7 Preliminary Analysis of the Physical Stratigraphy,
Depositional Environment, and Paleoecology of the Miocene
Non-Marine Deposits, Stewart Valley, Nevada
A thesis submitted in partial fulfillment of the
requirement for a Master of Science in Geology
by
Thomas P. Lugaski, Ph.D. Ul
JULY 1986 1
MINES bIBRARY i The thesis of Thomas Peter Lugaski is approved: T J t e 3 1 0 8
sis Advisor ACKNOWLEDGMENT S
I wish to express my sincerest appreciation to my committee members: Dr. James Firby, Dr. Joseph Lintz and
Dr. James Hulse for their efforts on my behalf. I also wish thank Drs. Don Savage and Harvey Scudder for their help in the field. A special thanks is due Dr. James
Firby, Dr. Jack Kepper and Howard Schorn for their help in the field and discussions on the origin and formation of the Stewart Valley beds. This study was funded in part by a grant from the Bureau of Land Management.
A very special thanks and appreciation is due my wife,
Lhly and daughter, Kate for their support and understanding throughout the study. ABSTRACT
The Late Tertiary lake beds of Stewart Valley, Mineral
County, Nevada were examined. The lake sediments represent
the remnants of Miocene Lake Savage, a chemically
stratified, meromictic lake. Deep-water, fine-grained,
laminated, silicified and non-silicified paper shales
accumulated in the anoxic hypolimnion of the lake. Within
the laminae are preserved complete leaves, insects,
articulated fish skeletons and other fauna. Early in its
depositional history, Lake Savage was a net siliciclastic
depositional system, later Lake Savage became a net
carbonate depositional system. X-ray diffraction analysis
of the shales reveals they are made up of cristobalite.
Shallow-water bedded diatomaceous clays and mudstones, gypsiferous mudstones, elastics, marlstones, ostracodal
limestones, stromatolites, tufa domes, and oolites were also formed. Molluscs, ostracodes, and pertified logs are abundant. Fluvial portions of the sediments consist of numerous cut-and-fill structures, and point bar deposits.
Trace fossils are locally abundant. Pyroclastic (volcanic mudflows, surge deposits and air-fall ashes) units are interbedded with the sediments. These sediments preserve an excellent and unique record of changes in depositional rates, lake level fluctuation, lake chemistry, flora and fauna through time. TABLE OF CONTENTS
PAGE
ACKNOWLEDGMENTS...... ^
ABSTRACT...... i±i
TABLE OF CONTENTS...... v
LIST OF FIGURES...... vii
LIST OF TABLES...... viii
LIST OF PLATES...... ix
INTRODUCTION...... 1
Present Topography, Climate and Environment. 5
Objective and Scope of Work...... 10
Present and Future Status of the Fossil Beds 11
Previous Work...... 12
General Mesozoic Geologic History...... 16
General Cenozoic Geologic History...... 18
RESEARCH METHODS AND RESULTS...... 26
X-ray Diffraction Analysis...... 96
Sedimentary Structures...... 97
DISCUSSION...... 106
Ancient and Modern Comparisons...... 113
Green River Formation...... 113
Miocene Wassuk Group 129 Oligocene Florissant Lake, Colorado...... 132
Eocene Freshwater Deposits of British
Columbia......
Pleistocene Rita Blanca Lake Deposits, Texas 136
Deep Springs Lake, California...... 140
Fayetteville Green Lake, New York...... 142
Pluvial Pleistocene Lake Lahontan...... 144
Lake Classification...... 149
Siliceous Beds...... 155
Sedimentary Pyrite Formation...... 166
Lacustrine Carbonates...... 171
Carbonate Sediments-Ooids...... 182
Varves and Varving Mechanisms...... 184
Trace Fossils or Lebensspuren...... 190
CONCLUSIONS...... 194
LITERATURE CITED...... 201 vi i
LIST OF FIGURES
FIGURE PAGE
1 Location map of the Stewart Valley
study site, Mineral County, Nevada.... 2
2 Landsat 5 Thematic Mapper false color
image (bands 4, 3, 2 RGB) of west-
central Nevada...... 3
3 Landsat 5 Thematic Mapper false color
image (bands 4, 3, 2 RGB) of Stewart
Valley, MineralCounty, Nevada...... 4
4 Landsat 5 Thematic Mapper false color
image (band ratios 5/7, 5/6, 4/2 RGB)
of Stewart Valley, Mineral County,
Nevada...... 7
5 Landsat 5 Thematic Mapper false color
image (band ratios 3/6, 5/6, 4/2 RGB)
of Stewart Valley, Mineral County,
Nevada...... 24
6 Lineation map made from Landsat 5
Thematic Mapper image (Figure 5),
showing the major structural blocks
in Stewart Valley...... 25 vi i i
7 Photograph of Pacific Union Canyon where
Stewart Valley Stratigraphic Section
27 was measured...... 93
8 This illustration graphically displays
the variation in lithology and bed
thickness in Stewart Valley
Stratigraphic Section 27 from Pacific
Union Canyon...... 94
9 The common x-ray diffraction pattern
found during the analysis of shales
from Stewart Valley, Nevada...... 98
LIST OF TABLES
TABLE PAGE
1 Results of the x-ray diffraction
analysis on selected rocks from Stewart
Valley, Nevada...... 100
2 A listing of the sedimentary structures
found in the Stewart Valley beds by
stratigraphic section...... 102 IX
LIST OF PLATES
PLATE
1 Stewart Valley, Mineral County, Nevada
Stratigraphic and geologic cross section locations
2 Stewart Valley Section III.
3 Stewart Valley Sections IV and V.
4 Stewart Valley Sections VI, VII, and VIII.
5 Stewart Valley Sections XI and XII.
6 Stewart Valley Section XXVII.
7 Stewart Valley Generalized Composite Stratigraphic
Section.
8 Stewart Valley Cross Sections. 1
INTRODUCTION
Stewart Valley is located in the northeastern portion
of Mineral County, Nevada. It is a northwest trending
valley located between the Pilot Mountains and the Gabbs
Valley Range, on the southwest, and the Cedar Mountains, on
the northeast ( Tonopah 1:250,000; Ross, 1961 Plate 1,
Figure 1 and 2). Stewart Valley is accessible via Nevada
State Route 23 (the Gabbs to Luning, Nevada highway) which
crosses the valley on its nothwestern end and via a county
road from Mina to the Simon mine in the southern end of the
valley. Secondary county roads from the north also
intersect the Stewart Valley road. The area of interest in
Stewart Valley lies some 12 miles (19.3 km) southeast of the Stewart Valley-State Route 23 intersection, at a point starting at the Rawhide ranch (Granny Goose Well Quadrangle
1.24,000 map, Plate 1, Figure 1 and 2) and continuing down the valley for some 10 miles (16 km) to just below the junction of the Stewart Springs road with the Stewart
Valley road (Stewart Springs Quadrangle 1:24,000 map, Plate
1) .
This study was undertaken, in part, to provide a detailed description of the physical stratigraphy and NEVADA
Figure 1. Location map of the Stewart Valley study site,
Mineral County, Nevada. Figure 2. Landsat 5 Thematic Mapper false color image
(bands 4, 3, 2 RGB) of west central Nevada. The arrow is at
the junction of Fingerrock Wash and Omco Wash.
? i ixM 4
Figure 3. Landsat 5 Thematic Mapper false color image
(bands 4, 3, 2 RGB) of the Stewart Valley, Mineral County,
Nevada area. The arrow is at the junction of Fingerrock
Wash and Omco Wash. The light colored beds in the center
of the image are the "Esmeralda" beds of Stewart Valley.
The Cedar Mountains are to the right and Gabbs Valley Range to the left. 5
depositional environments of the Miocene non-marine
deposits in the areas for the Carson City District of the
Bureau of Land Management as part of their project to set
aside this area as an area of critical environmental
concern (ACEC) in order to protect the paleontological
resources found in these beds. While other Miocene and
Pliocene non-marine beds in the area were examined, those
reported here are only those units found in Stewart Valley
proper from below the Rawhide ranch to just below Stewart
Spring, which contain the paleontological resources to be
preserved. Future reports by the author will include
detailed examination of the rest of the Miocene and
Pliocene non-marine beds in southern Stewart Valley and
lone Valley to the east.
Present Topography, Climate and Environment
Stewart Valley has an average elevation of 5,500 feet
(1676.4 m) and is flanked by the Pilot Mountains, Gabbs
Valley Range, and Cedar Mountains which average 7,000 feet
(2133.6 m) in elevation with some areas exceeding 8,000
feet (2438.4 m) in elevation. Stewart Valley drains northwestward via Fingerrock Wash (In this report the term
Fingerrock Wash will be used as a single word, which is the 6
traditional usage as opposed to the usage of Finger rock
Wash on U.S.G.S. topographic maps.) to the Gabbs Valley
playa, some 35 miles (56.33 km) northwest. This playa is
the local base level of erosion and is some 1,550-2,000
feet (472.4-609.6 m) lower than Stewart Valley.
A series of older (Pleistocene?) erosional surfaces
and pediments can be seen in the valley, especially along
the eastern front of the Pilot Mountains and Gabbs Valley
Range. This series of older surfaces are covered in many
areas with desert pavement has been incised with numerous
gullies and stream beds that have exposed the buried
Miocene and Pliocene non-marine sediments below. In Figure
4, these surfaces appear on the left hand side (western) of
the image, between the Gabbs Valley Range and Fingerrock
Wash. In addition tectonic activity has uplifted various blocks in the valley and erosion has stripped away the younger alluvium and exposed the Miocene and Pliocene non- marine beds. In general the topography in the valley proper varies from sandy wash deposits, small alluvial fans and alkali playas to rolling and rapidly ascending hills mainly of Miocene lacustrine and fluvial beds, rhyolitic volcanic caps, dikes, and agglomerate to breccia deposits. 7
Figure 4. Landsat 5 Thematic Mapper false color image
(band ratios 5/7, 5/6. 4/2 RGB) of the Stewart Valley,
Mineral County, Nevada area. This is the same scene as
Fig. 2. The dark green beds immediately to the left of
Fingerrock Wash are the older Buwalda terrace deposits. 8
The annual precipitation ranges from 4 to 8 inches
(10.16-20.32 cm) annually with a 30-year average of 4.58
inches (11.63 cm) being reported by the National Weather
Service through the period ending in 1970. Annual
temperature ranges from -25 degrees F (-31.67 degrees C) to
100+ Ag r e e s F <37.78+ degrees C) with an annual average of
70.4 degrees F (21.23 degrees C)(National Weather Service,
1973, Vreeland, Vreeland and Lugaski, 1978; Lugaski,
Vreeland and Vreeland, 1979).
Vegetation in Stewart Valley is dominated by the sage
(Artemisia tridentata), shadscale (Artiplex confertifolia),
snakeweed (Gutierrezia sarthrae), rabbitbrush
(Chrysothamnus nauseosus), hopsage (Gravia spinosa),
juniper (Juniperus osteosperma), and a variety of forbs and grasses. Vegetation cover in the southern part of Stewart
Valley ranges from 10.4% to 23.3% with an average of 18.02% cover, while in the northern portion of Stewart Valley the vegetation cover ranges from 8.6% to 30.4% cover with an average of 20.5%. The grass/forb content of the cover is two to three times greater in the northwestern end of
Stewart Valley as compared to the southeastern end
(Vreeland et al., 1978; Lugaski et al., 1979). Cover by 9
shrubs is equally proportionally greater in the
southeastern portion of Stewart Valley as compared to the
northwestern end. Shrub cover in the northwestern portion
of Stewart Valley is dominated by shadscale (Atriplex sn.t
and in the southeastern portion of Stewart Valley is
dominated by sage (Artemisia sp.). The trees and shrubs
with the highest relative cover which are found in the
southeastern end of Stewart Valley are more hydrophytic in
nature (Artemisia sp. , Juniperus s p .) ; while those found in
the northwestern end of Stewart Valley (Atriplex s p . ) are
more xerophytic in nature. The southeastern end of Stewart
Valley is vegetationally more representative of Billing's
(1949) classification of a sagebrush zone which is altitudinally higher and wetter than the northwestern end of Stewart Valley which is classified as being more of a shadscale zone, is lower in elevation and more xeric. Much of Stewart Valley is transitional between the two zones
(Vreeland, et al.f 1978; Lugaski, et al., 1979). The high abundance of rabbitbrush (Chrvsothamnus nauseosns) throughout Stewart Valley is an indication of an area of transition between the shadscale and sagebrush zones that has been either burnt-over or over-grazed (Young,Evans and 10
Major, 1972).
Only a few seeps and active springs are known in the
area. Stewart Springs is a natural phenomenon that has
been developed for the local ranching interest as have been
several water wells. Fingerrock Wash, Omco Wash and the
other drainages in Stewart Valley are usually dry, with the
exception of annual localized and regional cloud-bursts,
which do a great deal of erosional damage to the landscape
and local roads.
Objective and Scope of Work
The known location of the paleontological resources of
Late Tertiary age ("Esmeralda formation") in Stewart Valley starts just below the Rawhide ranch (Section 7, T9N, R35N
(Tonapah 1:250,000 map) and occur within one mile (1.6 km) on either side of Fingerrock Wash to a point approximately one mile (1.6 km) south of the intersection of the Stewart
Valley road and the Stewart Springs road (Section 3, T8N,
R36E (Tonapah 1:250,000 map). Several other localities are known in the Cedar Mountains, Stewart Valley and lone
Valley but are only briefly referred to here.
The objective of this research was to do measure and describe in detail the beds originally named as being the 11
"Esmeralda formation" that are found in the above described
area, determine the paleodepositional environment of these
beds and correlate the units.
The study area is 7.70 miles (12.39 km) long and 2.0
miles (3.22 km) wide, totaling approximately 15.5 square
miles (39.88 km ) of exposures to examine and map. A large
portion of Stewart Valley is covered with pre-"Esmeralda"
volcanics, post-"Esmeralda" volcanics and Quaternary
alluvium.
Present and Future Status of the Fossil Beds
In the past the knowledge that this paleontological
resource existed was rather limited. In the last ten years
this information has become widely disseminated throughout
the academic and non-academic community to a point that has
threatened the loss of the resource. Through the efforts of Dr. Don Savage of the University of California, Berkeley and Dr. Harvey Scudder, California State University,
Hayward, action is being taken by the Bureau of Land
Management to restrict access to the paleontological resources. It is after Dr. Don Savage that Miocene Lake
Savage in Stewart Valley, Mineral County, Nevada is named. 12
Currently the area has been designated an ACEC (Area of
Critical Environmental Concern) and fossil collecting
permits are required. Those individuals wishing to obtain
more information on the permits should contact the BLM
office in Carson City, Nevada.
Previous Work
Turner (1900a, 1900b, 1902, 1909) was the first to
designate the Late Tertiary non-marine deposits of west-
central Nevada in the vicinity of Silver Peak, Esmeralda
County, Nevada as being the Esmeralda formation, with
Knowlton (1900) describing the fossil plants and Lucas
(1900) describing the fossil fish. Subsequent geologic
studies have expanded the usage of the term Esmeralda fm.
to include most of the Miocene non-marine beds in west-
central Nevada. Robinson, McKee and Moiola (1968) and
Moiola (1969) redefined the Esmeralda fm. and restricted its usage to a sequence of rocks described from the type locality in the southern end of Big Smoky Valley, Esmeralda
County, Nevada with generically related units in Clayton
Valley, Fish Lake Valley and the Silver Peak Range. This redefining follows Firby’s (1963, 1966, 1969) questioning of the usage of the term Esmeralda fm. in areas outside 13
Turner's (1900) type locality. Everdan, Savage, Curtis,
and James (1964) and Robinson et al. (1968) have
radiometrically dated the oldest Esmeralda beds in the type
locality (as redefined by Robinson et al., 1968) as being
13.1 m.y.B.P. and the youngest (as defined by Robinson et
al., 1968) as being 4.3 m.y.B.P. This makes the Esmeralda
fm. as redefined and restricted by Robinson et al. (1968)
as being Clarendonian to Hemphillian and possibly Blancan
in North American Land-Mammal Age.
Additional works on the "Esmeralda fm." in its type
locality area includes: Stock (1926) on Anchitheriine
horses in Fish Lake Valley; Ferguson (1927) on the Gilbert
mining district; Berry (1927) on fossil plants; Stirton
(1929, 1931, 1932, 1936) on fossil rodents in Fish Lake
Valley and their correlation with the fossil mammals of the
Cedar Mountains and Stewart Valley; Axelrod (1940) on fossil plants; Ferguson, Muller and Cathcart (1953) mapped the Coadale junction region; Van Houten (1956) included the
Esmeralda fm. (non-restricted) in his reconnaissance of
Cenozoic sedimentary rocks of Nevada; Clark, Dawson, and
Wood (1964) described new fossil rodents from the Esmeralda fm. of Fish Lake Valley; Everdan and James (1964) who 14
published additional K-Ar dates on the fossil flora
localities of the Esmeralda fm. (non-restricted); Axelrod
(1966) who also published K-Ar dates on fossil flora
locations; La Rivers (1966a, 1966b) who described two new
fossil fish from the Alum mine and Coaldale-Blair junction
area; Albers and Stewart (1972) who included the redefined
Esmeralda fm. in their geology of Esmeralda County, and
Lugaski (1977) who redefined the age and relationship of
the Blair junction fossil fish described by La Rivers
(1966a) in relation to the redefined Esmeralda fm.
Buwalda (1914) first described the Late Tertiary non-
marine rocks in Stewart Valley and assigned them to the
"Esmeralda fm." His rationale was two fold; first the beds
he found in lone Valley that he correlated with the beds in
Stewart Valley were described as being the "Esmeralda fm."
by Turner (1902) and secondly he claimed he could trace the
lacustrine beds from Stewart Valley to and through the
Monte Cristo Range and on to the Silver Peak Range.
Merriam (1916) agreed with this assesment and wrote the
first descriptions of the fossil vertebrates found in the
Cedar Mountains. Knopf (1921) discussed the ore deposits of the Cedar Mountains; Clark (1922) the Santa Fe mining 15
district in the Pilot Mountains; and Ferguson (1927) the
Gilbert mining district in the Monte Cristo Range; all of
which included references to the Esmeralda beds. Gianella
and Callaghan (1934) presented field studies on the 1932
earthquake in the Cedar Mountains and its bearing on the
genesis of Basin and Range structure. Ross (1961) briefly
described the geology of Stewart Valley and Cedar Mountains
in the geology of Mineral County. Firby (1963a, 1963b,
1966, 1969) described some 34 taxa of non-marine molluscs
from Stewart Valley, seven of which were new and discussed
the relationship of the molluscan fauna to the environment
in which they lived and the make-up of the Miocene lake in
Stewart Valley; Wolfe (1964) described the Stewart Springs
and Fingerrock Wash floras; Mawby (1965, 1968a, 1968b) described some new fossil vertebrates; and Smedman (1969) described some fossil diatoms from Stewart Valley and the
Cedar Mountains. Everdan et al. (1964) would include several Stewart Valley and Cedar Mountain localities in their list of K-Ar dates which indicate the oldest units of the non-marine sequence in Stewart Valley to be 16.1 m.y.B.P. and the youngest some 10 m.y.B.P. in age. These dates make the Stewart Valley non-marine beds to be 16
Barstovian to Clarendonian in ages which is older than the
restricted and redefined Esmeralda fm. and closer in age to
the Siebert Tuff of Nye County (Silbermann and McKee, 1972;
Bonham and Garside, 1974, 1979; Albers and Stewart, 1972;
Lugaski, 1978). Lugaski, Firby and Schorn (1984) have
presented a first discussion on the physical stratigraphy,
depositional environment and paleontologic resources found
in Stewart Valley. Howard Schorn of the University of
California, Berkeley has finished a geologic map of the
general units in Stewart Valley and is describing a series
of new fossil plants. Lugaski, Schorn and Firby are
currently working together on several publications dealing
with the stratigraphy and paleontology of Stewart Valley.
General Mesozoic Geologic History
The Mesozoic tectonic history of Nevada is a complex
series of events consisting largely of compressional
folding and thrusting. Post-Sonoma orogenic activity is recorded in Mid-Jurassic rocks of western Nevada. The evidence for this is based in part on coarse locally derived material that is abundant in the Lower Jurassic to perhaps Middle Jurassic Dunlap formation. This abundance of coarse detritus is characterized by abrupt lateral 17
variation in lithology, abundant amount of volcanic rock,
and unconformable relations with other units. This has
been suggested by Ferguson and Muller (1949), and Stanley
(1971) to be an active orogenic belt in which the Dunlap
fm. was deposited.
The Tnassic and Jurassic rocks of western Nevada also
occur in complexly deformed imbricated thrust slices which
Speed (1978) and Oldow (1978) consider to have been in
place by Mid-Jurassic time. The classic area of these
structures is in the northern Pilot Mountains. They were
first studied by Ferguson and Muller (1949) and have
recently been restudied by Oldow (1978). In the Pilot
Mountains shelf-type carbonate and terrigenous clastic
rocks of the Luning formation allochthon have moved over
subaerial volcanic and sedimentary rocks. Thirteen
imbricated thrust slices make-up the allochthonous rocks, which are in part recumbently folded before being in turn cut by planar thrusts. The Luning fm. unconformably underlies, in part, the Late Tetiary "Esmeralda fm." beds in the Stewart Valley area of western Nevada (Stewart,
1980). 18
General Cenozoic Geologic History cw J The topography of much of Nevada consists of typical
north-northeast trending ranges in the northeastern portion
of the state, typical north-northwestern trending ranges or -I Sfc 9 the Sierra Nevada in the western and southwestern portion 3 1 rj of the state. The areas between these two physiographic >D i l l jrj/JB areas, which has been called the "Walker line" or "Walker
lane" and more recently the Walker belt (Stewart, 1980) is
a series of northwest trending belts of discontinuous and
commonly arcuate ranges of diverse orientation which
extend from the western border of Nevada and parts of
adjacent eastern California. The northeastern boundary of
the Walker belt is considered by Gianella and Callaghan
(1934), Stewart (1980) and named by Locke, Billingsley and
Mayo (1940) to be a physiographic lineament characterized
by numerous valleys and general low relief mountains. The
Cedar Mountains on the the eastern side of Stewart Valley
rise less than 1,000 feet (304.8 m) above the valley floor
(Gianella and Callaghan, 1934). The Walker belt is
considered to be a fundamental structural unit that is for
the most part poorly understood (Stewart, 1980). Within
the Walker belt there is a diverse topography, which is 19
considered by some (Albers, 1967; Stewart, 1968) to be
related to large scale right-lateral displacement (80-120
miles (128-193 km.) that has occurred for the most part as
slippage along well-defined faults and as preserved large-
scale folding in rocks ranging in age from Precambrian to
Mesozoic. Timing of these right-lateral events ranges from
Early-to-Middle Jurassic (Albers, 1967; Speed, 1974) to
Late Tertiary (Hardymann, Ekren and Byers, 1975; Fleck,
1970) in age. Albers (1967) has noted that the average
trend of the Sierra Nevada is N25-30 degrees W, where as
the average Great Basin range is North to N20 degrees E.
These two trends converge for at least 300 miles (482.8 km)
and are expressed by not only topography which reflects
Cenozoic Basin and Range faults, but are also expressed by
the bend of fold axes and by the strike of pre-Middle
Miocene (mainly pre-Cretaceous) formations. This convergence as previously mentioned has been labeled the
Walker belt by Stewart (1980), which also has a smaller zone that is marked topographically by discontinuous and commonly arcuate ranges. On the eastern side of this zone is a physiographic lineament characterized by numerous valleys and generally low relief (Gianella and Callaghan, 20
1934; Locke et al.f 1940), which is an area of right-
lateral displacement. Albers (1967) also concludes that
the right-lateral movement on these faults may have
continued in dimishing degree through the Tertiary and into
the present. Evidence found in the Monte Cristo Range of
Esmeralda County shows rather tightly folded rocks of the
Late Miocene to Pliocene "Esmeralda fm." These rocks are
within a mile (1.6 km) or so of the Soda Springs Valley
fault zone, which is thought to be a major component of the
right-lateral displacement of the Walker Lane (Stewart,
1967). Albers (1967) further concludes that the dominately
north-south trending Basin and Range dip-slip faults formed
during Middle to Late Tertiary time was consistant with a
right-lateral movement pattern. These faults could be extension features of a broad-scale right-lateral shear zone. Finally, he concludes (Albers, 1967) that deformation along the mobile belt began as long ago as late
Early Jurassic time and some movement has probably persisted ever since, but that the bending (oroflexing) and major right-lateral strike-slip faulting was completed not later than Early to Middle Miocene time and that movements since Middle Miocene time has been restricted to faulting 21
■ t >; f (dip-slip). Speed (1974) basically believes the Tertiary
faulting generally followed a trend of minimum fracture Jfh strength m the basement Paleozoic and Mesozoic or earlier
rocks and that the direction of slip on Tertiary faults
depended upon the orientation of the fault plane with
respect to the Tertiary Great Basin extension direction. Oi J Ekren, Byers, Hardyman, Marvin and Silberman (1980), based
I n .1 h upon their mapping of the Mid-Tertiary tuffs of the Gillis 'S ;>ir; Range and Gabbs Valley Range, have shown that the X-.jrf.'lpl] paleotopography in the area was subdued and consisted of
eroded Mesozoic basement rocks when the first ash-flows of
Oligocene age swept over the terrane. They have also noted
that these tuffs occurred in a regionally extensive
northwest-trending topographic trough which indicates the
Walker Lane was a distinctive rift zone prior to the advent
(pre-Oligocene to Oligocene) of the ash-flow tuff
eruptions. Faulting and rifting continued during the ash-
7 i 163 flow tuff eruptions. These tuffs were deposited in a
12-i s a fault-controlled structural and topographic trough; faults
that bound the tuffs (Poinsettia Tuff Member of the Hu-pwi
'iS jr» J Rhyodacite in the Gabbs Valley Range) indicate a N 65
■ n: 2 degree W orientation that parallels the axes of 22
distribution of the tuffs. Landslide debris in the tuff of
Redrock Canyon suggest strike-slip faulting commenced some
23 m.y.B.P. and may have continued into Quaternary time
(Ekren et al., 1980).
Through out the rest of Late Tertiary time fluvial and
lacustrine deposition along with volcanic sequences would
continue to be deposited in various areas of western
Nevada. In some areas the combined non-marine and volcanic
sequences would be in excess of 9,000 feet (2750 m) , while
in other areas the sequence would be only 1,500 feet (450
m) or less in thickness. The difference in thickness of
the Late Tertiary fluvial and lacustrine deposits is a
direct reflection of the tectonic activity, mainly Basin
and Range extension, that has gone on in that particular
valley.
The general climate during Pliocene time is considered
to be more arid. This aridity resulted in the desiccation
of many of the existing lacustrine systems in western
Nevada, while others continued to exist in the extreme western portion of Nevada, feed by drainage from the Sierra
Nevada (Axelrod, 1958). During this time of reduced precipitation alluvial deposits were developed in various 2 3
areas (Buwalda, 1914; Gilbert and Reynolds, 1973; Goila and
Stewart, 1984). The resurgence of Basin and Range faulting
coupled with an increase in precipitation during the
Quaternary (Russell, 1884, 1885; Jones, 1925; Morrison,
1964, 1965) caused the development of pluvial lakes of uoe-i. various sizes through out the western Great Basin.
Figure 5 is a false color Landsat 5 Thematic Mapper )B V -)i1 image of the Stewart Valley area showing lineations found . -*j;p9g in the valley and Figure 6 is a graphic representation of
i -j o : u these lineations.
Ofifi v 9 1 X fc ' Figure 5. Landsat 5 Thematic Mapper false color image
(band ratios 3/6, 5/6, 4/2 RGB) of the Stewart Valley,
Mineral County, Nevada area. This is the same scene as
Fig. 2. This scene was used primarily to make the lineation map of the Stewart Valley area 25
ruyi'5 Figure
>n jd) Mapper
\ nxM in the
. pH
-j ill 2 6
RESEARCH METHODS and RESULTS
This research project was undertaken to determine in
detail the physical stratigraphy, depositional environments
and paleoecology of the Miocene non-marine deposits found
m Stewart Valley, Mineral County, Nevada. This study is
restricted to those deposits found in the central portion
of the valley that are known to contain an abundant
paleontologic resource. The author has been collecting a
variety of fossils in the area for some ten years and was
fairly familiar with the general location of the various
fossil localities and the rocks in which they occurred.
An initial reconnaissance of the area was undertaken
in the Fall of 1981 with detailed mapping and measuring of
stratigraphic sections beginning in Spring of 1982.
Further measuring of sections was carried out during the
Summer of 1983. Analysis and publication of these sections i u\ 'i was begun during the Summer of 1984. s q Q f . M Existing aerial photographs of the area were examined -it . i during the field study segment of the research. However,
in general the poor quality of the photographs precludes
their use for the lithologic mapping.
A variety of outcrops were located during the 27
reconnaissance period and marked on topographic maps
(Granny Goose Well and Stewart Springs, 1:24,000
topographic maps). Starting in the 1982 field season these
stratigraphic sections were measured at random. The random
method was chosen mainly as a matter of convenience since
the fluvial sections are rather sandy in nature and dry out
sooner than the clay and silty lacustrine section. In
addition the lacustrine sections, which contain the
majority of the known paleontologic resource, were often
crowded with out-of-state geologic classes on Spring and
Summer field trips which with its resulting destruction of
the out crops made measuring of the sections difficult in
several areas.
The sections were measured using a brunton and metric tape following the methods and corrections of Compton
(1960). Each section was given a roman numeral (I, II, etc.) at the start of the measuring and then each sample taken was given an arabic number (1,2, etc.) with repetitive sampling of a given unit given a letter (a, b, etc.) so the final sample number would be I-l-a. Each measured section was then plotted on the topographic maps.
Each measured section that has been fully illustrated in 28
this report retains its original roman numeral, those
sections described here but not illustrated were changed
from roman numerals to their corresponding arabic numeral.
Each sample was briefly described in the field and recorded
in note books.
Samples returned to the laboratory were then examined
in detail. All rock descriptions were developed using
Lundegard and Samuels (1980) for field classification of
shales; Fisher and Schmincke (1984) for pyroclastic rocks;
Pettijohn, Potter and Sevier (1973) for elastics; Tucker
(1982) and Collison and Thompson (1982) for sedimentary
structures; Bathurst (1975) for carbonates; and Reading
(1978) and Galloway and Hobday (1983) for terrigenous
depositional system.
The results of this research will be presented in
several phases. The first phase is a narrative description
of all of the measured stratigraphic sections. Each narrative includes basic rock types, sedimentary structures, fossils, possible depositional environment, etc. as observed and noted in the field. This will be followed by detailed descriptions and illustrations of selected measured stratigraphic sections. Next will be 29
presented information on the x-ray diffraction analysis
undertaken on selected shales to determine their
composition. A section on the sedimentary structures found
m each stratigraphic section follows the x-ray diffraction
analysis.
Stewart Valley Stratigraphic Section 1
Stewart Valley strastigraphic section 1 was located
some 100 ft (30 m) northeast of the junction of the Stewart
Valley road and Omco Wash road (Plate 1). Traverse was
vertical to N52E. This stratigraphic section consists of a
series of outcrops measured from the base of the hill at
the road junction. The lower portion of the section is a
covered interval of brown mud, the first observable unit is
a reddish-brown, iron stained, calcareous, fine to medium
sand clast, highly indurated bed. This is followed by
alternating layers of mud and sand, some units are
channelized, others are cross-bedded. A silicified shale
layer is found to contain ostracodes, then there are more
alternating beds of mud and sand until the base of the
biotitic white ash. Above the white ash is a sequence of
clastic and mud beds. The clastic beds vary from fine to very coarse sand grains, some are cross-bedded, some iron- 30
stained, some calcareously cemented. Erosion of the beds
form a highly coralline-looking surface. Erosional
unconformities are frequently seen, along with scour and
fill structures. The mud units contain load casts and
concretions. The concretions are so numerous in some beds
they almost completely cover the slopes formed by the
eroding mudrock. This portion of the section is topped by
a blue vitric ash layer that has load casts, ripples and
occasionally cut and fill structures. Above the blue ash
layer are more fine to coarse sand clast beds and mud rock,
some iron stained, some with cut and fill structures, some
cross-bedded and several fining upward sequences. Top of
the section is alluvium. This section represents fluvial
and near-shore ancient alluvial environments.
Stewart Valley Stratigraphic Section 2
Stewart Valley stratigraphic section 2 is located
some 2,500 ft (762 m) due west of the junction of the
Stewart Valley road and Omco Wash road (Plate 1). Traverse was vertical. A similar section (2b) was measured some
1,000 ft (304 m) northwest (around the base of the hill) of this site. This section includes a large channel deposit
(cut and fill) and several erosional unconformities. The section consists of a series of fine to medium grained sand
clasts and mud rock. Fish vertebrae were found in several
of the mud and sandstone units, as were mud balls, some
large clasts, and iron staining. Cut and fill structures
are common. In many cases mud, volcanic ash and sand clasts
are mixed in a single unit. Both fining upward and
coarsening upward sequences are seen. This section
represents mainly a fluvial environment.
Stewart Valley Stratigraphic Section 3
See detailed and illustrated Stewart Valley
Stratigraphic Section III for details (Plate 1 and 2).
Stewart Valley Stratigraphic Section 4
See detailed and illustrated Stewart Valley
Stratigraphic Section IV for details (Plate 1 and 3).
Stewart Valley Stratigraphic Section 5
See detailed and illustrated Stewart Valley
Stratigraphic Section V for details (Plate 1 and 3).
Stewart Valley Stratigraphic Section 6
See detailed and illustrated Stewart Valley
Stratigraphic Section VI for details (Plate 1 and 4). 32
Stewart Valley Stratigraphic Section 7
See detailed and illustrated Stewart Valley
Stratigraphic Section VII for details (Plate 1 and 4).
Stewart Valley Stratigraphic Section 8
See detailed and illustrated Stewart Valley
Stratigraphic Section VIII for details (Plate 1 and 4).
Stewart Valley Stratigraphic Section 9
Stewart Valley stratigraphic section 9 is located
some 1,000 ft (304 m) due east of the Stewart Valley road
and due south of the Two Tips fossil insect locality (Plate
1). Traverse was N50W. The section was measured starting
m the wash and ended at the top of the hill. This section
was trenched by T.P. Lugaski and L.A. Lugaski. Lower
portions of this section contained a series of sandstone,
shale, and mudrock beds. The sandstone beds are well
indurated and form the outcrop units. The sandstone beds
occur in a variety of colors and grain sizes, are highly
indurated, contain fossil fish bones, are occasionally
iron-stained, and have some desiccation cracks on top.
Interbedded mud and paper shale was seen, paper shale 33
gradded upward into more finely laminated, silicified unit
with syneresis or desication cracks and fish bones. The
series of sandstone, mud and paper shale repeats numerous
times. Occasionally some pumaceous material is found as
are several tuffaceous sandstones. Diatomaceous mud and
claystones are also found. The top of this section is a
massive sandstone that grades laterally into a stromatolite
bed which has been cut by channels filled with sand and
mud. This is followed by a series of smaller sandstone
beds, massive shale beds, some paper shale, some silicified
shale and some diatomaceous mudstone, all containing fossil
plants, fish and occasionally molluscs. This measured
section represents near-shore and beach lacustrine
environments followed by deeper water lacustrine environments.
Stewart Valley Stratigraphic Section 10
Stewart Valley stratigraphic section 10 is located due east of Stewart Valley stratigraphic section 9 (Plate
1). Traverse was N35W. This section includes a sandstone unit in the lower portion of the section followed by a series of mudrocks, fossiliferous paper shales (plant, fish, ostracodes), some units are silicified, burnt plant 34 • n i w j material found in some units and volcanic ash is xi** occasionally seen. A layer above the sandy shale with 9n; I ostracodes contains large carbonate oncolites and is followed by a diatomaceous mud unit. This measured section ^ represents near-shore lacustrine environment and possibly a beach environment followed by deep water environment which at times was greatly agitated. Stewart Valley Stratigraphic Section 11 See detailed and illustrated Stewart Valley Stratigraphic Section XI for details (Plate 1 and 5). I f i q Stewart Valley Stratigraphic Section 12 See detailed and illustrated Stewart Valley ■ v nf’ Stratigraphic Section XII for details (Plate 1 and 5). ■'/ns Stewart Valley Stratigraphic Section 13 Stewart Valley stratigraphic section 13 is located some 1.5 miles (2.3 km) southeast of the junction of the BUD Stewart Valley road and Omco Wash road about 1,000 ft (304 . (i m) west of the road at the blue vitric ash cliff (Plate 1). rin : Traverse was N40W. The lower portion of the section contains sandstone with volcanic glass, mud beds with 35 channels, an erosional unconformity, coarse grained sand, some thinly laminated mudrock, mudstone with desiccation cracks, cross-bedded sandstones and blue vitric ash. The blue vitric ash is blocky and swirled below and thinly laminated with fine ripples in the upper portion. A series of alternating shale and mudrock beds with sandstone (containing blue ash. conglomerate and/or cobble clasts) is also seen. Tufa deposits are found on top of the unit. This section represents a near-shore lacustrine environment to fluvial environment. Stewart Valley Stratigraphic Section 14 Stewart Valley stratigraphic section 14 is located in the hills northeast of measured section 12 and south of measured 13, starting behind the large gypsiferous brown mud hill (Plate 1). Traverse was N65W. The lower portion of this measured section begins in mudrock which is followed by a series of mud, ash mud deposits and then very fine ash. This sequence is repeated with the inclusion of an ostracodal sandstone containing abundant fossil trout bones and a single layer of gypsum. The sequence of mud, ash and well indurated shale continues and repeats itself several times until coming to a conglomerate bed. Beyond 36 l5.io the conglomerate bed a seriea o£ mud and sandstone units are repeated. Some fossil wood and plant remains are seen. *d Tufa surrounding fossil wood is encountered in the upper portion of the section. Dendritic and thinolitic tufa mounds occur in the upper mud units as does a large heavily iron-stained conglomerate filled channel deposit. This section represents a flucuating near-shore lacustrine and r riT beach environment, ending in a fluvial environment. Stewart Valley Stratigraphic Section 15 Stewart Valley stratigraphic section 15 is located in Fingerrock Wash some 2,450 ft (750 m) south of the junction fi J .< of the Stewart Valley road and Omco Wash road and some 1,600 ft (500 m) due west of the Stewart Valley road (Plate • : 1). Traverse was N45E. Base of the measured section starts in the wash and consists of blue vitric ash. This is followed by mudrock, shale and sandstone, some of the lol mudrock is thin bedded to thinly laminated and calcareous. ■nil Massive reddish-brown shales occur on top, with some large I j channels and conglomerate. Some trace fossils (burrows) ■nod are present and intraformational fracturing occurs. fter, Desiccation cracks are abundant in some layers. 37 Alternating coarse sandstone, s e e cross-bedded and mud with large clasts of red shale are present in the upper portion. Large-scale ripples are seen and the top of one red shale unit is covered with thinolite tufa. A small layer of blue tuff is seen near the top. This section represents near-shore lacustrine and fluvial environments. Stewart Valley Stratigraphic Section 16 Stewart Valley stratigraphic section 16 is located in Fingerrock Wash some 3,280 ft (1,000 m) south of the junction of the Stewart Valley road and Omco Wash road and some 820 ft (250 m) west of the Stewart Valley road (Plate 1). Traverse was N55E. Base of the measured section starts in the wash with a blue vitric ash, then mudrock, tuffaceous sand and sandstone. Several erosional unconformities can be seen. A variety of muds, mudrock, and sandstones occur above the unconformities ending at the biotitic white ash cliff. This section represents a fluvial environment. Stewart Valley Stratigraphic Section 17 Stewart Valley stratigraphic section 17 is located 650 ft (200 m) south and east of Stewart Valley section 16 38 (Plate 1). Traverse was vertical. This measured section is mainly a covered interval and a cliff of biotitic white ash, representing volcanic deposition. Stewart Valley Stratigraphic Section 18 Stewart Valley stratigraphic section 18 is located some 3,280 ft (1,000 m) south of the junction of the Stewart Valley road and Omco Wash road on the east side of the Stewart valley road across from Stewart Valley sections 16 and 17 (Plate 1). Traverse was N5W. This measured section starts above the biotitic white ash and it includes mudrock, pebbly sandstone, well-indurated shale with desiccation cracks, blue vitric ash, a white shale with raindrop impressions and desiccation cracks, several more mudrock and sandstone layers. The blue vitric ash laterally (east) becomes more channelized and is covered by a highly iron-stained sandstone. This measured section represents a near-shore lacustrine to fluvial environment. Stewart Valley Stratigraphic Section 19 Stewart Valley stratigraphic section 19 is located 2,460 ft (750 m) east of the Stewart Valley road at a point some 6,230 ft (1,900 m) north of the junction of the 39 Stewart Valley road and Stewart Springs road (Plate 1). Traverse was HIM. This section starts in the wash and includes a series of well indurated fossil mollusc beds alternating with mudrock, in some cases the mollusc beds contain pebbles. Several beds of coquina (mollusc hash) are also present. Some large channels were also seen, which in turn are refilled with sand, mud, and shell fragments. Some of the mollusc beds are highly indurated. This measured section represents the interface between a fluvial environment and lacustrine deltaic and/or near- shore lacustrine environment. Stewart Valley Stratigraphic Section 20 Stewart Valley stratigraphic section 20 is located some 1,640 ft (500 m) due south of the junction of the Stewart Valley road and Omco Wash road (Plate 1). Traverse was vertical. This measured section starts in Fingerrock Wash with the biotitic white ash. The lower portion of the ash is laminated and to some extent cross-bedded, the upper portion is massive with an erosional unconformity on the surface. This erosional surface is then filled with a blocky, swirled white ash. This measured section represents volcanic depositional environment. 40 Stewart Valley Stratigraphic Section 21 Stewart Valley stratigraphic section 21 is located some 100 ft (30 m) due south of Stewart Valley section 20 (Plate 1). Traverse was N50E. This measured section starts where section 20 leaves off and consists of a series of tuffaceous sandstones to non-tuffaceous sandstones before being covered with alluvium. This measured section represents volcanic depositional environment and fluvial environment. Stewart Valley Stratigraphic Section 22 Stewart valley stratigraphic section 22 is located some 100-200 ft (30-60 m) due south of Stewart Valley section 21 (Plate 1). Traverse was vertical and N40E. This measured section starts in Fingerrock Wash and also consists of a series of mudrock, tuff, ash, and sandstones. Some of the sandstones and volcanic layers are cross- bedded, and some are channelized. Layers of pumice are seen. The sandstone varies from fine to very coarse sand clasts (fining upward sequences). Large silicified nodules are seen. Reddish-brown shale layers also also found. The section is topped with a well indurated, well rounded and 41 well sorted beach deposit. This measured section represents a Eluvial to near-shore lacustrine environment. Stewart Valley Stratigraphic Section 23 Stewart valley stratigraphic section 23 is located 9,185 ft (2,800 m) north of the junction of the Stewart Valley road and Stewart Springs road and some 3,600 ft (1,100 n) east of the Stewart Valley road where the BLM fence crosses the wash (Plate 1). Traverse was vertical and N64E. This measured section contains a series of cross-bedded and planar sandstones interbedded with mudrock and shale. Some units are tuffaceous, some contain fossil wood fragments. The sandstones and shales tend to be well indurated. Desiccation surfaces and raindrop impressions are seen. Several layers of tufa and oncolite-to- stromatolite-like structures are seen on top of the section. This measured section represents a deltaic to near-shore lacustrine emvironment. Stewart Valley Stratigraphic Section 24 Stewart valley stratigraphic section 24 is located some 10,000 ft (3,100 m) due south by southeast of the junction of the Stewart valley road and Omco Wash road and some 820 ft (250 m) due east of the Stewart Valley road, across the road from Stewart Valley sections 11 and 12 (Plate 1). Traverse was N45E. This measured section starts at the base of the brown mud cliff in laminated mudshale, includes blocky diatomaceous mud to claystone, other laminated mudstones and mudshales, blue vitric ash layers (5), shales, ostracodal sandstone with fossil trout bones and oncolites. This section repeats the sequence of mudrocks, laminated mudshales to mudstones several times. This measured section represents off-shore to near-shore and beach lacustrine environments. Stewart Valley Stratigraphic Section 25 Stewart Valley stratigraphic section 25 is located some 5,100 ft (1,550 m) north of the junction of the Stewart Valley road and Stewart Springs road and some 1,100 ft (350 m) due west of the Stewart Valley road at the large cliff (Plate 1). Traverse was N62W. The measured section starts in Fingerrock Wash in mudrock and then into blue- gray calcareous sandstone. Numerous sandstones are then seen as is a blue ash conglomerate. Several mudrock beds follow, some containing fossil fish bones and shells. A 43 blue vitric ash bed comes next as do several silicified shale beds that are well bioturbated. a white ash and sandstone bed with U-shaped burrows and a well indurated, silicified ostracodal sandstone bed with feeding trails comes next. This sequence is repeated with numerous sets of trace fossils (burrows) seen. This measured section represents a deltaic to near-shore lacustrine environment and possibly the interface between deltaic-fluvial environments. Stewart Valley Stratigraphic Section 26 Stewart Valley stratigraphic section 26 is located some 900 ft (300 m) due south of Stewart valley section 25, starting in Fingerrock Wash (Plate 1). Traverse was N65W. This is the lateral equivalent of Stewart valley section 25 but also includes a very large blue ash layer that is a channel fill rather than a laterally continuous bed. This channel fill contains lapilli layers and fossil gastropods. Fossil ostracodes and trace fossils are abundant. The section continues into blocky diatomaceous mud to claystone, sandstone and thinly laminated to papery shale. This measured section represents a near-shore to deltaic environment with a fluvial channel changing rapidly to an 44 off-shore lacustrine environment. Stewart Valley Stratigraphic Section 27 See detailed and illustrated Stewart Valley Stratigraphic section XXVII of Pacific Union Canyon (Plate 1 and 6) . Stewart Valley Stratigraphic Section 28 Stewart Valley stratigraphic section 28 is located at the junction of the Stewart Valley road and Stewart Springs road on the east side of the Stewart Valley road and south side of the Stewart Springs road (Plate 1). Traverse was N20W. This measured section in its lower portion consists of a series of mudrocks and sandstones, some tuffaceous. Fossil wood, plant remains and fish bones abound. Mud lumps are seen as is a large mound of silicified fossil fish bones. The upper portion of the section contains more sandstone beds, some diatomaceous claystone and some shale beds. The sandstone beds contain fossil fish bones, gastropods, small channels, ripples, tufa, fine rill and ripple structures, desiccation cracks, and breccia cemented with calcite. This measured section represents in the lower portion an off-shore lacustrine environment and in 45 the upper portion a near-shore to beach lacustrine environment. Stewart Valley Stratigraphic Section 29 Stewart Valley stratigraphic section 29 is located in Owl Canyon some 6-7 miles (10-12 km) due east of Stewart Valley along the Omco Wash road (Plate 1). Traverse was N25W. This measured section consists of a series of non- carbonate mudstones and volcanic ash deposits that starts with carbonate ooids. Sandstone, paper shale, diatomaceous claystone, mudstone occur near the top of the section. The top portion of the section contains mudstones (non- calcareous) with fossil clams and gastropods, and then calcareous mudstones and sandstones with fossil gastropods, a blue vxtrie ash bed and oolitic limestones in several layers, ending in a series of rippled sandstones and marl deposits. This measured section represents an off-shore lacustrine environment changing to a near-shore, highly agitated lacustrine environment. Stewart Valley Stratigraphic Section 30 Stewart Valley Stratigraphic section 30 is located up section from Stewart Valley section 23 (Plate 1). Traverse 46 was N83E. This measured section is the upper tufa dome beds of section 23 and includes a series of sandstone and mudstone beds. Each of the lower set of tufa domes tends to be well rounded, almost oncolite-like in appearence but massive, multi-layered with center of the domes showing a series of water conduits. A great deal of fossil wood is found around the base of each dome. Oncolite or sandstone base is associated with each tufa dome. The upper tufa dome is separated from the bottom set of tufa domes by a series of cross-bedded sandstones and calcareous mudrocks. The upper tufa domes are made up of thinolite tufa and rest on a base of highly bioturbated marlstone to calcareous mudstone. All the burrows are U-shaped and similar to chironomid midge larvae burrows. This measured section represents a deltaic to non-deltaic near-shore lacustrine environment. Stewart Valley Stratigraphic Section 31 Stewart Valley stratigraphic section 31 is located some 10,100 ft (3,100 m) southeast of the junction of the Stewart Valley road and Omco Wash road and some 3,100 ft (950 m) west of the Stewart Valley road in a side wash off 47 of Fingerrock Wash (Plate 1). Traverse was vertical ^ N90W. This measured section contains a series of silicified shales, paper shales, mudrocks, sandstones, ash, and turbidite deposits similar to the Pacific Union Canyon locality, Stewart Valley section 27. This section also shows evidence that a large block of sediment has slide into other sediments leaving an unconformity in this part of the section. The upper portion of the section grades into diatomaceous mud or claystone and gypsiferous mudrock. This measured section represents deep-water lacustrine environments to near-shore lacustrine environments in the upper portion. Stewart Valley Stratigraphic Section 32 Stewart Valley stratigraphic section 32 is located some 1,500 ft (450 m) due west of the junction of the Stewart Valley road and Stewart Springs road (Plate 1}. Traverse was N97W. This measured section starts in Fingerrock Wash. The lower portion of the section includes mudrock with medium sand clasts, chert beds, and a variety of other sandstone, shale and mudrocks. The mudrock and shale beds seem to alternate with an occasional inclusion of the sandstone beds. This lower portion may represent 48 part of the lower most Stewart Valley stratigraphic section. The upper portion of this section consists of paper shales, silicified shales and mudrock, some with small beds of calcite. The paper shales contain abundant fossil fish, leaves and insects. This section represents a near-shore lacustrine environment in its lower portion gradding into a deep-water lacustrine environment similar to Stewart Valley sections 27 and 31. The upper portion of the section reflects a change from deep-water lacustrine environment to a near-shore or possibly beach lacustrine environment. Stewart Valley Stratigraphic Section 33 Stewart Valley stratigraphic section 33 is located in Pacific Union Canyon some 4,300 ft (1,300 m) southeast of the junction of the Stewart Valley road and Stewart Springs road and across the gully from Stewart Valley section 27 (Plate 1). Traverse was vertical and N20W. This measured section consists of a series of alternating silicified shales, mudrocks, some paper shales, and ash beds. This measured section represents a deep-water lacustrine environment. Stewart Valley Stratigraphic Section 34 Stewart Valley stratigraphic section 34 is located at Two Tips Hill, located some 6,200 ft (1,900 m) north of the junction of the Stewart Valley road and Stewart Springs road and some 320 ft (100 m) due east of the Stewart Valley road (Plate 1). Traverse was N30W. This measured section represents a continuation of Stewart Valley section 9 and consists of thick beds of sandstone, paper shale, mudrock, and silicifled shale. This section while similar in composition to Stewart Valley sections 27, 31, and 33, has at the same time many differences which indicate it was in a more flucuating environment. This measured section represents a near-shore lacustrine environment changing into a moderately deep-water lacustrine environment before going back to a more near-shore lacustrine environment. Stewart Valley Stratigraphic Section 35 Stewart Valley stratigraphic section 35 is located some 5,000 ft (1,550 m) north of the junction of the Stewart Valley road and Stewart Springs road and some 650 ft (200 m) due east of the Stewart Valley road at the base °f a cliff known as Snail Cliff (Plate 1). Traverse was 50 N75E. This measured section has as its base a pebbly conglomerate that closely resembles the upper portion of a lahar found 650 ft ,200 m> up the canyon and may represent the base of the Stewart Valley lacustrine deposits. This section consists of a deep channel cut that grades laterally (east) into channelized sandstone and mudrock of Stewart Valley section 6. To the west the channel deposit becomes thin beds of silicified shale with trace fossils (burrows), all of which are filled with fossil ostracodes. Massive beds of fossil ostracodes also fill the channel. The top portion of the channel is mudrock and gypsiferous mudrock that contains small amounts of tufa, tufa attached to fossil wood, small stromatolites, and fossil logs. The logs are not usually upright and from the evidence of the tufa deposit, were rafted into place where they were exposed to warm temperature, carbonate water that facilitated the growth of the tufa. This measured section represents an interface between the fluvial and deltaic lacustrine environment along with a silty over-bank deposit of a channel. 51 DESCRIPTIONS OF MEASURED AND ILLUSTRATED STRATIGRAPHIC SECTIONS Preceeding each description of an illustrated stratigraphic section will be a description of its location and any special notes on the section. The description w i n include the bed number (BN) , starting at the bottom and going up section, the description, and bed thickness (BT) in meters with a total thickness listed at the end of the description. Stewart Valley Stratigraphic Section III Location: Stewart Valley stratigraphic section III was measured on Hill 5451, Granny Goose Well Quadrangle 7.5 minute sheet (1:24,000). It is located 1.2 km southwest of the junction of the Stewart Valley road and Omco Wash road (Plate 1 and 2). Traverse was N9W. Special Notes: None. BN Description BT(meters) 1 Blue ash (fresh surface) to darker blue-gray weathered surface. .335 2 Covered interval of brown mud. 1.342 3 Gray sandstone, very course sand grains (50% quartz, 20-30% feldspar, 20-30% lithics (mainly obsidian)), <15% matrix, angular to subrounded, considerable iron staining and coating on quartz (mainly) and other clasts, some gray to bluish-gray volcanoclasts, where it is the dominate clast the rock is bluish- gray color, where quartz is dominate clast color is reddish-orange. Top of the unit is coarse grained to medium grained clasts; 40-50% quartz, 40-50% feldspars, 10% lithoclasts (mainly obsidian), blue-gray tuffaceous clasts rare to missing, <15% matrix, iron staining, well sorted. .195 4 Covered interval of soft, brown gypsiferous mud. 1.817 5 Slope covered with crusty mud, coarse clasts (1-2 cm), weathers beneath surface to rusty-orange. 53 Subsurface sample fine sand clast (50-60% quartz, 20-30% feldspar, <10% lithics (mainly obsidian), <15% matrix, to medium sand clast; (60-70% quartz, 20-30% feldspar, 10% lithics, all subangular to rounded, high sphericity, whitish-gray to very light gray color, localized reddish-orange iron staining, moderately to well indurated, well sorted. 4.920 6 Very fine sand clast, >15% but <75% matrix (mainly ash), 85- 90% lithics (5% bluish-gray pumice, 5% obsidian), 5-10% quartz, extremely well indurated, iron stained on top, some throughout. .084 7 Covered interval of light brown, gypsiferous mud. 1.762 8 Coarse grained to fine grained sandstone, 50% quartz, 20% feldspars, 30% lithic clasts (33% obsidian, rest 54 other lithics), very angular to subrounded, low sphericity, light gray, whitish-gray to black with iron staining. ,017 9 Covered interval of coarse grained sand and mud, clasts l-2cm cover slope. .710 10 Fine to coarse grained sandstone (less than 5% very coarse grained), 40- 50% quartz, 20% feldspar, 30% lithic clasts (20% chert), 5% biotite, < 15% matrix, well indurated, iron stained, light gray to brownish gray, poorly sorted, very angular to sub- rounded, medium sphericity. .056 11 Covered interval of coarse grained sand and mud, clasts 1-2 cm cover slope. 1.565 12 Medium grained (5% coarse grained) mainly quartz, 10% lithic clasts (chert), >15% and <75% matrix, light gray to gray-brown with red-orange iron staining, subangular to rounded, medium to high sphericity, poor to medium induration. *note: beds 13-23 in vertical section. 13 Mudstone, gray-white to light brown, more grit in upper portion. 14 Mudstone, yellowish-white to gray- white, some grit and very fine sand, trace of biotite. 15 Tuffaceous coarse grained sandstone, 60% coarse grained sand (30% quartz, 30% olivine, 20-25% feldspar, 10% lithic clasts (50% obsidian, <5% biotite, 40% tuffaceous material (75% very fine grained tuff, rest equal parts blue-gray pumice and very coarse sand size lithics), cross-bedded, yellow-orange to reddish-orange iron staining, fairly well sorted, some fining upward gradded-bedding, angular to subrounded, medium sphericity, moderate induration. 56 16 Brown mudrock. .120 17 Pebbly, coarse grained sandsone to fine grained sand stones with coarse clasts, 2cm pebbles, lenses of scour and fill with fine sand and silt, very angular to subangular, 90% lithic clasts, 5% quartz, <5% biotite and feldspar, <15% matrix, 2cm pebbles are angular to subrounded dacite, outcrop has patchy iron staining, weathers white-gray, fresh surface greenish gray to white-gray. .710 18 Brown mudrock. .150 19 Pebbly sandstone to conglomerate on bottom gradding to coarse sandstone layer, then to medium and fine sandstones layers, cross-bedded, some scour and fill (silt), poorly indurated. .140 20 Medium sandstone, gray, some large pebble size clasts, poorly indurated. .200 21 Brown mud rock. .160 22 Conglomerate to pebbly, coarse grained sandstone, cross-bedded. 23 Extremely well indurated, calcareous cemented, dark red-brown weathered surface, white-gray fresh surface, base is fine grained sandstone, occasional coarse grained sand clast, occasional biotite, grades upward into coarse grained sandstone with ripples (10% quartz, 90% lithic clasts, <15% matrix), angular to subrounded and well sorted. Middle portion pebble size gravels (<5%), 60% coarse grained to very coarse grained sandstone, 40% medium to fine grained sandstone (10% quartz, 90% lithics), subangular to subrounded, poorly sorted. Top is 90% fine grained sandstone and 10% medium grained sandsstone, mainly lithics of obsidian. 24 Poorly sorted brown mudrock with coarse sand size clasts and 1cm 58 pebbles. .648 25 Very fine grained sand to silt size clasts to ash layer on top with medium to large size, fine grained sand balls and lumps, yellow-white, weathers red-orange, shows algal mat traces on top of ash layer. .249 26 Coarse grained to very coarse grained sandstone with 1cm volcanic pebbles, <5% quartz, >95% lithic clasts (mainly volcaniclastic), poorly sorted, whitish-gray to light brown with patchy red-orange iron staining. Pebbles are very angular to angular, low sphericity, cross-bedded and well indurated. .748 27 Extremely well indurated, calcareous cemented dark red-brown weathered bed similar in composition to bed 23. .080 28 Brown mudrock. .069 29 Extremely well indurated coarse 59 grained sandstone, red-brown weathered surface similar to beds 23 and 27. .069 30 Brown mudrock. .249 31 Medium size sand clast, cross- bedded with channel cuts, 6-7cm long mud lenses in bed with orange colored iron stained base. 1.247 32 Covered interval, 15 degree slope of easily eroded mudrock, some red shale with fossil ostracodes found half way up the slope but no outcrop was seen. 9.039 33 Light brown to whitish mud ash layer. .050 34 Biotitic white ash •25m blocky ash •55m thin-bedded with some channel cuts filled with blocky ash bed, thicker on bottom, thinner on top. • 75m Blocky ash with small shallow channels or slump 60 structures. 1.500 36 Covered interval of Pleisitocene soil and gravel. 14.750 TOTAL 44.461 m Stewart Valley Stratigraphic Section IV Location: Stewart Valiey stratigraphic section IV was located 3,900 ft (1,200 m) southwest of the junction of the Stewart Valley road and the Omco Wash road, starting in Fingerrock Wash. Traverse was vertical. Special notes: Nearly vertical (89 degrees) section with minor dip (<3 degrees) on beds (Plate 1 and 3). BN DESCRIPTION BT(meters) 1 Well indurated, medium to fine grained sandstone, dark red-brown. .010 2 Brown gypsiferous mudrock. .060 3 Biotitic white ash •5m Blocky gray-white ash •46m thin bedded ash, thicker beds on bottom, thinning upwards . 21m two white-gray chert and fine ash layers gray-white blocky ash layer with highly convoluted soft sediment deformation All ash outcrops have a pinkish-red to red—orange stain on them. 3.700 4 Brown mudrock, gypsiferous. .365 5 Very fine, gray to light brown, cross-bedded tuffs. 2.000 6 Fine to medium very dark reddish- brown weathered surface to light gray to light olive gray fresh surface (50% quart, 50% lithics) tuffaceous matrix, extremely well indurated. .500 7 Yellowish-gray to bluish -gray, medium sand clasts (80-90% quartz, 10- 20% lithics) layers of dark lithic clasts (obsidian), subangular to rounded, high sphericity, well sorted, poorly indurated, charred fossil wood. .500 8 Gray-white to white, well indurated tuff, some laminations and cross-bedding of very fine sand size to silt size obsidian. 1.200 9 Calcareous, very fine ash with glass shards, half bubbles, light grayish-white to white. 0.010 10 Brown gypsiferous mudrock and covered interval. 13.800 11 Brown resistant mudrock. .770 12 Calcareous very fine ash, weathers dark red-brown, light grayish-white to white fresh. .0300 13 Brown mud with remnants of red layers (see above, bed 12), large blocks and mud balls. .1500 14 Medium sand clasts, 50% quartz, 50% lithics (mainly, 90% obsidian), <5% matrix, weathers red-orange, yellow- gray, very light gray, yellow and reddish-orange fresh, two mud filled erosional unconformities seen, clasts are angular to subrounded, mediun-high sphericity. .100 15 Brown mud, poorly indurated. .150 16 Brown mudrock, well indurated. .150 17 Brown mudrock, poorly indurated. .100 18 Brown mudrock, well indurated. .100 19 Brown mudrock, poorly indurated. .050 20 Brown mudrock, well indurated. .230 21 Bottom portion is very fine ash 1 medium sand clast glass and quartz (10%), top has erosional surface with desiccation cracks, calcareous, whitish-gray to light gray. Top is medium to coarse sand clasts, calcareous, red-orange iron staining, 50% quartz, 50% lithics, nearly 100% obsidian. .270 22 Brown mudrock. .300 23 Light whitish-gray to white, calcareous tuff, 25% clasts (90% quartz, 10% lithics (obsidian)), some reddish-orange iron staining on weathered surfaces. .100 64 24 Brown mudrock. .800 25 Agglomerate cap. 2.000 26 Covered interval to top of the hill. 10.500 TOTAL 37.950 m Stewart Valley Stratigraphic Section V Location: in canyon behind white ash cliff (sv Section IV), some 800 ft (250 m) south of section IV. (Plate 1 and 3). Traverse was N80E. Special notes: None. Description BT(meters) 1 White to gray-white blocky ash. 1.510 2 Whitish-gray to yellowish-gray or brown ash with large pumice clasts, large amount of lithics (10-20%), very coarse to coarse sand clasts, remaining medium sand clasts, poor to moderate sorting, 20-30% lithics are quartz and feldspars, white chert or silicified volcaniclasts present, very angular to subrounded, extremely well indurated. .1259 65 3 Brown mudrock, covered interval. .600 ^ Yell°w-white on bottom to light bluish-gray top, bottom is fine ash, top is fine sand clasts, 80% quartz, 15% lithics (66% obsidian), 5% feldspar, well sorted, angular to subrounded, well indurated, channels and cross-bedding, some reddish-orange iron staining, definite layers of yellow limonitic clay. .913 5 Brown mudrock, covered interval. .440 6 Whitish-gray to dark reddish-brown with bluish-gray streaks in the lighter color, very fine sand clasts, mainly lithics, glass fragments, calcareous, well indurated, well sorted, several layers of ostracodes. .377 Brown mudrock, covered interval. 2.454 Whitish-yellow to yellow-brown, fine sand clasts (80% quartz, 20% lithics (50% obsidian)), calcareous, well sorted, subangular to subrounded, well indurated, some thin laminations. .188 9 Brown mudrock, covered interval. 1.385 10 Dark red brown to yellow-gray, very coarse sand clasts (90% lithics, 10% quartz), angular to subrounded, well sorted, well indurated, calcareous. .377 11 Brown mudrock, covered interval. 4.091 12 Light gray to whitish-gray with reddish-orange iron staining, coarse sand clasts, 80% quartz, 20% lithics, biotite, hornblende, well indurated, well sorted, angular to subrounded, normal bedding. .315 13 Brown mudrock, covered interval. 1.510 14 Whitish-gray, reddish-brown, reddish-orange very fine to fine sand clasts to volcanic tuff (multiple layers of colored sand and tuff), calcareous, extremely well indurated. .188 Brown mudrock, covered interval. 3.461 Light gray, yellowish-gray with 67 reddish-orange iron staining, very fine sand clasts and tuff, clasts mainly quartz, tuff silicified, erosional surface on top of unit, weathered surface shows differential cementation. .126 17 Brown mudrock, covered interval. .755 18 Yellow-orange to light gray silicified very fine tuff. .063 19 Blue vitric ash, with glass shards, weathers to powder blue, channel cuts, some interbedded cream- yellow ash, top is denser steel-gray, silicified tuff, current ripples (symmetrical) on top. 2.077 20 Reddish-orange vitric tuff. .094 21 Green-gray sandstone. .283 TOTAL 21.334 m Stewart Valley Stratigraphic Section VI. Location: In the canyon to the east of Snail Cliff, some 4,600 ft (1,400 m) north of the junction of the Stewart Valley road and the Stewart Springs road on the . 68 east side of the road, southern most canyon. Traverse was N5E. (Plate 1 and 4). special notes: This section is the lateral equivalent of the lebensspurren location described at Snail Cliff Stewart Valley section 34. BN Description BT(meters) 1 Covered interval measured from gully to first exposure. Base is either a lahar or erosional breccia/conglomerate from a lahar. This covered interval was partially trenched and found to contain interbedded blue-gray tuffaceous sand channel fills; brown mudrock; large fossil tree stumps and logs. Channel fills are 10% very fine to medium pebble lithic clasts, 10% very coarse sand lithic clasts, 80% coarse sand (10% quartz, 90% lithics (50% obsidian)), pale blue-gray clay coated color when dry to earth brown when wet, well indurated, not calcareous, no ’ 69 fossils, grades upwards into black to dark brown, well indurated,non- calcareous, coarse sand clasts (10% quartz, 90% lithics (50% obsidian) to fine sand clasts, all subrounded to rounded, spherical, near top is a white to whitish gray very fine volcanic ash, well indurated; medium to coarse sand with fine sand clasts of obsidian, well indurated, white to whitish-gray with 3mm clasts of pumice. 7.52 2 White-gray mudrock with yellow limonitic staining, numerous pebbles ( in numbers, near top the pebbles are of volcanic ash and pumice, pebbles appear to be in distinct beds, mudrock has cracks and joints parallel to bedding. 400 3 Fine grained sand clasts (high % lithics, mainly obsidian and biotite), some coarse sand clasts (lithics). .100 70 4 Gray-white mudrock with algal mud balls. .200 5 Massive light brown, blocky mudrock, at base some fine grained sand filled channel cuts. .500 6 Dark red-brown, highly siliceous, (nearly completely replaced with silics near contact with dacite) to calcareous, clastic, with iron staining and silica cementation. Bottom is fine ash matrix supported pebbles (100% lithics), subrounded to well rounded, iron-silica cemented, iron stained dark orange-red, very coarse sand clast layered of lithics (dacite), channel cuts; central portion is siliceous, whitish-gray to dark brown, yellow- brown, coarse to very coarse sand lithics, bottom 40-50% obsidian and ash; top of unit similar to first part described above with large pebble conglomerate bed. .800 71 7 Whitish-gray to dark reddish-brown to greenish-gray, well indurated, ostracodal, fish bone, fossil wood debris bearing, very coarse, medium to coarse sand clasts coated with calcium carbonate, rock calcareous and silicified in part, <15% matrix, subrounded to well rounded, 10% quartz, 90% Ixthics on bottom. Top is medium to coarse sand clasts (100% lithics) to 50% bioclasts (ostracodes) and 50% obsidian, well indurated, well sorted, subrounded to rounded, light gray to whitish-gray, reddish-brown, light brown, black when fresh; iron stained and iron coated grains. .100 Top contains abundant fish bones, large pieces of fossil wood, great deal of lateral variation, silification increases towards contact with andesite, color becomes darker to black, slump blocks seen in the unit and stromatolite-like structures also found. .300 TOTAL 10.22 m Stewart Valley Stratigraphic Section VII Location: Outcrop behind Snail cliff, SOme 4,500 ft (1.400 .) north of the junction of the Stewart Valley road and Stewart Springs road. Traverse was N70E. (Plate 1 and 4) . Special notes: Outcrop contains fossil molluscs, fish vertrebrae, and mammals. BN Description BT(meters) 1 Covered interval from top of bed #8, Stewart Valley Stratigraphic section VI. 7.98 2 Brown mudrock gradding upward into sandstone, numerous channel cuts and fill, cross-bedding, blue-gray sandstone, some channels with reverse gradding, black organic matter, wood debris present; blue -gray weathered, dark brown when wet, fine to medium sand clasts (50% quartz, 50% lithics (30-40% obsidian)}, some lithics are lumps of volcanic ash and lithic clasts, blue-gray color is clay coating on grains, >15% <75% matrix, subrounded to rounded, spherical to discoidal bottom unit; top is blue-gray weathered, light to dark brown fresh, >15% <75% matrix, fine ash diagenesis to blue clay, medium to coarse sand clasts, <5% very coarse sand; (10- 20% quartz, 80-90% lithics (20% obsidian)), subangular to rounded, spheroidal to discoidal, well indurated. 3 Whitish-gray to bluish-gray fine sand clasts (10% quartz, 90% lithics (20% obsidian), >15% <75% fine ash matrix, rounded to well rounded, spheroidal to discoidal, well indurated. 74 4 Whitish-gray to bluish-gray very fine to medium pebbles (mainly fine pebbles, 100% lithics includes pumice), >15% <75% fine ash matrix to coarse sand to very coarse sand (10% quartz, 90% lithics (20% obsidian) in >15% <75% matrix of fine ash to clay, fine ash and obsidian on top. .065 5 Whitish-brown to dark red-brown, fine sand clasts (obsidian and other lithics), coarse sand clasts (100% lithics), very coarse clasts (100% lithics), >15% <75% matrix of fine ash, well indurated. .010 Whitish-gray to light brown, well indurated, mud ash, layers of pumice, occasional pebble size lithic clast to very fine ash and lithic clast (20% obsidian) includes blocks of whitish- gray to light brown, well indurated, calcareous, calcite cemented, coarse to very coarse sand clasts, some fine 75 pebbles (10%), few medium pebbles; >15% <75% matrix, (10% quartz, 90% lithics (andesite pebbles), angular to subrounded, spheroidal to discoidal. .040 7 Repeats mud ash layer of bed #6. .110 8 Light brown, brownish-gray, light bluish-gray, calcareous, medium to coarse sand clasts (100% lithics, 20- 30% obsidian) some mica, >15% <75% fine mud ash matrix, angular to subrounded, sub-prismoidal to sub-discoidal. 035 Lt. brown mudstone, lower portion poorly indurated, top well indurated. ,070 10 Whitish-gray to light brown, very gritty, laminated, some plant debris, very fine sand clasts (90% quartz, 10% lithics), >15% <75% mud matrix; very dark gray to blackish-brown, silicified, dark reddish-brown weathered surface contains 10-20% medium to coarse sand clasts, very angular to subangular, prismoidal to 76 spherical, some burnt plant debris in channel cuts and fill. .015 11 Brown mudrock. .020 12 Whitish-gray, dark gray to dark brown, silicified, fine sand clasts, >15% <75% mud matrix, few large black plant remains, numerous small pieces plant material, well indurated shale. .020 13 Brown mudstone, some channel cuts and fill. .070 14 Dark reddish-brown, iron stained weathered surface, light brown to grayish-brown fresh, highly calcareous mud with some very fine sand clasts (5%), some fossil mollusca, well indurated. .010 15 Brown mudrock with some coarse sand clasts and great deal of molluscan debris. .020 Gray to gray-brown, reddish-orange iron staining, medium to coarse pebbles (10%), very fine to fine pebbles (80%), 77 medium to coarse sand clasts (10%) all lithics, fossil molluscs. <15% matrix of fine ash and clay, few subangular, subrounded to rounded, moderate sorting, not well indurated, coarse pebbles on erosional surface, fining upward sequence and then layer of coarse pebbles again. .020 17 Brown mudrock with 5mm clasts and molluscan shell fragments on the bottom. .050 18 Intermixed mudrock and coarse sand clasts in channel cuts, medium gray to dark gray, gray-brown, some reddish orange iron staining, calcareous, abundant fossil molluscan debris, >15% <75% mud matrix, 20% fine pebbles (100% lithics), subrounded to rounded, spherical, very coarse sand (100% lithics), subangular to rounded, spherical to prismoidal, moderate to good induration. .040 78 19 Gray-brown, light brown, calcareous, 5% fine pebbles (lithics), 10% very coarse sand (100% lithics), very fine to fine sand clasts (lo% quartz, 10% feldspar, 80% lithics (10% obsidian), very angular to subrounded, prismoidal to spherical, <15% mu(j matrix, well indurated. .010 20 Grayish-brown to light brown, very calcareous, abundant fossil molluscan material, very coarse sand to coarcoarse sand clasts (10-20% quartz, 80-90% lithics), >15% <75% mud matrix, subangular to subrounded, well indurated on bottom; whitish-gray to light brown, calcareous, abundant fossil molluscan material, molluscs filled with mud; medium to very coarse sand clasts, >15% <75% mud matrix grades upward into abundant fossil molluscan material, matrix supported, coarse sand clasts (100% lithics), calcareous and very well indurated. 21 Gray-brown to light brown, very coarse sand clasts (5 %) , very fine pebbles «5%) , coarse sand to medium sand clasts (10% quartz, 90% lithics (high % obsidian), >15% <75% matrix of fine ash and mud, very calcareous, well indurated, some reddish-orange iron staining, some fossil molluscan debris, fossil fish bones, subrounded pebbles, subrounded to rounded sand clasts, spherical. 23 Light brown to pinkish-white (molluscan debris), abundant fossil molluscan debris, calcareous, >75% mud, some grit, claystone. 24 Grayish-white to light golden brown, 95% molluscan debris and 5% very coarse sand clasts (lithics), extremely calcareous, extremely well indurated, silicified layers to well silicified layer s, grades upward into 80 coarse to very coarse sand clasts (20% quartz, 20% feldspars, 60% lithics), subrounded to rounded. Top is whitish-gray to light brown, calcareous, extremely well indurated, abundant fossil molluscan debris, molluscs filled with sand grains (coarse grained sand clasts (20% quartz, 20% feldspar, 60% lithics (10% obsidian)), subangular to rounded, spheroidal. .025 25 Alluvium mainly from near by volcanics. 10.00 TOTAL 20.325 STEWART VALLEY STRATIGRAPHIC SECTION XI LOCATION. Stewart Valley section was located half way between the junction of the Stewart Valley road and the Omco wash road and the junction of the Stewart Valley road Stewart Springs road, on the west side of the road at brown gypsiferous mud hill (Plate 1 and 5) . Traverse was N85W. Special Notes: None. BN Description BT(meters) 1 Yellowish-brown to light brown, blocky, diatomaceous claystone, moderately indurated, contains small pieces of woody material. 1.075 2 Vitric blue ash. .107 3 Light brown, blocky, diatomaceous claystone, moderately indurated, contains some plant debris, some burnt material. 2.11 4 Light brown, thinly laminated, diatomaceous claystone, moderately indurated, numerous layers of plant debris, some burnt material. 2.150 White-whitish gray weathered to brown (unweathered), blocky, mud rock, fossil leaves and plant debris, some burnt material. 824 6 Blue to blue-gray fine vitric ash, Moderately well indurated. some reddish-orange iron staining. .072 7 Light brown to brown, blocky mudstone. .394 8 Yellowish-orange weathered to greenish-blue when wet, fine vitric tuff, moderately well indurated with gypsiferous material. .036 9 Thinly bedded to laminated mud rock with fossil plant material. .932 10 Light brown (fresh) to white (weathered), blocky diatomaceous claystone, some plant debris, some burnt material. .538 11 Light to medium brown diatomaceous claystone. 5.527 12 Light brown, blocky, claystone with Plant debris, some reddish-orange iron staining. .436 13 Brown mud rock. 1.212 14 Brown, blocky mudstone, highly limonitic. .242 15 Light brown, blocky claystone with 83 plant debris, some reddish-yellow iron staining. .436 16 Reddish-orange to light brown, calcareous, blocky claystone with plant debris. .097 17 Very light brown to gray-brown, blocky claystone with plant debris and seeds. .073 18 Brown blocky mudrock. .300 19 White to whitish-gray, blocky mudstone. .100 20 Brown gritty mudstone. .080 21 Gray to blue vitric ash with reddish-orange iron staining on top and bottom. .030 22 Brown, bedded mudrock. 1.090 23 Blue-gray five vitric ash. .010 24 Brown, blocky mudrock. .090 25 Light gray vitric ash. .010 26 White to whitish gray claystone. .090 27 Whitish-gray to light brown, locky, claystone with algal traces. .100 2288 Light brown claystone. 1.700 29 Whitish-gray five vitric ash. .100 30 Light brown claystone. .800 31 Reddish-brown to brownish red, shale, well indurated. .200 32 Light brown claystone to mudstone. .800 33 Light brown mudrock. 2.431 34 Whitish-gray to yellow-gray laminated siltstone to non-laminated siltstone with flat lying eliptical concretions, calcareous. .156 35 Laminated to bedded brown mudrock. .313 36 Light blue-gray vitric ash with some reddish-orange iron staining. .080 TOTAL 24.75 m Stewart Valley Stratigraphic Section XII Location: Stewart Valley section 12 is the upper Portion of Stewart Valley section 11 (Plate 1 and 5) . Traverse was N42E. Special notes: This section begins where Section XI 85 leaves off. This hill was tranche, by T . P . Lugaski M d A. Lugaski in order to obtain the stratigraphic information. BN Description BT(meters) 1 Whitish-gray, light gray to light brown, claystone to clayshale, some laminations, poorly indurated, some burnt plant debris. 4.603 2 Darker brown gypsiferous, claystone to interbedded claystone and mudstone. . 414 3 Gray—brown to medium brown claystone to clayshale, small mud balls and lumps, some reddish-orange iron staining, top and bottom have no grit but dark gray interlayers have grit. .218 4 Brown claystone to clayshale, lapilii near top, laminated, heavy in comparison to other layers. .817 5 Light gray to light brown, medium to coarse sand clasts (70% lithics, 30% quartz, 5% biotite), clay balls (1- 86 2cm), dark gray and very gritty, burnt plant debris, fossil plant remains, reddish-orange iron staining. .436 6 Light gray to light brown siltstone to claystone with some reddish-brown iron staining, organic material in matrix, very gritty to non-gritty, small 7 Light brown to yellowish-brown claystone, white l-2mm lapilli near top, some grit, well indurated, dark brown mud balls 1—3cm with no grit, desiccation cracks toward top. .273 8 Light brown to yellow-brown, no grit, claystone, wavy laminations, 1mm mud lumps on bottom. .381 Light brown, light gray to reddish brown, mudrock, some plant debris. .272 10 Yellow, red, to light whitish-brown (no grit, very bitter; very gritty; laminated, little grit), partially silicified, thin lapilli and ash layer 87 on top. .327 11 Yellowish-gray to white fine ash, <5% biotite, no other clasts, poorly indurated, faint bedding, interbedded with light brown clay, top partially silicified, fine scale ripples. .436 12 Light brown claystone to clayshale, blocky to wavy laminations. .490 13 Yellowish-gray to white fine ash, <5% biotite, no other clasts. .109 14 Light brown claystone to mudrock, thin layer of fine sand clasts. 3.70 15 Sandstone and a few stromatolites. .080 TOTAL 13.877 m STEWART VALLEY STRATIGRAPHIC SECTION XXVII Location: Stewart Valley section 27 was located in Pacific Union Canyon on the west side (for location see Stewart Valley Stratigraphic Section 33 (Plate 1 and 6; Figure 7 and 8) Special notes: Lower portion of the section was trenched by T. P. Lugaski and L. A. Lugaski but the top portion was not trenched. BN Description BT(meters) 1 Covered interval, bottom of wash. .700 2 Light brown laminated paper shale. .100 3 Dark brown to black or blue-gray bottom to yellow top, bottom is very fine sand clasts (60% quartz, 40% lithics (60%ash, 40% obsidian), top is zeolitized ash, 1cm of gypsum, middle is silicified shale, well indurated. .100 4 Dark brown, blocky, gritty, some laminations, mudrock to mudshale, fossil plants and fish bones. .100 5 Whitish-gray to light bluish-gray very fine sand clasts, lithics of ash (mainly) and obsidian, layered, well indurated. .050 Light brown to dark brown mudrock. .100 Black gradding into dark brown silicified shale. .250 Brown, poorly indurated but laminated mudshale of various types. .600 9 Brown silicified, laminated shale. 300 10 Brown laminated mudshale, barely indurated. .100 11 Brown, silicified, laminated shale. <100 12 Brown, laminated mudshale. 050 13 Dark brown, laminated silicified shale. , .100 14 Gray-white, silicified, blocky mudstone. .050 15 Brown, layered, some laminated and papery mudstone to mudshale, layers of fossil wood debris, some burnt material. 100 16 Brown mudrock with fossil wood debris. .i50 17 Brown silicified, laminated shale. .150 18 Blue-gray to brown mudrock with yellow (bottom) layer of gypsum. .020 19 Brown, silicified, laminated shale. .080 20 Brown mudrock with gypsum. .030 21 Brown, silicified, laminated shale. .090 22 Light brown to gray, gritty 90 mudstone to siltstone. .030 23 Brown, silicified, laminated shale. .170 24 Brown mudrock, gypsum on bottom. .030 25 Brown, silicified, laminated shale. .130 26 Whitish to whitish-gray, gritty, tuffaceous, moderately indurated mudstone. .040 27 Brown, silicified, laminated shale. .460 28 Light brown, gritty mudstone to siltstone. .040 29 Brown, silicified, laminated shale. .060 30 White ash with blue—to-blue-gray tuff. .030 31 Brown, laminated mudshale to clayshale. .020 32 Brown, silicified, laminated shale. .090 33 Brown, silicified, cross-bedded with channel cut and fill, very gritty, fine sand, yellow-white mudstone. .230 Debris flow material. 34 Brown, silicified, laminated shale. .650 35 Whitish-yellow, gray ash. .100 36 Brown, silicified, laminated shale. 150 37 White ash with yellow (sulphurous) colored bottom. .010 38 Dark brown, laminated mudshale. 060 39 Yellow ash. 40 Dark brown, laminated mudshale. >050 41 White, blocky ash. 050 42 Dark brown, laminated mudshale with fossil plant and fish remains. .200 43 Brown, silicified, cross-bedded with channel cut and fill, very gritty, fine sand, top includes a layer of calcite and ash. Debris flow material. .200 44 Brown, laminated mud to silicified, laminated shale. 1.150 45 Light gray to grayish-white to light brown paper shale. .282 46 White ash. .047 47 Light gray to grayish-white to light brown paper shale. 16.150 48 Yellow to red-brown, calcareous claystone with fossil fish bones. .469 92 49 Light gray to grayish-white to light brown paper shale. 13.802 50 Covered interval, brown mudrock. 9.389 TOTAL 43.440 m 94 Figure 8. This illustration graphically displays the variation in lithology and bed thickness found in Stewart Valley Stratigraphic Section 27 from Pacific Union Canyon. Note the cyclic alteration of silicified shale with mud rock over time and the periodic intrusion of volcanics. Towards the top of the section paper shale and diatomaceous mud rock are found. m D 4.0 .39r 9 3 9. L Stratigraphic Up w F P L IC ION STRAT IC ION VII X X N O TI C E S C HI P A R G TI A R T S N O Y N A C N O NI U C FI I C A P a C - - - TH IN S R E T E M N I S S E N K C HI T D E B f L w Ff. . 5 5 . 1 f w Ff, 0 Cx C S 5 . Sl iied le i tex ra/co r s e g n a h c or ol c al/ ur xt e t h wit e al h S d e cifi Sili f = ls i c -bedd g n di d e b s- s o cr h wit s el n n a h C = x C = lcareous u o e r a c al C = a C = ia m d mu s u o e c a m o at Di = m D = iicf sha e al h s d d o e o w cifi = Sili w d F e = h at s n =fi S mi Ft a s L sil s = o F L = F = lcan c ni a c ol V = V = sha e al h s r e p a P = P r ina Sha wth tura/co changes e g n a h c r o ol c al/ r u xt e t h wit e al h S d e at n mi a L er/ p a P Rock i tex ra/co r s e g n a h c or ol c al/ ur xt e t h wit k c o R d u M lcan Rock l ash r il pilli a l or h s k c a o R y all u c s u sti a k c Cl o R c ni a c ol V .6 2. .0 3. .6 3. 5 9 6 d 5 1 6. 1 f 38 P 13.80 .0 4. 96 X-RAY DIFFRACTION ANALYSIS In the examination of the lacustrine deposits in Stewart Valley, especially the silicified and paper shales, which contain a variety of well preserved fossils, and a review of the literature on the formation of silicified and paper shale, it became apparent that some x-ray diffraction analysis of selected samples was needed. This was not an exhaustive sampling but rather a random sample picked to offer an insite into the origin of the silicified shales and paper shales. Some nine samples were chosen: three from Pacific Union Canyon, site of Stewart Valley section 27, from silicified shales; one sample from northeast Savage Canyon, site of Stewart Valley section 26, in the Paper shales that contain fossil fish; one sample from Savage Canyon west of the site of Stewart Valley section 26, from a silicified to paper shale that contained Catalina Ironwood and fossil insects; one sample from the site of Stewart Valley section 32 (across from Tedford's Pocket) in the fossil fish bed; one sample from the site of tewart Valley section 31 that contained fossil insects; ne sample from the older Fingerrock flora paper shale sediments; and one sample from the diatomaceous claystone 97 at the site of Stewart Valley section 11. Figure 9 shows the x-ray diffraction pattern that was found at all sites with some sites having other minerals present in sufficient quantities (Table 1) to show up. Pyrite was absent from all of the x-ray diffraction patterns, even though it can be seen in thin section of the Pacific Union Canyon samples, indicating its presence is <5% of the total volume. SEDIMENTARY STRUCTURES The sedimentary structures found in the Miocene, non- marine beds of Stewart Valley are listed in Table 2 by measured stratigraphic section number. These sedimentary structures were used in determining facies relationships and depositional environments of the beds in Stewart Valley. 98 Figure 9. The common x-ray diffraction pattern found during the analysis of shales from Stewart Valley, Nevada. 9 9 X-RAY DIFFRACTION ANALYSIS OF SILICIFIED SHALES PAPPo oua, , APER SHALES, DIATOMACEOUS SHALES, and CHERTS in STEWART VALLEY, NEVADA. QC < CL N CO h~ G DC -J < UJ I) LL O 100 Table 1. Results of the x-ray diffraction analysis on selected rocks from Stewart Valley, Nevada. SAMPLE LOCATION ROCK TYPE MINERALS PRESENT DEPOSITIONAL ENVIRONMENT 001 Pacific Union Canyon, Stewart Silicified Shale Cristobalite* Deep-water Lacustrine Valley Section 27 Quartz 002 Pacific Union Canyon, Stewart Silicified Shale Cristobalite* Deep-water Lacustrine Valley Section 27 Quartz Feldspar 003 Pacific Union Canyon, Stewart Silicified Shale Cristobalite* Deep-water Lacustrine Valley Section 27 Quartz Feldspar 004 Savage Canyon, Stewart Valley Paper Shale Cristobalite* Deep-water to near-shore Section 26 Quartz Lacustrine Feldspar* Amphibole 005 Savage Canyon, due west of Paper Shale Cristobalite* Deep-water to near-shaore Stewart Valley Section 26 Quartz Lacustrine Feldspar 006 Stewart Valley Section 32 Silicified Shale Cristobalite* Deep-water lacustrine (Across from Tedford's Quartz pocket) Feldspar Montmorillonite 007 Stewart Valley Section 31 Paper Shale to Cristobalite* Deep-water lacustrine (Canyon west of Fingerrock Silicified Shale Quartz Wash) Feldspar 1 0 1 TABLE • Con't. 008 Fingerrock Flora site Paper Shale Cristobalite* Paludal Quartz 009 Stewart Valley Section 11 Diatomaceous Base of Brown Top Hill) Cristobalite* Deep-water to near-shore Claystone Quartz Lacustrine Feldspar indicates abundant mineral, other minerals in much lesser amounts. 10 2 TABLE 2. A listing of the sedimentary structures found in the Stewart Valley beds by stratigraphic section. SEDIMENTARY STRUCTURES STEWART VALLEY STRATIGRAPHIC SECTIONS lllllllllli t n -j t Bedding surface structures Ripples X X X X X X Shrinkage cracks Desiccation cracks XX XX X X X X Synaeresis cracks X X Rain and hail pits X Bedding undersurface structures Load casts X X Scour and fill X X X X X X X X X X X X X X X X X Small scale X X X X X X X X X X X X Large scale X X X X X X X X X X Gutter casts Rill, harrow marks X Flame structure III. Internal sedimentary structures Normal bedding xxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxx Massive bedding X X XXX X X X XX X Graded bedding x X X X X X X X X X X X 103 w ^ 't in U3 N Table 2. Con't. ’ * 1 1 ’ I <—1 1—I 1—I sassssaiss&sasssRss 11111111111 111111 iiiuiiiiim m n Normal x X X X X X X X X Reversed X X X X X X X X Cross-bedding X X X X X x X X X X X X X X Convoluted bedding, laminations X X X X Laminations X X X X X X X X X X X X X X X X X X X X X X X X Slump bedding X Nodules, concretions X X X X X Conglomerate XXX X Breccia X X Armoured mud and clay balls X Mud and clay balls xx X XX X Hardgrounds x x x XXX XX Pyrite X Evaporite, gypsum X X x xxx Geopetal structure X r- Pyroclastic material Lapilli X X X X X Pumice x x x x Base surge X X X X X X X X Lahar X X 104 Table 2. Con't. iiiiiiiiiiiiiii i Subaerial to sublacustrine 1 111111111111111111 fallout tephra x x x x x XXXXXXXXX XX XXXXXX X X X X Volcaniclastic sediments x X X X XXXXXX X XXXXXX xxxxx V. Unconformities Non-conformity- X X Angular conformity X X Disconformity XX X XXX xxxxxxxxxxxxxx xxxxxxxxx Paraconformity X X X XX x X X X x xxxxxxxxx VI. Biogenic structures Plant root traces X X Algal structures X X X X Burrows XX X X Organism tracks and trails X VII. Organosedimentary structures Tufa XXX Stromatolites X X X Oncolites ? X Oolites VIII. Fossils Plants Diatomaceous material XXX X XXX X 105 Table . Con't. iJ-iiiiiiiiiiiiiiiiiiillllllllllllll Whole material XXX X xx X X X X Debris XX X X X X X X xxxxxx X xxxx Burnt XXX X XX Fish Cyprinid X X Salmonid X X X Debris X X X X X X X X xxxx Ostracodes X X X X X X Molluscs xxxx X XX XX Abundant X X XX XX Occasional X X X X X X X X Insects X X ? X Other X XX XX X x = present ? = may be present 106 DISCUSSION In the discussion of the results of this investigation major areas need to be explored. The first area to be discussed is the topic of paleolimnology/limnology, where a discussion of past and present lakes is needed in order to determine what physical and chemical characteristics of the paleolake in Stewart Valley were like and how this lake could have evolved. Based upon an examination of the literature and actual observations on the ground, Lake Savage most likely began its existance in a linear trough that roughly paralleled the present day valley and ran parallel to the Cedar Mountains and the Gabbs Valley Range. Pre-Lake Savage deposits appear to be more paludal in nature and perhaps a change in precipitation, evaporation or both occurred during the Middle Miocene that coalesced these paluadal areas into a larger more coherent body of water. Vail, Mitchum and Thompson (1977) in their paper on global cycles of sea level changes report an increase in relative sea level during the Middle Miocene (17-19 m.y. B.P.). Glaciers and glaciation are the only well understood mechanism for relatively rapid sea level change (although 107 tectonic mechanisms are becoming better understood) and since early and late Miocene glaciation is known there is most likely a correlation. Haq and Lohmann (1976) have also shown that low sea level stands generally represent climatically cool conditions and highstands represent climatically warm conditions. Low sea level stands were also reported by (Vail et al.f 1977) at about 13 m.y.B.P. and at 11 m.y. B.P. Ekren et al. (1980) report the Oligocene tuffs seen in the Gabbs Valley Range appear to be deposited in a pre-Oligocene trough that ran N65W-S65E, which is roughly equivalent to the direction of the northern end of Stewart Valley and similar to the trough that existed during the deposition of the biotitic white tuff at 10.4 m.y.B.P. The Oligiocene (pre-volcanics) erosional surface was most likely the result of a rift formed during the Mesozoic, now known as the Walker Lane (Stewart, 1980; Ekren et al., 1980). This topographic low would then become the ideal catchment basin for surrounding run-off. In his classification of alluvial basins, Miall (1981) proposed twelve (12) major plate tectonic settings in which the non-marine-paralic depositional environments are 108 subdivided into nine catagories: 1) lacustrine, 2) alluvial fan, 3} low sinuosity fluvial, 4) high sinuosity fluvial, 5) river, dominated deltaic, 6) wave-dominated deltaic, 7) tide dominated deltaic, 8) non-deltaic coastal (beach and barrier system), and 9) estuarine. From cross comparing this data set he (Miall, 1981) define nine (9) major basin- fill models. Two of these models concern us here, the first (model 1) is composed of transverse fan and braided plain system with discharge inadequate to develop a major delta system. Rivers terminate in a shallow distributary network or as sheet-floods on a lake margin or tidal flat. This type of basin fill model is typical of intermontane basins, especially graben basins of the southwestern United States. The second type (model 6) involves an ephemeral river terminating along a playa lake margin or permanent lake. Sediment load is inadequate to form a major delta. This type of system is also common in the intermontane basins of California and Nevada that have rivers that terminate in the permanent or playa lake. Such as the Truckee River and Pyramid Lake, Walker River and Walker / Lake, and the Carson River and Carson Lakes. Tectonic settings in which these two types of basin-fill models occur includes pre-drift rift grabens, intermontane, successor basins and pull-apart and fault-flanked depression basins (Miall, 1981). The sediments formed in Lake Savage come closest to matching the pre—drift rift graben concept in that the sediments are entirely non-marine, consist of alluvial fan and other fluvial deposits, and lake sediments that are interbedded with abundant volcanic flows (Miall, 1981). This is, however, where the similarity ends because the rift system in Stewart Valley did not continue to develop and the structural low did not continue any major growth in depth and breath. The next step in the evolutionary development of the Lake Savage basin should have followed Late Tertiary (N65W-S65E) and Quaternary (east-west) extension tectonics (Zoback et al., 1978; Stewart, 1980). If this had happened the sedimentary basin would have continue to grow in depth and breath, resulting in more in- filling and a greater total stratigraphic thickness. A similar evolution of such a basin can be seen in the Wassuk Group (Goilia and Stewart, 1984), the Esmeralda formation (Robinson et al., 1969) and the Siebert Tuff (Bonham and Garside, 1979) . The coarse clastic nature of much of the sediments found in these groups can be contrasted with the very fine-grained Lake Savage lacustrine sediments. This general lack of coarse grained elastics in the Stewart Valley lacustrine sediments is an indication of general tectonic quietesence. While fluctuations can be seen in clastic grain size changes in the near-shore sediments, which reflect oscillation of lake levels; the deep water clastic particles size remains extremely fine, indicating little change in the tectonic regime until quite late in the lake cycle. There are no known intraformational faults within the Lake Savage lacustrine or fluvial units and these units appear to be continuous across Stewart Valley, only interrupted by range-front fault scarps. A single large fold can be found in the Lake Savage fluvial units below the biotitic white ash. This fold is completely intraformational and may have resulted from sliding or slumping of these unconsolidated units in place before lithification. The beds above and below the fold appear to be unaffected. Some folding is known and can be observed in the southern end of the study area where a number of small synclines and anticlines are also observed. These folds, synclines and anticlines occur only in the Miocene lake sediments and do not incorporate the over-lying Plio- Plexstocene gravels, indicating a period of warping after the lake sediment formation (17-10 m.y. B.P.) and prior to the deposition of the gravels. Although little off-set (strike-slip) of the Stewart Valley formation units can be seen in the study area, several miles to the south some post-Lake Savage volcanic flows are right-laterally displaced 100-200 m (328-656 ft). The lack of such structures in the lake sediments indicates a period of quiescence followed by a brief period of right-lateral movement. Numerous dip-slip and some oblique-slip faults of Basin and Range type can be seen in the study area. These faults as with the folds become more numerous and intense as one approaches the are of right-lateral offset proposed for the Walker lane in the southern end of Stewart Valley. This basically agrees with Albers (1967) who Postulated the formation of the Walker Lane with its major bending and right-lateral strike-slip faulting begining as long ago as the Jurassic and being completed by early to middle Miocene time with the more recent movement restricted to Basin and Range dip-slip faulting. 112 Following the demise of Lake Savage a period of erosion and alluvial deposition followed, at some point a Quaternary basaltic lave flow covered part of the area and would become interbedded with the gravels. This older series of gravels appear to be cover in several places by a series of younger gravels and alluvium. At some point during the Quaternary a major reactivation of the Stewart valley drainage occurred which has truncated the alluvial fan development with a new drainage system. The older terrace can prominately be seen along the Gabbs Valley Range side of Stewart Valley. This terrace was first described by Buwalda (1912) and should hence forth be known as the Buwalda terrace. The new drainage system is 180 degrees off the system that formed Lake Savage. The Miocene drainage came from the north, northwest and northeast to flow southward parallel to the Walker lane rift structure into Lake Savage. The new drainage pattern flows north and northwest from the surrounding structural highs of the Cedar Mountains, Gabbs Valley Range and the south end of Stewart Valley. The structural high at the south end of Stewart Valley coincides with the Walker Lane fault zone. In addition while Stewart Valley has very little typical Basin and Range valley fill, the Monte Cristo valley on the other side of the Walker lane structural high is more typical Basin and Range in nature. This valley contains several thousand feet of valley fill (from well-log information), a playa, and is rimmed with the Miocene lake sediments that are fault blocked upwards along the ranges. Ancient and Modern Lake Comparisons There are a number of well documented examples of Tertiary, Quaternary and recent lakes to which the sediments of Lake Savage can be compared. The best documented from the stand point of number of papers and number of researchers is the Eocene Green River formation of Wyoming, Colorado and Utah. The next best documented Cenozoic lake sediments include but are not limited to the Oligocene Florissant deposits, Oligocene sediments of British Columbia, a Pleistocene lake in Texas, the Lake Lahontan system in Nevada, Searles Lake , California, and modern lakes such as Pyramid and Walker Lakes, Nevada, Deep Springs Lake, California and Green Lake, New York. Green River Formation 114 Although the Eocene Green River formation beds were known for some time, Bradley (1929a, 1929b) was the first to do an in-depth study of the sediments and propose a model for their origin. Basically, the Green River fm. was contained in two intermontane basins, one in Wyoming and the other in Colorado and Utah. Both basins contained fluvial and lacustrine components that deposited sediments in the form of marlstone, oil shale, fine-grained sandstones and varved sediments. Bradley (1929a) first proposed a thermally stratified lake with a depth of 75-100 ft (23-30 m). Bradley (1929a) could distinguish three cycles of deposition in the lakes other than the varve cycle. Some of the cycles contained salt mold indicating an evaporative phase in the lake cycle. Large portions of the Green River fm. went unstudied after Bradley’s (1929a, 1929b) pioneering efforts. Picard and High (1972) described rapid facies changes in Green River fm. lithology from fluvial to "deep" water lacustrine units in the Parachute Creek Member, Unita Basin, Utah. Eugster and Surdam (1973) proposed a new model for the depositional environment of the Wilkins Peak and Tipton Members of the Green River fm. in Lake Gosiute of Wyoming. 115 The lake was fringed by a large playa flat where alkaline brines evolved through evaporation and precipitation. Dolomitic mudstones, marlstones, and calcareous and siliciclastic sandstones were the products of occasional floods on the playas. Organic material (algae) and dolomite were formed on the playa and then carried off into the lake where it was deposited along with other material. This model differs greatly from the stratified lake model suggested by Bradley. Bradley (1973) points out that the Wilkins Peak Member of the Green River fm. differs from most of the other Green River fm. oil shale in that it has very few varved sediments and numerous mud cracks and local desiccation breccias, unlike the rest of the oil shale members. The algae indicate that sediments were formed in a very shallow, spring fed lake made up of a flocculent ooze rather than water. He also reported that in tropical shallow lakes (1-10 m (3-32 ft)) the algae form a living layer of ooze which is readily penetrated by fish, aquatic insect larvae and aquatic worms. Aquatic larvae of the midge flies (Chironomidae) live in and on the ooze 116 converting much of it into fecal pellets. The prevention of decay of this organic mass can be accomplished in only a few ways, one of which is to dry it out (heat fixation) which preserves the algae and insects. Such an environment can be found in a playa lake environment where rapid water level fluxuations occur as indicated by mud cracks and brecciation. He also suggested that thin, regular microlamination can occur only in the quite hypolimnion of lakes whose waters are permanently stratified and the bottom depleted of oxygen. In most lakes abundant numbers of many kinds of animals (Chironomidae, Oligochaeta, etc.) constantly churn-up the bottom and de—stratify it. Consequently, in only hostile (non-oxygenated) environments can non-glacial varves be formed. This would make this portion of the Wilkins Peak Member a highly oxgenated body of water whereas the other parts of the member that have varves were non-oxygenated. Desborough, Pitman and Donnell (1973) used microprobe analysis of the biotites found in the tuffs to correlate these beds in the Green River fm. Williamson and Picard (1974) discussed the petrology of the Green River fm. carbonate rocks as they related to several lacustrine subenvironments: mudflats, beach-bar, lagoonal, shoal, transitional and offshore. They also described locally present algal stromatolite bioherms. Wolfbauer and Surdam (1974) described the origin of non—marine dolomite from the Laney Member of Lake Gosiute of the Green River fm. as a result of evaporative concentration of subsurface Mg-rich brines on mud flats. They use this to re—enforce previous conclusions about the nature of the origin of the Green River fm. as a playa-lake complex in an arid environment. In 1974 the Rocky Mountain Association of Geologists produced a guidebook to the Piceance Creek Basin in which Roehler (1974) identified ten basic depositional environments for the Green River fm. as described from Wyoming, Colorado and Utah. Smith (1974) described the geochemistry of oil-shale genesis in the Piceance Basin and believed it was deposited in a stratified lake with a reducing bottom which produced seasonal varves composed of organic matter. Desborough and Pitman (1974) discussed the significance of applied mineralogy to the oil shale member in the upper part of the Parachute Creek Member of the Green River fm. and it application to retorting oil shale. Robb and Smith (1974) discussed the mineral profiles in a 118 core hole in Piceance Creek Basin where they observed a direct correlation between silicate minerals and organic matter. Beard, Tait and Smith (1974) examined the nahcolite and dawsonite resources in the Green River fm. and while they made no correlation between these saline minerals and the environment of deposition it is evident from the thick accumulation of these minerals (1,130 ft. or 344 m.) that a very large saline lake was present in the area. Dyni (1974) also examined the stratigraphy and nahcolite resources of the saline facies of the Green River fm. Surdam and Wolfbauer (1975) reexamined the suggestion by Eugster and Surdam (1973) that the Green River fm. was a playa-lake complex and proposed a new playa-lake model. They basically divide Lake Gosuite into three distinct facies: (1) marginal silt and sand, (2) carbonate mud flat, and (3) lacustrine. By mapping the proposed positions of shorelines, stromatolite deposits and carbonate units they proposed a progressive increase in salinity and alkalinity of the lake. They believe the stratified lake model for the Wilkins Peak and Tipton Shale Members is untenable, in contast with the playa lake model. Eugster and Hardie (1975) examined the sedimentation of the Wilkins Peak Member of the Green River fm. and were able to identify seven rock units on the basis of their sedimentary structures: (1) flat-pebble conglomerate, (2) lime sandstone, (3) mudstone, (4) oil shale, (5) trona- halite, (6) siliciclastic sandstone, and (7) volcanic tuff. Their respective subenvironments were: (1) rapid transgression of a shallow lake, (2) lake shore oscillating over a mud flat (slow transgression), (3) playa mud flats, (4) shallow lake with occasional desiccation, (5) seasonally dry salt lake, (6) braided stream, and (7) not specific. The Wilkins Peak Member was pictured as a playa- lake complex which consists of a shallow, central playa lake and a vast surrounding exposed mud flat fringed by alluvial fans. They came to the conclusion that the Wilkins Peak model also applies to the other members of the Green River fm. and proposes that all the Green River fm. was deposited in a dynamic playa-lake complex. Lundell and Surdam (1975) came to the same conclusions as Eugster and Hardie (1975) that the Green River fm. was deposited in a playa-lake complex. Desborough (1978) investigated the chemical composition and mineralogy of the Ca-Mg-Fe carbonates in the Green River fm. oil shale and found that they are not entirely primary or detrital minerals as proposed by the playa-lake advocates but are chiefly the result of authigenic and diagenetic processes that derive their higher magnesium content, in kerogen-rich rocks from blue- green algae which accumulate high Mg/Ca ratios from water much lower in Mg/Ca ratio. Enrichment of magnesium with respect to calcium in high-kerogen rocks is derived by largely biogenic-chemical means rather than strictly chemical means. Desborough's model for a biogenic-chemical origin of the oil shales uses the model employed by Gebelein and Hoffman (1973) to explain the algal origin of dolomitic laminations in stromatolitic limestone. Desborough believes the Green River fm. was deposited in an open lacustrine environment that was strongly influenced by climatic conditions, such as rainfall and temperature, which largely determined evaporation rates and in part controlled the salinity and depth of water. The vertical water column has significant chemical difference between the surface and the mud-water interface (chemically stratified). Photic zone photosynthesis resulted in the 121 consumption of carbon dioxide and production of oxygen, while at greater depths the decay of algae would reduce the available oxygen. The death of algae could be caused by location below the photic zone, excessive temperatures, high salinity, high pH or general depletion of some essential mineral such as phosphorus. The reducing environment on the bottom would prevent decay of the organic material (and prevent its utilization by bottom dwelling animals!) . Smoot (1978) described a new model to explain the carbonate sediment production in a playa-lake complex. He proposed that groundwater draining in limestone-dolomite- orthoquartzite terrain precipitated low Mg-calcite cements within the sediments of the alluvial fans surrounding the playa, thus increasing the Mg/Ca ratios. Where the ground waters surfaced as springs, high Mg-calcite and protodolomite travertine mounds (tufa or chemical stromatolites) were formed. Perennial storms would move the limestone and dolomite clasts out into the playa where they would be deposited. The standing water on the playa would produce the organic and fine-grained laminates found in the Wilkins Peak Member of the Green River fm. Smoot (1978) favors this model because it explains the detrital intraclasts, crustose precipitates are the only obvious source for the material, modern dolomite crusts, caliches and travertine form under similar circumstances,and mass budget suggest that adequate amounts of carbonate minerals can be produced by precipitating crusts along the basin edge and without a standing lake. Buchheim and Surdam (1978) reported on the recent discovery of numerous and widespread fossil catfish from the Laney Member of the Green River fm. Since the catfish were discovered in the oil shales, this would appear to place severe constraints on some of the chemical parameters characterizing the water column in which the catfish lived. They there by concluded that the fossil catfish discovery would put an end to the discussion on the type of lake environments the Green River beds were formed. Evidence used was: (1) catfish cannot tolerate even small amounts of hydrogen sulfide but can withstand low concentrations of oxygen and high turbidity, (2) the abundance and widespread occurrence of catfish skeletons indicate they were a resident population, (3) it was highly improbable that the fish were transported very far because experiments have 123 shown that fish decompose and disarticulate after only a short distance of transport, and (4) the abundance of fish coprolites indicates a large fish population in the area. Buchheim and Surdam (1978) conclude that Lake Gosiute had no permanent stagnent hypolimnion, that the development of temporary or periodic anoxic conditions may have killed the catfish and that oil shale deposition does not depend upon the existance of a permanently stratified lake with a toxic, hydrogen sulfide rich hypolimnion. Surdam and Stanley (1979) re-examined the Laney Member of the Green River fm. and have proposed that during this final period of Eocene Lake Gosiute the lake evolved from a saline, alkaline lake into a freshwater lake. Grande (1980) in his review of the fossils known to occur in the Green River fm. described a maximum of 22 species from 14 families. He concluded that while finding a high number of non-marine fish species, Lake Gosiute has a relatively low specific richness, likely resulting from a highly fluctuating environment where conditions were never stable long enough for teleost radiation to occur as it has in African lakes. He attributed some of the fish beds to nav-ing-lieen killed by periodic plankton bloora$ resulting in 124 a stratified, anoxic, hydrogen sulfide-rich hypolimnion. In addition Grande (1980) lists over 100 families of insect found in the Green River beds. Boyer (1982) asked the question "does the playa-lake model really invalidate the stratified-lake model?" He has shown that the playa-lake model is untenable for the fossil fish bearing, kerogen rich laminates found in the Green River fm. These fossil fishes are mainly brine-intolerant but some brine-tolerant species were also found in the laminates. Lacustrine salmonid are not found in the Green River fm. and this may be a function of their need for an oxygenate and cool aquatic environment. Boyer proposed an ectogenic meromixis (chemical stratification) model for the Green River fm. which includes an early playa-lake stage followed by inflow that created an ectogenic-meromictic lake. That is, a lake with a chemocline between a saline monimolimnion on the bottom and a less saline mixolimnion on top. As various vertebrates (fish, mammals such as bats) and invertebrates such as flying insect die, they will follow through the mixolimnion rapidly and into the monimolimnion that is depleted of oxygen where they would be preserved. Fossil snails could not live in the 125 monimolimnion portion of the lake and hence are rarely preserved in the laminates but do live along the shore of the mixolimnion portion of the lake and are preserved in large numbers in those sediments. Smoot (1983) studied the Wilkins Peak Member of the Green River fm. in the Bridger basin which contains abundant evaporite and fewer organic-rich shales then the underlying Tipton Member or the overlying Laney Member. He basically studied the southeastern basin edge where the dolomite facies interfingered with the siliciclastic facies. This data which included sedimentary structures, vertical and lateral facies relationships indicates a complex depositional environment that includes sheet flood deposits on a dry playa mudflat, and transgressions and regressions of a large, shallow lake. He also believes the data supports the syndepositional clastic origin for the dolomite in the Wilkins Peak Member (Eugster and Hardie, 1975; Smoot, 1978) and that the sedimentation was controlled by cyclic variations in climate, which were similar to the large scale changes from the pluvial Tipton Member to the arid Wilkins Peak Member and back to the Pluvial Laney Member. In summary Smoot (1983) believes the 126 playa-lake model proposed for the Tipton and Laney Members of the Green River fm. are similar to the Wilkins Peak Member in that they are: (1) cyclic, reflecting transgression and regression of a lake; (2) contain mudcracked dolomitic mudstones; and (3) have small portions of their evaporites in the central portion of the basin. The gradational contacts between the Wilkins Peak Member, overlying and underlying members is consistant with shallow lake conditions rather than deep stratified lake conditions. The oil shales have less lateral extent in the Wilkins Peak Member and freshwater fossil evidence indicates the Tipton and Laney Members were much fresher bodies of water. The laminae of the oil shales represents each flooding event on the playa but the accumulation of mudstone laminae was much faster then the sub-millimeter layers of equivalent thickness of the oil shales and that the nearshore subenvironment facies and tuff beds are non- parallel in nature indicating lateral shifting of adjacent environments (based upon Walter's Law), in doing so he was able to indentify some 40 major climatic cycles in the Wilkins Peak Member. Sullivan (1985) compared subsurface data, mainly in 127 the form of geophysical logs, that was not available to previous investigators with existing stratigraphic relations and depositional models to form a more complete picture of the environments of deposition in the Green River fm. He found that during the more humid subtropical periods widespread deposition in predominately open meromictic system with rivers entering the basins from the north took place. These depositional conditions were present for the formation of the Tipton Shale Member and Laney Member, The Wilkins Peak Member was deposited during a more arid climatic phase and along the southern margin of the basin, shallow, saline-alkaline playa lakes were formed. Fluvial clastic studies indicate the playa lakes were frequently flooded with freshwater (more frequently vW then proposed by other investigators). The lakes were continually undergoing modification by climate and possibly tectonic processes. His study basically supports the ecotgenic meromictic lake model of Boyer (1982) with the closed drainage playa-lake model of Eugster and Surdam (1973) being restricted to certain portions of the Wilkins Peak Member. In summary the proposed origin of the lacustrine rocks 128 of the Green River fm. have evolved from an open drainage, meromictic lake model in which the lake (Lake Gosiute) was a large, chemically and thermally stratified body (Bradley 1929, 1931, 1948, 1964; Bradley and Eugster, 1969; Picard and High, 1968; Desborough, 1978) to a closed drainage, Playa l^ke model in which the lakes were shallow and saline-alkaline bodies surrounded by playa flats (Eugster and Surdam, 1973) to one in which the lake is considered to be an ectogenic, meromictic lake (Boyer, 1982; Sullivan, 1985) with periods of more arid conditions (Wilkins Peak Member) during which time playas and playa deposits were formed. No single modern analog of Lake Gosiute is known to exist and most likely Lake Gosiute, due to its complex climatic and possibly tectonic history combines a variety of existing modern analogs. Lake Savage in Stewart Valley and Lake Gosiute of the Green River fm. have a number of characteristics in common. They both contain marlstone, calcareous sandstone, siliciclastic sandstone, varved sediments, stromatolites, tufa domes, and fossil fish, insects, snails, mammals, etc. The chironomidae midge fly adult and larvae appear^ to have occurred in abundance in both lakes (much the same as they 129 do in modern lakes). Lake Gosiute does contain evaporite deposits and dolomitic varved sediments where as Lake Savage does not. Lake Gosiute has been shown to contain the silica minerals magadiite and kenyaite (see section on silica) where Lake Savage does not but instead contains the silica mineral cristobolite which indicates a significant difference in the origin and paleoecology of the two lakes. MIOCENE WASSUK GROUP The Miocene fluvial and lacustrine deposits exposed in Coal Valley, Nevada have been described by Axelrod (1956) as the Wassuk Group. The Wassuk Group is composed of three formations which are in ascending order, the Aldrich Station, Coal Valley, and Morgan Ranch. The Aldrich Station formation unconformably overlies a 15 m.y.-old andesite (Gilbert and Reynolds, 1973) and is composed of diatomite and claystone with a 12 m (39 ft) thick white rhyolite tuff near the middle of the formation that provides a marker bed for stratigraphic correlation. This white rhyolite tuff in the Aldrich Station fm. has yielded a K-Ar date of 13-11.4 m.y.B.P. (Everdan et al., 1964; Gilbert and Reynolds, 1973; Golia and Stewart, 1984). The 130 Coal Valley formation conformably (in most areas) overlies the Aldrich Station fm. The Coal Valley fm. is mainly sandstone^ and elastic-supported conglomerate^’ that interfinger with diatomite, claystone, siltstone, and carbonate siltstone with a rhyolite tuff bed of 11.4-9.3 m.y.B.P. K-Ar date (Everdan et al.f 1964; Gilbert and Reynolds, 1973; Golia and Stewart, 1984). The Morgan Ranch formation conformably overlies the Coal Valley fm. (Axelrod, 1956; Golia and Stewart, 1984) and consists mainly of matrix-supported conglomerate. Fossil flora and fauna collected from the Morgan Ranch fm. suggests a Hempilian age (Axelrod 1956; Everdan et al., 1964). The Morgan Ranch fm. is covered by alluvium, within the alluvium is a basalt flow that has a K-Ar date of 7.6 m.y.B.P. (Gilbert and Reynolds, 1973). Golia and Stewart (1984) have characterized the Wassuk Group as a ^lacustrine, braided alluvial plain , and alluvial fan environment^. Syndepositional faulting as reported by Gilbert and Reynolds (1973) and Golia and Stewart (1984), and volcanism indicates the Wassuk Group was formed in a tectonically active intra-arc basin. The Aldrich Station and Coal Valley fms. (Gilbert and Reynolds, 1973; Golia and Stewart, 1984) were deposited in grabens or half grabens that were produced by northwest-trending faults. These faults were produced by northeast-southwest extension in the northern Basin and Range as reported by Zoback, Anderson and Thompson (1981) -rior to 10 m.y. ago and then, subsequently changed to east-west extensionalism. The total thickness of the sediments of the Wassuk Group as reported by Gilbert and Reynolds (1973) and Goila and Stewart (1984) is 2,400-2,500 m (7,874-8,200 ft). The Wassuk Group of west-central Nevada and the sediments of Lake Savage were deposited during the same time period, with the onset of deposition of Lake Savage sediments being several million years prior to the onset of deposition of the Wassuk Group and with the Wassuk Group's depositional interval lasting several million years beyond Lake Savage. The Wassuk Group has a biotitic white tuff unit similar but older in age than the Lake Savage biotite tuff. Similar fossil flora and fauna can be found in the two systems but only Lake Savage has fossil insects. The total thickness of deposition in the Wassuk Group was considerably greater (2,400-2,500 m) than that found in Lake Savage (500 m) for approximately the same number of years indicating the Wassuk Group was deposited in a tectonically active intra-arc basin while the Lake Savage sediments were not deposited in a tectonically active basin. OLIGOCENE FLORISSANT LAKE, COLORADO McLeroy and Anderson (1966) report more than 226 publications can be found on the Florissant Lake beds, with a majority being on the fossil insects and leaves found in the Florissant shale. The following is not intended to be a review of these publications but a review of the McLeroy and Anderson (1966) paper on the laminae. The Florissant Lake beds consist of predominately fragmental volcanic material which includes micaceous, pumaceous tuffs, andesitic lava flows, volcanic conglomerate, fluvial deposits and the lacustrine shales. The maximum thickness of the beds is 50 ft (15 m). The lacustrine shales consist of primarily volcanic debris and a minor biogenic component of diatomite and sapropel in a complex laminated structure. The predominate type of lamination is the alternation of diatomite and sapropel which averages 1 mm in thickness; a second type is a graded tuff laminae that is overlain by one-or-more 133 diatomite/sapropel couplet with an 8 mm average thickness; a third type is an inversely graded yellow pumice with sporadically interbedded biogenic couplets (diatomite/sapropel) which has an average thickness of 1.5 cm. The biogenic couplet (diatomite/sapropel) represents a one year (varved) process. The diatomite laminae are spring diatom bloom accumulations; the sapropel is a late summer and early fall accumulation of organic material. Florissant Lake lamination indicate they were formed in a chemically stratified lake environment that most likely had a circulating, slightly alkaline epilimnion and a stagnant, acid, oxygen-depleted hypolimnion (McLeroy and Anderson, 1966) . In comparing the Oligocene Florissant beds and laminae to the the Miocene Lake Savage beds and laminae we find both have alternating light and dark laminae representing diatomite or silica and sapropel (organic matter) and both contain various tuff beds. Both systems were most likely chemically similar in that they represent chemically stratified lakes with oxygen-depleted hypolimnions. EOCENE FRESHWATER DEPOSITS OF BRITISH COLUMBIA 134 Wilson (1977a, b, c; 1978c; 1980) has reported on a series of Eocene freshwater deposits in British Columbia and near-by Washington state. Wilson (1978a, b; 1982) has also reviewed the Paleogene insect faunas of western North America, their evolutionary significance, and paleoenvironmental biases in limiting which insects will be preserved. The Middle Eocene lacustrine sediments that crop out in the valley of the Horsefly River, British Columbia contain abundant fossil material. This fossil material consists of fishes, fish scales, fish coprolites, insects, leaves, and diatoms. The fish scales, leaves and insects are preserved in laminae of alternating light tuff and dark sapropel. Wilson (1977a) proposed the depositional environment of the varves to be in the deeper portion of a stratified monomictic or meromictic lake in a warm temperate climate. This deeper portion of the lake was anaerobic, contained hydrogen sulphide, was free of turbulence and benthos fauna. The fish were entombed mostly during the spring and summer and the deciduous leaves during the late summer and autumn. Stratification of the lake was most likely caused by seasonal temperature variations which created density differences in the lake 135 water. Ruttner (1963) has shown that lakes in a warm climate are more likely to be thermally stratified for a given amount of seasonal temperature change than lakes in a cool climate because the density of water varies more for each degree of temperature change in warm lakes than in cool lakes. In addition the Eocene lacustrine deposits described at Horsefly, B.C. were formed in a long, narrow lake sheltered by surrounding hills. Ruttner (1963) has shown that long, narrow, and sheltered bodies of water stratify more easily because the maximum depth of mixing in meromictic lakes is proportional to the fourth root of the surface area or the square root of the radius. The preservation of thinly laminated sediments also indicates the Horsefly Eocene lake was stratified and had an anaerobic, turbulence-free hypolimnion. The absence of calcite in the sediments indicates high carbon dioxide concentrations, low oxygen concentration and/or deep water. Sulphur is present in the form of finely disseminated Pyrite and as gypsum crystals along bedding planes, suggesting hydrogen sulphide in the hypolimnion. Shallow water sedimentary structures have not been observed in the fossiliferous sequences. 136 Wilson (1980) in a later study found lake depth and distance from shore were important in the preservation of the flora and fauna in lake environments. Deep water/off— shore assemblages are more uniform and taxonomically less diverse than shallow water/near-shore assemblages. The sorting of terrestrial fossils into near—shore/off—shore components by flying ability, wind dispersion, or degree of flotation is suggested. In comparing the Eocene freshwater deposits of British Columbia to Miocene Lake Savage we find they both have fossil insects,leaves, fish, and fish copolites; alternating light and dark laminae; were meromictic in nature; have no calcite deposited in the deep-water sediments; have sulphur present in the form of pyrite and gypsum crystals along bedding planes; and were most likely sheltered bodies of water confined to long, narrow basins which helped to create the stratification of the lake. In short, the Eocene freshwater deposits of British Columbia are a good model for part of the developmental history of Miocene Lake Savage. PLEISTOCENE RITA BLANCA LAKE DEPOSITS, TEXAS Anderson and Kirkland (1969) examined the plant, vertebrate and invertebrate fossils from the laminated clays of the Rita Blanca lake deposits, Texas. They found the initiation of the pluvial conditions marked a break between the Pliocene and Pleistocene in the southern High Plains. The process of basin formation was by deflation during the dry periods and the formation of lakes and lacustrine deposits during the humid periods. The sinking of the surface of the land is thought to be caused by solution of Permian evaporites. The lake beds are light gray to white in color and contain dolomite, limestone, clay and sandstone units. Slump structures are common in the calcareous claystone and argillaceous limestones. The laminated sediments are nearly horizontal. The carbonate- f . A rich varves' light-colored laminae are mainly calcium carbonate with smaller amounts of clay and silt, and very little organic material. The dark-colored laminae of the carbonate-rich varves are mainly clay, some calcium carbonate and very small amounts of organic material and silt. The carbonate-poor varves', light-colored laminae are mainly clay with lesser amounts of calcium carbonate, organic material and silt. The dark-colored laminae of the 138 carbonate-poor varves are almost all clay with small amounts of calcium carbonate, organic material and silt. The coarser allochthonous organic material consists of non aquatic insects and parts of terrestrial plants. The coarse autochthonous organic material consists of some aquatic plants, fish remains, ostracodes, and aquatic insects. The carbonate-rich clay zone contains abundant -amounts of- ostracodes. The most common insect is the member of the family Chironomidae. Both adults and midge pupae of the genus Chironomus are relatively abundant. Anderson and Kirkland (1969) also mention the fossil midge pupae found at the Florissant lake deposits, Colorado, whose death was attributed to volcanic ash. They have quoted unpublished studies that indicate such deaths are most likely caused by sudden climatic changes rather than volcanic ash. Warm weather initiates metamorphosis in midge larvae which then leave the bottom of the lake and swim to the surface where sudden storms can decrease the water temperature and cause mass mortality. Adult midges emerge during the spring, summer and fall. Anderson and Kirkland (1969) also presented data on varve formation in modern non-glacial lakes. They found that most light- 139 colored laminae are formed of calcium carbonate during the summer but some are formed of diatom silica, and most dark-colored laminae are formed of organic material the rest of the year. Modern non-glacial varve forming lakes are northern temperate lakes. Numerous fossil plants were also found in the Rita Blanca deposits with one of the most important being Ruppia or ditch grass which indicates water of greater then-normal salinity and alkalinity. The ostracodes mentioned earlier indicate a moderately large and deep lake with seasonal droughts, the lakes were also brackish to saline, Some sixteen different aquatic insects and forty-five different terrestrial insects were found in the Rita Blanca deposits with the chironomids being the most numerous. Since only adult chironomids were found (for the most part) in the profundal sediments, this indicates that no macroscopic form of life occurred in the profundal area of the lake, for if it did the chironomid remains would have been quickly fragmented. Also since some delicate chironomid pupal remains were also found in the sediments this indicates the lack of scavengers in these areas. Two species of fish, Fundulus zebrinus or Plains killifish and Lepomis meqalotis or longear sunfish 140 were found in the deposits. Basically, the Rita Blanca lake deposits represents a cyclic series of non-varved to varved clays or a period of time the lake was a non- meromictic to a meromictic lake, a change that occurred in six varved cycles and lasted 1,150 varves. As the varves increased in thickness to 1 cm, this is an indication the lake was filling in with silt, sand, and clay. In comparison of the Pleistocene Rita Blanca lake deposits to the Miocene Lake Savage deposits we find they both have fossil fish, leaves, ostracodes, insects, etc. and varved sediments. The formation of the lake basins is quite different, deflation by dissolution of evaporites in the case of the Rita Blanca lake deposits and tectonically derived basin in the case of Lake Savage. The discussion of the preservation of the chironomid midges in the Rita Blanca lake deposits has a direct importance in relation to, and understanding of the preservation of insects in the Lake Savage deposits. Anderson and Kirkland (1969), also dispell the myth that volcanic ash killed the insects. DEEP SPRINGS LAKE, CALIFORNIA Deep Springs Lake is a small, intermittent saline lake 141 that occurs in Deep Springs Valley, California and was once part of a valley wide, larger pluvial Pleistocene lake. Data on Deep Springs Lake has been presented by Jones (1965) and is reviewed here. The present playa lake has three basic lacustrine deposits: (1) a saline crust, (2) carbonate muds, and (3) fine-grained well-sorted silts and sands, with other lacustrine deposits of the older pluvial lake being recognized elsewhere in the valley. Annual water contribution to the playa lake comes from surrounding streams and springs along with direct annual precipitation. The chemical composition of most of these waters is towards increased alkali, sulfate, chloride, and total dissolved solids. Data on the hydrochemistry and saline mineralogy of Deep Springs lake indicates it is basically a sodium sulfo-carbonate system with the aerial sequence of mineral zones from playa margin to center being calcite and/or aragonite, dolomite, gaylussite, thenardite, and burkeite. The log of a shallow hole drilled in the carbonate mud portion of the plays found a series of alternating light and dark laminae varying in size from 1-3 mm thick, which may represent varves. A similar shallow core hole was drilled in the playa's margin and yielded dark and light 142 fine sand to silty mud. No other sedimentary or biogenic structures were found. In comparison of the Miocene Lake Savage sediments to the holocene Deep Springs lake sediments we find little in common other than both are varved. Chemically the Deep Springs playa lake and its resulting lacustrine deposits are distinctly different from the Miocene Lake Savage deposits, indicating the Lake Savage deposits are not from a playa lake. FAYETTEVILLE GREEN LAKE, NEW YORK Fayetteville Green Lake is a modern north temperate hard-water plunge lake in New York State which has been extensively studied by Brunskill and Ludham (1969), Brunskill (1969), Ludham (1969), Brunskill and Hariss (1969), Culver and Brunskill (1969), Ludham (1974), Eggleston and Dean (1976), and Dean (1981). Green Lake's morphometric parameters have been reported by Hutchinson (1957) and will not be reported here except for the following. The lake has a total drainage area of 4.328 km S (170 ft), mean depth of 28 m (91 ft), and a chemocline at 18 m (59 ft). Green Lake is a meromictic marl lake, 143 chemically stratified with a relatively stable moniolimnion, has active carbonate precipitation, laminated sediments, and organosedimentary structures. The laminated sediments contain both light and dark alternating laminae with occasional turbidites. The light-colored laminae are higher in calcium carbonate and lower in organic material than the dark-colored laminae, with 80% of the couplet mass being assigned to the light-colored laminae which is deposited from June through October and the remaining 20% of the couplet mass being assigned to the darker-colored laminae for the deposit formed during the rest of the year. Some 80% of the total sedimentation mass from June through October was calcite. Stromatolitic biohermal lenses are common throughout the lake and are made up of sediment derived from the lake banks, mainly calcite that is trapped and bound by the mucilaginous sheaths of blue-green algae. The biohermal growth begins on solid objects such as tree branches, pebbles, etc., with maximum depth of occurrence controlled by the chemocline, below which there is not enough adequate light for algal growth. In comparing Fayetteville Green Lake to Miocene Lake Savage we find little resemblence between the earlier 144 stages of Lake Savage and Green Lake. Lake Savage, in its earlier stage, is dominated by net siliclastic sediments with carbonate sediments only rarely occurring. Those carbonate deposits that do occur in the early stages of Lake Savage are calcium carbonate deposits (tufa) on logs an other pieces of wood in the fluvial and deltaic portion of the lake and some shoreline stromatolites. Later, a change in the chemistry of Lake Savage becomes apparent as more and more calcium carbonate deposits and calcareous sediments are found, until a point is reached when all the sediments are calcareous. In this later stage, Lake Savage resembles Fayetteville Green Lake in its water chemistry and chemical precipitates. Lake Savage goes from being a net siliclastic depositional system to a net carbonate depositional system. PLUVIAL PLEISTOCENE LAKE LAHONTAN Pluvial Pleistocene Lake Lahontan existed in northwestern Nevada and has been studied in various degrees by Hague (1877), Russell (1883, 1885), Jones (1925), and Morrison (1964, 1965). Russell (1885) made the first and most comprehensive study of the geological history of Lake Lahontan until Morrison's (1964) work. Remnants of Lake Lahontan include Pyramid Lake, Walker Lake, the Carson Lakes and Soda Lake. Perhaps the most conspicuous deposit in all areas of Lake Lahontan are the tufa deposits. These calcareous, organosedimentary deposits include the three types of tufa as described by Russell (1883, 1885): the thinolite, dendritic, and lithoid tufas. Many extremely large and spectacular tufa domes can be found in the Pyramid Lake, Winnemucca Lake, Smoke Creek desert and Carson desert areas of Nevada, with large tufa domes not being present in the Walker Lake area, only massive beds of tufa and areas of stromatolites (Osborne, Licari and Link, 1984) . Both Pyramid Lake and Walker Lake are thermally stratified but oxygenated to the bottom for the most part throughout the year, experience at least one and usually two overturns per year, have a pH in the 9.1 to 9.5 range, total dissolved solids range from 5,600 ppm (Pyramid Lake) to 10,700 ppm (Walker Lake). Both lakes support fish populations of Lahontan chub (Gila bicolor obesus), Lahontan cutthroat trout (Salmo clarki henshawi) and Tahoe sucker (Catostomus tahoensis) for example, with Pyramid 146 Lake supporting a unique relict lakesucker, the Cui-ui lakesucker (Chasmistes cujus) whose presence proves the theory that Pyramid Lake has never completely dried up as Walker Lake probably did. The lack of a greater fish diversity in both Pyramid and Walker Lakes is an indication of the relative^^newness of the fauna in comparison to African rift lakes and Lake Biakal in the Soviet Union which has dozens of fish species. Spencer (1977) and Koch, Cooper, Lider, Jacobson and Spencer (1979) found the formation of precipitates of calcium carbonate in the waters of Walker Lake and also discovered the dissolution of the same precipitates took place in the upper few meters of the sediments due to biologic reduction activity producing a lower pH. Core samples yielded primarily quartz, feldspar and clay minerals as silicate minerals, and diatom frustules in abundance. All cores were black in color and contained free hydrogen sulfide throughout,,six of the seven cores contained portions of volcanic ash layers (five or six per core, each 1-2 mm thick). While the pH of the lake was 9*5, the core pH was 7.05. All cores showed an increase in SiO in solution with depth. The silica content of the 147 lake water varied from lowest concentration during the summer, in conjunction with low abundance of diatoms, but after maximum diatom production. The highest surface lake water silica concentrations were found during the winter and spring when diatom production was the highest. Silica stratification of the lake water was also evident with the bottom water concentration increasing through the summer and fall which was attributed to the precipitation of diatom frustules from the epilimnion. Big Soda Lake occupies a volcanic crater rimmed by basaltic debris and lacustrine sediments on the southwestern margin of the Carson Sink. Until 1907 the lake was a single mixed water mass. A sudden rise in the lake level occured between 1907 and 1930 resulting in a chemically stratified desert lake with a chemocline at 35 m (115 ft) separating the highly anoxic moniolimnion and oxygenated mixolimnion (Russell, 1885; Hutchinson, 1937; Cloern, Cole and Oremland, 1983; Kharaka, Robinson, Law and Carothers, 1984). Inflow is almost entirely as a result of local irrigation practices and groundwater flow. Studies of the moniolimnion reveal it is an alkaline, saline environment, highly reduced, contains abundant ammonia, 148 reduced sulfur compounds and methane, it harbors an active methanogenic flora. Core samples from the upper two meters (6.5 ft) of sediment reveal visable laminations, interspersed on a dark green colored core were 1-5 mm thick multicolored (red, brown, white, yellow, black) layers composed mainly of diatoms, indicating seasonal aspects to diatom production (Cleorn et al., 1983). The mixolimnion of Big Soda Lake also exhibits seasonal changes similar to a temperate monomictic lake in that: (1) there is a summer- fall period of thermal stratification, (2) is a winter circulation of the mixolimnion, and (3) during the spring there is the initiation of stratification. Purple sulfur bacteria are the most abundant phototrophic producers during the summer and fall with winter phototrphic production being by diatoms. Studies of the origin of the sediments must take into account both of these processes and their proportional influence on the sediment composition. In comparing Miocene Lake Savage to pluvial Pleistocene Lake Lahontan and its remnant lakes (Pyramid Lake, Walker Lake, Big Soda Lake, etc.) we find the two system share many common morphological and ecological features. First off, both systems have extensive tufa and other calcium carbonate deposits, although Lake Savage's tufa and other calcium deposits come later in the lakes development and is not as extensive as the tufa deposits found in the Lake Lahontan system. Secondly, both system share similar fish fauna, since both apparently contained members of the Family Salmonidae and Family Cyprinidae, and had extensive fish populations. Both the fish and gastropod populations indicate external basinal connections at some point in the lake’s development. Big Soda Lake, while offering a unique insight into the formation of a chemically stratified lake from one that only a few years before was not chemically stratified, does not appear to represent a phenomena directly similar to Lake Savage, but does show on a smaller scale chemical stratification can rapidly (geological time wise) develop under the right environmental conditions. LAKE CLASSIFICATIONS Hutchinson(1957) in his classification of lake basins described some 76 different types, most of which have not bearing on the sediments described here. Two and possibly three of his general types include: (1) tectonic basins, (2) lakes associated with volcanic activity and (3) lakes formed by landslides. The third type (landslide formed lakes) is mentioned as only a possibility and for the most part is to be ignored as a possibility since no known major landslide deposits have been found to date in the Stewart Valley deposits but are known in the Oligocene0- deposits (Ekren et al., 1980). Tectonic basins as described by Hutchinson (1957) included some nine types, ranging from lakes formed by epeirogenetie uplift of sea bottom or marine surfaces (Caspian Sea); lakes formed by upwarping all around a basin (Lake Victoria); lakes formed by local subsidence due to earthquakes; lakes in basins tectonically dammed synclines; basins on tilted fault blocks (Abert Lake, Oregon) to basins in grabens between faults (Lake Biakal, Pyramid and Walker Lakes, Nevada). Lakes associated with volcanic activity included lakes in relatively unmodified craters in cinder cones, maars, crater lakes (Big Soda Lake, Nevada), caldera lakes (Crater Lake, Oregon), lakes in large basins formed by slow collapse with step faulting over an empied magma chamber (perhaps Mono Lake), lakes formed by lava damming of valleys, and lakes formed by volcanic mud flows or lahars. 151 Miocene Lake Savage in many ways may be a combination of both tectonic basins type and lakes associated with volcanic activity. Earlier studies on Oligocene tuffs in the Gabbs Valley Range (Ekren et al., 1980) show that an active trough (basin?) existed in the Stewart Valley area and while data indicated that this trough was basically inactive throughout the rest of the Tertiary and possibly earliest Quaternary, the basic structure of the valley was formed by tectonic activity. Within the Stewart Valley sediments, several of the units, including several major stratigraphic indicators are subaerial to subaqueous volcanic fallout tephra. Two volcanic mudflow or lahars (a major one at the base of the Stewart Valley units described here and a minor one near the top of the stratigraphic section) are found in Stewart Valley. The earlier, major lahar apparently forms the surface upon which some of the earliest Miocene sediments were deposited. Many of the sedimentary units (especially the fluvial units) contain volcanic clasts. This indicates that the fluvial-lacustrine system that formed Lake Savage occurred in a tectonically active terrane. Cole (1975), and Hakanson and Jansson (1982) classified 152 lakes based upon their annual circulation patterns and periods of mixing. Direct stratification of lake water is caused by dense cold water lying beneath lighter, warmer water. The upper, warmer region is called the epilimnion. This region is mixed to a more or less uniform temperature by wind action. The colder region or hypolimnion is little affected by wind. The area of the lake that separates the cold water from the warmer water in the case of thermally stratified lakes is called the thermocline. In the case of chemically stratified lakes this region is called a chemocline. Lake classification based on thermal types range from amictic lakes (those lakes that never circulate and are covered by a permanently- ice cover) to holomictic lakes (where wind-driven circulation mixes the entire lake) . Wetzel (1975) has produced a modified classification of thermal lake^/* types with latitude and altitude. Based upon the latitude at which Lake Savage would have likely occupied during the Middle Miocene, the most likely types of thermal lakes are warm monomictic, dimictic and meromictic. Monomictic lakes have only one period of circulation, 153 where as the warm monomictic lakes circulate during the winter months and stratify (thermally or chemically) during the summer. Dimictic lakes usually have two mixing periods, one vernal and one autumnal overturn. Dimictic lakes will stratify during the warm months. In shallow areas they may become monomixic and exhibit stagnation. Cold weather usually cools the surface waters and eventually breaks up the statification and initiates complete circulation. The water often cools to a point of surface waters freezing which induces winter stagnation that is only broken by vernal overturn. Meromictic lakes (Hutchinson, 1937) circulate at times, but this circulation is incomplete. The dense bottom water remains for the most part stagnant and characteristically anaerobic. Meromictic lakes have their own unique terminology as defined by Findenegg (1935). The monimolimnion is a permanently stagnant layer containing a greater concentration of dissolved substances than the overlying water. The upper layer is the mixolimnion which is more dilute, mixed by the wind and shows seasonal changes. Between these two layers is a zone called the chemocline where salinity increases rapidly with depth. The vertical density gradient in the chemocline is similar to the temperature gradient in the thermocline. Meromictic lakes tend to be more stable than other lake types, primarily as a function of materials in solution. Classification of meromictic lakes is based upon the manner by which solutes accumulate in the monimolimnion. Biogenic meromictic lakes are those in which there is an accumulation of substances derived from bacterial decay, diffusion from sediments and from photosynthetic precipitation of carbonate (tufa). The water of biogenetic meromictic lakes is typically calcium bicarbonate. The Fayetteville Green Lake, New York is a example of a biogenic meromictic lake. Ectogenic meromictic lakes are lakes where contrast in density is brought about by the delivery of water from outside sources. The water is delivered first in a dilute superfical layer that comes to lie above a pre-existing saline body or a saline source that lies beneth a freshwater body. Ectogenic meromictic lakes have a stability based on a function of salinity differences, tend to be more saline than biogenic meromictic lakes, and are usually found in arid regions where water is concentrated by evaporation in closed basins. Crenogenic meromictic lakes are formed by the inflow of subsurface saline water into the basin. Saline water introduced into Lake Tanganyika and other African lakes causes the bottom layers to be 4.5 times more saline than the top-waters and anoxic. Siliceous Beds vU Bualda (1914) in his examination of the lithologies found in Stewart Valley found the paper shales to be barren of diatoms and described a series of chert beds. In his brief description of these beds he noted their color, varying from white, yellow, red, brown, blue to green; variation in thickness from 35 feet (10.6 m) to extreme thinness; and not distinctly bedded. This description would indicate he was discussing, for the most part, the chert beds that are considered to be older then the lacustrine units and interbedded in the pre-lacustrine volcanics. It is from these units the older Fingerrock flora has been described (Wolfe, 1964). Siliceous lacustrine units such as those at Pacific Union site appear to be lumped with the paper shales which they superficially 156 resemble. This study has found chert and/or chert-like beds in several areas of Stewart and lone valleys. First, those multicolored chert beds of Buwalda can be easily found in the northern end of the Cedar Mountains, interbedded in what has been called the Gilbert Andesites. Those layered sliiceous beds (cherts) that are located stratigraphically below the paper shales, appear to have not been described by Buwalda. Other cherts can be found interbedded in the upper portion of the deltaic units, informally known as Snail Cliff (Firby, 1965) and in the upper portions of the fluvial units to the northwest in Stewart Valley. These fluvial units appear to be channelized, in situ, volcanic air-fall tephra. The channels appear to be the same or similar reworked tephra, and over time diagenesis, resolution and migration of silica has taken place with the resulting chert being formed between the channels. X-ray analysis of the layered paludal or lacustrine, fossil plant-bearing beds that are interbedded with the Gilbert Andesites (=Fingerrock flora site), the layered siliceous (chert) beds found stratigraphically below the paper shales, the paper shales containing fossil fish, 157 plants, and insects at the Pacific Union site, paper shales containing fossil fish across the canyon from Snail Cliff which are interbedded with deltaic units, paper shales south of Snail Cliff which contain fossil fish and insects, and the fossil location at Two Tips reveal these units are composed of cristobalite. Other minerals can be detected in small quantities in some of the samples, especially those interbedded with the deltaic units or those units considered to be closer to the paleoshore line than the lake sediments found at Pacific Union. These other minerals include quartz, feldspar, montmorillinite and amphibole. Pyrite which can be seen in polished sections is in insufficient quanitity (less than 5%) to be detected by x-ray diffraction. No calcium or magnesium carbonates were detected by x-ray analysis but minute reactions with dilute HCL does occasionally occur. The question that then arises is how can cristobalite, which is usually considered to be a framework silica mineral which has a stable form at 1470 degrees C (Dietrich and Skinner, 1979) be formed in lakes or lake sediments. Opal is considered to be a form of cryptocrystalline cristobalite (Dietrich and Skinner, 1979). 158 In the examination of the literature we find that chert (microcrystalline sedimentary rock composed largely of quartz that was precipitated from aqueous solution) occurs in three different types of stratigraphic and tectonic settings: (1) as nodules in cratonic carbonate rocks; (2) as pure, bedded chert in geosynclinal tectonic regions; and (3) in association with hypersaline lacustrine deposits (Blatt, 1982). A review of saline lakes by Eugster and Hardie (1978) covers the complete range of mineral deposits known to occur in these types of lakes. Most of this work has involved lakes with hypersaline, alkaline brines. The best known of the authigenic silica minerals formed in saline lakes are the zeolites (Hay, 1964; Sheppard and Gude, 1968, 1969, 1973; Surdam and Eugster, 1976). X-ray analysis, to date, of the siliceous deposits have not found any significant amounts of zeolites in the Stewart Valley lacustrine deposits that are cristobaltic in nature, but some zeolitization does occur in the volcanic tephra beds. Another common silica product of saline.^ alkaline lakes are bedded cherts, specifically sodium magadii-type cherts (magadiite) that are precipitated from the alkaline 159 brines and later converted to kenyaite and eventually to chert (quartz) (Eugster, 1967, 1969, 1970; Jones, Rettig and Eugster, 1967; Jones, Eugster and Rettig, 1977; Eugster and Maglione, 1979; Blatt, 1982). This magadiite chert is finely laminated and precipitation is postulated to have taken place in annual increments in a chemically stratified lake at the hypersaline brine (hypolimnion-epilimnion) (dilute water) interface (Eugster, 1969). These cherts are interbedded with zeolites and clays. Magadiite cherts are also found in certain portions of the Green River fm. (Blatt, 1982). X-ray diffraction studies of these sodium silicates (magadiite and kenyaite) indicate their patterns are different and distinct from other silicates (Eugster, 1967). In subsequential studies of the magadiite type cherts (see references cited above) both in using field samples and laboratory studies there is no evidence of a cristobalite or opal-ct stage in the formation of magadiite, its conversion to kenyaite or subsequent conversion to chert (quartz). Based upon this information about the concurrence of zeolites and magadiite in hypersaline, alkaline lakes, and especially the lack of an cristobalite or opal-ct stage in 160 magadiite formation, one can say the formation of cristobalite in the lake sediments of Stewart Valley were not formed in the type of hypersaline, alkaline lake similar in composition to Lake Magadi. However, these studies do show that formation of silica mineral and their subsequent precipitation is possible at the epilimnion- brine interface found in chemically stratified lakes. As mentioned earlier the formation of cristobalite has been regarded by most as being a high temperature phenomona. White, Brannock and Murata (1956) based upon field work and observations at Steamboat Springs, Nevada, and laboratory analysis came to several generalized conclusions: (1) at ordinary temperatures, amorphous silica establishes a solubility equilibrium in water with 105-120 ppm of silica in true solution; (2) rates of reaction are slowed which permits supersaturation and undersaturation to exist for considerable periods of time, and (3) the solubility of silica is not much influenced by pH in the range 2-9, with solubility being greater in acid rather than weakly alkaline solutions. Krauskopf (1956) found: (1) that solubility of silica rose rapidly at pH's over 9.0 at normal temperatures; (2) silica is no more soluble in 161 very dilute alkali than in acid; (3) supersaturated solutions of silica ordinarily do not precipitate but become a colloid and the colloid may eventually precipitate in weakly basic solutions or may set to a gel in weakly acid solutions; (4) colloidal silica may be precipitate by evaporation, by co-precipitation with other colloids and by fairly concentrated solutions of electrolytes, but this precipitation is fastest in basic solutions and slowest in solutions with a pH less than 6; (5) silica is added to natural water by volcanoes and spring activity associated with volcanic activity and by dissolution of silica and silicates during weathering; (6) undersaturation of terrestrial water is a function of the slowness at which silica and silicates dissolve; and (7) the origin of chert in marine sediments cannot be accounted for by inorganic precipitation, except near volcanic centers. Bramlette (1946) reviewed the origin of chert, especially in relation to the opaline cherts of the Monterey formation and argued against direct inorganic precipitation from sea water. Peterson and von der Borch (1965) described the precipitation of chert in the form of gelatinous opal- cristobalite in ephemeral lakes associated with the Coorong 162 Lagoon of South Australia. Dolomite, magnesite, and magnesian calcite were also deposited. A high pH (9.5- 10.2) causes dissolution of detridal silicates, a general lowering of the pH (to 7.0 to 6.5) and/or desiccation of the lake causes precipitation of chert. The lake waters show a rather wide fluctuation of pH that follows a seasonal pattern. Dessication increases the brine concentration (electrolyte concentration) to saturation. Opal-cristobalite can be formed by direct inorganic precipitation at the time of deposition of the sediment. Mizutani (1970) discussed the consecutive transformation of amorphous silica to low-cristobalite and from low-cristobalite to quartz. This diagenesis and transformation of silica is in reality a thermal history of the sediment. He also believes that "since low- cristobalite is hardly precipitated immediately from solution, its presence suggests that the mineral was converted from amorphous silica and is being converted to quartz". He stresses the point that the change in disequilibrium state is essential in diagenesis of silica and is driven by the kinetics (thermal history) of the sediment. This "cristobalite stage" as he called it is roughly correlative to the earliest zeolite grade. He also mentions the work of Ernst and Calvert (1969) on the Monterey formation that is an example of silica diagenesis and the cristobalite stage at work. The lower part of the Monterey fm. contains chert (cryptocrystalline quartz) and porcellanite (low-cristobalite); whereas diatomaceous sediments composed of poorly ordered opaline silica increases in volume toward the upper portion of the formation. Henderson, Jackson, Syers, Clayton and Rex (1971) in their study of the authigenic origin of cristobalite in montmorillonite and bentonites, found cristobalite is not a common component of unaltered volcanic ash but forms by secondary processes and that low-cristobalite of hydrothermal origin does not have an appreciable oxygen isotopic exchange with meteoric water once crystallization has taken place, whereas opal-cristobalite of the Monterey fm. does have an oxygen isotopic exchange with meteoric water. O'Neil and Hay (1973) in their oxygen isotopic studies of cherts associated with saline lake deposits (Lake Magadi) concluded the cherts' precursors formed in association with lake waters of varying salinities. 164 Murata and Nakata (1974) have shown that with increasing depth of burial, cristobalite in the Monterey fm. shows a decrease in the d(101) spacing 4.115 to 4.040 angstroms. The spacing in porcellanite is 0.004 to 0.015 angstroms smaller than in associated cherts, indicating the cristobalite of porcellanite formed later (at higher temperatures=deeper burial) than that of the chert. The Monterey Shale is composed of alternating layers of dark compact chert and light porous porcellanite, both yielding x-ray diffraction patterns of low-cristobalite. Another stratigraphically higher and earlier diagenetic stage of the Monterey Shale consists of alternating layers of chert and diatomaceous mudstone; only the chert is cristobalitic with free silica of the mudstone consisting of unaltered biogenic opal that is amorphous to x-rays. Murata and Larson (1975), and Murata, Friedman and Gleason (1977) studied the oxygen isotope ratios in the silica phase of the Monterey Shale and found the change from disordered cristobalite to ordered cristobalite seemed to be a solid state reaction not involving interstitial water. Brueckner and Snyder (1985) in their study of the Monterey fm. have found the same silica diagenesis as previous studies. They found initial clay-poor sediments formed diatomite (opal-A) which changes to CT-chert (opal- CT) and then to quartz chert, while clay—rich diatomaceous shale changed to CT-porcellanite (porous opal-CT) and then quartz porcellanite. Silica diagenesis and petroleum catagenesis and migration appear to have occurred at the same time. Taking all this information into account, the Stewart Valley thinly laminated cherts and papershales are most likely the result of precipitation of amorphous silica gel from an alkaline, saline lake. The amorphous silica gel either penecontemporanously with or shortly after deposition underwent diagenesis to cristobalite. Since the sediments do not contain any precursors to the magadi-type chert and only cristobalite, the lakes in which they formed was most likely not hypersaline but saline and alkaline with a chemically stratified bottom (meromixis). The higher salinity brines were confined to the deeper portions of the lake. THe general abundance of low-cristobalite as opposed to quartz would indicate the silica diagenesis is only in its preliminary stages. The general lack of depth 166 of burial (maybe a few hundred meters) could account for the lack of an advanced diagenetic state. This is further enforced by the general lack of diagenesis of the clay-rich sediments which are interbedded with the cristobalitic sediments. Sedimentary Pyrite Formation The problem of sedimentary pyrite (FeS ) has been a discussed by Berner (1970; 1971; 1984) and others (Hutchinson, 1957; Howarth, 1978; Berner, Baldwin and Holdren, 1979; Postma, 1982; Berner and Raiswell, 1983; Cook, 1984; Howarth and Merkel, 1984; Davison, Lishman and Hilton, 1985). From these discussions there seems to be three basic problems about pyrite formation that arise; (1) sources of the iron and sulfur, (2) factors limiting formation, and (3) mechanisms of formation. The chief source of iron for pyrite formation in sediment is detrital iron minerals that are deposited in an anaerobic environment and become solubilized by bacterial or inorganic processes. Not all iron minerals react to form pyrite and those that do must for the most part be extremely fine grained. Pyrite is formed by the reaction of iron minerals with dissolved H2S. There are two major 167 sources of H2S: (1) bacterial sulfate reduction and (2) decomposition of organic sulfur compounds derived from dead organisms. The abundance of sulfate appears to be the limiting factor in pyrite formation. Most lakes are very low in dissolved sulfates, leaving organic sulfur compounds as the main source. Examples in the Great Basin are Pyramid Lake which is low in sulfate (280 ppm), and Walker Lake (2080 ppm) and Big Soda Lake (6700 ppm) which are high in sulfate (Galat and Robinson, 1983; Cloern, Cole and Oremland, 1983). The limiting factors, therefore, in the production of pyrite are: (1) concentration and reactivity of the iron compounds, (2) availability of dissolved sulfate, and (3) concentration of organic sulfur compounds that can be utilized by sulfate-reducing bacteria. For the most part aquatic environments especially those in terranes that contain an abundance of iron mineral containing rocks (i.e. volcanics), detrital iron availability is not a limiting factor. In most sediments dissolved sulfate is not completely removed by bacterial action and as a result the availability of dissolved sulfate is not usually a limiting factor in pyrite formation. The availability of organic 168 sulfur compounds is usually a function of organic productivity in and around the lake basin. For most lakes organic sulfur compounds are available in abundance. The mechanism of formation for pyrite does not allow for direct production of pyrite (for the most part but see Howarth, 1978) by reaction of bacteriogenic sulfides with detrital iron minerals but requires the formation of other metastable iron sulfides (mackinawite=tetragonal Fe„ s, 1+x greigite=cubic Fe,SJ first. Howarth (1978) has shown 3 4 pyrite will form rapidly under suitable acidic conditions (pH 5.0-6.5) whereas iron monosulfides (mackinawite, greigite) will form under more alkaline conditions and that there tends to be a direct competition between the two groups with the formation of iron monosulfides being favored. In a typical lacustrine environment the mineral phase of pyrite is usually in low concentration and difficult to quantify by ordinary means. Davison, Lishman and Hilton (1985) have shown reducing sediments high in acid volatile sulfur (AVS) do not favor the direct formation of pyrite. AVS tends to color sediments intensely black which changes to grey-brown with time and depth of burial. The color is associated with surface coatings of sulphide on iron oxide particles, only a very small amount of AVS is needed to turn sediments black. They have also shown that pyrite is most probably formed near the oxic-anoxic boundary at the sediment surface, chiefly from low concentrations of ferrous and sulfide ions with a partial oxidizing environment resulting from a low turnover of organic carbon or form a temporary phenomena such as seasonal anoxia (periodic incursions of oxygen). Diagenetic conversion of AVS into pyrite does not appear to be an important process in freshwater sediments. Therefore, it would appear that one could have dark black to gray-brown sedimentary rock (due to AVS) being deposited within a lake in addition to pyrite being formed and deposited on the surfaces of those sediments only during seasonal anoxic periods. The laminated silicified shales and papershales found in Stewart Valley vary in color from almost black or very dark brown to gray-brown and grayish-white, with a majority of the deep-water sediments being dark black-brown to gray- brown. Pyrite is found in these sediments in small quantities (not enough to be detected by x-ray diffraction) but can be seen microscopically in polished section. The 170 pyrite invariably lies on a sedimentary surface and is often associated with burnt organic remains (cinders) to the point of filling cavities within the remains. Berner (1981) in his discussion of a geochemical classification of sedimentary environments states the pH range of marine and non-marine sediments usually range from 6 to 8 with 90% of the values ranging from 6.5 to 7.5. Therefore, pH is not necessarily useful in characterizing sedimentary environments with the notable exception of alkaline lakes and freshwater peat bogs. His classification scheme links life processes occurring in the sediments with authigenic mineralogy because of the importance of the oxidation-reduction reaction to both. Aerobic organisms and oxidized minerals (hematite) can not tolerate traces of H^S without death of the organism or conversion of the mineral to sulfide minerals. Similarily dissolved oxygen and H2S cannot co-exist even at low measurable concentrations, which makes dissolved oxygen and H2S ideal variables in a geochemical classification scheme. His first division is oxic or anoxic depending upon the presence or absence of measurable oxygen (measurable -6 °xygen=10 molar). Oxic environments are those that 171 contain hematite, goethite, etc., and no organic matter. Anoxic environments can be further divided into sulfidic (H A S conc.> 10 6 molar and nonsulfidic (H 2 S conc.< 10 6 molar). Sulfidic sediments contain pyrite, marcasite, rhodochrosite, etc., and organic matter. Nonsulfidic sediments can be divided into post-oxic environments with glauconite, no sulfide minerals and minor organic remains, and methanic environments with siderite, vivianite, etc., early formed sulfide minerals and organic matter. Euxinic environments such as the Black Sea are better not classified based upon their authigenic pyrite formation but upon the presence of finely laminated sediments resulting from the lack of bioturbational mixing by benthic organisms. When oxygen is present the underlying sediments invariably are bioturbated to the point of destroying the lamination. Black Sea sediments in addition to being laminated are intensely black in color due to high amounts of AVS. LACUSTRINE CARBONATES In the Stewart Valley area the occurrence of Late Tertiary carbonate rocks is restricted vertically for the most part to the younger of the sediments, with the exception of the molluscan rich mud of the deltaic deposits at Snail Cliff. Few carbonate rocks can be found in the deep basin sediments. Early near-shore environments also contain little carbonate material, however during the later stages of lake development the carbonate sediments become more abundant and pervasive and the non-carbonate sediments are the exception. The earlier carbonate sediments as mentioned above are mainly made up of fragments to whole molluscs but thick beds of ostracodes also occur. As one moves up the geologic column stromatolites, tufa domes, oncolites and ooids are found, indicating a change in the chemistry of the lake from a primarily siliciclastic lake to one that allowed for the precipitation and deposition of carbonate material and diatomaceous material. Muller, Irion and Forstner (1972) in their study of inorganic calcium and magnesium carbonates in the lacustrine environment found three main groups of carbonate-deposit lakes. Those that produce: (1) calcite and/or high-magnesian calcite and aragonite; (2) high- magnesian calcite, aragonite, dolomite, huntite, and 173 magnesite; and (3) aragonite. They also found that primary carbonates were formed by; (1) loss or extraction of carbon dioxide which prevails in lakes found in humid regions; (2) evaporation concentration which prevail in lakes found in arid regions; and (3) mixing of different water bodies such as river water rich in calcium and bicarbonate ions at a pH of 7.5 entering a highly alkaline environment and precipitating aragonite as "whitings" on rocks in the mixing zone. Dean (1981) in his summary of carbonate minerals and organic matter in sediments of modern north temperate lakes, Eugster and Hardie (1978) in their study of saline lakes, and Kelts and Hsu (1978) in their study of freshwater carbonate sedimentation have shown that the four most important components of lacustrine carbonate sedimentation are: (1) the detrital clastic material involved; (2) biogenic silica; (3) carbonate minerals formed; and (4) organic matter. The detrital clastic material is derived from the surrounding drainage basin. The biogenic silica is from diatom frustules produced in the lake. The carbonate minerals involved are mainly low- magnesian calcite, some high-magnesian calcite, dolomite 174 and/or aragonite. The organic matter is: (1) herbaceous allochthonous material; and (2) lipid rich autochthonous material. The herbaceous material is described as being pollen grains, leaves, needles, seeds, and woody material in general from the surrounding drainage basin. The lipid- rich autochthonous material is described as being large debris from planktonic algae and aquatic macrophytes from the lacustrine system. Since newly formed lakes usually have very little organic material in them because organic production is very low, there is little initial accumulation of precipitated carbonates in the lake basin unless the surrounding drainage basin is contributing extremely large amounts of dissolved calcium carbonate. As the lake increases in calcium carbonate content and reaches saturation, one or more carbonate mineral will precipitate in varying proportions with clastic material to form a carbonate rich clastic deposit called marl, and when the deposit is totally dominated by the organic material and carbonate combination a deposit called gyttja is formed. If the organic productivity of the lake increases, more organic debris is either incorporated into the sediments or decays 175 and releases more nutrients. The source of carbonate in lacustrine sediments is: (1) inorganically precipitated calcium carbonate; (2) photosynthesis-induced, inorganically precipitated calcium carbonate (also known as bio-induced calcium carbonate precipitation); (3) biogenic calcium carbonate consisting of debris from calcareous plants and animals; and (4) allochthonous (detrital) material derived from carbonate rocks in the surrounding drainage basin. Most lake carbonates are inorganic precipitates or bio-induced precipitates. The chemistry of carbon in a lacustrine system as it applies to the precipitation of calcium carbonate has been described by Wetzel (1975), Berner (1971), Kelts and Hsu (1978), and Dean (1981). The precipitation of calcium carbonate is limited to the concentration of four other carbon compounds in solution. These carbon compounds are: (1) gaseous carbon dioxide (C02) ; (2) undissociated carbonic acid (H2CC>3); (3) - -2 bicarbonate ion (HCC>3 ) ; and (4) carbonate ion (C03 ) . The concentration of the hydrogen ions (pH) is important on -2 the relative abundance of CC>2, HCC>3 and CC>3 in . the solution. In most freshwater systems there is no free C03 carbonate precipitation will only occur if the lacustrine system is saturated with respect to both the calcium ion and the carbonate ion. The most effective way to increase the concentration of the carbonate ion is to remove the hydrogen ions from the system (i.e. increase pH). For each mole of carbon dioxide removed there will be two moles of hydrogen removed. The most effective way of obtaining a supersaturation of solution with respect to calcium carbonate is to remove carbon dioxide (and therefore the hydrogen ion) from the solution. Carbon dioxide solubility in water can be decreased by increasing temperature and/or decreasing pressure. The balance between carbon dioxide consumption by photosynthesis, carbon dioxide production by respiration and decay, carbon lost to the sediments and carbon dioxide replacement from the atmosphere is critical in the formation of lacustrine carbonates. Depletion of carbon dioxide from the lacustrine environment favors a predominance of phytoplankton that can utilize carbon from 177 carbon dioxide (Wetzel, 1976). This process of utilizing the bicarbonate ion will result in an increase in the pH (decrease in hydrogen ion) and lead to the supersaturation and precipitation of calcium carbonate and is the basis of the bio-induced form of calcium carbonate precipitation. In the study of lacustrine systems one finds that water temperature decreases with depth, producing warm but carbon dioxide deficient water on top and colder but higher content carbon dioxide water on the bottom. The pH of most lacustrine systems decreases towards the bottom, which supports the retention of carbon dioxide in solution and calcium carbonate saturation therefore decreases with depth. In most lacustrine systems there is a net photosynthesis in the epilimnion and a net respiration in the hypolimnion during a summer day which will cause a markedly decrease in oxygen content of the bottom water to a point anoxia occurs in the hypolimnion. In saline- alkaline lakes and temperate marl lakes the removal of carbon dioxide from the epilimnion may increase the pH to 9.0 or more which causes the epilimnion to become supersaturated with respect to calcium carbonate and precipitation of the calcium carbonate occurs. 178 If the lake is highly eutrophic and has a hypolimnion that is anoxic or nearly so the lake may become permanently stratified (meromictic) with a permanently anoxic bottom (monimolimnion) that can contain hydrogen sulfide and/or methane. Dean and Gorham (1979) have shown that the calcium carbonate precipitate may never reach the lake bottom because dissolution may take place in the hypolimnion in response to the increased carbon dioxide content (and resulting high hydrogen ion concentration or lower pH) and an excess of organic material. Dean and Gorham (1979) found that once the organic matter content of the sediment reached 39% no carbonate material was found in the profundal sediments. Megard (1968) found that calcium carbonate generated by photosynthesis in the epilimnion during the summer and might survive dissolution in the undersaturated hypolimnion but would be dissolved from the sediment in the following winter, hence no net carbonate sedimentation took place in the profundal area of the lacustrine system, leaving only siliciclastic sedimentation. Littoral carbonate production in lacustrine systems is a different matter, inorganic precipitation rates are great because of greater diurnal and seasonal 179 temperature fluctuations and are greater because of high photosynthesis rates of littoral algae and rooted aquatic vegetation. The algae and rooted aquatic vegetation remove carbon dioxide at a rate much faster than it can be replaced causing supersaturation of the littoral areas with respect to calcium carbonate. In studying the formation of carbonates in a lacustrine system one will find that some facies will be time-dependent so that facies boundaries are essentially time lines (i.e. the beginning of calcium carbonate precipitation, the formation of organic-carbonate rich muds) while other facies will be time transgressive, increasing in age towards the center of the basin as the lake fills in or outward from the basin center as the lake expands (vertical sequence in the stratigraphic record). These facies relationships may be extremely complex and care must be taken in trying to separate the time-dependent profundal facies sequence that reflects changes within the lake from their laterally equivalent time-transgressive littoral facies of the lake margin that change as the lake expands or fills in. There are several organosedimentary carbonate 180 structures that are formed in lacustrine systems by the trapping and binding of carbonate sediment particulates or chemical precipitation. These structures are: (1) lacustrine stromatolites; (2) tufa domes; and (3) oolites and oncolites. The formation of non-marine stromatolites appears to be limited to lacustrine shoreline areas of Lake Gosiute, Green River fm. (Bradley, 1929; Surdam and Wray, 1978), Pliocene Ridge Route formation of California (Link, Osborne and Awramik, 1978), the Pliocene-Pleistocene Lake Turkana, Kenya (Abell, Awramik, Osborne and Tomellini, 1982) and the Pleistocene-Holocene Walker Lake, Nevada (Osborne, Licari and Link, 1982). In most cases the lacustrine stromatolitic carbonate deposits are small generalized columnar forms, found in areas of strong wave activity and in association with facies interpreted as being lake shore deposits. The formation of tufa domes was originally described by King (1878), Russell (1883, 1885, 1889) and Dana (1884). King (1878) originally describe all tufa as being thinolite but Russell (1883, 1885) defined three basic types: (1) thinolite, (2) dendritic, and (3) lithoid. Dana (1884) 181 originally thought thinolite (describe from Soda Lake, Nevada) as being a pseudomorph after gaylussite (Na2Ca(CC>3 )2 5H2°) but redefines it as an unknown double salt of perhaps chloro-carbonate of calcium. Jones (1925) said the original material was aragonite and was the first to associate blue-green algae with its formation but Dunn (1953) and Radbruch (1957) showed that individual crystals of thinolite were calcite. La Rivers (1962) presented the following summary of how tufa formed: (1) through the agency of algae, (2) by mechanical precipitation against shores and headlands, largely by wave action, (3) by precipitation from supersaturated waters entering the cold lake waters from hot springs, (4) by precipitation in bottom muds through the action of hot, supersaturated waters rising from below, and (5) by precipitation throughout the lake waters as they concentrate during periods of general desiccation. In all cases the tufa, especially thinolite crystals are found in areas of high organic-carbonate muds where springs (usually hot springs but also cold springs) and/or groundwater mix with the saline-alkaline and usually colder lake water. Linear alinement of tufa domes indicate spring activity along a fault zone and can usually be 182 utilized to help delineate lake margins. Lacustrine marl formation has been studied in detail by Murphy and Wilkinson (1980) for Littlefield Lake, Michigan, by Brinkley,Wilkinson and Owen (1980) for Ore Lake, Michigan, Brunskill and Ludham (1969), Brunskill (1969), Ludham (1969), and Dean (1981) for Green Lake, New York, and Muller, Irion and Forstner (1972) for European and Near Eastern lakes in general. Carbonate Sediments-Ooids Marine ooids exhibit little morphologic variation in comparison to ooids from areas of both lower and extremely higher salinity settings. Non-marine and hypersaline ooids are volumetrically insignificant from a stratigraphic point of view in comparison to marine ooids but because they show a great deal of mineralogic and morphologic variation owning to their formation under a variety of different physical and chemical environmental conditions are important in the study of modern and ancient non-marine low to high salinity lacustrine systems. Only a few non-marine lacustrine and fluvial ooids have been described. These non-marine systems include the Great Salt Lake, Utah (Mathews, 1930; Eardley, 1938; Kahle, 1974; Sandberg, 1975; Halley, 1977), Lake Lahontan and Pyramid Lake, Nevada (Jones, 1925; Popp and Wilkinson, 1983), Higgins Lake, Michigan (Wilkinson, Pope and Owen, 1980), the Pliocene Glenns Ferry formation, Idaho (Swirydczuk, Wilkinson and Smith, 1979, 1980), and fluvial oolites (McGannon, 1975). Popp and Wilkinson (1983) in their study of ooids from Pyramid Lake, Nevada found both aragonite and low-magnesian calcite ooids. They basically agreed with Jones (1925) that the ooid formation resulted from the mixing of calcium-rich spring waters with bicarbonate-rich lake waters in near shore agitated environments. The restriction of Holocene ooid occurrence to the Needles area (an area of intense tufa building activity at the northwestern end of the lake), and the formation of ooids only in the areas of spring discharge indicated an intimate association between hot spring activity and ooid formation. Wilkinson, Pope and Owen (1980) described the formation of low-magnesian calcite ooids growing on shallow water lake margin bench platforms in a temperate region marl lake, Higgins Lake, Michigan. Swirydczuk, Wilkinson and Smith (1979, 1980) described the formation of massive beds of ooids (up to 12 meters deep (40 ft)) from the Pliocene Lake 184 Idaho's Glenns Ferry fm. These ooid beds exhibited horizontal bedding, vertical burrows, some large scale foreset bedding, locally abundant fish, wood and molluscan debris, and has been interpreted as a transgressive beach and nearshore sand environment. The oolitic sands grade upward into an overlying sequence of lake center silts and ashes. They postulated the ooid formation was on a near- shore bench and ooid deposition was on lakeward, steeper dipping bench slopes. VARVES AND VARVING MECHANISMS In the examination of the very thinly laminated sediments found in Stewart Valley, we find they consist of a series of alternating light and dark laminae. Such thinly laminated sediments occur as distinct units and have been shown by x-ray diffraction analysis to be composed of mainly cristobalite. Bradley (1929) in one of his first papers on the Green River fm. discusses the term varve in relation to thinly laminated annual sediments and traces the definition through Antevs to de Geer (see lit. cite, in Bradley, 1929). De Geer (1912) proposed the international term varve for distinctly marked annual deposit of a sediment regardless of its origin (Flint and Skinner, 185 1977). Older texts on physical geology (Longwell, Knopf and Flint, 1932) use this basic definition, however, since that time the term varve has been applied primarily and mistakenly only to glacial deposits by many authors (Potter, Maynard and Pryor, 1980) with some authors recognizing de Geer's original definition and more utilitarian application of the term (Pettijohn, 1975; Friedman and Saunders, 1978; Leeder, 1982). Blatt, Middleton and Murray (1980) , for example, in describing varves use as part of their definition "this type of lamination is the type most typical of glacial varves" which most people intrepret as being found "only in" glacial deposits, which they are not. Reineck and Singh (1980), and Davis (1983) get around this problem by using the term rhythmites as originally described by Bramlette (1946) in refering to repetitous sequence of generally thin and alternating types of sediments, The cyclic nature of rhythmites when caused by season changes are varves. Proving the cyclic seasonal nature of the laminations can be a difficult problem, whereas showing the cyclic nature (alternating light and dark bands) can be seen visually. In studying the origin of varves in non-glacial, 186 non-marine lacustrine settings, most authors have shown the alternating light and dark bands to be caused by biogenic production in the lakes coupled with chemical precipitates (calcium carbonate, silica,,etc.), clastic sediments, and/or volcanic ash (Bradley, 1929; McLeroy and Anderson, 1966; Eugster, 1967, 1969, 1970; Eugster and Hardie, 1978; Jones, Rettig and Eugster, 1967; Anderson and Kirkland, 1969; Friedman and Saunders, 1978). Dickman (1979) in reviewing the possible varving mechanisms for meromictic lakes discussed rhythmically laminated lake sediments and their possible use as indicators of chronology based upon annual events. He reviewed the common explanation that the varve formation in meromictic lakes is the result of biogenic calcium carbonate precipitation and that the Black Sea light colored laminae were caused by seasonal plankton blooms of coccolithophorids. Anthony (1977) found the light-colored laminae in a meromictic iron-rich lake to be caused by the precipitation of iron oxides during periods of partial oxygenation of the lower mixolimnion. Calvert (1963) in studying the Gulf of California found the light laminae are a result of diatom deposition. Strom (1939) attributed the 187 formation of dark-colored laminae to sedimentation of algal matter and Culvert (1963) to dark clays. Dickman (1979) proposed the formation of meromictic dark laminae to anaerobic plankton bacteria. He compared two lakes: Crawford Lake, Ontario, which has dark laminae, and Pink Lake, Ontario, which has no dark laminae. He came to the conclusion that the process associated with alke meromixis and anaerobic bacterial deposition are also responsible for the formation of the rhythmically laminated light and dark bands in profundal and subchemocline sediments of meromictic lakes. Pink Lake in October became homeothermic in the deepest portion of the mixolimnion which was anaerobic during the summer. This sudden oxygenation by wind mixing coincided with massive sedimentation when 60% of the annual seston load accumulated; some 87% of this sediment was bacteria, mainly purple sulfur bacteria. The bacterial biomass of the lake never exceeded 15% to the total biomass. Crawford Lake has dark laminae composed of pyrite framboids, blue-green algal heterocysts, fungi, plant fibers, diatoms and tests of euglenoid alga; the light laminae consist of calcite, clasts, few dark granuals and 188 pine pollen. Dickman (1979) proposed the following seasonal changes in an hypothetical meromictic lake. First, during spring, summer and early fall, decomposition of organic matter entering the lower mixolimnion of the lake produces a gradual elimination of the oxygen of that layer. The anaerobic zone which exists at the chemocline in early spring moves upward during the summer, engulfing most of the lower mixolimnion by late fall, there-by creating an anaerobic zone in an area that is normally aerobic. During the late fall the mixolimnion destratifies and mixes with oxygenated surface waters. This intrusion of oxygenated water into the lower mixolimnion kills all the anaerobic and photosynthetic bacteria found in that area. The water column in temperate zone meromictic lakes in early spring has an oxygenated mixolimnion with the exception of a thin layer above the chemocline. The dead bacteria can serve as nucli for pyritization ( or for that matter the nucli of iron monsulphides). As decomposition of the organic tripton removes dissolved oxygen from the underlying water mass, the anaerobic region increase in thickness and the photosynthetic bacteria move closer to the lake surface where light intensities are greater 189 allowing for greater biogenic production. Dead bacteria settle to the bottom and form a thin organic-rich layer which also contains ferrous sulphides. Pink Lake in the spring has a rise in temperature and to a lesser extent a rise in pH which triggers the precipitation of calcium carbonate from the epilimnion. Photosynthetic bacteria for a dense plate on top of the anaeronic zone in the mixolimnion and supplant the other phytoplankton as major primary producers. Crawford Lake also undergoes similar changes but dark laminae are formed in the fall unlike Pink Lake. The difference between the two lakes is Pink Lake is essentially free of dissolved iron in its moniolimnion. Although dark clays and other sulfides of metals may produce dark laminae in anaerobic sediments, the dark laminae in most anaerobic sediments are the results of ferrous sulfides. In studying the reduction of iron sulfates to H^S (Takahashi, Broecker and Thurber, 1968) has shown for each mole of organic carbon decomposed in the moniolimnion there are .55 moles of H S produced. H S and ferrous ions are 2 ^ mutually incompatable in solution and lakes with high concentrations of ferrous ions in there moniolomnion will 190 2 - rarely produce H2S. Carbonate rich waters with CC>3 may 2+ compete successfully with H a S for Fe so sediments may contain FeCO rather than FeS or FeS . Carbonate-poor, H S -3 2 2 -poor lakes have substantial soluble iron in their moniolomnion. Biogenic accumulations of iron concentrations can even cause a holomictic lake to undergo permanent stratification. Lakes that are poor in sulfur and rich in iron also fail to produce light-dark laminated sediments. Dark sediments also fail to form in lakes that are poor in other sulfide forming metals. Therefore, for alternating light and dark laminae to form in lake sediments, a meromictic lake is needed in which the mixolimnion becomes anaerobic due to biogenic decomposition and repopulation by anaerobic, photosynthetic bacteria ( mainly purple-sulfur bacteria) and upon remixing in the fall and winter the mixolimnion is reoxygenated, killing the anaerobic bacteria. Secondly, the lake must have a moniolimnion rich in iron and sulfur to react with the organic, dead bacteria to form a layer of dark sediment. TRACE FOSSILS or LEBENSSPUREN 191 Non-marine trace fossils or lebensspuren are for the most part rare in the fossil record and are virtually unstudied in comparison to those trace fossils found in the marine environment and terrestrial vertebrate trace fossils (e.g. tracks). Recent studies include Chamberlin (1975), Fisher, Lick, McCall and Robbins (1980), Ratcliffe and Fagerstrom (1980), McCall and Tevesz (1982), Fisher (1982), Cohen (1984), Ekdale, Bromley and Pemberton (1984), Brucken and Picard (1984), Retallack (1984), and Squires and Advocate (1984). Chamberlin (1975) presented the most comprehensive discussion of recent non-marine aquatic trace fossils to date. The meager published research of Holocene and fossil invertebrate lebensspuren of non-marine aquatic origin has greatly hindered the use of such structures in the intrepretation of non-marine paleoenvironments. Some trace fossils such as feeding trails of molluscs and the U- shaped burrows of chironomid midge larvae are distinct enough to be identified and enough environmental characteristics of each taxon is known to be useful in paleoenvironmental reconstruction. The presence of non-marine trace fossils and bioturbated sediments are thought to occur together where 192 as fine-grained laminated lacustrine beds are usually interpreted as developing under anoxic bottom water conditions where 0 C m levels are too low to support invertebrate burrowing and filter-feeding metazoans. See Boyer (1982) for a discussion of the origin of various types of non-marine lacustrine laminated sediments. The preservation of the laminated sediments is believed (in part) to be due to low 0 availibility or high salinity but as Cohen (1984) has pointed out, the preservation of laminates in lakes or the lack of bioturbation of lake sediments in general may be be caused by carbon-poor sediments. Thus the productivity of off-shore benthonic invertebrates in well oxygenated water is limited by the organic detritus deposited in the sediments. So the lack of bioturbation if off-shore sediments can be caused by anoxia, salinity, or lack of organic detritus and thus models of laminae formation in ancient lake deposits must take these three factors (oxygen content of the hypolimnion, salinity of the hypolimnion, and organic carbon content of the sediments) into account. McCall and Tevesz (1982) in their study of the effect of benthonic organisms on the physical properties of non-marine freshwater sediments have shown (after Sly, 1979) that most alkes vary from marine environments in a variety of ways and as a consequence, the benthonic organisms and activities are different. Primarily lakes have clastic or carbonate near-shore environments and fine- grained sediments in deeper water. That bioturbation in lakes is caused primarily by chironomid midge larvae and oliogchaetes with amphipods, bivalves, and molluscs in a secondary role. Fine-grained lacustrine sediments are ideal for bioturbation by most benthonic organisms. 194 CONCLUSIONS 1) The Miocene sediments in Stewart Valley represent a set of lacustrine deposits that are considerably older than the now restricted Esmeralda fm. The depositional sequence in Stewart Valley was coming to an end about the time the Esmeralda fm. depositional sequence was starting. 2) The lacustrine sediments in Stewart Valley range in age from more that 10 m.y. (10.1 m.y.) to less than 17 m.y. (16.1 m.y.) in age. 3) These sediments in Stewart Valley, if found not to synonymous with other existing, described formations, should be described as a new formation, the Stewart Valley formation. 4) The Miocene Stewart Valley lacustrine sediments represent a range of depositional environments from deep-water, near-shore, beach, deltaic, to fluvial. 5) The well preserved deep-water lacustrine sediments are unusual and possibly unique for the Basin and Range province. In most areas of the Basin and Range, the deep-water lacustrine Late Tertiary sediments usually become part of a graben and are buried under several 195 thousand feet (hundreds of meters) of alluvium. Once buried they under go diagenesis. 6) As shown by studies on pre-Oligocene and Oligocene tuffs in the Gabbs Valley Range, a N65W trough existed in the area prior to the Miocene and during the Miocene became the main depositional basin for Lake Savage. 7) Lake Savage was a chemically stratified meromictic lake that originally formed from a series of older paludal environments. 8) The earlier Lake Savage was a siliclastic lake in which the epilimnion had a pH range of from 7 to the low 8's. The lake produced abundant diatom flora which in turn produced diatom frustule deposits on the lake floor, which in turn made up the silicified sediments. Portions of that same lake produced abundant molluscan populations along the lake margins. 9) The lake has an anoxic bottom (at least in the begining) which may have been facultative rather than permanent. This anoxic environment produced some pyrite but mainly produced iron monosulphides (which along with organic matter produced the various brown to black colors of the deep-water sediments). 196 10) The lack of oxygen (at least part of the time) in the deep-water lacustrine environment resulted in the preservation of the fossil fish, leaves, and insects. 11) The fish remains were the result of normal mortality (individual fossils) and the occasional presence of hydrogen sulfide gas in the hypolimnion (mass mortality) being mixed with the epilimnion water. Since hydrogen sulphide and dissolved iron in the water are mutually exclusive, hydrogen sulphide gas usually did not occur in abundance but instead iron monosulphides were produced. 12) The well preserved insect remains are a result of (1) the chemically stratified meromictic lake with an anoxic hypolimnion, (2) little diagenesis of the silicified shale and paper shale (amorphous silica to low cristobalite grade diagenesis), and (3) the lack of metazoan scavangers in the deep-water lacustrine environment which may have been caused by lack of oxygen and/or lack of organic material for them to feed on. 13) The insect preserved in the silicified shale and paper shale are the result of natural mortality, and the proper preserving environment and not the result of a volcanic blow-down. There is no evidence what so ever that links the insect demise with volcanic action and all attempts in the past at other localities to show such a link have been show to be erronous. This does not mean that volcanic action was not important in the formation of Lake Savage and its sediments; for Lake Savage was situated in a active volcanic province and the volcanic action did contribute silica and iron to the sediments which was important in creating the laminated shales. 14) The chemistry of the lake waters changed over time and the lake went from a net siliclastic depositional system to a net carbonate depositional system. During this time the pH of the lake water increased past pH 8 and into the low pH 9 range. The molluscan fauna disappeared from the lake waters (much like the molluscan fauna of present day Pyramid Lake) and relict populations inhabited refugia in fresher bodies of water such as springs and streams, with a massive but species restrictive population emerging late in the lake's history. 15) The change of Lake Savage from a net siliclastic depositional system to a net carbonate depositional system is indicated in the sediments as calcareous mudrocks and sandstone, marlstones, oolites, and tufa deposits become prominant. The tufa domes occur in several cycles (two or three) within the upper portion of the lake sediments. Fossil trout make their appearence in large numbers in Lake Savage during the net carbonate depositional cycle. The trout, cyprinid fish and molluscs are all indicators Lake Savage was at one time connected to other lacustrine systems that contained these species. 16) Lake Savage most likely became shallower, warmer, more saline, and had a high pH during its later depositional period as indicated by the vertical and lateral succession of sediments. Thinly laminated deep- water sediments are replaced by diatomaceous mudstone and claystone, which in turn are replaced by gypsiferous mudrocks. 17) Volcanic activity played an extensive role in the development of Lake Savage. The lake appears to have been formed on an older lahar deposit, and contains numerous beds of vitric ash and biotite ash. These rocks contributed silica, iron, calcium, etc. to the lake system which made the alternating light and dark laminae possible as well as other deposits (e.g. tufa). This volcanic activity, while making the life of the lake possible, also 199 contributed to the death of the lake. The massive deposits of biotitic white ash and blue vitric ash late in the lake's history filled in many of the deeper portions of the lake in the region studied. 18) The total depth of sediments formed during the existance of Lake Savage was slightly greater than 500 m, which is considerably less than other Miocene-Pliocene deposits of comparable age longevity. The Wassuk Group, which is slightly younger and is found on the western side of the Walker Lane accumulated some 2,400 to 2,500 m of sediments in the same time interval as Lake Savage. This indicates the western side of the Walker Lane was a tectonically active intra-arc basin while the Stewart Valley portion on the eastern side of the Walker Lane was a very quiescent intra-arc basin. No intraformational faults have been found in the Lake Savage sediments, although indirect evidence exists (series of tufa domes in a linear array) that faults were present, as can be seen in the Wassuk Group sediments. Some later lateral movement can be seen in some younger volcanic flows and dip-slip movement with some strike-slip components can be seen in the more recent faults in Stewart Valley. Some intraformational folding (3 areas) plus one large block of laminated sediments that has slide into others, can be seen in the Lake Savage sediments indicating some tectonic activity. Also one area to the southeast of the study site has some folded Lake Savage sediments overlain by unaffected Pliocene and Pleistocene deposits indicating some localized tectonic activity. 19) The fact that the laminated and paper shales are mainly cristobalite indicates the Lake Savage sediments have undergone very little diagenesis and were most likely never buried very far below the surface (several hundred meters maximum). If the sediments had been buried deeper and undergone more diagenesis, more of the cristobalite would have been converted into quartz. 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