sea fan complex. Dark, laminated mudrocks with DE Bouma sequences represent over-bank deposits. Contained within the mud lithosome are channelized debris flows with consistent pa 1eocurrents (trending north-northeast or south-southeast); immature turbidites and lenticular(?), thick to very thick calcarenites representing channel or fan-lobe deposits. The entire section along the northeast wall of Alaska Canyon is lithologically characterized by an abundance of angular to subrounded, silt-sized to course-grained quartz; a lack of feldspar and non-sedimentary lithic clasts; replacement of micritic clasts and burrows by chert and some authigenic pyrite. One thin, coarse-grained to 60
granule-sized grainstone contains feldspar, rounded to well-rounded lithic clasts and agal material. The lithic component contains volcanic, plutonic and metamorphic quartzite clasts. This bed may contain beach derived material from a distant or normally isolated site.
Ammonites of the genera Arcestes (A. pacificus) and Discotropites (D. mojsvarensis) are contained in thin to thick-bedded, immature, wackestone-packstone turbidites and debris flows. These are Upper Karnian forms belonging to the Tropites we 11eri zone. Two pelecypods Halobia cf. HaI°bia superba and Halobia cf. Halobia austriaca are present in certain dark gray mudrocks along the northeast wall of Alaska Canyon. These also belong to the Tropites welleri zone. All these fossils are present in the so-called Hosselkus in Shasta County, California. The Alaska Canyon section is also in part correlative with the Tri assi c-Jurassic (? ) limestone across the valley in the Pine Forest Range based on a contemporaneous ammonite (Arcestes carpenteri) found there.
The sequence at Alaska Canyon and north to Bliss Canyon is highly folded and thrust faulted. Work by Russell (1981) indicates two phases of compressive deformation during the Mesozoic. The earliest phase deformed Jurassic and older rocks forming open northwest-striking folds of multikilometer wavelengths 61
deformation. The second, better documented phase produced northeast-striking folds and axial plane cleavage in Cretaceous and older rocks and may be related to Sevier deformation (Oldow, 1983). This phase of compressive deformation produced major thrusting which resulted in the deformed stack of thrust nappes seen between Alaska and Bliss Canyons (Russell, 1981), The structural data in the Boulder Creek beds at this location although scattered displays a probable second phase fabric. The scattering probably reflects non-cy1indricaI deformation of a lithologically non-homogeneous section. 62
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Schweickert, R.A., Bogen, N.L., Girty, G.H., Hanson, R.E. and Merguerian C., 1983, Timing and structural expression of the Nevadan orogeny, Sierra Nevada, California: Geol. Soc. America, Rocky Mountain Section, 36 annual meeting, abstracts with programs, 15(5), p. 293, March 1983.
Selwood, B.W.. , 1978, Jurassic: in McKerrow, W.S., ed., The Ecology of Fossils, publ. by M.I.T. Press, Cambridge, Mass., p. 204-279.
Silberling, N. J. and Roberts, R. J., 1962, Pre-Tertiary stratigraphy and structured of northwestern Nevada: Geol. Soc. Amer. Spec. Paper 72, 58p. Silberling, N.J. and Wallace, R.E., 1969, Stratigraphy of the Star Peak Group (Triassic) and overlying lower Mesozoic rocks, Humboldt Range, Nevada: U.S. Geol. Surv. Prof. Paper 592, 50p. Skinner, J.W. and Wilde, G.L., 1965, Permian 68
biostratigraphy and fusulinid faunas of the Shasta Lake area, northern California: University of Kansas Palontological Contributions-Paper, Protozoa, art. 6, 98p.
Smith, J.P., 1927, 1927, Upper Triassic marine invertebrate faunas of North America: U.S. Geol. Surv. Prof. Paper 141.
Snyder, Walter S. and Brueckner, Hannes K., 1983, Tectonic evolution of the Golconda allochthon, Nevada: problems end perspectives: in Stevens, C., ed., Pre-Jurassic rocks in western North American suspect terranes; Soc. Econ. Paleont. and Mineral., Pacific Section, May 18, 1983, pp. 103-123.
Speed, R. C., 1978a, Basinal terrane of the early Mesozoic marine province of the western Great Basin: _i_n Howell, D. and McDougall, K., eds., Mesozoic pa 1eogeography of the western United States; Soc. Econ. Paleont. and Mineral., Pacific Section, Pacific Coast Paleogeography Symposium 2, p. 237-252. Speed, R. C., 1978b, Pa 1eogeography and plate tectonic evolution of the early Mesozoic marine province of the western Great Basin: _i_n Howell, D. and McDougall, K. , eds., Mesozoic pa leogeography of the western United States; Soc. Econ. Paleont. and Mineral., Pacific Section, Pacific Coast Pa leogeography Symposium 2, p. 59
253-270.
Speed, R. C., 1979, Collided Paleozoic microplate in the western United States: Jour. Geology, v. 87, p. 279-292.
Speed, R.C., 1983, Precenozoic tectonic evolution of northeastern Nevada: Geothermal Resources Council, Special Report No. 13, p. 11-24
Stevens, C.H., 1977, Permian depositional provinces and tectonics, western United States: _i_n Stewart, J., Stevens, C. and Fritsche, A. , e d s., Paleozoic paleogeography of the western United States; Soc. Econ. Paleont. and Mineral., Pacific Section, Pacific Coast Paleogeography Symposium 1, p. 113-135. Stewart, J.H., MacMillan, J.R., Nichols, K.M. and Stevens, C.H., 1977, Deep-water upper Paleozoic rocks in north-central Nevada - a study of the type area of the Hava 11 ah Formation: rn Stewart, J., Stevens, C. and Fritsche, A., eds., Paleozoic pa leogeography of the western United States; Soc. Econ. Paleont. and Mineral., Pacific Section, Pacific Coast Paleogeography Symposium 1, p.337-347. Tozer, E.T., 1980, Triassic Ammonoidea: geographic and stratigraphic distribution: House, M.R. and Senior, J.R., eds., Systematics Assoc. Special Vol. No. 18, The Ammonoidea, p. 397-431, Academic Press, London and New York.
Walker, R.G., 1967, Turbidite sedimentary structures an
their relationship to proximal and distal depositional environments: J. Sed. Petrol., 37, p. 25-43. Walker, R.G., 1979, Turbidites and associated coarse
clastic deposits: j_n Walker, R.G., ed., Facies Models, Geoscience Canada, reprint series 1, p. 91-104. Walker, R.G. and Mutti E., 1973, Turbidite facies and
racies associations: _i_n Turbidites and Deep Water Sedimentation; Soc. Econ. Paleont. Mineral., Pacific Section, Short Course, Anaheim, p. 119-157.
Wilden, R., 1958, Cretaceous to Tertiary orogeny in Jackson Mountains, Humboldt County, Nevada: Amer. Assoc. Petrol. Geo 1. Bull., v. 42, p. 2376.
Wilden, R., ,1963, General geology of the Jackson Mountains, Humboldt County, Nevada: U.S. Geol. Surv. Bull., 1141-D, 65p.
Wilden, R., 1964, Geology and mineral deposits of Humboldt County, Nevada: Nevada Bur. Mines Bull. 59, 154p. 71
APPENDIX 1
LITHOLOGIC DESCRIPTIONS 72
200 meters
AA: dark gray, massive pelmicrite
A: dark gray, weathering mottled tanish-gray, massive, crinoidal biopelmicrite containing rounded, redish-gray to black chert grains (JM 018)
B: massive biopelmicrite (JM 017) C: dk. gray, bedded, calc, siltstone D: dark gray, biopelmicrite
E: dk. gray, quartzose intramicrite
F: dark gray, thin to thick bedded, bioclastic intramicrite; Arcestes, Discotropites (JM 001)
G: dark gray, weathering light tan to brown, thin to thick bedded, quartzose micrite; Halobia (JM 041)
H: dk. gray, bioclastic intramicrite; debris flows (JM 007)
I: very dk. gray, weathering tan, thin to thick bedded, slightly fissile, laminated, quartz poor, calcareous mudstone
J: very dk. gray, silicified intra micrite; thin interbeds of lamin ated mudstone (JM 004A, JM 004C)
K: gray to dark gray, massive, crinoidal biomicrite
FIGURE 16: Graphic representation of described section along the northeast wall of Alaska Canyon. Detailed description follows. Numbers refer to thin section photographs. 73
DETAILED DESCRIPTIONS FROM FIGURE 16
AA: dark gray, massive, pelmicrite (packstone) with flattened peloids to a micrite with slight bioturbation and rare crinoidal bioclasts. A: dark gray, weathering mottled tanish-gray, massive, fractured; spar filled biopelmicrite (packstone); some rounded, coarse to granule-sized, reddish-gray (hematitic/pyritic) to black chert; peloids mostly small (less than 1mm), some medium to course-grained; bioclasts mostly crinoidal, medium to course-grained; burrowed and burrows generally containing scattered finely crystalline pyrite and chert, some with silt-sized angular quartz (JM 018). B: dark gray, weathering gray, massive biopelmicrite (packstone); peloids small (less than 1mm), some pyritized; some crinoidal bioclasts; minor silt-sized to medium-grained, angular to subrounded quartz; some clasts partially replaced by chert and microcrysta11ine quartz (JM 017). C: dark gray, weathering greenish-gray or rusty-gray to light brown, very thin to thin-bedded, quartz rich, slightly calcareous siltstone to a silty to medium-grained quartzose micrite (wackestone) ; containing angular to subrounded, silt-sized to 74
medium-grained quartz with some bioclastic fragments; bioturbated containing pyritized micrite filled burrows or burrows replaced by chert (1-2mm d i ameters).
dark gray, very thin bedded to massive, fractured, biopelmicrite (mudstone) with minor silt-sized to very fine-grained, angular to subrounded quartz; scattered crystalline pyrite; burrowed; one 10 cm pebbly bed. dark gray, weathering tanish-gray, fractured quartzose intramicrite (wackestone-packstone); contains silt-sized to very fine-grained, angular to subrounded quartz and silt-sized to very fine-grained, subrounded to rounded intraclasts replaced by chert, dark gray, weathering rusty brown to brown, thin to thick bedded, graded, sediment gravity flows of intramicrite (wackestone-packstone); containing fine to course, angular to subrounded quartz and very fine-grained to pebble-sized, subrounded to rounded intraclasts completely or partially replaced with chert and polycrystalline quartz; some shelly and crinoidal bioclasts; some beds contain ammonite molds (JM 001). dark gray, weathering light tan to brown, thin to thick bedded, highly fractured quartzose micrite (wackestone) ; containing silt-sized to very 75
fine-grained, angular to subrounded quartz and chert; scattered crystalline pyrite and finely disseminated carbonaceous material; burrowed; contains rare
pelecypod (Halobia) impressions on weathered surfaces and complete valves in thin section (JM 041). dark gray, weathering tan, thin to thick bedded, sole scoured, debris flow intramicrites (wackestone to f1oatstone) ; containing silt-sized to very fine-grained, angular to subrounded quartz;
medium-grained to cobble-sized subrounded to rounded intraclasts usually nearly replaced by chert (and some po1ycrysta 11ine quartz) and medium-grained to pebble-sized intraformationa1 lithic clasts; pebbly beds display longitudinal clast orientation; some bioclasts of ammonites, pelecypods and occassional coraline remains in cobble-sized clasts (JM 007). very dark gray, weathering tan (with rusty spherical voids (averaging about 1cm in diameter), thin to thick-bedded , slightly fissile, highly fractured, laminated, non-bioturbated, carbonaceous, calcareous mudstone with laminae variable from less than 1mm to greater than 1cm resulting from grain size differences and variations in carbon content; pyrite diffused throughout and occasional concentrated as hairline stringers or irregular finely crystalline masses several millimeters in size; rusty spherical voids on the surface are probably due to mud lumps incorporated during deposition; contains less than 10% silt-sized to very fine-grained, angular quartz; occasional pelecypod impressions on weathered surfaces, very dark gray, weathering dark gray to tanish gray, massive intramicrite (wackestone-packstone) containing silt-sized to course-grained, angular to subrounded quartz and fine to very course sand-sized, subangular to rounded intraclasts partially or completely replaced by chert and polycrysta11ine quartz; some subangular to rounded clasts of carbonaceous algal mat material; contains carbonaceous material as masses and stringers along with finely crystalline pyrite; unit contains interbeds to 0.5m of very dark gray, laminated, slightly fissile, highly deformed mudstone containing considerable silt-sized angular to subrounded quartz; laminations due to both change in grain size and carbon content; light colored laminae containing quartz, chert and micrite; limonitic stringers following fractures and disrupted zones; one thin, granule-sized grainstone at the top of this unit contains subangular to rounded, fine to coarse-grained feldspar and quartz and subrounded to well rounded, fine sand-sized to pebble-sized chert, intraclasts 77
replaced by chert, algal mat and lithoclasts of volcanic, plutonic and metamorphic origin (JM 004A, JM 004C). gray to dark gray, weathering gray to tanish-gray, massive, fractured, spar filled crinoidal biomicrite (wackestone) to a micrite highly fractured and with limonite staining along some fractures. JM 018: lOx thin section photomicrograph (plane polarized light). Bioclastic pelmicrite. Bioclasts mostly crinoidal(c), monocrystalline calcite; burrows(b) have been partially replaced by chert and contain finely crystalline pyrite; one flattened burrow contains detrital quartz(dg). JM 017: lOx thin section photomicrograph (plane polarized light). Biopelmicrite (packstone); darker peloids pyritized; crinoidal bioclasts(c) of small monocrystalline calcite; burrows(b) filled with sparry calcite; minor guartz(q). JM 001: lOx thin section photomicrograph (plane polarized light). Poorly sorted, silicified, bioclastic(bc) intramictrite (wackestone). Subangular to rounded intraclasts(i) in various stages of silicification by microcrystalline (chert) and polycrystalline quartz; angular, monocrystaline, detrital quartz(q) is common; matrix and some intraclasts contain black, carbonaceous material, amorphous hematite and finely crystalline pyrite. Beds contains whole and crushed ammonites. 81
JM 041: lOx thin section photomicrograph (plane polarized light). Bioturbated micrite (wackestone); containing Halobia valves(h); pyritized burrows(b); generally angular guartz(q) and chert(c); partially silicified peloids(p). JM 007: lOx thin section photomicrograph (crossed polars). Poorly sorted, silicified intramictrite (wackestone). Subangular to rounded intraclasts(i) in various stages of silicifaction by microcrystalline (chert) and polycrystaline(pc) quartz; angular, monocrystaline, detrital quartz(q) is present but not abundant; matrix and some intraclasts contain black, carbonaceous material, amorphous hematite and finely crystalline pyrite. 83
JM 004C: lOx thin section photomicrograph (crossed polarized light). Grainstone contains felspar, chert(c), quartz along with clasts of micrite(m) and algal(a) material and lithoclasts of volcanic(v), plutonic(p) and metamorphic origin; metamorphic polycrystalline quartz grains(pq). JM 004A: lOx thin section photomicrograph (crossed polars). Quartz rich, silicified intramictrite (wackestone-packstone). Intraclasts(i) in various stages of silification; pyrite crystals(p) are present in some silicified intraclasts; polycrystaline guartz(pc) has replaced many intraclasts; angular, monocrystaline, detrital quartz(q) is abundant; some black, carbonaceous stringers and voids contain amorphous hematite and pyrite.
86
Class CEPHALOPODA Subclass AMMONOIDEA Order CERATITIDA Family TROPITIDAE Mojsisovics, 1875 Genus DISCOTROPITES Hyatt and Smith, 1905 Pi scotropites mojsvarensis Smith (plate 1, figures a-c)
Discotropites mojsvarensis Smith, 1927, Upper Triassic marine faunas of North America, U.S. Geol. Survey
Professional Paper 141, p. 42, pi. 8, figs. 1-18. Geologic Age: Karnian (early Upper Triassic); Juvavites subzone of the Tropites subbulIatus zone (Smith, 1927). The Tropites we 11eri zone is the western North American equivalent of the Tropites subbul latus zone (Tozer, 1980).
Occurrence: Upper Triassic so-called Hosselkus limestone of Bear Cove at the north end and east side of Brock Mountain between Squaw Creek and the Pit River, Shasta Co-> Ca.; Juvavites subzone of Admiralty Island, Alaska at locality 10180 between Chapin and Herring Bays (Smith, 1927); Boulder Creek beds (this report) on the north wall of Alaska Canyon in the Jackson Mountains, Humboldt County, Nevada. Material: One complete specimen, no. JM-AC1; and one partial specimen, no. JM-AC2.
Description: Discoidal, laterally compressed, moderately involute, open but moderately narrow umbilicus, flattened sides, strong ventral shoulders, prominant keel, strong falcate dichotomous ribs bending sharply forward at the venter, fine spiral lines forming tuberac1es on crossing the sides of the ribs, fine radial striae, nodes present on ribs primarily as a double row of nodes at the ventral and umbilical shoulders.
Observations: The dimensions of specimen JM-AL1 compares well with the type (plate VIII, Smith, 1927). Below are some ratios of specimen JM-AC1 compared to approximate ratios of the type taken from plate VIII.
Ratio of:______JM-AC1______Type
Height of whorl to dia.: 40% 38% Width of preceding
whorl to last whorl: 73% 72.5% Width of umbilicus to dia.: 25% 20% 88
Family APCESTIDAE Mojsisovics, 1875 Genus ARCESTES Suess, 1865 Arcestes (Proarcestes) pacificus Hyatt and Smith (plate 2, figures a-f)
Arcestes (Proarcestes ) pac i f i cus. Hyatt and Smith, 1 905, the Triassic cephalopod genera of America: U.S. Geol. Survey Prof. Paper 40, p. 75, pi. 81, figs. 1-9; pi. 37, figs. 1-9.
Arcestes (Proarcestes) pacificus. Smith, 1927, Upper Triassic marine faunas of North America, U.S. Geol. Survey Professional Paper 141, p. 68-69, pi. 23, figs. 12-23; pi. 37, figs. 1-9, pi. 81, figs. 1-9. Geologic Age: Karnian (early Upper Triassic): Juvavites subzone of the Tropites subbullatus zone (Smith, 1927 ). The Tropites we 11eri zone is the western North America equivalent of the Tropites subbul latus zone (Tozer, 1980).
Occurrence: Common in the Upper Triassic so-called Hosselkus limestone in the upper horizon (Juvavites subzone) of the Tropites subbullatus zone on the divide between Squaw Creek and the Pit River, Shasta County, California (Smith, 1927); Boulder Creek beds (this report) in the north wall of Alaska Canyon in the Jackson Mountains, Humboldt County, Nevada. 89
Material: Four nearly complete specimens (JM-AC3 through JM-AC6) plus numerous incomplete and/or less well preserved specimens.
Description: Involute, subglobose, whorls broad, deeply embracing and deeply indented by inner volutions; umbilicus very narrow, almost closed; venter a low arch, broadly rounded abdominal shoulders; maximum width is about 75% diameter in mature specimens and whorl height is approximately 20% of diameter; surface ornamented with fine radial growth striae; usually several constrictions per revolution, curving gently forward on ventral surface, fairly straight and slightly (approx. 20%) oblique on flanks; septa are ammonitic but not deeply digitate.
Observations and discussion: A. pacificus was described by Hyatt and Smith in 1 905. A. shastensis was described by Smith in 1927. Both species are common in the same horizon in the so-called Hosselkus limestone of Shasta County, Calif. Morphologically the two forms are very similar. According to Smith (1927) A. shastensis is closely related to A. pacificus, but differs in a more compressed form and more oblique constrictions. According to Smith the septation in the two forms is exactly alike.
Four specimens (JM-AC3 through JM-AC5) from 90
Alaska Canyon are plotted below in figure 17a for
their maximum width/maximum diameter to maximum diameter. Smith's (1927) description lacks
quanitative data so for the purposes of discussion specimens of A. shastensis and A. pacificus are
plotted with approximate dimensions taken from figured specimens in Smith. Note that the specimens plotted are not necessarily of the same stage and may vary due
to ontogenic variation. Three of the specimens from Alaska Canyon plot similar to A. pacificus. The other specimen plots along with JA. shastens i s. Further
inspection of the plot reveals that there is really very little difference in form with respect to compression in these two forms. There is only about 10% difference between the average values for the two forms.
Another parameter is plotted in figure 17b. When the whorl height/maximum diameter to maximum diameter is compared no clustering is displayed for A. shastens i s and /\. pac i f i cus while the forms from Alaska Canyon display some clustering. This plot certainly gives no indication that A. shastensis and A. pacificus are different forms.
Smith (1927) further states that the constrictions on A. shastens i s are more oblique than on A. a m 92
pacificus, but close examination of figured specimens, including the types, from his paper (1927) shows that the angle the constriction makes with the tangent to the whorl are nearly equal in the two forms, about 24 degrees for both. This angle on the specimens from Alaska Canyon is about 20 degrees as nearly as it can be measured in these partially crushed specimens.
In conclusion, I believe that since there is only a slight difference of about 10% in the average lateral compression of the two forms, A. shastensis ar)d A* pacif icus, and no other apparent morphological differences there is little justification to consider them separate species. The range in lateral compression would likely fall within the morphological variation of a single species. Therefore, without a more complete statistical evaluation of these forms, I believe that Arcestes shastensis and the specimens from Alaska Canyon fall within the morphological range of Arcestes pacificus. Arcestes (Proarcestes) carpenteri Smith (p1 ate 3, figures a-c)
Arcestes (Proarcestes) carpenteri, 1927, Upper Triassic marine faunas of North America, U.S. Geol. Survey Professional Paper 141, p. 68, pi. 23, figures 1-11. Geologic Age: Karnian (early Upper Triassic): Juvavites subzone of the Tropites subbullatus zone (Smith, 1 927 ). The Tropites we 11eri zone is the western North American equivalent of the Tropites subbull atus zone (Tozer, 1980).
Occurrence: Common in the Upper Triassic so-called Hosselkus limestone, at the upper horizon of the Tropites subbullatus zone on the North Fork of Squaw Creek and also in the same horizon on Brock Mountain, between Squaw Creek and the Pit River, Shasta County, California (Smith, 1927); in dark gray, Triassic-Jurassic(?) (Wilden, 1964) limestone (McDaniel, 1982; this report) in an unnamed canyon immediately south of Dike Hot Spring (sect. 36 of T.43N., R.30E.) along the eastern front of the Pine Forest Range, Humboldt County, Nevada. Material: One complete specimen, PFR-DHS1, and several incomplete specimens. 94
Description: Laterally compressed with gently convex flanks and narrowly rounded ventral margin; whorl high and compressed, hardly adding any width; umbilicus narrow but not closed with a gentle slope to the umbilical walls: constrictions nearly straight, slightly oblique (approx. 20 degrees); septa moderately digitate. Much like Arcestes pacificus but more compressed laterally and possessing more digitate septa. Observations: Specimen PFR-DHS1 compares well with the approximate dimensions taken from figures of the type specimen and a smaller specimen figured in Smith (1927). They are compared below:
_____ Ratio of______PFR-DHS1 Type______Sm. spec. Width/diam.: 0.57 0.56 0.59 Increase in diam. per 180 deg.: 125% 121% 128% Class BIVALVIA Order PTERIOIDA Family POSIDONIIDAE FRECH, 1909 Genus HALOBIA Brown, 1830
Halobids are present in Alaska Canyon in a highly fractured, dark gray, weathering brown, non-1 aminated calcareous mudrock along the northeast wall of Alaska Canyon. Specimens occur as impressions on the weathered surfaces. Because of this rock's lack of fissility and highly fractured nature, no complete specimens were collected. Because of the lack of abundant specimens and their incomplete nature it is not possible to assign them with complete certainty to a particular species. It is evident, however, that at least two species are present in the rocks from Alaska Canyon.
Ha I obia cf. H. superba Mojsisovics (plate 4, figures a and b)
Halobia superba. Mojsisovics, 1874, Ueber die triadischen Pelecypoden-Gattungen Daonella und Halobia: K.-k. geol. Reichsanstalt Wien Abh., Band 7, Heft 2, p. 30, 96
pi. 4, figs. 9, 10.
Halobia superba. Smith, 1904, The comparative stratigraphy of the marine Trias of western America: California Acad. Sci. Proc., 3d ser., Geology, vol. 1, No. 10, p. 403, pi. 48, figs. 1,2.
Halobia superba. Diener, 1908, Ladinic, Carnic, and Noric faunae of Spiti: India Geol. Survey Mem., Palaeontologia Indica, ser. 15, vol. 5, Mem. 3, p. 94, pi. 16, fig. 7.
Halobia superba. Kittl, 1912, Materialien zu einer Monographie der Halobiidae und Monotidae der Trias: Resultate der wissenschaftlichen Erforschung des Ba1atonsees, Band 1, Teil 1, Pa 1eonto1ogie, Band 2, p. 151, pi. 7, figs. 17, 18.
Halobia,superba. Smith, 1927, Upper Triassic marine faunas of North America, U.S. Geol. Survey Professional Paper 141, p. 118, pi. 98, figs. 1-4.
Geologic Age: Karnian (early Upper Triassic): Tropites subbullatus zone (Smith, 1927). The Tropites welleri zone is the western North American equivalent of the Tropites subbullatus zone (Tozer, 1980). Occurrence: Very common in the Upper Triassic so-called Hosselkus limestone on Broke Mountain, on the divide between Squaw Creek and the Pit River in Shasta County, California; present at many other places in 97
Shasta and Plumas counties, California and in Alaska [the best locality is in the Yukon Valley one quarter mile above the mouth of the Nation River (Smith, 1927)]; first described in the Tyrolian Alps of
Austria; occurs throughout the Mediterranean region and in the Himalayas of India (Smith, 1927); present in pelitic rocks of the Boulder Creek beds (this report) on the north wall of Alaska Canyon in the Jackson Mountains, Humboldt County, Nevada.
Material: Two nearly complete specimens (JM-AC11, JM-AC12) and numerous fragments.
Description: (Note that in the following description that portion in parentheses is from Smith (1927, page 118) and pertains to features not observable in the incomplete specimens from Alaska Canyon.) (Form elongate, wider than high, rounded in front and rear; hinge line long and straight); beak slender, conical, symmetrical, projecting slightly above hinge line; anterior ear set apart by distinct furrow and lack of ornamentation; shell with definite concentric wrinkles parallel growth lines below beak, fading after first centimeter; radial ribs several times as broad as interspaces, tending to bifurcate and occasionally trifurcate away from origin; ribs suddenly bend forward at one of the last prominant wrinkles and then 98
resume radial orientation, (this sudden change of
direction happens again once or twice at greater age, giving a zigzag appearance to the ornamentation), anterior and posterior ribbing is straight lacking zigzag appearance.
Halobia cf. _H. austri aca Mojsisovics (p 1 ate 5)
Halobia austriaca. Mojsisovics, 1874, Ueber die
triadischen Pelecypoden-Gattungen Daonella und HaIobia: K.-k. geol. Reichsanstalt Wien Abh., Band 7, Heft 2, p. 26, pi. 4, figs. 1-3, pi. 5, fig. 14.
Halobia ,austriaca. Arthaber, 1906, Lethaea Geognostica, Band 2, Tei1 1, pi. 45, fig. 2.
Halobia austriaca. Kittl, 1912, Materialien zu einer Monographie der Halobiidae und Monotidae der Trias: Resultate der Wissenschaftlichen Erforschung des Balatonsees, Band 1, Teil 1, Pa 1eonto1ogie, Band 2, p. 101, pi. 6, figs. 11-14.
Halobia austriaca. Smith, 1927, Upper Triassic marine faunas of North America, U.S. Geol. Survey Professional Paper 141, p. 113-114, pi. 99, figs. 10-13. 99
Geologic Age: Karnian (early Upper Triassic); Juvavites subzone of Tropites subbullatus (Smith, 1927). The
IH°Pites -w e I Ieri zone is the western North American equivalent of the Tropites subbullatus zone (Tozer, 1980).
Occurrence: Common in the Yukon Valley, Alaska, one mile north of the Nation River and in the same valley on Trout Creek about three miles above its mouth; on Herring Bay, Admiralty Island, Alaska,; in the so-called Hosselkus limestone on Brock Mountain, Shasta County, Calif. (Smith, 1927); present in pelitic rocks of the Boulder Creek beds (this report) on the north wall of Alaska Canyon in the Jackson Mountains, Humboldt County, Nevada.
Material: One partial specimen, JM-AC10, and several fragments.
Description: (Note that in the following description that portion in parentheses is from Smith (1927) and pertains to features not observable in the incomplete specimens from Alaska Canyon.) (Form nearly symmetric, somewhat longer than high, with the beak slightly anterior to the middle of the straight hinge line) and projecting above it; (anterior ear arched and sharply divided from the body of the shell by a distinct furrow. Posterior portion not developed as 100
an ear but differs from the rest of shell in its weak sculpture); earlier surface ornamented with strong concentric wrinkles parallel to growth lines, these wrinkles extend about a centimeter from beak; (entire surface) with strong, straight radial broadly flat ribs, these being stronger on the anterior portion of shell; ribs increasing in width and tending to bifurcate away from origin to accommodate growth of shell. PLATE 1: Piscotropites mojsvarensis Smith. Specimen JM-AC1. All views enlarged 2X. Oneof two specimens collected from a sediment gravity flow on the northeastern wall of Alaska Canyon, Jackson Mountains, Humboldt Co., Nevada. Figure a is an aperatural view. Figure b is a lateral view. Figure c is a top view.
PLATE 2: Arcestes pacificus Hyatt and Smith. Three specimens collected from a sediment gravity flow on the northeastern wall of Alaska Canyon, Jackson Mountains, Humboldt Co., Nevada. Figures a and b are aperatural and lateral views, respectively, of JM-AC3. Figures c and d are lateral and aperatural views, respectively, of JM-AC6. Figure e is a lateral view of JM-AC4. Figure f is the suture pattern of JM-AC4. All figures are enlarged 2X.
PLATE 3: Arcestes carpenteri Smith.
Specimen PFR-DHS1 collected from an unnamed canyon immediately south of Dike Hot Spring along the eastern front of the Pine Forest Range, Humboldt Co., Nevada. Figures a and b are apertural and lateral views, respectively. Figure c is the suture pattern. All figures are enlarged 2x. 106
PLATE 3: Arcestes carpenteri Smith. PFR-DHS1 and sutures enlarged 2x PLATE 4 : Ha 1 ob i a c f. PL superba Mojsisovics.
Two specimens collected from fractured mudrock on the northeastern wall of Alaska Canyon, Jackson Mountains, Humboldt Co., Nevada. Figure a is JM-AC11. Figure b is JM-AC12. Both figures enlarged 3X.
109
PLATE 5: Ha 1 ob i a cf. _H. austr i aca Mojsisovics
Specimen JM-AC10 collected from mudrock on the northeastern wall of Alaska Canyon, Jackson Mountains, Humboldt Co., Nevada. Enlarged 3X.