Geology of Paleozoic rocks in eastern Sierra Nevada roof pendants, California

Abstract

Rocks in the major roof pendants of the eastern Sierra Nevada have been mapped in various degrees of detail to bet- ter understand their stratigraphy, internal structure, and geologic history, and their relationships to other rock assem- blages in the region. Ten formations ranging in age from Middle(?) Cambrian to Middle(?) Permian are recognized in these pendants, which along with other minor pendants, constitute a tectonostratigraphic unit called the Morrison block. Rocks of the Morrison block were first deformed by north-northwest-trending thrust faults and footwall syn- clines involving strata as young as Early or Middle Permian. We designate this event, which correlates with a simi- lar pre-middle Early Triassic event recognized in rocks near Tinemaha Reservoir, the Morrison orogeny. Structures produced during this orogeny include a probable cryptic thrust fault separating rocks assigned to the Morrison block from those in the Big Pine Creek pendant, which may belong to the White-Inyo block, and the Nevahbe thrust, which separates lower from upper Paleozoic rocks in the eastern part of the Mount Morrison pen- dant and may separate the Pine Creek and Bishop Creek pendants. In the Mount Morrison pendant structures produced during the Morrison orogeny apparently were later refolded twice prior to sinistral displacement on the Laurel-Convict fault, which cross-cuts older structures and is intruded by a pre-latest Late Triassic dike. Other thrust faults in the eastern Sierra Nevada include the Golconda thrust of early Middle Triassic age and the Lundy Canyon thrust of Late Triassic age. The Golconda thrust system apparently overprints the Roberts Mountains thrust and separates rocks of the Morrison block from those of the Golconda and Roberts Mountains allochthons in the Saddlebag Lake pendant, and perhaps from those of the Roberts Mountains allochthon in the Northern Ritter Range and Log Cabin Mine pendants. After thrust-faulting, but prior to intrusion of the Late Triassic Wheeler Crest Granodiorite, dextral move- ment on the Tinemaha fault displaced Paleozoic facies and structural belts in the Sierra Nevada northward, pro- ducing most of the present complicated paleogeographic patterns apparent in the region. Other less important structures, such as the Laurel-Convict fault, have further complicated the geology of the Morrison block.

Full text

237
Geological Society of America
Field Guide 2
2000
INTRODUCTION
Paleozoic metasedimentary rocks are present in numerous
roof pendants and wall rock septa in the eastern Sierra Nevada of
east-central California. In our mapping of the major pendants in
this region, we have been able to show that a consistent stratigra-
phy, differing from that of surrounding areas, is present (Stevens
and Greene, 1999). The purpose of this paper is to describe the
major structures in the eastern Sierra Nevada roof pendants and
to interpret the nature of the tectonic events in the region.
Paleozoic rocks in the eastern Sierra Nevada pendants are
interpreted to be separated from all other outcrops of rocks of
this age by faults and therefore to form a discrete structural
block we have referred to as the Morrison block (Fig. 1; Stevens
and Greene, 1999). The Morrison block includes the Mount
Morrison and Bishop Creek pendants, which we have mapped
and studied in moderate detail, and the Gull Lake and Log Cabin
Mine pendants, which were studied in less detail. The Morrison
block also includes the Pine Creek pendant, part of the North-
ern Ritter Range pendant, the Deadman Pass pendant, and various
other smaller outcrops (Fig. 1). Thus, the Morrison block, as we
recognize it, extends from Chocolate Peak at the southern end
of the Bishop Creek pendant
&
115 km northwestward to just
south of Conway Summit (Fig. 1).
Stevens, C.H., and Greene, D.C., Geology of Paleozoic rocks in eastern Sierra Nevada roof pendants, California, in Lageson, D.R., Peters, S.G., and Lahren,
M.M., eds., Great Basin and Sierra Nevada: Boulder, Colorado, Geological Society of America Field Guide 2, p. 237–254.
Geology of Paleozoic rocks in eastern Sierra Nevada
roof pendants, California
Calvin H. Stevens
Department of Geology, San Jose State University, San Jose, California 95192 USA
David C. Greene
Department of Geology and Geography, Denison University, Granville, Ohio 43023 USA
ABSTRACT
Rocks in the major roof pendants of the eastern Sierra Nevada have been mapped in various degrees of detail to bet-
ter understand their stratigraphy, internal structure, and geologic history, and their relationships to other rock assem-
blages in the region. Ten formations ranging in age from Middle(?) Cambrian to Middle(?) Permian are recognized in
these pendants, which along with other minor pendants, constitute a tectonostratigraphic unit called the Morrison block.
Rocks of the Morrison block were first deformed by north-northwest–trending thrust faults and footwall syn-
clines involving strata as young as Early or Middle Permian. We designate this event, which correlates with a simi-
lar pre-middle Early Triassic event recognized in rocks near Tinemaha Reservoir, the Morrison orogeny.
Structures produced during this orogeny include a probable cryptic thrust fault separating rocks assigned to the
Morrison block from those in the Big Pine Creek pendant, which may belong to the White–Inyo block, and the
Nevahbe thrust, which separates lower from upper Paleozoic rocks in the eastern part of the Mount Morrison pen-
dant and may separate the Pine Creek and Bishop Creek pendants. In the Mount Morrison pendant structures
produced during the Morrison orogeny apparently were later refolded twice prior to sinistral displacement on the
Laurel–Convict fault, which cross-cuts older structures and is intruded by a pre-latest Late Triassic dike.
Other thrust faults in the eastern Sierra Nevada include the Golconda thrust of early Middle Triassic age and
the Lundy Canyon thrust of Late Triassic age. The Golconda thrust system apparently overprints the Roberts
Mountains thrust and separates rocks of the Morrison block from those of the Golconda and Roberts Mountains
allochthons in the Saddlebag Lake pendant, and perhaps from those of the Roberts Mountains allochthon in the
Northern Ritter Range and Log Cabin Mine pendants.
After thrust-faulting, but prior to intrusion of the Late Triassic Wheeler Crest Granodiorite, dextral move-
ment on the Tinemaha fault displaced Paleozoic facies and structural belts in the Sierra Nevada northward, pro-
ducing most of the present complicated paleogeographic patterns apparent in the region. Other less important
structures, such as the Laurel–Convict fault, have further complicated the geology of the Morrison block.
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Figure 1. Regional index map (after Bateman, 1992). CD—Casa Diablo; CP—Chocolate Peak; CS—Conway Summit;
DP—Deadman Pass pendant; JF—Jackass Flats; L-Cf—Laurel-Convict fault; Mi—Miller Mountain; Rma and Ga—
interpreted margin of Roberts Mountains and Golconda allochthons; TR—Tinemaha Reservoir. Small numbers refer to
field trip stops.
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Geology of Paleozoic Rocks in Eastern Sierra Nevada Roof Pendants, California 239
REGIONAL SETTING
The Morrison block, which contains rocks ranging in age
from Middle(?) Cambrian to Middle(?) Permian, is located be-
tween the mostly shallow-water miogeoclinal rocks to the east
in the White–Inyo Range and coeval, deep-water eugeoclinal
rocks of the Antler belt to the northwest. Rocks of the Morrison
block are transitional between those of the eugeocline and the
miogeocline in that many of the rocks are eugeoclinal in char-
acter, but are intercalated with beds or sequences of beds com-
posed of sediment derived from the miogeocline.
The pendants of the Morrison block generally are relatively
small, representing only fragments of original depositional sys-
tems mostly destroyed by intrusion of the Sierra Nevada
batholith. In the Mount Morrison pendant, many of the rocks
have been metamorphosed to hornblende hornfels (Rinehart and
Ross, 1964). This alteration, however, has not obliterated the
original character of the rocks, and original bedding features
generally are preserved.
PREVIOUS WORK IN THE MORRISON BLOCK
Rocks in the Morrison block have been mapped previously
in various degrees of detail. Of the four major pendants we have
studied, the Bishop Creek pendant was mapped by Bateman
(1965) and Bateman and Moore (1965), the Mount Morrison
pendant was studied by Rinehart and Ross (1964) and Wise
(1996), and the Log Cabin Mine and Gull Lake pendants were
mapped by Kistler (1966a).
Ages of most rocks in the Morrison block are now rela-
tively well known from the work of Rinehart and Ross (1964),
Willahan (1991), Moore and Foster (1980), Frazier et al. (1986),
Stewart (1985), and Stevens and Greene (1999). Data on all
identified fossils from the roof pendants and their locations were
summarized by Stevens and Greene (1999).
The nature of deformation of the pendants has been con-
troversial. Russell and Nokleberg (1977) recognized several pe-
riods of folding in the Mount Morrison pendant, the oldest of
which was interpreted by them to have been due to the Devo-
nian to Mississippian Antler orogeny. More recently Wise (1996)
analyzed the structure of this pendant and concluded that only
one folding event of post-Early Permian age, which developed
across an angular unconformity formed in the Late Devonian(?),
is present. Most recently, Stevens and Greene (1999) found that
Mississippian rocks in three of the pendants conformably over-
lie Devonian rocks, and that no structures in the Morrison block
can be demonstrated to be of Antler age. They indicated that
folds and thrust faults in the Mount Morrison pendant involve
rocks as young as Early Permian and that these structures are
truncated by the Laurel–Convict fault, which is intruded by a
225 ; 16 Ma dike (Greene et al., 1997b), showing that the ma-
jor deformation was post-earliest Permian and pre-latest Trias-
sic in age.
STRATIGRAPHY
Introduction
The stratigraphy of the Morrison block was worked out by
Stevens and Greene (1999) who recognized, thoroughly de-
scribed, and interpreted 10 stratigraphic units of formational
rank, one of group rank, and several members (Fig. 2). Fossils
show that the stratigraphic sequence is relatively complete with
ages ranging from possibly as old as Middle Cambrian to as
young as Middle Permian.
Regional correlations
Rocks temporally correlative with those of the Morrison
block are exposed to the east in west-central Nevada and
the White–Inyo Range, and to the west in the Saddlebag Lake,
western Log Cabin Mine, and Northern Ritter Range pendants
(Fig. 1). Correlations made by Stevens and Greene (1999) are
shown in Figure 3.
Middle(?) Cambrian through Ordovician rocks of the
Morrison block compare closely with rocks exposed south of
Miller Mountain in west-central Nevada (Fig. 1), suggesting
original paleogeographic continuity. In both areas rocks of
this age are represented by deep-water, continental-margin se-
quences that contrast significantly with the coeval, dominantly
platform deposits in the White–Inyo Range to the southeast.
Devonian rocks, which in the Morrison block are represented
by a submarine-fan system, can be traced into submarine chan-
nels in the western Inyo Mountains and thence onto the shelf
in the eastern Inyo Mountains, providing a tie between these
areas (Stevens and Greene, 1999). Eugeoclinal rocks now com-
posing the Roberts Mountains allochthon are distal equivalents
of the lower and middle Paleozoic rocks in the Morrison block
that were deposited and deformed far to the west during the Late
Devonian–Early Mississippian Antler orogeny (Stevens and
Greene, 1999).
Mississippian rocks in the Morrison block constitute a pos-
sibly shallowing-upward sequence overlain by shallow-water
carbonates of Pennsylvanian to Early Permian age, showing that
by that time, at least, deposition was in relatively shallow water.
Coeval deposits in the Inyo Mountains include deep-water shale
with superjacent limestone turbidites. Thus, the elevation dif-
ference between these two regions was reversed in the early late
Paleozoic from what it had been in the early Paleozoic. In the
Antler belt, where intensely deformed lower Paleozoic rocks are
overlain by a thin Permian conglomerate and Early Triassic silt-
stone and sandstone, late Paleozoic time apparently was marked
primarily by erosion or nondeposition.
STRUCTURAL GEOLOGY
The four pendants studied in some detail all have major
structural features that trend north–northwestward. There are,
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240 C.H. Stevens and D.C. Greene
Figure 2. Stratigraphy of the Morrison block (from Stevens and Greene, 1999).
however, large differences in the character of these structures,
and other structures are present in some of the pendants. Below
we describe the major structural features of each pendant based
on our recent work and observations of Kistler (1966b) in the
Gull Lake and Log Cabin Mine pendants and those of Russell
and Nokleberg (1977) and Wise (1996) in the Mount Morrison
pendant. Earlier structural work on many of the Sierran pen-
dants was summarized by Nokleberg and Kistler (1980).
Mount Morrison pendant
Most of the Mount Morrison pendant was remapped by
Stevens and Greene (1999), using Rinehart and Ross’s (1964)
map as an outline. The central and eastern parts of the pendant
were mapped concurrently by Wise (1996). Here we employ the
names suggested by Wise for structures not previously named
by Rinehart and Ross (1964).
Our mapping (Fig. 4) shows that the structure of the Mount
Morrison pendant is complex and that there has been extensive
overprinting of structures of different generations. In general,
we interpret the Mount Morrison pendant to consist of complex
structural repetitions of a relatively simple stratigraphy in con-
trast to Rinehart and Ross (1964), who interpreted the pendant
to consist of a relatively simple homoclinal sequence with a
complex stratigraphy.
Folds and faults. Both map-scale folds and faults are well
represented in the pendant (Fig. 5). Here, five synclines, one
anticline, and eight faults will be considered.
Sevehah Cliff syncline. This large faulted fold is exposed
in Sevehah Cliff. It strikes slightly west of north, is overturned
to the west, and is cored by highly deformed beds of the
Squares Tunnel Formation. It has been refolded producing a
small anticline with an east-trending axis. This anticline, which
is visible from the shores of Convict Lake, is cored by the
Mount Morrison Sandstone and plunges moderately to the east.
The Sevehah Cliff syncline is bounded by the Laurel Mountain
fault on the west and the Convict Creek fault on the east. To the
south this fold is displaced by a splay of the Laurel–Convict
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Geology of Paleozoic Rocks in Eastern Sierra Nevada Roof Pendants, California 241
Figure 3. Regional correlations (from
Stevens and Greene, 1999).
fault, and farther south it is cut off by the major trace of the
Laurel–Convict fault.
Horsetail Falls syncline. This structure, named for a waterfall
in the vicinity, is exposed in upper McGee Creek. It is overturned
to the west and plunges steeply to the south. At the south end,
where it is cored by the Mount Morrison Sandstone, it trends
northward, but to the north it bends to the northwest. This syncline
is bounded on the west by a splay of the Laurel–Convict fault.
McGee Creek syncline. This small syncline is exposed
along the south bank of McGee Creek and is bounded by the
McGee Creek West fault on the west. It plunges very steeply to
the north and is cored by the Squares Tunnel Formation.
McGee Mountain syncline. This spectacular structure under-
lies McGee Mountain. It is northwest-trending, upright, and plunges
almost vertically, opening to the northwest. It is cored by the Squares
Tunnel Formation. This syncline is bounded by McGee Creek East
fault on the west, and the Esha Canyon fault on the east.
Tobacco Flat anticline and syncline. This fold pair, exposed
on Tobacco Flat, is very steeply plunging and is bounded on the
west by the McGee Creek fault.
Laurel–Convict fault. This fault with its several splays was
studied in detail recently by Greene et al. (1997b). It dips very
steeply, cuts bedding in the upper Paleozoic rocks on the south-
west side of the fault at a low angle, and cuts obliquely across
folds and faults in the dominantly lower Paleozoic rocks on the
northeast side. A large splay of the Laurel–Convict fault, which
cuts deeply into the lower Paleozoic rocks in the central part of
the pendant (Figs. 4 and 5), also cuts earlier folds and faults.
This splay shows
&
0.5 km of sinistral offset of the minimum
of 5 km of sinistral offset on the entire Laurel–Convict fault
system (Greene et al., 1997b). The main trace of the Laurel–
Convict fault and small splays at both ends of the main fault
trace have been intruded by the Laurel–Convict felsite dated at
225 ; 16 Ma (Greene et al., 1997b). This fault separates rocks
to the southwest deformed by gently plunging folds from
rocks to the northeast deformed by mostly very steeply plung-
ing folds and steeply dipping faults.
Laurel Mountain fault. This fault (Fig. 5), as well as all sub-
sequently described faults, is here considered to be a thrust fault
(see following discussion). The Laurel Mountain fault places
Squares Tunnel Formation on the west against the Convict Lake
Formation and is cut off by the Laurel–Convict fault. The Seve-
hah Cliff syncline lies immediately east of this fault.
Convict Creek fault. The Convict Creek fault emplaces the
Convict Lake Formation on the west against the Mount Morri-
son Sandstone. This fault is cut by both the Laurel–Convict fault
and its prominent eastern splay. Wise (1996) measured the fault
plane as striking north-south and dipping 70°E. Along the east
side of Convict Creek, quartzites in the Convict Lake Formation
near the fault have been separated into large isolated blocks sur-
rounded by argillite.
Mount Morrison fault. This fault emplaces the Convict Lake
Formation on the west against the Mount Morrison Sandstone
and Squares Tunnel Formation on the east. The fault strikes
about north-south and dips 85°E. It is cut both by the Laurel–
Convict fault and its eastern splay.
McGee Creek fault. Wise (1996) treated the McGee Creek
fault as a zone consisting of two main strands north of McGee
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Figure 4. Geology of the Mount Morrison pendant (modified from Stevens and Greene, 1999).
Great Basin-Ch.12 9/19/00 10:11 AM Page 242

Figure 5. Major folds and faults in the Mount Morrison pendant. (1)—Sevehah Cliff syncline;
(2)—Horsetail Falls syncline; (3)—McGee Creek syncline; (4)—McGee Mountain syncline;
(5)—Tobacco Flat syncline and anticline; CC—Convict Creek fault; L-Cs—major splay of Laurel–
Convict fault; LM—Laurel Mountain fault; MM—Mount Morrison fault. a-b, etc.—lines of struc-
ture section shown in Figure 6.
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244 C.H. Stevens and D.C. Greene
Creek and several strands south of the creek. Here, we consider
the two main strands as separate faults that probably merge into
one to the north. We call them the McGee Creek West fault and
the McGee Creek East fault. The McGee Creek West fault
places the Mount Aggie Formation on the southwest against the
Mount Morrison Sandstone and other units on the northeast.
This fault also juxtaposes the sandstone and conglomerate fa-
cies of the Mount Morrison Sandstone, suggesting that it may
be one of the most important faults in the pendant. Wise (1996)
observed kinematic indications of primarily left-lateral shear
along the fault zone. Near where the fault merges with the
McGee Creek East fault, however, Wise (1996) observed both
right- and left-lateral shear indicators. We interpret both struc-
tures as thrust faults (see following section). Along the McGee
Creek East fault, the Mount Morrison Sandstone on the west is
emplaced against the Mount Aggie and Convict Lake Forma-
tions. The McGee Mountain syncline and folds involving the
Mount Aggie and Convict Lake Formation south of McGee
Creek lie east of this fault.
Esha Canyon fault. The Esha Canyon fault places the
McGee Mountain syncline against the Mount Aggie Formation
north of McGee Creek. South of McGee Creek it separates
Mount Aggie and Convict Lake formations on the west from an
intensely deformed part of the Mount Aggie Formation farther
east. The more calcareous parts of the latter unit form discon-
tinuous beds, lenses, and large phacoids surrounded by argillite,
evident from U.S. Highway 395 a distance of several kilometers
to the northeast.
Nevahbe fault. The Nevahbe fault places Convict Lake and
Mount Aggie formations on the west against highly altered
Bright Dot Formation and Mount Baldwin Marble on the east.
This fault represents a greater amount of stratigraphic separa-
tion than any other fault in the Mount Morrison pendant.
Relationship between folds and faults. The Laurel–Con-
vict fault and its splays clearly truncate the Laurel Mountain,
Convict Creek, and Mount Morrison faults, and the Sevahah
Cliff syncline. Other major structures described above that are
not actually cut by the Laurel–Convict fault are interpreted
to belong to the same older tectonic episode because both the
folds and faults have strikes similar to those cut by the Laurel–
Convict fault, and most of the faults and axial planes of the folds
dip similarly, steeply eastward.
The older folds and faults could have originated in their
present orientation or they could have been rotated later. The
synclines, with the exception of the Horsetail Falls syncline, are
bounded on the southwest by the older faults (Fig. 6). This rela-
tionship, combined with the apparent thinning of the western
limbs of some of the synclines, suggests that the folds are foot-
wall structures under thrust faults and that vergence originally
was eastward. If so, the entire pendant has been tilted down-to-
the-northwest, overturning all older structures. Strike-slip kine-
matic indicators on the McGee Creek West fault (Wise, 1996)
suggest that this fault may have been reactivated later, perhaps
simultaneously with displacement on the Laurel–Convict fault.
As noted above, the Horsetail Falls syncline is bounded on the
southwest by a major splay of the Laurel–Convict fault rather
than by one of the older faults. We suggest that this splay also
has overprinted part of an earlier thrust fault and that both the
fault southwest of Horsetail Falls syncline and the McGee Creek
West fault are old thrust faults that continued to be zones of
weakness along which displacement occurred when the Laurel–
Convict fault was active.
Structures on opposite sides of the Laurel–Convict fault are
of very different character. In contrast to the rocks northeast of
the Laurel–Convict fault, which are cut by thrust faults and very
steeply plunging synclines, the upper Paleozoic rocks south-
west of the fault are deformed by northwest-striking, very gen-
tly plunging folds with few faults. Stevens (1998) suggested
that the large north-northwest–trending folds and faults were
formed during an early contractional event and that later the
folds northeast of the Laurel–Convict fault were refolded about
east-trending axes produced in a major restraining bend in a
large scale, right-lateral shear zone of which the Laurel–
Convict fault was a part (Fig. 7). Later, displacement was
transferred to the Tinemaha fault to the east. In this scenario,
sinistral displacement on the Laurel–Convict fault occurred
later. That the large synclines have been refolded is based on the
fact that the synclines open in opposite directions and the
plunges are too great to be the result of depressions and culmi-
nations. One such east-trending anticline in the Sevehah Cliff
syncline was referred to above. In addition, the McGee Moun-
tain and the Tobacco Flat synclines may be refolded parts of the
same fold that opens in opposite directions. Apparently because
the hingelines of the refolded folds were perpendicular to the
Figure 6. Diagrammatic cross-section of the Mount Morrison pendant. L-C—Laurel–Convict fault; MCW—
McGee Creek West fault; MCE—McGee Creek East fault; EC—Esha Canyon fault; N—Nevahbe fault. D—
Devonian rocks of the Mount Morrison Sandstone and Squares Tunnel Formation. For other letter designations,
scale, and location of section, see Figure 5 and its caption.
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Geology of Paleozoic Rocks in Eastern Sierra Nevada Roof Pendants, California 245
Figure 7. Diagrammatic representation of a restraining bend inferred to
explain refolding northeast of the Laurel–Convict fault.
planes of the older thrust faults, these faults do not show devi-
ations from their original trends.
Stevens (1998), originally following Wise (1996), recog-
nized only one period of north–northwest folding, but here we
accept the interpretation of Russell and Nokleberg (1977) that
there were at least two periods of such folding. The thrust faults
and large footwall synclines formed during the first event. These
structures have a mostly northerly trend and probably corre-
spond with the first generation folds reported by Russell and
Nokleberg (1977) to have an orientation of N8°W. The second
generation north-northwest–trending folds are much smaller
and trend more northwesterly, averaging N30°W (Russell and
Nokleberg, 1977). The large scale synclines appear to have been
refolded by the northeast-trending folds, as stated above, but it
is uncertain if the second generation northwest-trending folds
are refolded by, or if they refold, the northeast-trending folds.
The interpretation of Wise (1996) that the lower Paleozoic rocks
in the pendant had been tilted steeply prior to deposition of the
late Paleozoic rocks seems unlikely as Devonian and Mississip-
pian rocks are parallel and apparently conformable in a se-
quence north of Laurel Mountain (in northwest corner of Fig. 2)
and in two other pendants, as noted previously.
Bishop Creek pendant
We remapped most of the Bishop Creek pendant east of
Bishop Creek (Fig. 8) showing that the major structure, also pre-
viously mapped by Bateman (1965) and Bateman and Moore
(1965), is a large, open, horizontal syncline. The west limb of
the syncline, exposed on the slopes above Bishop Creek, has
been deformed by tight, east-verging folds. East of Coyote
Ridge, the east limb of the syncline forms the west limb of an
anticline cored by the Mount Aggie Formation. Northward the
anticlinal axis and all units in both limbs of the anticline appear
to have been pushed aside around the eastern margin of a large,
200 Ma pluton (Bateman, 1992). A northeast-southwest–trend-
ing, apparently left-lateral fault, which also may be related to the
emplacement of this intrusion, cuts across the area south of
Lookout Mountain.
Gull Lake pendant
For this work (Fig. 9), we used the mapping of Kistler
(1966a) in most areas as a base. The rocks in this pendant are ex-
tremely altered. However, critical lithologic characteristics are
preserved so that we have been able to recognize the Aspen
Meadow Formation, Mount Morrison Sandstone, and the Squares
Tunnel Formation with a thin McGee Mountain Member. The
Squares Tunnel Formation appears to be unusually thick in this
pendant. Near Silver Lake, however, sequences of beds appear
repetitious so the section may be repeated by faulting.
The structure of the Gull Lake pendant is enigmatic. The
major mapped structure in the southeastern part of the pendant
appears to be a large northeast-trending, gently southward-
plunging anticline that has been refolded on a large scale about
very steeply plunging axes, a structural style not recognized
elsewhere in the Morrison block. In the western part of the pen-
dant, units trend northward. The relationship between these two
different structural patterns is unknown.
In the eastern part of the Northern Ritter Range pendant im-
mediately west of the Gull Lake pendant, we have recognized
units ranging from Convict Lake Formation to Squares Tunnel
Formation that are repeated by thrust faults. Farther west in the
pendant, Greene (1995) mapped rocks as belonging to the
Antler belt.
Log Cabin Mine pendant
We have remapped only the eastern part of the Log Cabin
Mine pendant and its extension southward into Williams Butte
(Fig. 10). In the Log Cabin Mine pendant a thick upright strati-
graphic section, consisting of Convict Lake Formation, As-
pen Meadow Formation, Mount Morrison Sandstone, Squares
Tunnel Formation, and Bright Dot Formation, apparently rests
on a series of thrust slices containing repetitions of the Convict
Lake and Aspen Meadow formations, and Mount Morrison
Sandstone. These slices have been thrust over an upright section
of calcareous sandstone of the Sevehah Group at the eastern
margin of the pendant. The Squares Tunnel–Bright Dot contact
appears conformable. West of the lower beds of the Bright Dot
Formation, faults have completely dismembered the section.
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Figure 8. Geology of the Bishop Creek pendant (modified from Stevens and Greene, 1999).
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Geology of Paleozoic Rocks in Eastern Sierra Nevada Roof Pendants, California 247
Figure 9. Geology of the Gull Lake pendant (modified from Stevens and Greene, 1999).
Several fault slices in this area are composed of calcareous sand-
stone, and thus are parts of the Sevehah Group, and one shale
outcrop has yielded Ordovician graptolites typical of the Con-
vict Lake Formation. In the western part of the pendant, rocks
that have been totally disrupted are considered to represent the
Roberts Mountains allochthon (Stevens and Greene, 1999).
On the eastern side of Williams Butte, an inverted strati-
graphic sequence consists of the Convict Lake Formation,
Aspen Meadow Formation, and Mount Morrison Sandstone
faulted over the Aspen Meadow Formation, a structure some-
what similar to that in the eastern part of the Log Cabin Mine
pendant to the north.
A short distance north of the Log Cabin Mine pendant, a
large outcrop of Mount Baldwin Marble belonging to the
Morrison block is closely associated with, but almost certainly
is in fault contact with, rocks of the Antler belt. All of these
rocks underlie the Golconda allochthon (Schweickert and
Lahren, 1993).
Great Basin-Ch.12 9/19/00 10:12 AM Page 247

Figure 10. Geology of the Log Cabin Mine pendant (modified from Stevens and Greene, 1999). Geology in south-
eastern corner of pendant has been simplified.
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Geology of Paleozoic Rocks in Eastern Sierra Nevada Roof Pendants, California 249
COMPARISON OF MAJOR PENDANTS
The stratigraphy of the four pendants studied is quite simi-
lar although there are differences in lithology and thickness. The
structures in the pendants also are similar, especially in the pres-
ence of north-northwest–trending reverse or thrust faults. These
faults are best represented in the Mount Morrison pendant
where they involve Early Permian rocks and are cut by the
Laurel–Convict fault, which is intruded by a 225 ; 16 Ma
dike. Similar faults probably of similar age are responsible for
the complicated structure of the rocks on the west side of the
Bishop Creek pendant along Bishop Creek, for the repetition of
stratigraphic units in the Northern Ritter Range pendant west
of the Gull Lake pendant, and for the repetitions of section along
the ridge north of Highway 120 in the eastern part of the Log
Cabin Mine pendant and in Williams Butte. The prominent syn-
clines associated with the thrust faults in the Mount Morrison
pendant may have a counterpart in the Bishop Creek pendant,
but similar synclines have not been recognized in other pen-
dants. The upright, refolded fold in the Gull Lake pendant ap-
parently does not have counterparts anywhere else.
MAJOR TECTONIC EPISODES
Interpretation of the tectonic development of the eastern
Sierran roof pendants is difficult because of the paucity and iso-
lation of outcrops. Based on the data available and that known
or inferred from nearby areas, however, we have created a tec-
tonic model that is internally consistent. The major Paleozoic to
early Mesozoic orogenic episodes recorded in the roof pendants
of the Sierra Nevada are: (1) the Antler orogeny of Late Devon-
ian to Early Mississippian age, (2) an Early Permian to earliest
Triassic event, which we name the Morrison orogeny, (3) the
Middle(?) Triassic Sonoma orogeny, and (4) a major dextral
faulting event of Middle to Late Triassic age.
Antler orogeny
The Antler orogeny is the oldest structural event repre-
sented in the region. The results of this orogeny are evident in
the pervasively deformed rocks in the Saddlebag Lake, western
Log Cabin Mine, and Northern Ritter Range pendants. These
rocks are interpreted to have been deformed far to the west
(Stevens and Greene, 1999) and brought into their present posi-
tion during emplacement of the Golconda allochthon. The ef-
fects of the Antler orogeny in the Morrison block were subdued.
No structures of Antler age have been recognized, and in the
Mount Morrison, Bishop Creek and Log Cabin Mine pendants,
Devonian and Mississippian rocks are conformable. During
Late Devonian to Early Mississippian time, however, there was
a change in the character of sediment deposited and there appear
to be large differences in the thickness of the Upper Devonian
Squares Tunnel Formation from pendant to pendant, perhaps re-
flecting differential uplift.
Morrison orogeny
Study of the Mount Morrison pendant has revealed that the
major, mostly north-trending thrust faults with footwall synclines
developed between Early Permian and Late Triassic time. The
Pennsylvanian to very early Permian Mount Baldwin Marble is cut
by the Nevahbe thrust in the southeastern part of the Mount Mor-
rison pendant, and as this unit is conformably overlain by a very
thick Permian section in the southwestern part of the pendant, the
orogenic event evidently is late Early Permian or younger. The
faulted synclines in the Mount Morrison pendant correspond quite
closely in shape, orientation, and age with a cryptic thrust fault and
footwall syncline in the Tinemaha Reservoir area, which formed
prior to deposition of the Early-Middle(?) Triassic Union Wash
Formation. Thus, the age of the Morrison orogeny probably is late
Early Permian to early Early Triassic. Folds in the Bishop Creek
pendant and thrust faults in the Log Cabin Mine pendant are in-
terpreted to have formed at this time.
Sonoma Orogeny
The Golconda thrust system, the lowest fault of which ap-
parently carried Antler belt rocks as well as the Golconda
allochthon into the eastern Sierra Nevada (Stevens and Greene,
1999), is in a higher structural position than any of the faults in the
Mount Morrison pendant. The Golconda fault system has been
recognized only in the Saddlebag Lake pendant (Schweickert and
Lahren, 1987, 1993), but it may extend across the Northern Ritter
Range pendant and the western part of the Log Cabin Mine pen-
dant. This fault system apparently is early Middle Triassic in age.
Middle-Late Triassic dextral displacement
Rocks in the northwestern Mount Morrison pendant are
considered to be displaced
&
65 km in a right-lateral sense from
those in the Tinemaha Reservoir area along a cryptic fault called
the Tinemaha fault (Fig. 1; Stevens et al., 1997). This interpre-
tation is based on the apparent offset of a major Devonian sub-
marine channel and similar structures in the two areas (Stevens
and Greene, 1999), and displacement of the
87
Sr/
86
Sr = 0.706
isotopic isopleth (Kistler, 1993). Because this fault displaces
folds that involve the Union Wash Formation in the Tinemaha
Reservoir area, the Tinemaha fault can be no older than Early to
Middle(?) Triassic. As the inferred trace of the fault is crossed
by the late Late Triassic Wheeler Crest Granodiorite of Bateman
(1992), which apparently is not offset, the age of the fault is con-
sidered to be Middle or early Late Triassic.
REGIONAL TECTONIC INTERPRETATIONS
Offset of facies from the eastern Sierra Nevada to the Miller
Mountain area (Stevens and Greene, 1999) has been explained
by displacement on the Tinemaha fault and the Death Valley–
Furnace Creek fault zone. Convergence taken up on thrust
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250 C.H. Stevens and D.C. Greene
faults, however, has not received much attention. That the east-
ern Sierra Nevada and the White–Inyo Mountains to the east
consist of a stack of thrust plates was first recognized by Schwe-
ickert and others (1988). They proposed the following sequence
of thrust faults, from structurally lowest to highest: Last Chance
thrust; a Bishop thrust, carrying rocks of Casa Diablo, and the
Big Pine Creek, Bishop Creek, and Pine Creek pendants; and a
Mount Morrison thrust carrying rocks of the Mount Morrison
pendant. Our studies similarly suggest that there is a stack of
thrust plates, but we assign the pendants to thrust plates differ-
ently (Fig. 11).
In our model, the Last Chance thrust is the structurally low-
est thrust. As it is interpreted to be a very Early Permian fault
(Snow, 1992; Stevens and Stone, 2000), it probably predated the
Morrison orogeny, which involved a very thick Permian section.
The upper plate rocks of the Last Chance thrust (White–Inyo
plate) are interpreted to include the Cambrian and Precambrian
rocks in the main part of the southern White and northern Inyo
Mountains, younger Paleozoic rocks near Tinemaha Reservoir,
and the Lower Cambrian rocks at Casa Diablo and the Big Pine
Creek pendant (Fig. 11). These latter rocks are assigned to the
White–Inyo plate because the rocks in these areas are very simi-
Figure 11. Major thrust faults, thrust plates, and dextral strike-slip fault in the region. The cryptic thrust
in Owens Valley is shown as truncated by the Tinemaha fault in Figure 12. BC—Bishop Creek pendant;
BPC—Big Pine Creek pendant; GL—Gull Lake pendant; LC—Log Cabin Mine pendant; MM—
Mount Morrison pendant; NR—Northern Ritter Range pendant; PC—Pine Creek pendant; WB—
Williams Butte; A-B and C-D—lines of cross-section shown in Figure 12. For other letter designations
see caption for Figure 1.
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Geology of Paleozoic Rocks in Eastern Sierra Nevada Roof Pendants, California 251
lar to those of the same age in the Inyo and White Mountains
(Stevens and Greene, 1999; Stevens and Stone, in prep).
Structurally above the White–Inyo plate is a cryptic thrust
in Owens Valley west of Tinemaha Reservoir, which was active
during the Morrison orogeny. This thrust probably is cut by the
Tinemaha fault (Fig. 12). Above this thrust are the Nevahbe and
Morrison plates, which were emplaced during the Morrison
orogeny (Figs. 11 and 12). The Nevahbe thrust separates highly
altered upper Paleozoic rocks of the Nevahbe plate, exposed near
the mouth of McGee Creek, from the remainder of the Paleozoic
rocks in the Mount Morrison pendant that constitute the Morri-
son plate. The Mount Baldwin Marble in the Nevahbe plate in
the Mount Morrison pendant can be traced along a discontinu-
ous string of outcrops to the Pine Creek pendant (see Bateman,
1992),
&
20 km to the south, suggesting that the highly altered
rocks of this latter pendant also belong to the Nevahbe plate.
The Morrison plate is interpreted to include all of the Paleo-
zoic rocks in the Mount Morrison pendant above the Nevahbe
plate, all rocks in the Gull Lake and Deadman Pass pendants,
rocks at the southern end of the Northern Ritter Range pendant,
and rocks in the eastern part of the Log Cabin Mine pendant. Strata
of the Bishop Creek pendant are provisionally assigned to this
plate rather than the Nevahbe plate because the rocks in this pen-
dant include much older and less altered rocks than those in the
latter plate. As shown earlier, rocks in the Morrison plate in the
Mount Morrison pendant are complexly faulted. None of these
faults, however, can be correlated with faults in the other pendants.
As shown by Schweickert and Lahren (1993) the Golconda
allochthon, probably emplaced in the early Middle Triassic,
overlies an outcrop of Mount Baldwin Marble (belonging to the
Morrison block and probably to the Morrison plate) near Con-
way Summit, and thus is in a structurally higher position than
the Morrison plate. The Lundy Canyon plate, which has only
been identified in the Saddlebag Lake pendant (Schweickert and
Lahren, 1987, 1993) is at an even higher structural level. Thus,
the sedimentary rocks composing the eastern Sierra Nevada and
western Inyo Mountains consist of remnants of at least five im-
portant thrust plates emplaced during Permian and Triassic time.
SUMMARY
Rocks in roof pendants of the Sierra Nevada stretching from
the Bishop Creek pendant northwestward to north of the Log
Cabin Mine pendant share a similar stratigraphy and structural
style that contrasts markedly with that of most other rocks of sim-
ilar age in the region. These pendants comprise the Morrison
block. The lower Paleozoic rocks in this block and those in west-
central Nevada are very similar, and are deep-water equivalents of
the well-known miogeoclinal rocks of the White–Inyo Range to
the east. During the early part of the late Paleozoic, the difference
in elevation between these two regions was reversed so that depo-
sition in the Morrison block was then in shallow water, whereas
coeval sediment the Inyo Mountains accumulated in deep water.
Major tectonic events recognized in the region are the Late De-
vonian to Early Mississippian Antler orogeny, the late Early Per-
mian to earliest Triassic Morrison orogeny, the Middle(?) Triassic
Sonoma orogeny, and Late(?) Triassic dextral faulting; several ad-
ditional events of less significance also can be recognized. Antler
deformation took place far to the west of the Morrison block and
apparently had little effect on the rocks of the Morrison block. The
Morrison orogeny resulted in generally north-trending thrust faults
with footwall synclines. Later, during the Sonoma orogeny, rocks
of the Golconda allochthon and Antler belt were emplaced over the
Morrison block. Finally, in Late Triassic time, the Lundy Canyon
thrust was emplaced and dextral strike-slip displacement on the
Tinemaha fault tore apart the original facies and structural belts,
producing much of the present perplexing distribution of these fea-
tures in east-central California and west-central Nevada.
ACKNOWLEDGMENTS
We appreciate the ideas and observations of Richard Schwe-
ickert and James Wise, especially during the field work. Robert
B. Miller and Paul Stone provided continuing inspiration and en-
couragement throughout this work and Paul critically reviewed
the manuscript. We also are grateful for comments provided by an
undisclosed outside reviewer. The project was supported by Na-
tional Science Foundation grant EAR-9218174 to Stevens.
ABBREVIATED FIELD GUIDE (see Figure 13 for
location map)
Stop 1
Northeast shore of Convict Lake, Mount Morrison pendant
(see Fig. 4). Most of the stratigraphic units in the pendant crop
Figure 12. Cross section from roof pendants in the Sierra Nevada to the Inyo Mountains. See Figure 11 for location
of section.
Great Basin-Ch.12 9/19/00 10:12 AM Page 251

252 C.H. Stevens and D.C. Greene
out around the lake and in Sevehah Cliff (Fig. 14). Also visible
are the Sevehah Cliff syncline, the Tobacco Flat anticline and
syncline pair, and the Laurel–Convict, Laurel Mountain, Con-
vict Creek, and Morrison faults.
Stop 2
Parking lot at end of road, McGee Creek, Mount Morrison
pendant (see Fig. 4). Along McGee Creek the McGee Creek,
McGee Mountain, and Horsetail Falls synclines, and the McGee
Creek West, McGee Creek East, Esha Canyon, and Nevahbe
faults can be observed.
Stop 3
Trailhead at Pine Creek, Pine Creek pendant. The Missis-
sippian Bright Dot Formation and Pennsylvanian–Early Per-
mian Mount Baldwin Marble compose this pendant.
Figure 13. Field trip route along U.S. Highway 395. Numbers refer to field trip stops.
Great Basin-Ch.12 9/19/00 10:12 AM Page 252

Geology of Paleozoic Rocks in Eastern Sierra Nevada Roof Pendants, California 253
Stop 4
Four Jefferys Campground in Bishop Creek Canyon,
Bishop Creek pendant (see Fig. 8). Convict Lake to Bright Dot
formations, as well as east-vergent overturned folds, are visible
on the east side of the canyon.
Stop 5
Overlook of Tinemaha Reservoir above Los Angeles Water
and Power facility
&
8 mi south of the center of Big Pine. The
conglomerate facies of the Mount Morrison Sandstone, inter-
preted to be the major channel of a Devonian deep-water fan
complex, occurs only here at the base of the Inyo Mountains and
on McGee Mountain in the Mount Morrison pendant.
Stop 6
West edge of Gull Lake,
&
2.7 mi from southern intersection
of Highway 158 with U.S. Highway 395, Gull Lake pendant
(see Fig. 9). Observe Mount Morrison Sandstone and large re-
folded fold with Aspen Meadow Formation in the core.
Stop 7
Nature Trail on south side of June Mountain ski area, 0.1 mi
beyond Stop 6, Gull Lake pendant (see Fig. 9). Outcrop of
Squares Tunnel Formation shows the characteristic black chert
with light gray phosphatic lenses.
Stop 8
Mountain Rose Restaurant parking lot
&
0.9 mi beyond Stop
7 Gull Lake pendant (see Fig. 9). The cliff in the distance is com-
posed of Convict Lake to Squares Tunnel formations repeated
by thrust faults (Fig. 15).
Stop 9
Mono Craters scenic view near junction of the northern in-
tersection of Highway 158 and U.S. Highway 395. In the dis-
tance is a fault probably correlative with the Roberts Mountains
thrust (Greene, 1995), but overprinted by a Sonoman fault that
places rocks of the Roberts Mountains allochthon over rocks of
the Morrison block.
Stop 10
Trail to Bennettville from junction of Highway 120 and
Saddlebag Lake road, 2.1 mi east of Tioga Pass, Saddlebag Lake
pendant. Exposed here are highly disrupted rocks of the Roberts
Mountains allochthon.
Stop 11
Mono Lake Visitors Center, Log Cabin Mine pendant (see
Fig. 10). The Convict Lake to Bright Dot formations are ex-
posed in the mountain front to the west. The Mount Baldwin
Marble overlain by the Golconda allochthon is visible in the dis-
tance to the north.
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am—Aspen Meadow Formation; mm—
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Figure 15. Line drawing of Morrison
block rocks north of Agnew Lake look-
ing west from Mountain Rose restaurant
parking lot in the town of June Lake.
Trlv—Granite of Lee Vining Canyon of
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nations see caption for Figure 14.
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