RICE UNIVERSITY
SEDIMENTOLOGY AND STRATIGRAPHY OF THE
TRIASSIC CHINLE FORMATION,
EASTERN SAN JUAN BASIN, NEW MEXICO
by
DENNIS DARL KURTZ
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
Master of Arts
Thesis Director's Signature:
Houston, Texas
April, 1978 ABSTRACT
SEDIMENTOLOGY AND STRATIGRAPHY OF THE
TRIASSIC CHINLE FORMATION,
EASTERN SAN JUAN BASIN, NEW MEXICO
Dennis Dari Kurtz
Upper Triassic continental ëtrata of the Chinle Formation, in the Nacimiento Mountains, New Mexico, represent two cycles of fluvial to fluvial-deltaic to marshland-lacustrine sedimentation. Each cycle was initiated by uplift in the southern Ancestral Rocky Mountains and deposited on the local palaeoslope. However, the influence of this highland on sediment transport and supply diminished with time. Depo- sitional trends exhibited up-section are: 1) fluvial regime changed from a high energy braided stream to a very low energy meandering stream system; 2) palaeotransport directions shifted from southwestwardly to north-northwest¬ wardly; 3) the initial crystalline and metasedimentary source terrain was gradually replaced by a widespread sedimentary one; and 4) mean and maximum grain size decreased. Decreases in fluvatile energy and grain size upwards reflect the lowering of uplands by erosion and the evolution of a con¬ tinental drainage system on a gentle gradient. Also, the uplift which generated the lower fluvial sequence probably had a steeper initial palaeoslope and was of greater magnitude than the second uplift. Shifts in palaeotransport and provenance are not due to active tectonism, but rather, are a passive response to the waning influence of local highlands on regional sedimentation patterns.
The locations and distributions of sedimentary copper deposits, found in the fluvial channel sequences of Chinle sandstones, are chiefly dependent upon the shape and porosity of the channel deposits,and the amount of organic matter that was present to reduce copper-bearing fluids.
Porosity of these channels, as it affects copper mineral¬ ization is controlled by the amount and' time of formation of calcareous cement. ACKNOWLEDGMENTS
I should like to thank my major thesis advisor,
Dr. John B. Anderson, for his encouragement, constructive criticism,and confidence in me throughout this study.
Special acknowledgments are also due to Glen Vague, my able field assistant during the summer of 1976, without whose organization and companionship this thesis would have been a much more difficult task, and to Drs.
H. C. Clark and Donald R. Baker for their help. This project was supported by grants from the New Mexico Bureau of Mines and Mineral Resources and the American Association of Petroleum Geologists. TABLE OF CONTENTS Page INTRODUCTION 1
METHODS 3 STUDY AREA 5
Location 5 Accessibility 5 Climate 8 Physiography 9 Structure and Tectonics 10
STRATIGRAPHY 12
Permian Cutler Group (Wolfcampian - Leonardian) 12 Upper Triassic Chinle Formation (Karnian - Norian) 15 Agua Zarca Sandstone (Karnian) 17 Salitral Shale tongue (Karnian) 21 Poleo Sandstone lentil (Kami an-Norian) 23 Upper Shale - Petrified Forest Member (Norian) 26 Red Mesa Sandstone 28 Jurassic Entrada Sandstone (Upper Jurassic) 32 Palaeomagnetic Stratigraphy 32
PALAEOTRANSPORT DIRECTIONS 33 Types of Palaeocurrent Structures Measured 33 Statistical Analysis 33 Reliability of Statistical Measurements 41 Permian Cutler Group - Palaeocurrent Directions 43 Triassic Chinle Formation - Palaeocurrent Directions 43 Agua Zarca Sandstone 43 Salitral Shale tongue 47 Poleo Sandstone lentil 48 Lower Poleo Sandstone 48 Upper Poleo Sandstone 52 Upper Shale Member 56 Red Mesa Sandstone 56 Page SEDIMENTARY PETROLOGY 61
Agua Zarca Sandstone 68 Poleo Sandstone lentil 69 Petrified Forest Member 74 Red Mesa Sandstone 74 C/M Diagram 75 Significance of Micaceous Material in Sediments 77
PROVENANCE 79
Permian Cutler Group 79 Triassic Chinle Formation 80 Agua Zarca Sandstone 80 Salitral Shale tongue 82 Poleo Sandstone lentil 83 Lower Poleo Sandstone 83 Upper Poleo Sandstone 84 Upper Shale - Petrified Forest Member 84 Red Mesa Sandstone 85 Origin of Volcanic Material 86
ENVIRONMENTS OF DEPOSITION 88
Agua Zarca Sandstone 88 Salitral Shale tongue 88 Poleo Sandstone lentil 90 Lower Poleo Sandstone 90 Upper Poleo Sandstone 91 Upper Shale - Petrified Forest Member 93 Red Mesa Sandstone 94
GEOLOGIC HISTORY 95
Summary - Geologic History 103 Palaeoclimatology 108 ECONOMIC GEOLOGY 111
Copper 111 Primary Copper Deposits 112 Secondary Copper Deposits 115 Genesis of Copper Deposits 115 Economic Potential 118 Prospecting 118 Uranium 121 Petroleum 122
REFERENCES 123 Page APPENDIX Is
Locations and Descriptions of Measured 132 Sections-Upper Triassic Chinle Formation North-central New Mexico
APPENDIX II: 167 Locations and Lithologic Descriptions of Samples Used in this Study
APPENDIX Ills 178 Locations of places referred to in this study
APPENDIX IV: 181 Summary of Characteristics of Stratigraphic Units FIGURES Page 1. Index map showing location of study area 6 2. Map of the field area showing structural 7 provinces; numbered locations and sample sites 3. Lithologies of measured sections — Upper 13 Triassic Chinle Formation 4. Fence diagram showing distribution and thicknesses of Upper Triassic Strata (in back pocket) 5. Stratigraphic relationships between Upper 16 Triassic units in north-central New Mexico and elsewhere in the southwestern United States 6. Map showing approximate northeastern limits 18 of deposition for the Agua Zarca Sandstone and the Salitral Shale tongue
7. Sharp erosional contact between the Agua 20 Zarca Sandstone and the Permian Cutler Group
8. Convolute laminations and pebble conglomerate, 25 Poleo Sandstone lentil
9. Thick exposure of Upper Triassic strata - 27 Abiquiu Dam, New Mexico 10. East-west stratigraphic cross-section of 29 Upper Triassic strata, North-central New Mexico
11. North-south'stratigraphic cross-section of 30 Upper Triassic strata, North-central New Mexico 12. Palaeocurrent rosettes for a hypothetical 42 linear channel and graded stream and from a modern braided stream
13. Map showing mean and total palaeocurrent 44 vectors and trends of bi-directional structures - Agua Zarca Sandstone Page 14. Palaeocurrent rosettes for various sample 45 sites - Agua Zarca Sandstone 15. Map showing mean and total palaeocurrent 49 vectors - Lower Poleo Sandstone
16. Palaeocurrent rosettes for various sample 50 sites - Lower Poleo Sandstone
17. Map showing mean and total palaeocurrent 53 vectors and trends of bi-directional structures - Upper Poleo Sandstone
18. Palaeocurrent rosettes for various sample 54 sites - Upper Poleo Sandstone and Petrified Forest Member
19. Palaeocurrent rosettes - site 12 - Red 58 îîesa Sandstone
20. Cumulative frequency curves of Upper 66 Triassic Sandstone beds throughout the field area
21. Cumulative frequency curves of Upper 67 Triassic Sandstone beds on French Mesa
22. Triangle plot of sand-silt-clay - Lower 71 Poleo Sandstone
23. Triangle plot of sand-silt-clay - Upper 72 Poleo Sandstone
24. Carbonaceous plant fragments - Upper 73 Poleo Sandstone
25. C/M plot of Upper Triassic Sandstone Chinle 76 Formation, North-central New Mexico
26. Map showing maximum measured pebble sizes - 81 Agua Zarca Sandstone; Lower Poleo Sandstone
27. Straight foresets of a point bar Sequence - 92 Upper Poleo Sandstone
28. East-west cross-section of Chinle Formation 105 North-central New Mexico
29. North-south cross-section of Chinle Formation 106 North-central New Mexico TABLES Page
1. Locations of sample sites referred to in 4 this study
2. Summary of stratigraphic relationships 14
3. Statistical formulas used in calculations 36 4. Comparison of bi-directional vs. 39 uni-directional data
5. Mean palaeocurrent vectors and mean vector 46 magnitudes determined from palaeocurrent measurements in the Agua Zarca Sandstone
6. Mean palaeocurrent vectors and mean vector 51 magnitudes determined from palaeocurrent measurements in the Lower Poleb Sandstone 7. Mean palaeocurrent vectors and mean vector 55 magnitudes determined from palaeocurrent measurements in the Upper Poleo Sandstone 8. Mean palaeocurrent vectors and mean vector 57 magnitudes determined from palaeocurrent measurements in the Upper Shale Member
9. Mean palaeocurrent vectors and mean vector 59 magnitudes determined from palaeocurrent measurements in the Red Mesa Sandstone
10. Samples analyzed and statistics generated 62 by RUASA
11. Relative amounts of quartz, quartzite, and 65 chert in upper Triassic Chinle Conglomerates
12. Explanation for Figures 28 and 29 107
13. Production of strata-bound copper in ng New Mexico as of 1956 1
INTRODUCTION
Along the eastern margin of the San Juan Basin, in the Nacimiento Mountains, a complex group of upper
Triassic continental deposits is exposed. The members of this sequence, collectively called the Chinle Formation, range in texture from pebble conglomerates and medium- to coarse-grained sandstones to siltstones and shales.
Elsewhere in the southern Rocky Mountains contemporary rocks are predominantly shales and siltstones. Only on the Western side of the San Juan Basin and in central- northern Utah do upper Triassic strata contain widespread sandy units.
Lithogenetic sequences and paleotransport patterns of the Chinle Formation in the vicinity of the Nacimiento
Uplift offer a unique opportunity to refine our under¬ standing of late Triassic tectonic activity in the Ancestral
Rocky Mountains. Exposures elsewhere on the Colorado
Plateau make it possible to relate this tectonism to the early development of the San Juan Basin and to examine the effects of regional geography upon late
Triassic palaeoclimate. Sedimentary factors influencing the genesis of potentially valuable strata-bound copper ores can also be evaluated.
Upper Triassic environments of deposition were determined from the sedimentologic character of the
Chinle units. They provide the basis for a discussion of the early Mesozoic geologic history of the eastern 2 margin of the San Juan Basin. This history is then related to the early development of the entire basin to the dis¬ tribution of sedimentary copper deposits, and to a regional interpretation of late Triassic history. 3
METHODS
Stratigraphic sections were measured using a Brunton compass and a Jacob's staff, or a 100 foot steel tape.
Locations of the sections are shown in Figure 4; sections descriptions are in Appendix I. Palaeocurrent directions and trends were measured with a Brunton compass. The attitude of the bed in which the measurement was taken, the bed's location in the section, and the type and size of structure measured were also noted. Measurement precision was approximately i 5° in most cases. Locations of palaeocurrent measurement sites are listed in Table 1. Sandstone and conglomerate specimens were petro- graphically described from, thin sections and/or hand specimens. These petrographic descriptions emphasized texture, mineralogy, sedimentary strucutres, and the characteristics and orientation of micaceous minerals. The rock naming scheme and classification of Folk (1974) were used throughout this study. Shales and mudstones were described from hand specimens but no detailed analyses of these rocks were performed. SITE # NAME OF LOCATION TOWNSHIP AND RANGE LATITUDE AND LONGITUDE * c 0 0
Location
2 The field area comprises approximately 2000 km , and is located in and around the western portion of the Santa Fe National Forest in north-central Sandoval County and southern Rio Arriba County, New Mexico. Though not rectangular, the area lies entirely within 35o45'00" and
36°37'30M north latitude and 107°00,00" and 106°15'00" west longitude (Fig. 1). Structurally it is located along the eastern border of the San Juan Basin and includes parts of the Nacimiénto and San Pedro Uplifts, the southern half of the French Mesa-Archuleta Arch, and the southwestern portion of the Chama Basin. The Rio Grande Rift lies just to the east; the Jemez Volcanics are to the east and south¬ east; and the Continental Divide runs through the north¬ western part of the area (Fig. 2).
Locations of the places referred to throughout this thesis are listed in Appendix III.
Accessibility
A large part of the field area is readily assessible by
Highways 44, 126, 96, 84 and 112, and the numerous Forest
Service roads. All of the highways and Forest Service
'all weather' roads are suitable for 2-wheel drive vehicles, but a larger number of unmaintained roads are passable only with a 4-wheel drive vehicle. Unpaved roads become FIGURE 1 Index map showing location
of the study area 6 FIGURE 2: Map of the field area showing structural provinces ; numbered
locations are sample sites
(Table 1) 7 8 impassable during and immediately following torrential summer rains, and at times during the winter months.
Climate
The field area lies on the border between two major climatic zones :
1. Tropical semi-arid climate — dominated by tropical and equatorial air masses, and
2. Mid-latitude semi-arid climate — dominated by
tropical and polar air masses (Critchfield, 1966).
Consequently, depending on elevation, exposure, and season,
the vegetation and weather typical of either or both of these climates is present.
An average of approximately 250 mm of precipitation occurs each year (U.S. Dept, of Commerce, 1974), but this may be locally exceeded, particularly at higher elevations, where warm air masses from the Gulf of Mexico cause
torrential thunderstorms nearly every afternoon during the
rainy season (Pettit, 1975). This rainy season occurs in
the late summer (July through September) with this region having the second highest frequency of violent thunderstorms
in North America (Pettit, 1975). These storms cause flash-
floods, rock slides, and mass movement in clayey soils -
all effective sediment transporting agents.
Annually the area receives an average of 75 per cent
possible sunshine and has a mean temperature of 13.8°C
(56.8°F). The coldest months are January and February and 9 the warmest months are July and August. The average relative humidity is less than 50 per cent.
Physiography
A wide variety of topographic features are present in the field area, which ranges in elevation from approximately 2040 m (6700 ft) to 3050 m (10000 ft). In the southern part of the area (south of 36°15'N. latitude and west of 106°45'W. longitude) highlands consist of rounded knobs of Precambrian crystalline rock. Rugged hogbacks of resistant, steeply dipping, sandstones separated by canyons and deep valleys characterize the western margin of the Nacimiento Uplift.
Some near-horizontal beds form mesas which are cut by steep, v-shaped stream valleys.
At lower elevations the landscape is composed of hills and northwestwardly sloping terraces with eastward and southward facing cliffs. The terraces are separated by broad valleys with deeply incised arroyos.
In the northern part of the study area, highlands are composed of extensive, relatively flat mesas capped by gently dipping sediments, basalt flows, or tuffs. These mesas are separated by steep, high cliffs and are dissected by steep- walled canyons.
Except for the Rio Chama, Salitral Creek, and the Rio
Puerco, streams are small and intermittent. The Rio Chama and its tributaries drain the entire eastern portion of the field area. 10
Structure and Tectonics
Four major tectonic provinces exist within the study area (Fig. 2). These include the San Juan Basin, the San
Pedro-Nacimiento Uplift, the French Mesa-Archuleta Arch, and the Chama Basin. The western boundary of the study area lies within the San Juan Basin and is characterized by westward dipping late Cretaceous and Cenozcic strata. Older sediments, including Triassic rocks, are exposed along the structurally raised eastern boundary of the basin. This boundary is the San Pedro-Nacimiento fault (Woodward and others, 1972). The uplift itself is an upthrust (Woodward and others, 1972) consisting of several fault blocks with varying degrees of offset relative to one another. Indi¬ vidual blocks are separated by tear faults trending nearly perpendicular (eastward) to the trace of the upthrust.
Displacement of the entire uplift relative to the San Juan Basin may be as much as 3000 m (10000 ft) (Woodward and others, 1972). Maximum offset occurs just north of Site 3 (Fig. 2) where Precambrian basement rocks are in fault contact with upper Cretaceous Mancos shales. The amount of offset diminishes northward from this point. Tuffs of the
Jemez volcanic district cover the eastern border of this uplift but no structure of the magnitude of western upthrust exists on this margin (Wood and Northrup, 1946).
The French Mesa-Archuleta Arch and the San Pedro-
Nacimiento Uplift are parts of the same major tectonic province. A boundary between them has been drawn on the 11 basis of surface exposures of basement rock (Woodward, 1974) . No Precambrian crystalline rocks are exposed in the
French Mesa-Archuleta Arch. The arch is a broad anticlinal feature whose western limb is faulted, forming a boundary with the San Juan Basin (Wood and Northrup, 1946;
Lookingbill, 1953, and Fitter 1958), and whose eastern limb forms the western margin of the Chama Basin (Woodward, 1974, and Gibson, 1975). Many of the sedimentary rocks exposed across this anticline are late Triassic in age (Wood and Northrup, 1946).
The eastern part of the field area lies within the
Chama Basin, and is partly covered by tuffs and basaltic flows from the Jemez volcanic field (Smith and others,
1970) . Most of the rocks exposed in this basin are moderately flat lying Triassic sediments (Wood and Northrup, 1946). The basin is a north-south trending synclinal feature associated with the anitclinal uplifts to the west. On the east the Chama Basin is bound by the Rio Grande Rift, and on the northeast by the Brazos Uplift (Woodward, 1974). 12 STRATIGRAPHY
Eleven stratigraphic sections (Appendix I) were measured in Triassic strata throughout the field area. Figure 3 is a graphic presentation of ten of these sections, and the fence diagram (Fig. 4) shows the distribution and thicknesses of members of the Chinle Formation in the region. Stratigraphic nomenclature follows that of Wood and Northrup (1946) and
Stewart and others (1972). However, in this study, the Poleo
Sandstone lentil has been informally divided into recog¬ nizable subunits. A summary of stratigraphic relationships is given in Table 2.
Permian Cutler Group (Wolfcampian - Leondarian)
Permian rocks in the Nacimiento Mountains are composed of the Wolfcampian Abo Formation and the Leonardian Yeso Formation (Baars, 1962). They were mapped by Anderson
(1970) as the undifferentiated Cutler Group. In the southern part of this range both formations are present, but in the northern part only the older, Abo Formation occurs (Baars, 1962)
The Permian strata are "redbeds" consisting primarily of crossbedded sandstones and conglomerates interbedded with shales. Locally, limestone and gypsum units are exposed (McKee, 1967). Palaeotransport directions are consistently towards the southwest.• FIGURE 3: Lithologies of measured sections —
Upper Triassic Chinle Formation, north
central New Mexico 13 SYSTEM SERIES STRATIGRAPHIC THICKNESS ENVIRONMENT UNIT (meters) OF DEPOSITION * iH < CO ■H HJ W •H P OJ O G S fd CO G u (d CMP a G d) eu u fd CO CO O U fd CO 0 0 O G d) N • •H CO CO ü •H .GCO rH fd O SH -U ncJG HJ COG •H •H G tns fd
m • •H fd -K rH O * MH 'O rH d) rH •rj CUCU U HMË •H HJ Pu •H T3 CU0) *G CDM > HJ cd -H G rH *H eu d) eu G SH g | 0) -H M fd G O O o d) g fd CD N 1 fd • G Ht * rH O a\ vo m CH r^ J* n eu ea JH fd d) CO ÎH fd * iH
TABLE 2: SUMMARY OF STRATIGRAPHIC RELATIONSHIPS 14 15
Upper Triassic Chinle Formation (Karnian-Norian)
Wood and Northrup (1946) correlated strata in the
Nacimiénto Mountains with the upper Triassic Chinle Formation named by Gregory (1917) from exposures in Chinle Valley, Arizona. The Chinle Formation is the only representative of the Triassic period in the field area. Reeside and others
(1957) placed this formation in the Karnian and Norian stages, and McKee and others (1959) assigned it to U.S.G.S.
Paleotectonic Map Interval C for Triassic rocks in North
America. Figure 5 shows the correlations between Triassic lithostratigraphic units in the eastern San Juan Basin and strata of similar age elsewhere in southwestern North
America. Stewart (1969) divided upper Triassic rocks on the Colorado Plateau into three lithogenetic sequences and put the Chinle Formation of north-central New Mexico in his lowermost "bentonitic sequence".
Paléontologie evidence for the upper Triassic age assignment consists of vertebrate remains (Colbert and
Gregory in Reeside and others, 1957) and plant fossils
(Daughterty, 1941). Vertebrate bones are found in the upper shale member of the Chinle Formation near French
Mesa and Ghost Ranch (Colbert, 1947). Plant fossils are found in Triassic sandstones in the Arroyo del Cobre (U.S.G.S. fossil plant sites 10144 and 10145) and in the Nacimiento
Mine, near Cuba, New Mexico.
Wood and Northrup (1946) divided the Chinle Formation FIGURE 5: Stratigraphie relationships
between Upper Triassic units in
north-central New Mexico and elsewhere in the southwestern United States
(after Reeside and others, 1957) 16
« * £’• m » • l ‘ € Î i ! 17 in the eastern San Juan Basin into four lithostratigraphic units. In ascending order these are:
1. Agua Zarca Sandstone 2. Salitral Shale tongue
3. Poleo Sandstone lentil
4. Upper Shale Member (Petrified Forest Member of Stewart and others, 1972)
Outside the study area two other stratigraphic units locally overlie the Petrified Forest Member. Along the southern margin of the San Juan Basin, in the San Lucero Uplift, the Correo Sandstone Member named by Kelly and Wood (1946) is exposed; in the Chama Basin, an unnamed siltstone member described by Stewart and others (1972) outcrops. This siltstone unit is placed in Stewart's (1969) "redbed" lithogenetic sequence.
Agua Zarca Sandstone (Karnian)
The Agua Zarca Sandstone was named by Wood and Northrup
(1946) for exposures in Agua Zarca creek, near Coyote, New
Mexico. It is present in a northeast-southwest trending belt (Fig. 6) but is thickest and is best exposed in Senorito Canyon (Site 3, Fig. 2). Maximum thickness of this member is approximately 30 meters (100 feet). In the
Nacimiento Mountains near Cuba, New Mexico, resistant beds of the Agua Zarca Sandstone are prominent ridge formers and mesa caprocks. North from Senorito Canyon, across the
French Mesa-Archuleta Arch, this member becomes progressively thinner and is absent on French Mesa and Gallina Mountain FIGURE 6: Map showing approximate
northeastern limits of deposition
for the Agua Zarca Sandstone and
the Salitral Shale tongue 18
Salitral Shale
• Agua Zarca Sandstone
12 19
(Lookingbill, 1953). In the southwest Chama Basin the
Agua Zarca Sandstone is in most places thin or non-existent. However, a few, thick, erosional remnants of this member do crop out indicating that it was once more widespread.
Wood and Northrup (1946) recognized the Agua Zarca Sandstone
as far south as San Ysidro, New Mexico, where they placed
the basal Triassic sandstone of that area in this member.
Stewart and others (1972) have questioned this identi¬
fication on the basis of palaeotransport directions and
sedimentary textures.
The basal contact of the Agua Zarca Sandstone with the
Permian Cutler Group is sharp and erosional at all
localities (Fig. 7). Commonly , large channels have been
cut into the underlying strata. On a regional scale the
contact is angular, since the Agua Zarca Sandstone directly overlies the Abo Formation in the northern Nacimiento
Mountains, but rests on the younger Yeso Formation in the
southern part of this range (Baars, 1962). Typically,
the Agua Zarca Sandstone consists of an irregular (0-5m)
basal channel overlain by a much thicker (up to 60 m)
sequence of tabular or broadly lenticular beds. The basal
unit is not everywhere exposed but where present is composed
of numerous cross-cutting lenticular channels with large
scale foresets and festoon bedding ($ 50 cm). Channel
lithologies range from well-indurated pebble conglomerates
to indurated siliceous, pebbly, coarse-grained orthoquartzites.
Dispersed gray clay lenses are also present. The upper FIGURE 7: Sharp erosional contact between
Agua Zarca Sandstone and the
Permian Cutler Group 20 21 sequence consists of medium- to coarse-grained quartz arenite beds with laminar bedding, 15-50 cm straight
foresets, and convolute bedding; as well as cross-cutting channels. Sandstones become finer grained towards the top of the Agua Zarca Sandstone and are interbedded with persistent shale and claystone units. Contacts within the Agua Zarca are commonly sharp and erosional. Conglomerates and
sandstones are petrographically mature, containing well over 95% quartz and quartzite, however, locally derived angular and well-rounded clay clasts are important con¬
stituents in some beds. The Agua Zarca Sandstone contains very little diagenetic hematite but substanital copper mineralization has occurred, particularly in the lowermost channel sequence. Palaeotransport directions are generally towards the southwest.
Salitral Shale tongue (Karnian)
The Salitral Shale tongue was named by Wood and Northrup
(1946) for exposures in Salitral Creek, a tributary of the
Rio Chama in the northern part of the field area. This member lies stratigraphically between the Agua Zarca
Sandstone and the Poleo Sandstone lentil. Where the Poleo
Sandstone is absent, however, the Salitral Shale underlies
the Petrified Forest Member. Figure 6 shows the northern depositional limits of the Salitral Shale. The southern
limits are impossible to define since one cannot distinguish
between the Salitral Shale and the Upper Shale Member south
of the Arroyo de los Pinos (site 1, Fig. 2), where-the Poleo 22
Sandstone lentil pinches out. Like the Agua Zarca Sand¬ stone, the Salitral Shale tongue is thin or absent across the French Mesa-Archuleta Arch and thickens to the east of that arch. This shale is thickest (90 meters, 300 feet) near San Miguel Canyon (site 2, Fig. 2) but localized thick exposures exist in the Chama Basin, near Abiquiu, New Mexico
(Sears, 1953). Exposures are poor in most places since the
Salitral Shale weathers readily.
A gradational basal contact is present between the
Salitral Shale tongue and the Agua Zarca Sandstone. In many sections these two units show extensive interfingering
(section 4, Appendix I). The dominant lithology is a mottled red, maroon, and green bentonitic claystone but thin coarse silt to (rarely) medium sand sized quartz'arenite lenses are present. Chenoweth (1974) notes that limestone beds occur near the base of the Salitral Shale north of
Coyote, New Mexico. Virtually no sedimentary structures are preserved in the claystones, but laminae and small
scale (2-3 cm) symmetric ripples occur in the finer quartz
arenite lenses. Palaeocurrent indicators are not present in
the Salitral Shale tongue. Septarian concretions are common
in this member at the Nacimiento Mine and near San Pablo
Canyon. The "frothy" surface described by Stewart and others (1972) is present on weathered surfaces of the Salitral Shale
tongue throughout the field area. It is probably due to the
presence of large amounts of bentonite (Stewart and
others, 1972). Plant root casts, commonly preserved in the 23 vertical position, are present at locales near Cuba and Coyote.
Poleo Sandstone lentil (Kamian-Norian)
The Poleo Sandstone lentil was first described on
Poleo Mesa, near Coyote, New Mexico by von Huene (1911) who called it the "Poleo top sandstone". Wood and Northrup
(1946) later redefined this unit as the Poleo Sandstone
lentil. It is recognizeable throughout the field area north of the Arroyo de los Pinos (section C, Fig. 4) and is
thickest across the French Mesa-Archuleta Arch and in the
southeastern Chama Basin where it is a prominent ridge
former. This lentil is probably equivalent to the basal '
upper Triassic sandstone which crops out on the north¬
eastern side of the Chama Basin in the Brazos Uplift
(Bingler, 1968). The Poleo Sandstone is the only
Triassic sandstone present across the French Mesa-
Archuleta Arch. The basal contact of the Poleo Sandstone lentil, whether with the Permian Cutler Group or with the Salitral Shale
tongue, is everywhere sharp, irregular and erosional. In
many localities the Salitral Shale tongue contains a zone
of light green claystone just below this contact. This
claystone zone ranges in thickness from 2-40 cm (1-15 inches)
and appears to be related to the erosional surface rather
than to a particular depositional unit.
In this study the Poleo Sandstone lentil has been
divided into two identifiable submembers, designated 24
Tr , (lower Poleo Sandstone) and Tr (upper Poleo pi pu Sandstone). The lower Poleo Sandstone is recognized everywhere north of Senorito Canyon. Like the Agua Zarca
Sandstone, this unit consists of a thick, locally prominent, conglomeratic channel sequence (up to 12 m thick) overlain by approximately 25-30 m of tabular or broadly lenticular calcareous medium- to fine-grained micaceous quartz arenites.
Gray, red and purple claystones are commonly interbedded with the sandstone beds. Conglomerates and pebbly sandstones are we^ll-indurated, and typically massive. In a few exposures, however, they do exhibit large (150 cm) crossbeds. Quartz arenite beds are in some places massive but do exhibit large scale (35 - 45 cm) crossbeds, laminar beds, cut and fill structures, and convolute bedding (Fig. 8a); and may contain a basal pebbly zone (Fig. 8b). Sandstone beds fine upwards and in may places (eg.•Abiquiu Dam) in claystone is present at the top of this unit. Contacts between beds in the lower Poleo Sandstone are either sharp and erosional or gradational. Palaeotransport was to the southwest and southeast.
The upper Poleo Sandstone is present throughout the vicinity of the Nacimiento Mountains. Tabular, crossbedded
(34-45cm) , calcareous, micaceous quartz arenites comprise this unit and few conglomerates or lenticular channels are present. The upper Poleo Sandstone fines upwards and persistent siltstone claystone beds are interbedded with fine-grained quartz arenites near the top. Calcareous FIGURE 8:
a) Convolute laminations - Poleo
Sandstone lentil b) Pebble Conglomerate - Poleo Sandstone
lentil, Abiquiu Dam 25 26 sublitharenites are also present towards the top of this submember. Maximum observed thickness of the upper Poleo
Sandstone is about 45 meters, but its contact with the lower
Poleo Sandstone is not always recognizable. Where evident, this contact is sharp and erosional, however, a transitional contact might somewhere exist. The upper
Poleo Sandstone is characterized by northward palaeo- transport directions. This difference in palaeoslope dis¬ tinguishes the upper and lower portions of the Poleo Sandstone and is commonly the only means by which they can be differentiated.
Figure 9 is a photograph of the thick (75 m) exposure of the Poleo Sandstone lentil at Abiquiu Dam (site 11,
Fig. 2). Many of the features described above are exhibited there and both the upper and lower Poleo Sandstone are present.
Upper Shale - Petrified Forest Member (Norian)
The Upper Shale Member was defined as the uppermost part of the Chinle Formation along the eastern margin of the
San Juan Basin by Wood and Northrup (1946). Since then it has been referred to as both the Red Shale Member (Smith,
1961) and the Petrified Forest Member (Stewart and others,
1972). This unit is present throughout the study area and is approximately 170m(550-650 ft) thick. Exposures are poor, however, since the Upper Shale Member weathers to form valleys or badlands topography. Contact relationships of the Petrified Forest Member FIGURE 9: Thick exposure of Upper Triassic
strata - Abiquiu Dam, New Mexico ■* rf. 27 28 with the Poleo Sandstone lentil are everywhere gradational and conformable. Its contact with the Salitral Shale tongue is unknown, due to lithologic similarities between the two members. In the southern Nacimiento Uplift, where the
Upper Shale overlies the basal Triassic sandstone near San
Ysidro, the contact relationship is likewise unknown. The
Upper Shale Member is composed of red-green, purple, and maroon claystones and shales with interbedded fine sand to medium silt sized quartz arenites. Quartz arenite beds are thin (8-20 cm) and broadly lenticular whereas shale unite appear tabular and persistent.- No sedimentary structures are perserved in the shales and claystones, but sandstones and siltstones contain small (2-3 cm) symmetric ripples, 8-10 cm crossbeds, and laminar bedding. Fresh water pelecypods (Cope, 1875) and vertebrate bones
(Colbert, 1947) are found in the shales. Figures 10 and 11 illustrate the regional geometric relationships between upper Triassic and underlying units.
Red Mesa Sandstone
The basal Triassic sandstone near San Ysidro, New
Mexico, herein informally denoted as the Red Mesa
Sandstone, was mapped as the Agua Zarca Sandstone by Wood and Northrup (1946). This unit attains a maximum thickness of approximately 60 m (200 ft) on Red Mesa, north of San Ysidro, where it is the prominent light- colored ridge former. The Red Mesa Sandstone extends north an unknown distance from San Ysidro and seems to FIGURE 10: East-west stratigraphic
cross-section of Upper Triassic
strata, north-central New
Mexico - Illustrating geometric
relationships between Chinle
sub-divisions 29 FIGURE 11: North-south stratigraphie cross-section of upper Triassic
strata, north-central New Mexico - Illustrating geometric
relationships between Chinle
sub-divisions 30
3 O O
2 Tl
o * — ^*o o CO O—:*e o c Z, o * o c te o «/>mu XV o c « 'O • m £2 x te O Z «n o E wï 31 overlie the Agua Zarca Sandstone (Stewart and others, 1972). These authors recognize it as far north as Senorito Canyon though it could not be identified there in this study. Parts of the Red Mesa Sandstone may correlate with the Poleo Sandstone lentil. Figure 11 shows these possible stratigraphic relationships. The Red Mesa Sandstone overlies the Permian Cutler Group with a sharp erosional unconformity identical to the Permo-Triassic unconformity in the northern Nacimiento Mountains. Near the base of this sandstone the predominant lithologies are iron-cemented conglomerates and indurated fine-grained pebbly quartz arenites. Conglomerates are less prominent up section, as claystone and siltstone units become more common. In the upper part of the Red Mesa Sandstone, sequences of pebbly sandstones overlain by claystones crop out. Contacts between these sequences are sharp, irregular, and erosional; and commonly exhibit load structures. Lithologic units are tabular or broadly lenticular, being massive and structureless or exhibiting large scale (30-50 cm) straight foresets and laminar bedding. Convolute bedding is locally present. Sedimentary structures decrease in size upwards in this member. Micaceous minerals are common throughout the section but the Red Mesa Sandstone contains well over 95% quartz. Palaeotransport directions vary greatly from the base to the top of this sandstone. The average palaeocurrent direction near the base is south-southwesterly; this 32 average changes to a mean north-northwesterly palaeotransport direction towards the top. Section 11 in Appendix I gives a more detailed description of the Red Mesa Sandstone. Jurassic Entrada Sandstone (Upper Jurassic) Throughout the eastern San Jaun Basin the Jurassic Entrada Sandstone disconformably overlies the Triassic Chinle Formation. The Entrada Sandstone was first described by Gilluly and Reeside (1928) for exposures on Entrada Point in southeastern Utah. It is composed of thick, well-sorted fine-grained quartz arenites,. These sandstone beds are massive and structureless or contain large scale (> 30 cm) cross-stratification. Palaeomagnetic Stratigraphy Approximate palaeomagnetic correlations between sections in the Chinle Formation and other Triassic fom mations have been proposed (Graham, 1955, Reeve and HeIsely, 1972) but no palaeomagnetic stratigraphic work has been on Triassic rocks in the Nacimiento Mountains. This technique may provide the means to understand facies relationships between the Salitral Shale tongue, the Poleo Sandstone lentil and the Upper Shale Member; and to refine the correlation between members of the Chinle Formation in the northern Nacimiento uplift, in the Brazos Uplift, and on Red Mesa in the southern Nacimiento Mountains. 33 PALAEOTRANSPORT DIRECTIONS Types of Palaeocurrent structures measured Palaeocurrent measurements were taken on both uni¬ directional and bi-directional structures. Uni-directional structures measured were small (<4 cm), medium (4-15 cm) and large (>15 cm) planar and trough foresets, large (>15 cm) festoon crossbedding, and directional sole markings. Bi¬ directional features included channel azimuths and fossil wood and pebble orientations. Sample site locations are given in Table 1. Statistical Analysis Palaeocurrent measurements acquired in tilted strata were, when necessary, plotted on stereonets and rotated to horizontality. Since deformation in these strata was caused almost totally by the San Pedro- Nacimiento Upthrust (Woodward and others, 1972) no rotation about a vertical axis was necessary. These data were then weighted in a manner adopted from Miall (1974), plotted on rose diagrams, and statistically analyzed using the von Mises or circular normal distribution (Curray, 1956, and Till, 1974). Miall (1974) suggests weighting palaeocurrent data using a volumetric measurement equal to the height of the measured structure cubed as the weighting factor. Thus, larger structures which theoretically represent a more persistent palaeotransport direction (Allen, 1966, 34 Pettijohn, Potter, and Siever, 1972, and Miall, 1974) receive more statistical weight than do smaller ones. Since precise volumetric measurements of palaeocurrent indicators were not, and often could not, be made, a weight based on field observations was assigned to each structure. Large and medium scale foresets,which generally show similar palaeocurrent directions,were assigned an equal weight of 10 (ten) relative to more variable small scale ripples. Similarly, channel azimuths rank higher than fossil wood and pebble orientations and so were assigned a weight of 10 (ten) with respect to them. The actual volumetric differences between these structures are orders of magnitude greater than this, thus the scheme is a conservative one. Though the general validity of this method may be questionable, it is believed to generate reliable palaeoslope directions when data from fluvial deposits are utilized. A summary list of palaeocurrent structures and their relative weights of shown below. Uni-directional Relative Statistical Structures V7eight Large and medium foresets, and large scale festoon crossbedding 10 Small scale foresets 1 35 Bi-directional Relative Statistical Structures Weight Channel azimuths 10 Orientations of long axes of pebbles and fossil wood fragments 1 Computational use of the von Mises distribution is easily adaptable to a modern hand calculator. Statistical values obtainable from the data are 1) the mean vector direction (0), 2) the mean vector magnitude (r) (r=1.0 if all the palaeocurrent vectors are in the mean direction (0); r=0.0 if the vectors are randomly distributed over 360°), and 3) the angular standard deviation (s). Formulas for the uni-directional and bi-directional cases are shown in Table 3. Uni-directional structures were treated as random variables varying from 0°-360°; bi-directional structures were either transformed to a nonsymetric periodic distribution with a range of 0°-360° (Krumbein. 1939; and Curray, 1956) or treated as uni-directional vectors varying from 0°-180° (i.e. all trends were either northeast or southeast). Where possible, mean bi-directional trends were referenced to the quadrant containing the mean uni¬ directional vector for that unit. Thus, a trend of N.20 E. would be called S.20 W. if foresets from that same section showed southwesterly transport. Where no sympathetic quadrants existed, or where no accessory palaeocurrent 36 UNI-DIRECTIONAL1 BI-DIRECTIONAL2 DATA DATA x^=iu cos 9^ xi=ni cos y^=n^ sin 0^ ^i=ni sln 2®i where 0^ is the individual vector direction, n^ is the weight for that vector, and x^ and y\ are the components of that vector in the x and y directions m m m m x = E (x.)/ Z (n.) x = E (x.)/ E (n.) i=l 1 i=l 1 i=l 1 i=l m m m m y = E (y.)/ E (n.) Ÿ = r (Yi>/ S (n i=l 1 i=l 1 i=l 1 i=l i> where x and y are the mean vectors in the x and y directions, and m is the number of measurements r = x2+y2 r = x2+y2 cos 0 - x/r, sin 0 = y/r cos 20 = x/r, sin 29 = y/r s = ( V 2 (1-r) ) (180/ir) 0=^ arctan (sin 20)/(cos 20) s = (V 2 (1-r) ) (180/ir) where r is the mean vector magnitude, 0 is the mean vector direction, and is is the angular standard deviation in degrees Is from Till (1974) 2. from Curray (1956) and Till (1974) TABLE 3: STATISTICAL FORMULAS USED IN CALCULATIONS 37 data was available, the mean-bi-directional trends were referenced to north. Statistical values obtained from palaeocurrent data measured are shown in Tables 5-9. Mean vectors (0) obtained in this study and from the literature are plotted on outcrop maps for various stratigraphic units (Figs. 13, 15, 17). For further reference, current rosettes for palaeocurrent measurements taken in different members are shown in Figures 14, 16, 18, and 19. In general, bi-directional palaeocurrent data had a much higher angular standard deviation (s) and a lower mean vector magnitude (r) than did uni-directional data. Two major factors are responsible for this: 1. Measurements taken from linear, bi-directional structures are less affected by outcrop sampling bias than are those taken from three-dimensional, uni¬ directional palaeocurrent features, and 2. The orientation of linear objects is not affected in any simple way by hydrodynamic processes, The first of these factors was especially important in this study. All measurements were affected somewhat by this bias which was manifested in several ways: 1. apparent dip; i.e. exposing a two-dimensional cut through a three-dimensional structure (linear structures are not affected by this), 38 2. preferentially exposing a greater number of structures in any given direction due to the strike of the outcrop, and 3. often preferentially exposing uni-directional structures in the same channel, thus eliminating inter-channel variations. Both 2 and 3 affect linear structures but do so less than they affect three-dimensional structures. Preferential exposure of structures in a given direction or channel were important biases since most measurements were taken in roadcuts or similar outcrops where the strike of the outcrop face varied only slightly. Interpreting the orientation of fossil wood fragments and pebbles is difficult since they do net necessarily align themselves parallel to the dominant flow. Modern wood fragments in the Rio Grande seem to be oriented prefer¬ entially perpendicular, or parallel to flow. Other authors have noted that approximately cylindrical wood fragments (i.e. larger in one direction, relatively straight, and free from protruding branches) tend to line up perpendicular to flow (Pelletier, 1958). Alignment perpendicular to flow is believed to be due to wood fragments being deposited in the troughs of current ripples. Ore (1963) notes that pebbles in fluvial deposits often show imbrication but records no preferential alignment of their long axes. Fossil wood orientations and the current directions obtained from other structures are shown in Table 4. It can be Location Bi-directional Uni-directional Difference Comments •H d «H P (P •3 d 04 P p O CO CO (D O -H O td d) TABLE 4: COMPARISON OF BI-DIRECTIONAL vs. UNI-DIRECTIONAL DATA •H 4-» PJ d 0 O fd O Bi-directional Uni-directional Difference Comments -P »H P -P -H P4 Û CQ Qj O P O -p fd £ d CD O p 0 fd -H fd CD O CQ CD H d O CQ ■H •H -P +J o *3 -P p O d H fd P d fd d o «H P rH 3 TJ ÆCD 4-4 P -P *d m vo IS ÿ4J CQ CD fd -Pr CQ «P*H g toO E CDA CD O-H d -P1 r- P CD (d CQVO on 0, O -PP o iH3-H & w VO CN H •H 0 VS -P £ CAP1 CD • • • o o P 1 iH cn 04 0 CD o CQ p CQ -P r-4 CQ CD d CD fd CQ d P E E CD 0 d fd iH d 1—I p H 4-4 P 4J •a ÆCD £ 5.p-p CQ CD 6 coo fd -Pi d -P1 CQ -PHH CD O-H P CD fd CQin & CU E CD VO rH o-H d 0 -PP s a o £ * 04 !S rH 'H (!< •H 04 WD CD • • . 0 o O P 1 i-H W CD 0 CQ P • CQ -P •H *H CQ CD £ 9 d CQ P CD fd d S 0 d fd Table 4 : continued * 40 41 seen that the measured orientations in this study have no preferred direction. Reliability of Statistical Measurements Shown in Figure 12 are two hypothetical current roses (Figure 12 a,b), the statistical transport values (0, r, and s) for each, and a rose diagram and statistical current values from foreset measurements made by Ore (1963) in a modern braided stream (Fig. 12c). Figure 12a represents the expected distribution of current directions measured from foresets in a linear channel (Allen, 1966); figure 12b represents similar data from a hypothetical braided stream (based on field observations of lunate ripples in the Rio Grande, New Mexico). The statistical values shown give an idea of the approximate minimum reliable mean vector magnitudes (r) and angular standard deviations (s) obtainable in a braided stream environment. The linear channel case approximates a set of measurements collected in a small area from one channel and the braided stream case represents data from many channels over a larger area, or with time. A bimodal distribution of current directions is expected in the latter environment because of the anastomosing channels and the development of foresets off the flanks of braid bars (Ore, 1963; Leopold and Wolman, 1957). Ore (1963) states that the difference in flow direction between these two modes will be approximately 90° in a straight main stream channel but increases to as much as 180° or more as the main channel FIGURE 12: Palaeocurrent rosettes for a hypothetical linear channel and braided stream, and from a modern braided river 42 N Hypothetical ideal linear channel (Allen, 1966) •0. = N. 90 E. r- .79 s= 37.8® o N-72 Hypothetical ideal braided stream •6- = N. 90 E. r = .84 *=32® N= 23 Modern braided stream (Ore, 1963) ê = N. 72 E. r = .20 *= 72.5° N = 26 C S. 85 E. 43 becomes more sinuous. Figure 12c shows such a bimodal distribution of foreset measurements in a modern stream. Even though the mean vector magnitude (r) is quite small (.20) the mean vector (0) is within 25° of the actual transport direction. This suggests that, if enough current measurements were gathered, a reliable interpretation of the paleoslope direction might still be possible. As seen in Tables 5-9 only one site had values sig¬ nificantly below the hypothetical minimums (site 3 Poleo Sandstone - Table 6). Statistical values are still substantially higher than those obtained from Ore's data in Figure 12c. Permian Cutler Group - Palaeocurrent Directions Though no formal palaeocurrent study of upper Permian fluvial deposits was done, large scale trough foresets in¬ dicating southwesterly transport were observed at sites 2, 3, 4, 11, and 13, in the Arroyo del Cobre. Gibson (1975) also noted southwesterly palaeotransport directions in Permian strata in and around the field area. This southward dipping regional slope persisted at least into Triassic time. Triassic Chinle Formation - Palaeocurrent Directions Agua Zarca Sandstone Mean palaeocurrent vectors (0) for braided stream de¬ posits of the Agua Zarca Sandstone are shown in Figure 13. All of the mean vector magnitudes (r) are greater than .87 FIGURE 13: Map showing mean and total palaeocurrent vectors, and trends of bi-directional structures - Agua Zarca Sandstone References: a) Kaufman, 1969 b) Poole, 1961 Exp lonation Site location Moan vector letter refers to reference Trend of bi-d i r ec tionai structures / 12 FIGURE 14: Palaeocurrent rosettes for various sample sites - Agua Zarca Sandstone 45 46 G O •H g Td 4-> g u 0 U ü 'O O C LO •H eu O LO rH rH > *H rH td •H -P 4-* C O AJ 'd en td Aj 0 w 0 M •H 3 C $ •H 44 H en 0 O o rH 0 C4 â &H & •H rH td d 0 CH Ï3 H 44 H M U w w td O •* 0 (Table 5). The combined mean vector direction for all Agua Zarca measurements is S. 41°W,,with a mean vector magnitude of .83 and an angular standard deviation of - 33.4°. The high r values and low s values are due in part to the previ¬ ously discussed outcrop sampling bias. This bias affected all the measurements made to some extent, but the resultant vectors are, nevertheless, considered to be reliable palaeoslope indicators. In general, this unit shows southwestwardly transport, but locally southeastward transport directions were observed (eg. site 3). Kaufman (1971), Poole.(1961) (Fig. 13), Poole and Williams (1956) and Stewart and others (1972) have made similar observations. As noted by Wood and Northrup (1946) and Stewart and others (1972), and shown in Figure 4, this unit thickens toward the south and southwest, in the direction of transport. It thins rapidly north of site 3 and is often missing or has a lenticular outcrop geometry in the northern and eastern parts of the study area (Woodward, pers. comm.). Southward from sites 1 and 2 the Agua Zarca thins gradually. Salitral Shale tongue The Salitral Shale is believed to have been deposited in a lacustrine environment (Kaufman, 1971). No sediment transport directions for this unit were obtained. In some silt and very fine sand beds small scale oscillation ripples were noted. Some of these seemed to have a current bias. 43 This possible bias, however, could represent a number of sediment transport mechanisms (eg. wind action or lacustrine longshore drift), therefore, the ripples are not considered reliable palaeoslope indicators. Poleo Sandstone lentil The Poleo Sandstone, largely deposited by meandering streams, can be subdivided on the bases of transport dir¬ ections into upper and lower units. Because of distinct differences in transport direction, the upper and lower parts will be discussed separately. Lower Poleo Sandstone Figure 15 shows mean palaeotransport vectors for the Poleo Sandstone: other statistical values are found in Table 6. All but one of these sites have mean vector mag¬ nitudes and angular standard deviations that compare fav¬ orably with the approximate minimums shown in Figure 12. One questionable site (site 3 - upper) nevertheless probably represents southwestward transport (~60% probablility of palaeotransport between S. 9° W. and N. 75° W.). No statistical analysis of palaeocurrent data was performed for site 4 (Fig. 15). The dashed vector shown represents an approximate transport direction based on a small number of field observations. The lower Poleo Sandstone lentil shows southward, generally southwestward transport, with a mean vector of S.22 W. and a mean angular standard deviation of + 45.1°. FIGURE 15î Map showing mean and total palaeocurrent vectors - Lower Poleo Sandstone; explanation same as Figure 13 12 FIGURE 16: Palaeocurrent rosettes for various sample sites - Lower Poleo Sandstone NORTH 50 51 'd £ g g fd O fs, ü en rH en g -P fd «"-“N. -P in O in CD £ g CD H H en 0 ü en A| A| CD •H CD V P -P in P O ü CO rH 0 CD CD MH CD Aj MH CP iH CP P B P rH P A •H .£ rd fd fd CP 'Ü g CD O' rH g rH £ 1 g Gn £ en O •H O P 0 g g P £ U fd P 0 0 -P £ rH AJ P en P £ O MH •P MH CP 0 B g CD g £ < 0 g en en en ü •H 0} P O -P CD •P en -P CO MH £ P £ LO £ £ pq CD 0 CD •H s V *G g MH g en A | 'd 1 P CD '■—* CD CD CD -P CD CD EH P -P P r£ P CD -P P O £ rH •£ £ CP £ en CD £ £ P en rH tTk en £ en CD CP en PS £ rH r* rH LO en rH rH rH CM LO iH «H 0 0 0 0 0 0 en a\ O O' in in W| in rH CM CM + 1 + 1 + 1 + 1 + l + 1 • • • • • • B B tt B B B | r- m r- O' O' Ml lO CP o> 00 O' LO • • • • • • £ £ £ £ £ 0 0 O 0 0 s •H •H •H •H •H «—s O -P AJ •P -P -P P H P P P P P 0 EH O Q 0 0 0 -P PU QA Û4 04 04 04 ü H r\ CD P P P P P CD > O CD CD CD CD CD rH CO QA & 04 ï 5 0 d w eu Q 04 0 0 PU CD Q P H P 4 P £ P •H FROM PALAEOCURRENT MEASUREMENTS IN THE LOWER POLEO SANDSTONE CD ,0 > % £ Q 0 O rH lH ü H en O' *H rH S CD CD CD fd U -P -P AJ -P O •H •H -H 0 l4 CO CO CO EH TABLE 6: MEAN PALAEOCURRENT VECTORS (0) AND VECTOR MAGNITUDES (r) DETERMINED 52 In one locality (site 9) southeastward dipping palaeocurrent transport directions were located high in the lower Poleo Sandstone and may represent a transition into the upper Poleo Sandstone. The thickness of the lower Poleo Sandstone is quite variable and is perhaps due to intraformational erosion. This lower portion thins rapidly south of site 3 and is not present at sites 1 and 2 (Fig. 15). East from site 9, it also thins and is missing at site 11. Thickness of the lower Poleo Sandstone north of sites 4 and 7 are not known. Upper Poleo Sandstone Mean palaeotransport directions for the upper Poleo Sandstone are shown in Figure 17 and statistical values for these measurements are listed in Table 7. All the mean vector magnitudes are greater than .75 and the angular standard deviations are less than i 40°. The upper part shows generally northwestward transport with a mean vector direction of N. 10° W. Poole (1961), Stewart and others (1972), and Gibson (1975) all measured palaeocurrent directions in the Poleo Sandstone similar to those found in the upper part (Fig. 17). The upper Poleo Sandstone is thickest at sites 10 and 11 (Fig. 17), and thins to the north, northwest, west and southwest. FIGURE 17 : Map showing mean and total palaeocurrent vectors, and trends of bi-directional structures - Upper Poleo Sandstone; Explanation same as Figure 13 References: a) Gibson, 1975, from Figure 15 b) Poole, 1961 c) Stewart and others, 1972 53 î„ \ 12 FIGURE 18: Palaeocurrent rosettes for various sample sites - Upper Poleo Sandstone and Petrified Forest Member • *d T3 & G Ui/A G g id X 0 0 0 0 0 0 O 0 0 00 LO CO CP rH CO o\ rH col CN CN CO in CN in + 1 + 1 X 1 + 1 + 1 XI XI X 1 X 1 • • • » • • • • • pa B w a a a 5 s a | OO rH 00 in CP in 00 0 in PI 00 C\ in rH PH rH c G id G id G id O O G CO 0 G CO 0 G CO a •H •H 0 CD *H O CD •H 0 CD P 0 X X •H P X *H P X •H P O H O P X 3 P X 3 p X 3 X EH CD 0 O X 0 O X 0 Ü X 0 fr CO & X CD U £4 CD u £4 CD Ü CD H 3 P 3 P 3 P 3 0 > P4 rH p 0 •H P P ■H P P •H P CD rH u id CD n3 *d X CD *d X CD *d X "d cn X £4 id 1 CO £4 1 CO £4 | CO 0 CD •H pa 0 £4 O •H Û4 •H £4 cu G •H Q EH a P$ CQ a a a a FROM PALAEOCURRENT MEASUREMENTS IN THE UPPER POLEO SANDSTONE p JQ MEAN PALAEOCURRENT VECTORS (0) AND VECTOR MAGNITUDES (r) DETERMINED CD £4 0 a £4 Ü 0 0 rH a H rH r- rH PH PH EH rtj CD CD CD CD CD id u X X X X X X O •H •H •H •H •H 0 HÏ cn cn cn cn cn EH 56 Upper Shale Member Since the Upper Shale Member was deposited in a marsh¬ land or lake environment (Wengerd, 1950; Smith, 1961) with a few scattered fluvial channels. Reliable palaeoslope indi¬ cators are rare. Mean vector magnitudes, directions, and angular standard deviations are shown for the few structures measured (Table 8). The palaeotransport directions are quite variable, but the exact stratigraphic positions of the sample points are unknown. However, the measurements are confidently known to be from the lower half of this member. All measuremements show southwestward transport, but since only a few measurements were taken these transport directions may not represent the true palaeoslope direction. These measurements are in accord with Wengard's (1950) conclusions that the Upper Shale Member was deposited on a gently dipping westward sloping plain. The thickness of this unit is difficult to determine since good exposures are rare, but seems to be fairly con¬ sistent throughout the area. Red Mesa Sandstone Palaeocurrent measurements were taken near the site of the measured section (Section 11, Appendix I) in this unit and were quite variable. They range from an average mean vector of S. 28° W. near the base to N. 39° W. near the top (Table 9). Figure 17 shows current roses for the lower and upper portions of this section and a combined rose for this entire section. The mean vector direction for the 57 -H g TJ fi rH o 0) U 0 id B (U •H fi a HJ 0 a V 0 OU •H -H w w -P P CQ HJ a CQ fi O H en rH rH HJ a) id a) s EH rH rH 0) 43 SH SH 3 a id (rt CQ HJ HJ -H p S g 0) TJ EH § CQ CQ P HJ |CD| in CM vc rH (N m V0 • • • • PS en en a en o P c*C_j en U PrT o\ 00 10 CM SH| Red Mesa Sandstone 58 * • • o tt w P * i i z i 59 a* d 'a d * a G o G iH 03 fO tO IT) p in G rH * CD >t H * 0 O Aj CO » rH CO *H p P 0 G w* p P 52 CD rH 0 CD Ü H cn CD CO CD CO CD 2 O' CD *» G O' CD P 2 52 P P CO 0 p P •H w as O -P •H <0 0 d EH § rH 4H G P rH MH 1 W CD Ü •H û o £ x: a CD a P G u 0 O' O' CO 0 fO G >I p—S P G O' (0 P G O' rH p a MH O G p CD MH as G O' G 52 p •H MH X! iH •H G 0 < CO p d P CO 04 d •H en P d d P d CO CO w cn G d CD 0 MH G d CD G p p w CD G p 0 O CD G P G p 2 e G 5 a to d CD EH P CD G G CD G CD a H P O i—1 0 P PG 0 p p 52 U G as O *H •H G O' 0 G G Ü P CO G P CO P CO G p & co Ctf «5 tO CO CO P to 0 CO a as S ÊH CD rH CD 0 0 CD P CD 0 SJ CO S 04 MH , Pu 04 S P MH u a 2 w EH U P > G co vo 00 rH w| 1—1 iH CO 1 rH g o 0 0 0 1 CM in 00 00 col IH ■H CM in D + 1 + l + 1 + l § • • • « S |CX> S 55 •*w* ICD| oo o\ V0 CM CM ro 00 • • • • 259 CO CO 52 52 o O TTT 00 00 i—1 s Pi cn S G 2 FROM P ALAEOCURRENT MEASUREMENTS IN THE RED MESA SANDSTONE •H d •G 52 CD g •# O CM 2 0 <7\ H rH ü EH rH H C CD as P a P P o •H 0 <5 p CO EH EH 60 entire section is N. 85° W. with a mean vector magnitude of .80 and an angular standard deviation of + 40°. The mean vector direction for the lower part of this section is shown in Eigures 13 and 15 because this part of the section is believed to correlate with the Agua Zarca Sandstone or the lower Poleo Sandstone lentil, or both. Since the upper part of the San Ysidro section probably correlates with the upper Poleo Sandstone its mean vector direction is plotted on Figure 17. Reasons for these correlations are discussed on page 100. Palaeocurrent directions in this section may reflect a continuous change in the regional palaeoslope during late Triassic time. They deserve to be studied in greater detail. 61 SEDIMENTARY PETROLOGY Sample collection and field description of units were done as the stratigraphic sections were being measured. Of particular interest were samples showing petrologic and textural, changes which seemed to correlate with changes in palaeotransport directions. However, specimens were also collected from the various depositional subenvironments wherever possible. All siltstone and sandstone samples were examined with a binocular microscope and classified according to Folk (1974). Relative amounts of sand, silt, and clay-sized material were estimated; sediment texture and small scale sedimentary structures were described. In addition, the kinds, amounts, size, orientation, and degree of weathering of micaceous minerals were determined. Qualitative size classifications were assigned to each sample by comparison with a sieved standard. Lithologic descriptions of samples, and sample locations are listed in Appendix II. A number of samples (Table 10), chosen to illustrate textural changes up-section, were analyzed in the Rice University Automated Sediment Analyzer (RUASA). Only sand¬ sized material (from -0.50 to 4.00) was included in this analysis. The samples were disaggregated in hydrochloric acid and wet-sieved to remove the mud fraction. Since most of the samples analyzed had a calcareous cement, only a minor amount of mechanical dissagregation was necessary to fully separate the grains. A few samples were more 62 u C ns O M -H rd -P p- 00 00 VÛfHOO^CMr^rHlDlDCO Tf rd co 00 »H C -H • • • •H rd > +> 4J "01 'SV ^QL 'SL 'SV "QV 'SV "GV ^SL *3* VO in in CM VO CM CO in c\ VO o CM CO rH a\ o^ o CM iH rH •H CM CM CM CO CM rH CM rH CM CM CM CO in r> 00 10 13 o rd U ni OJ in rH CP 1—1 CM CO H H H H H H in VO rH iH 1—1 H H H H H H rH 1 1 I 1 iH •*7» È à rd o P Q a 2 2 2 2 cd rd CO CO < < < EM PM PM PM PM PM CO CO TABLE 10: SAMPLES ANALYZED AND STATISTICS GENERATED BY RUASA 63 en CO 0) (NOïinr^cNr-cnvoo-'tfocninTr oo in o omrocMoin«HiH^fincMCMvoin co o ro G "d o U -H d «P CNvocoLno^oocnrHcoo^c^roco m co a\ *d d •H ininn(Ninin^fn^^^^^(M ro CN rr G -H 4J d > G +3 (U ^0 ^0 "0 'S "0 *0. 'QL *®L *0 *0 *0 *0 *0 'SL "SL 00 r- CN VO in VO o 00 CN oo o 00 rH 00 r- o rH a> CN in o rH 00 oo rH o • • • CN oo 00 00 CN rH rH 00 CN CN oo CN CN 00 00 00 oo o* cw =8= n. d *d 0 fa a> PQ 0 o o o o 00 CN rH vo 00 o rH rH rH in 00 rH rH rH 00 rH 00 0 1 £ A à Q O à A Q a a a a a a fa a a a d *< <3 <3 < < <3 fa fa cn < fa fa fa fa cn < fa fa Table 10: continued 64 forcibly disaggregated than the others; these are denoted with a question mark in Table 10. Prior to analysis in the settling tube each sample was scanned using a binocular microscope and re-disaggregated if needed. Only samples containing less than 5% aggregate grains were analyzed. The data generated by the RUASA was interpreted using a FORTRAN program supplied by Dr. Joel Gevirtz. This program outputs the frequency of cumulative percentages for each quarter-phi interval, as well as calculated values for median and mean grain size, standard deviation, and skewness. Results from this program are also tabulated in Table 10. Cumulative frequency curves (figs. 20, 21) are of several basic types which reflect both the fluvial sub-environment and the stratigraphic unit from which the sample was collected. Representative curves were selected and are presented in the figures below. Selected conglomerates and pebbly sandstones were dis¬ aggregated (either with HC1 or mechanically) and clast lithologies for particles coarser than -2.00 were determined. We11-indurated samples were cut into several slices and pebbles were identified and counted along transects. The results of the pebble count analysis are shown in Table 11. Triassic shales and siltstones were not analyzed tex- turally in this study. This situation arose chiefly through lack of time, equipment, and unweathered samples. 65 UNIT SAMPLE LOCATION RATIO QUARTZ : QUARTZITE : CHERT Agua Zarca Arroyo del Cobre 140 : 187 : 1 Sandstone Lower Poleo French Mesa 6 : 1 : 11 Sandstone Abiquiu Dam 3 1 : 11 Red Mesa San Ysidro 14 1 : 13 Sandstone TABLE 11: RELATIVE AMOUNTS OF QUARTZ, QUARTZITE, AND CHERT IN UPPER TRIASSIC CHINLE CONGLOMERATES FIGURE 20: Cumulative frequency curves of Upper Triassic Sandstone beds throughout the field area - Sample locations and descriptions in Appendix II €6 FIGURE 21: Cumulative frequency curves of Upper Triassic Sandstone beds on French Mesa - Sample descriptions in Appendix II 67 I i i I a. a. » * CO CM Q - o r o T o o 68 Agua Zarca Sandstone The Agua Zarca Sandstone Member is composed predominantly of subangular, siliceous, medium-grained quartz arenites. Pebble conglomerates and pebbly coarse quartz sandstones are common towards the base of this unit, but become less prominent up the section. Towards the top of the Agua Zarca Sandstone, slightly calcareous and siliceous, micaceous, fine quartz sandstones and silty sandstones become more common. Particle size generally decreases up-section and the sediments become more well sorted. Induration varies from friable to well-indurated. These sandstones are petrologically mature being composed almost entirely of quartz; pebbles are predominantly quartz and quartzite (Table 11). Small percentages of orthoclase feldspar and volcanic rock fragments occur in the basal portion of this member. Micaceous minerals are only locally an important constituent of these rocks, but they increase in amount higher in the section. Quartz overgrowths are common and most of the rocks have a siliceous cement. In general, no substantial intergranular matrix is present. Dispersed clay lenses are present throughout the Agua Zarca Sandstone but are most prominent near the top in the transition zone between the Agua Zarca Sandstone and the Salitral Shale. Copper minerals are found throughout the Agua Zarca but the largest deposits occur near the base of the section. In the upper part of this member copper minerals are found only as copper carbonate stains. 69 Poleo Sandstone Lentil The Poleo Sandstone lentil is composed predominantly of light-colored, subangular, calcareous, medium- to very fine-grained quartz arenites. Rocks range from semi-friable to well-indurated. Cobble and pebble conglomerates and pebbly sandstones containing well-rounded chert, limestone, quartz, and quartzite clasts are most prominent near the base of this lentil. Very fine-to fine-grained silty sandstones, siltstones, and claystones become more persistent up-section. Many of the sandstone beds exhibit normal grading and the Poleo Sandstone as a whole becomes finer and possibly more well sorted upwards (Figs. 20, 21, and Table 10). Like the Agua Zarca Sandstone, this unit is petrologically mature but contains a considerably higher percentage of micaceous material. These micaceous minerals increase from less than 1% in the lower portion of the Poleo Sandstone to approximately 5% in the upper part. The lentil also contains significant amounts of siliceous and calcareous sedimentary rock fragments, particularly near the base. Commonly, rocks of the Poleo Sandstone lentil have a calcareous cement; though beds with siliceous cement and abundant quartz overgrowths are found. A muddy matrix is a small but consistent proportion of these rocks. Unfortunately, it is difficult to determine if this matrix is depositional or diagenetic; some of it may be due to post-depositional tectonism. 70 The upper and lower Poleo Sandstone subunits, while readily distinguishable on the basis of palaeocurrent data, sedimentary structures, and depositional subenvironments, are very similar petrographically. Figures 22 and 23 are triangle plots of the sand-silt-clay ratios for the upper and lower Poleo Sandstone. Samples from the upper Poleo Sandstone have slightly higher percentages of clay and silt¬ sized material than those of the lower portion. None of the sandstones, however, has an appreciable amount of clay. Cumulative frequency curves from samples from these two subunits (Figs» 20, 21) show a fining upward trend but much of this change in average grain size occurs within the lower Poleo Sandstone. The petrographic similarity of sediments from the upper Poleo Sandstone to those of the top of the lower Poleo Sandstone probably reflects reworking of lower Poleo sediments by upper Poleo streams. Approximately 75% - 85% of the rocks in the upper Poleo Sandstone contain a calcareous cement. The lower portion of this lentil contains much less of this cement, though it is locally important. Unoxidized carbonaceous plant fragments were preserved in some Poleo Sandstone beds containing a calcareous cement (Fig. 24), suggesting the this cement probably formed soon after deposition. That calcite was available in this sedimentary environment is evidenced by septarian concretions and limestone beds in the Salitral Shale, and limestone fragments in Poleo Sandstone conglomerates. FIGURE 22: Triangle plot of sand (s) silt (z) - clay (c) Lower Poleo Sandstone 71 S C Z FIGURE 23: Triangle plot of sanâ (s) silt (z) - clay (c) Upper Poleo Sandstone 72 S C Z FIGURE 24: Carbonaceous plant fragments Upper Poleo Sandstone, Nacimiento Mine 73 74 The Poleo Sandstone is, in general, much finer and more well sorted (Table 10, Figs. 20, 21) than the Agua Zarca Sandstone. Agua Zarca sandstones contain lower percentages of micaceous minerals and less calcareous cement than the Poleo Sandstone units. Poleo Sandstone conglomerates contain much more chert, jasper, clastic rock fragments, and lime¬ stone pebbles than conglomerates in the Agua Zarca Sandstone. However, Agua Zarca conglomerates contain higher percentsge- of quartz and quartzite pebbles (Table 11). No copper mineralization has been observed in the Poleo Sandstone lentil. Petrified Forest Member Sandstones of the Petrified Forest Member are typically very fine-grained quartz sandstones and siltstones, commonly having a calcareous cement. The average particle size in this unit is much smaller than in underlying units (Table 10, Figs. 20,21) and the sandstones are more well sorted. Micaceous minerals are abundant. Red Mesa Sandstone The basal Triassic sandstone near San Ysidro, New Mexico, is predominantly a well sorted subangular siliceous to slightly calcareous fine-grained quartz arenite. Sandstones are for the most part indurated and conglomerates are well-indurated. In places near the base of the unit, chert pebble conglomerates and pebbly sandstones are present; 75 towards the top of the unit silty sandstones and claystones crop out. Some of the depositional units are micaceous, but this section as a whole has very little mica. Results of a pebble count on a sample taken from one of the basal con¬ glomerates are shown in Table 11. Trace amounts of malachite and azurite were noted in this member. C/M Diagram The coarsest and median grain sizes of samples from the Agua Zarca Sandstone, upper and lower Poleo Sandstone, and the Upper Shale Member were plotted on a modification of Passega's (1964) C/M diagram (Fig. 25). Though the samples did not cluster in discreet groups with respect to stratigraphic units they did fall in a linear band which partially overlaps with Passega's fluvial field. Furthermore,• fine-grained point bar and flood plain samples tended to fall near the suspended load portion of the field, while coarser channel deposits plotted near the traction sector. Though Passega'a fluvial field is empirical and need not include all fluvial samples, two effects of the disaggregation process may have shifted these Triassic sandstone samples farther from the field than they might otherwise have plotted. These factors are: 1) possible preferential grinding of coarse grains causing the coarsest 1 percentile (C) to be shifted upwards in Figure 25 (finer); and 2) wet-sieving the samples at 4.00, thereby shifting the median grain size (M) to the left (coarser). FIGURE 25; C/M plot of Upper Triassic sandstones, Chinle Formation, north-central New Mexico 76 O 77 Significance of Micaceous Material in Sediments Micaceous minerals are quite common in the Chinle For¬ mation in the Nacimiento Uplift, being found to some extent in almost every depositional unit. Muscovite is by far the most common micaceous mineral with biotite being the second most frequent. Particles of chlorite do occur but it is not known if these are primary or the result of diagenetic alteration of other micas. Fragments of micaceous minerals are most commonly aligned parallel to each other and to the bedding plane. Ordinarily they occur in discreet layers at a bedding plane surface. These fragments are, in some rocks, bent and deformed around quartz grains. Muscovite fragments appear unweathered chemically and may retain part of all of their original hexagonal outline. Biotite particles range from unweathered to very weathered, with unweathered biotite flakes commonly retaining a portion of their hexagonal outline. Chlorite fragments everywhere have a very weathered appearance with no suggestion of a hexagonal shape. A sudden increase in the amount of detrital mica in a sediment can indicate either initial weathering of a micaceous source area, a decrease in the energy of the environment of deposition, or both. If grain size decreases as the amount of mica increases, and no other igneous or metamorphic minerals appear, it is likely that the energy of the environment of deposition decreased. This 78 hypothesis is further supported if micaceous minerals are known from previous sediments in the same environment, or from downstream equivalents of the units in question. Increasing amounts of muscovite in the upper portions of the Agua Zarca and Poleo Sandstones probably reflect decreasing energy conditions. Since muscovite is stable enough to persist relatively unchanged through several sedimentary cycles, there is no need to invoke unroofing of a new igneous or metamorphic source area. Increased amounts of fresh, unweathered biotite may in¬ dicate an episode of volcanic activity. Figure 3 shows locations in several stratigraphic sections where biotite- rich zones were found. In these zones, biotite is commonly more abundant than muscovite. A tentative correlation can be drawn between these points, but much more work is needed to establish whether these biotite zones represent signifi¬ cant geologic events. The possibility of obtaining an exact date for these sediments and of establishing a time-strati- graphic horizon makes their study worthwhile. Should these zones prove to correlatable time horizons, their study will also shed considerable light on the preservation of sedimentary facies in fluvial environments. 79 PROVENANCE Permian Cutler Group Permian clastic sediments in north-central New Mexico were derived from crystalline and metasedimentary source terrains in the Ancestral Rocky Mountains to the northeast (McKee, 1967; Gibson, 1975; Baars, 1962). Minor amounts of clastic material may have been derived from limited exposures of pre-Permian Paleozoic sedimentary rocks. Palaeocurrent structures indicate southwestward transport during the Permian, and conglomerate clasts decrease in average size from boulders and cobbles in the northeastern part of the study area, to pebbles in the southwestern portion. Quartz and quartzite fragments in Permian rocks are, petrographically, very similar to Precambrian crystalline rocks and metasediments (eg. the Ortega Quartzite) presently exposed on the northeastern margin of the Chama Basin in the Brazos Uplift (Muehlberger, 1967; Bingler, 1968; Gibson, 1975). Along the eastern margin of the San Juan Basin the presence of arkosic units at several stratigraphic levels in the Permian section suggests that the crystalline source terrain was re-exposed several times. 80 Triassic Chinle Formation Agua Zarca Sandstone The entire Agua Zarca Sandstone was derived from a northeasterly source area located at the southern end of the Permo-Triassic Uncompahgre Highland (Beaumont and Reed, 1950; Lookingbill, 1953). The Uncompahgre Highland extended from north-central New Mexico to southern Wyoming during the upper Triassic (MacLachlin, 1972), but the restricted distribution of the Agua Zarca Sandstone (Fig. 6) suggests that only a small portion of the uplift supplied sediment to the rivers which deposited this member. Two major types of source terrains, Permian sediments and Precambrian metasediments and crystalline, were exposed. Isopleths of maximum pebble size (Stewart and others, 1972, Fig. 18; and this study, Fig. 26) and palaectransport directions (Fig. 13) indicate a southwestwardly dipping palaeoslope. Also, the Agua Zarca Sandstone thickens to the southwest, away from its source area. Erosional contact relationships of the Agua Zarca Sandstone with Permian sedimentary rocks (Fig. 7) indicate that the Agua Zarca Sandstone was in part derived from the underlying Permian strata. Sedimentary lithic fragments and chert contained in this sandstone, and its greater minéralogie maturity and smaller average grain size as compared to Permian rocks, also support this contention. Triassic sandstones uncon- formably overlie Precambrian metasediments (Muehlberger and others, 1960; Bingler, 1968). Since these Triassic FIGURE 26: Map showing maximum measured pebble sizes - Agua Zarca Sandstone (solid circles) - Lower Poleo Sandstone (open circles) 81 12 82 rocks and the lower Poleo Sandstone lentil (Muehlberger and others, 1960; Bingler, 1968) it follows that Precambrian metasediments were probably exposed in the upper Triassic. Trace amounts of feldspathic material in the basal Agua Zarca Sandstone suggest that minor amounts of Precambrian crystalline rocks were also exposed. Salitral Shale tongue The Salitral Shale tongue is composed of volcaniclastic and siliciclastics sedimentary material, probably derived from different source areas. Siliciclastics probably came ultimately from the same source terrain as those in the Agua Zarca Sandstone and the Permian Cutler Group, while a southerly volcanic source area may have contributed large quantities of volcaniclastic material. This volcanic terrain is thought to have been Mogollon Highlands in southern Arizona and New Mexico (Schultz, 1963; Stewart and others, 1972) Though no palaeoslope indicators are present in the Salitral Shale tongue, palaeotransport directions in both the conformably underlying Agua Zarca Sandstone, and the uncon- formably overlying lower Poleo Sandstone lentil, are to the southwest. Furthermore, the Salitral Shale thickens towards the southwest. These observations strongly imply that a major clastic source terrain still lay to the northeast while the Salitral was being deposited. The predominance of very fine sand and silt sized material in this member may be due to the low depositional energy of this lacustrine ; 7- ' 83 environment, or to multi-cycle reworking of sediments, or both. Additional detailed sedimentologic work must be done to resolve this. Perhaps as much as 50% of the Salitral Shale tongue in some locales is composed of bentonitic clays, which, in upper Triassic rocks are believed to have been formed from the alteration of volcanic ash (Stewart and others, 1972). Though little work has been done on the Salitral Shale per se, Schultz (1963) showed that the percent of montmorilIonite in the Petrified Forest Member of the Chinle Formation increased southwards, implying a southern source. Conceivably, this source could have supplied volcanic material which altered to bentonite in the Salitral Shale tongue. Poleo Sandstone lentil Lower Poleo Sandstone Like the Agua Zarca Sandstone, the lower Poleo Sandstone lentil had a northeastern source in the southern Ancestral Rocky Mountains. This is indicated by south- westwardly palaeotransport measurements (Fig. 15). The source terrain was a sedimentary one, as evidenced by chert, limestone, and sedimentary rock fragments in lower Poleo conglomerates. Chert and limestone clasts were prob¬ ably ' derived from Permian and Pennsylvanian strata, though no fossils have been found. Since upstream equivalvents of the Poleo Sandstone overlie Precambrian metasediments in the Brazos Uplift (Bingler, 1968) it seems that some 84 Precambrian rocks were exposed during the deposition of this unit. The Precambrian source terrain was probably of low relief, however, and contributed only minor amounts of material. Quartz and quartzite pebbles in the lower Poleo Sandstone are probably second cycle clasts derived from the Agua Zarca Sandstone or the Cutler Group. Upper Poleo Sandstone The upper Poleo Sandstone had a sedimentary source area located south and southeast of the present Nacimiento Uplift. This unit is in part probably derived from lower Poleo sediments and older Triassic and Paleozoic rocks. Palaeocurrent structures indicate northerly and north¬ westerly transport (Fig. 17), and the subunit thickens to the north (Figs. 4, 11). Pebbles consist predominantly of sedimentary chert (Table 11), limestone, and siltstone fragments, but few crystalline quartz or quartzite clasts. Sandstones are fine-grained and mineralogically mature suggesting reworking of previous sediments. More detailed knowledge concerning Triassic and late Paleozoic strata to the southeast of the field area is necessary to more fully understand the provenance of the upper Poleo Sandstone. Upper Shale - Petrified Forest Member Like the Salitral Shale the Petrified Forest Member had two major sources — a sedimentary source terrain and a volcanic source area. The few palaeocurrent measurements obtained in this study indicate that the sedimentary 85 terrain lay to the east, northeast and southeast of the San Juan Basin. The source area probably extended over much of northern New Mexico and possibly into eastern New Mexico and northern Texas. During late stages of deposition of this member some of the sedimentary material may have originated as far away as central Texas in the Ouachita Highlands. This source terrain must have been a sedimentary one, since all nearby Permo-Triassic highlands were covered in the early stages of deposition of the Upper Shale. The volcanic source area is also thought to have been the Mogollon Highlands in southern New Mexico and Arizona (Stewart and others, 1972). Bentonite content increases to the south in the Petrified Forest Member, suggesting a southerly source (Stewart and Smith, 1954; Schultz, 1963), but many have come from the easterly source (O'Sullivan, 1970) . Red Mesa Sandstone Palaeotransport directions indicate that the initial source area for the Red Mesa Sandstone lay to the northeast of Red Mesa but that the source shifted, gradually, to the south and southeast. Abundant chert fragments (Table 11) and texturally and mineralogically mature sandstones indicate a sedimentary source. Strata which were eroded to deposit this unit cannot be precisely identified, but probably consisted of Permian and older Paleozoic sediments. 86 Origin of Volcanic Material The presence of volcanic material in the Salitral Shale tongue and the Petrified Forest Member has been well documented (Stewart and others, 1972). In other Triassic units of the eastern San Juan Basin its presence is probably masked by fluvial elastics; but a few tuffaceous fragments have been found in the Agua Zarca Sandstone. Volcaniclastic material consists of volcanic glass, tuffaceous fragments, and bentonite formed from devitrified volcanic ash. Scattered biotite-rich layers in the upper Poleo Sandstone lentil may also have had a volcanic origin. The Mogollon Highland, in southern New Mexico and Arizona is usually named as the source for this volcanic material (Schultz, 1963; Stewart and Others, 1972). It is unclear, however, how large volcanic clasts could have transported into north-central New Mexico from this distant source. No known Triassic river system flowed northward from the Mogollon Highland into the field area. Poole (1962) has shown that palaeowind directions in the upper Triassic Wingate Sandstone were towards the southeast. Thus, aerial transport of volcanic ash would have been counter to prevailing winds if it came from the Mogollon region. An alternative source of Triassic volcanic material was a north-south trending geanticline in western Nevada, Idaho, and Utah (Wengerd, 1950; McKee, 1950; Seyfert and Sirkin, 1973). Volcanic ash could have been transported from this 87 northwesterly source into New Mexico via the prevailing winds but it remains difficult to reconcile the southerly increase in bentonite with a northern source area. This distant volcanic belt still could not easily have supplied the coarse volcaniclastics found in the Agua Zarca. 88 ENVIRONMENTS OF DEPOSITION Agua Zarca Sandstone The Agua Zarca Sandstone Member of the Chinle Formation was deposited on a southwestwardly dipping slope by high energy braided streams. This mode of deposition is indicated by medium to large (30 - 50 cm) scale foresets and festoon bedding, coarse particle size, and intraforma- tional clay clasts. In addition, numerous cross-cutting lenticular channels, ranging in width from 1 to 10 meters, suggest a fluvial environment wherein the channels were continually shifting. Towards the top of the Agua Zarca Sandstone the mean grain size decreases and, in places, partially developed point bar sequences exist. Thus the energy of the depositional environment decreased and a fluvial , system developed in which, at least locally, the channel was meandering. Throughout the section, sediments deposited in relatively quiescent fluvial sub-environments (overbank deposits, pools, or abandoned stream channels) are preserved. Salitral Shale tongue The Salitral Shale tongue is probably composed pre¬ dominantly of lacustrine sediments (Kaufman, 1971) with minor amounts of low energy delatic and paludal deposits. Symmetric wave ripples are found in discontinuous very fine sand and siltstone layers which probably represent nearshore or shallow water deposits above wavebase. Shales are finely 89 laminated; claystones are blocky and commonly contain rootcasts. Some of these rootcasts appear to be in place, suggesting that areas of standing water existed, shallow enough for bottom vegetation to survive. The Salitral Shale tongue, then, may represent a shallow area on the eastern side of a large lake. Palynographic studies of this member may yield more information concerning the palaeoecology of this area. Fresh water fossils have not been found in the Salitral Shale but the freshwater pelecypod Unio is known from the lithologically very similar Upper Shale Member in this area (Cope, 1875; Stewart and others, 1972). Regional palaeogeographic considerations prohibit the existence of a saltwater bay or seaway in north-central New Mexico during this time (see pp. 95—96), and, so, the proposed lacustrine origin for these sediments. However, the salinity of this lake cannot be determined without additional fossil evidence from the Salitral Shale itself. The Salitral Shale conformably overlies the Agua Zarca Sandstone but no well-developed deltaic sequence exists in the field area. Younger sediments of the San Juan Basin probably overly the existing delta (or deltas). Sections where the two members do interfinger were probably deposited after Salitral sediments began to onlap the fluvial Agua Zarca Sandstone. Sandstone beds at the base of the Salitral Shale probably represent sporadic influxes of elastics, perhaps during floods. The variable thickness of the Salitral Shale in the Chama Basin (Sears, 1953) is probably 90 due to differential subsidence. Poleo Sandstone lentil Sediments of the Poleo Sandstone lentil were deposited in a fluvial regime of lower energy than that of the Agua Zarca Sandstone and show a decrease in depositional energy up-section. The lower Poleo sandstone was deposited on a southwestwardly dipping surface by streams intermediate in regime between meandering and braided streams. The upper Poleo Sandstone was deposited by low energy meandering streams (with some braided stream features), but on a northwestwardly and westwardly dipping slope. (Changes and trends in sedimentary structures through this transition zone are similar to those observed by Schwartz (1976) in the braided- meandering transition in the Red River.) Very low energy fluvial-deltaic deposits are everywhere present at the top of the Poleo Sandstone lentil. Both the upper and lower parts of the Poleo Sandstone contain sediments deposited in subenvironments associated with the fluvial depositional system (eg. backwaters and pools, fluvial bars, overbank deposits, abandoned channels, and, possibly, windblown deposits). Lower Poleo Sandstone The lowest strata of the Poleo Sandstone lentil in the eastern part of the field area consist of one or a few major channel deposits. These channels outcrop only locally, and appear to have been restricted or entrenched for a long 91 period of time. Large scale (20 - 100 cm) festoon and trough bedding and pebble conglomerates with a medium- to coarse sand-sized matrix (Fig. 8b) indicate a high energy environment. Eventually the river shifted from this channel and deposited tabular or broadly lenticular medium-grained sandstone beds. These sandstone units contain a basal pebbly zone overlain by crossbedded strata. Sedimentary structures include large scale festoon and planar cross¬ bedding (Fig. 27) as well as laminar bedding, convolute bedding (Fig. 8a), and crosscutting lenticular channels. Structureless clay lenses deposited in locally quiescent areas are also preserved. The sedimentary structures and associations suggest that the lower portion of the Poleo Sandstone was deposited in a fluvial system with characteris¬ tics of both braided and meandering streams. Compared with the Agua Zarca Sandstone, the smaller mean grain size of the lower Poleo Sandstone indicates that the latter unit was deposited under lower energy conditions, possibly on a gentler slope, than the previous deposits. Upper Poleo Sandstone The upper and lower portions of the Poleo Sandstone lentil are very similar insofar as sedimentary structures, fluvial regime, and depositional energy are concerned. However, the upper sandstones are slightly finer grained (Figs. 20, 21, Table 10), and are more evenly bedded than sandstone of the lower portion. Almost no crosscutting FIGURE 27: Straight foresets of a point bar sequence - Upper Poleo Sandstone, Abiquiu Dam 92 93 channels are preserved in the upper Poleo Sandstone. Thus this part of the Poleo was deposited, for the most part, by meandering streams. Transitional sediments between the upper Poleo Sandstone and the Petrified Forest Member contain numerous small scale (3 - 8 cm) foresets, thinly laminated discontinuous, fine-grained sandstone and silt- stone beds interfingering with shales, and variable palaeo- current directions. These features suggest a low-energy deltaic environment. Upper Shale - Petrified Forest Member The Upper Shale Member was deposited under very low energy conditions on a gradually subsiding, westward sloping, gently dipping plain (Wengerd, 1950). No tectonic activity occurred in the Ancestral Rocky Mountains to in¬ fluence the deposition of this unit. Thinly laminated, multi-colored shales, and blocky clays deposited in marsh¬ land, lacustrine, and very low energy fluvial and deltaic deposits make up this member. Discontinuous, very fine sandstones and siltstones and siltstone lenses exhibit low amplitude crossbedding or oscillation ripples. These coarser lenses may represent areas of slightly higher energy in fluvial or deltaic environments (eg. channels, delta lobes). Freshwater pelecypods have been found in this member in north-central New Mexico (Cope, 1875). 94 Red Mesa Sandstone The Red Mesa Sandstone was deposited by meandering streams on a generally westwardly dipping palaeoslope. This depositional environment is indicated by the many well- developed point bar sequences throughout this section. A complete sequence consists of a basal pebble conglomerate (channel lag) overlain by several very clean, well sorted crossbedded (30 - 60 cm) sandstone beds (fluvial bars) which are in turn overlain by finer-grained sandstones and silt- stones exhibiting small scale (5-10 cm) cross-laminations (overbank deposits). Larger scale crossbeds are planar or concave and are associated in some units with convolute bedding. Few lenticular channels are preserved. In the basal portion of the Red Mesa Sandstone point bar sequences are only partially preserved, consisting of channel gravels and part of the sand bar deposits. Presumably, these incomplete sequences are due to extensive reworking and erosion of previous sediment by the meandering stream. Towards the top of the section the fine-grained floodplain deposits are more commonly preserved. The lack of conglomerates high in the section and the upward decrease in mean grain size indicate decreasing stream energy with time. GEOLOGIC HISTORY Prior to the late Triassic, from late Permian time through the middle Triassic, much of the southern Rocky Mountain region was positive, consisting of isolated highlands and uplands of low relief (Holmes, 1956; Elston and Shoemaker, 1960; MacLachlan 1972). With the retreat of the Permian seas marine deposition continued only in the miogeosyncline and eugeosyncline to the west (Moenkopi Formation, Thaynes Formation, Woodside Forma¬ tion) (Branson, 1927, 1929; Reeside 1929; Wheeler, 1952; Kummel 1954; MacLachlan, 1972). And, the scarcity of early to middle Triassic continental deposits indicates that positive areas were not being extensively eroded. The widespread "middle Triassic" unconformity, known throughout the Colorado Plateau, formed at this time. During the middle Triassic much of the southern Rocky Mountain region was actively uplifted (Smith, 1961). Rugged crystalline highlands were raised (eg. the Ancestral Rocky Mountains or Uncompahgre Range) in central Colorado and northern New Mexico (Holmes, 1956, Elston and Shoemaker, 1960), and the Zuni-Defiance Uplift along the northern Arizona-New Mexico border (Cadigan, 1961; McKee, 1963)'. Simultaneously, rivers began depositing sediment off the flanks of these highlands. A sedimentary terrain in east- central New Mexico, roughly coincident with the northern part of the Pennsylvanian Pedernal Uplift (Willis, 1929) and probably of low relief, was also being eroded at this time. Smith (1961) refers to this positive element as 96 the "Pedernal high". This area may not have been actively uplifted, however, and no Precambrian basement rocks were exposed in the upland during the late Triassic (Baars, 1962). The Mogollon Highlands in southern Arizona and New Mexico were also present during the late Triassic, and may have served as a source for both coarse-grained elastics and volcanic material (Hârshbarger, Repenning, and Irwin, 1957). Areas between the uplifts formed shallow sedimentary basins.1 *Isopach maps (Stewart and others, 1972) indicate such a basin to the north of the Zuni-Defiance Uplift and another one in the region encompassed by the modern San Juan Basin, northwest of the Pedernal high and between the Uncompahgre and Zuni-Defiance highlands. The existence of these basins probably reflects their intermontane location more than any active basin development. Deposition of the Chinle Formation in north-central New Mexico was, at first, largely controlled by the nearby presence of the Uncompahgre Highland. The coarse- to medium¬ grained sandstones ane conglomerates of the Agua Zarca Sandstone were derived primarily from metasedimentary rocks in this uplift. Though the rivers which deposited this sandstone flowed to the southwest, that palaeoslope was only a local one. Regional palaeocurrent data for upper Triassic strata (Poole, 1961, Fig. 199. 1-B) indicate that the major rivers flowed northwest, away from the Pedernal high and the remnants of the Permian Ouachita Mountains in central Texas. Other basal upper Triassic clastic units were 97 probably similarly influenced by nearby highlands. The Shinarump Member of the Chinle Formation was deposited off the flanks of the Zuni-Defiance Uplift (Stewart and others, 1972; Cadigan, 1961), and the Trujillo Sandstone in west Texas was laid down by rivers flowing northward from the Ouachita-Marathon orogenic belt (Asquith and Cramer, 1975). Such streams contributed to the continental drainage system which deposited a vast area of continental sediments extending from northern Wyoming (eg. the Popo Agie Member of the Chugwater Formation) to west Texas (eg. the Dockum Group and the Trujillo Sandstone) to central Arizona (eg. the Shinarump Member, Petrified Forest Member, Wingate Sandstone, and Glen Canyon Group). This drainage system emptied into a sea whose strandline was in Idaho, Nevada, and western Utah (McKee, 1951; Wengard, 1950'; MacLachlan, 1972) . Marshland and lacustrine sediments were deposited in interfluvial areas and in the low-lying basins. Units such as the Salitral Shale tongue and the Monitor Butte Member near the Zuni-Defiance uplift were probably deposited in such environments. Though they commonly overlie basal fluvatile sequences, these fine-grained low energy deposits were probably contemporaneous with the initial high energy braided streams that drained the uplifts. For instance, stratigraphic cross-sections (Stewart and others, 1972) and gradational, conformable contacts between the Agua Zarca Sandstone and the Salitral Shale tongue indicate that the Salitral Lake probably existed at the same time the Agua 98 Zarca sandstones were being deposited. Furthermore, lac¬ ustrine deposition may have occurred in this lake throughout the entire time span represented by the Agua Zarca Sandstone and the Poleo Sandstone lentil. The Salitral Shale tongue represents a lacustrine transgression owing to a local, or perhaps a regional, change in base level. At this time the southern Uncompahgre Highland had been lowered by erosion so that fluvial systems were probably of low energy. A rapid lacustrine transgression coupled with the presence of these low energy streams resulted in ^conformable contact, but not in a well-developed delatic sequence. Compared with the original braided rivers the streams draining the southern Ancestral Rocky Mountains at the time of maximum Salitral transgression were of low ; energy. Their drainage patterns, however, were still influenced predominantly by the nearby uplift rather than by the continental drainage patterns. The Poleo Sandstone lentil was deposited initally in response to renewed uplift in the southern Uncompahgre Highlands. That the deposition of this unit was due to tectonic activity rather than a benign lowering of base level is suggested by 1) the presence of coarse basal conglomerates in this lentil, conglomerates which have a sedimentary provenance, 2) renewal of braided stream depo¬ sition which presumably indicates a steep slope, 3) the entrenchment of channels in the Poleo Sandstone (eg. Abiquiu Dam) , perhaps due to continuing uplift, and possibly by 4) normal faulting in the Salitral Shale tongue near 99 Abiquiu, New Mexico (U.S. Army Corps, of Engineers, 1955). Though an unconformity, exists at the base of the lower Poleo Sandstone the deltaic sequence in the upper Poleo Sandstone at the Nacimiento Mine suggests that lacustrine deposition was continuing in the basin. This final major uplift in the Ancestral Rocky Mountains was of a lower magnitude than previous ones and caused warping over a large area. A rugged crystalline highland was probably not exposed and the source terrain was largely a sedimentary one. At first, the local palaeoslope was still southwesterly. But, with the erosion of this highland to one of low relief, the slope shifted gradually to the northwest. Local fluvatile deposits, the Pedernal high, and, perhaps, the remnants of the Ouachita orogenic belt, probably supplied sediment to the Poleo Sand¬ stone after this shift took place. Thus, the upper Poleo Sandstone, with its northwesterly palaeocurrent directions, reflects the waning influence of the Uncompahgre Highlands on sedimentation patterns in this region. The shift in palaeocurrent directions was a passive response to erosion of the local highlands, not a dynamic response to tectonic activity elsewhere. With increasing erosion and reduction in the regional palaeoslope, the rivers depositing the Poleo Sandstone decreased in energy until, at the end of Poleo deposition, they were sluggish meandering streams flowing across a marshy lowland. Similarly, the Moss Back Member of the Chinle Formation near the Zuni-Defiance range may reflect renewed uplift in that region. 100 Deposition of the Red Mesa Sandstone probably spanned the time represented by the Agua Zarca Sandstone, the Salitral Shale, and the Poleo Sandstone lentil to the north of it. This is suspected because of its apparent continuity with the Agua Zarca Sandstone, basal southwestward palaeo- transport directions, and the northwestward palaeocurrent directions in the upper portion of the section. No Salitral Shale transgression took place in this area. The change in transport direction in the Red Mesa Sandstone is complex and detailed study of this unit may^reveal that the palaeoslope was fluctuating due to the alternating influences of the Uncompahgre and Pedernal highlands. By the latest Triassic all remnants of the Pedernal high, the Zuni-Defiance Uplift, and much of the Ancestral Rocky Mountains were gone. Only a limited region in central Colorado remained high into the Jurassic (Peterson, 1972). Throughout the Colorado Plateau low energy marshland and fluvial sediments of the Petrified Forest Member and its equivalents were deposited. Occasional sandstone units, perhaps representing the final stages of tectonoism (Smith, 1961), occur, but most of the sediments are fine-grained. Sedimentation in north-central New Mexico was controlled primarily by the uplift and erosion of highlands which caused the construction and outward progradation of alluvial fans, and subsequent basin filling. One event, however, seems to run counter to this trend — the transgression of the Salitral Shale tongue. Mechanisms proposed to explain this 101 transgression must account for 1) the transgressive event itself, 2) its rapidity, 3) the close coincidence of the depositional limits of the Agua Zarca Sandstone and the Salitral Shale (fig. 6), and 4) why similar events may have occurred elsewhere on the Colorado Plateau. A great deal of information is lacking which would help solve this problem. In particular 1) time-stratigraphic relationships of upper Triassic lithogenetic units are not precisely known, 2) the configuration of the Salitral Lake prior to the trans¬ gression has not been determined, and 3) the status of the external drainage of the lake is still a mystery. Without this knowledge no definitive statements concerning the driving mechanisms behind this transgression or its areal extent can be made. Models to account for the transgression fall into roughly three overlapping categories: 1) regional transgression caused by a widespread tectonic event; 2) an overall increase in the amount of water in the basin, possibly concurrent with the construction and main¬ tenance of a barrier to external drainage; 3) a local transgressive event caused by localized tectonic activity or isostatic subsidence. These are discussed briefly below. 1) Tectonism on a regional scale could cause a rapid transgression. Similar transgressive events elsewhere in this region might then have been synchronous with the 102 Salitral Shale. The close coincidence of the Salitral Shale and the Agua Zarca Sandstone depositional patterns could be due to a pre-existing crustal flexure which caused channel¬ ization of the rivers depositing the Agua Zarca and a sub¬ sequent embayment of the Salitral Lake when base level rose. Though the basin centers did not rise, uplift in the Zuni- Defiance region could have caused a transgression on the opposite side of the intermontane basin, resulting in depo¬ sition of the Salitral Shale. The timing or existence of such an uplift is, at present, unknown. 2) Increasing the total amount of available water could have been done by tectonic re-routing of drainage patterns or by climatic change. The rapidity of the transgression, however argues against a solely climatic cause. Additionally, for either case, some sort of barrier to external drainage might have had to have been constructed and maintained to keep the base level high. As yet, insufficient data exists to support or refute these suppositions. 3) Local downwarping due to tectonism or isostacy could also give rise to the observations noted above. First order calculations indicate that the thickness of sediment deposited in the Agua Zarca Sandstone was enough to have caused isostatic adjustment. The close coincidence of depositional limits for the Salitral Shale and the Agua Zarca Sandstone (fig. 6) could easily have been caused by this adjustment in the immediate area of the Agua Zarca Sandstone. Only 20 to 30 meters of subsidence need have 103 occurred to have given rise to the observed sedimentary sequence and thicknesses. Analogous litholigic sequences elsewhere in the southern Rocky Mountains could have been caused by similar but independent isostatic events. Thus, the transgressions need not be time equivalent to the Salitral Shale tongue. Some degree of isostatic compensation almost certainly occurred. It is impossible to state, however, if this adjustment was indeed the primary cause of the Salitral Shale transgression. Summary — Geologic History Upper Triassic sediments of the Chinle Formation along the eastern margin of the San Juan Basin are continental fluvial-deltaic and marshland-lacustrine deposits which reflect a generally less energetic environment of deposition with time. The decreasing energy of this environment is indicated by the gradual development of the fluvial system from a youthful braided stream transporting coarse-grained material (Agua Zarca Sandstone), to a more mature meandering stream carrying finer sediment (Poleo Sandstone lentil), to a senile, sluggish drainage system flowing through marshy lowlands (Petrified Forest Member). Initially,the local palaeoslope dipped to the southwest but later shifted to the northwest. The source area, originally a relatively localized metasedimentary one in the southern Ancestral Rocky Mountains, shifted gradually until it encompassed a wide¬ spread sedimentary terrain to the east and southeast of the 104 Nacimiento Uplift. Sedimentation was continuous throughout the upper Triassic in this region, though intra-formational diasterns (in particular, between the Salitral Shale and the Poleo Sandstone) are present. Excluding the Petrified Forest Member the Chinle Formation shows an east-west facies progression of 1) fluvial-deltaic, to 2) interfingering fluvial-deltaic and marshland-lacustrine, to 3) marshland- lacustrine (fig. 28). The north-south section (fig. 29) indicates that the Agua Zarca Sandstone and the Salitral Shale are more restricted than ~later deposits. Appendix IV is a summary of characteristics of members of the Chinle Formation in the Nacimiento region. Deposition of upper Triassic sediments was controlled initally by the presence of nearby highlands, elastics were shed off the uplifts as prograding alluvial fans which gradually filled intermontane basins. Low energy depositional environments, marshlands and lakes, developed in the basins. Two tectonic pulses in the southern Ancestral Rocky Mountains are recorded in the upper Triassic sediments of north-central New Mexico. These events are reflected by the onset of elastics in the Agua Zarca Sandstone and the Poleo Sandstone. A transgressive event occurred between these two units (represented by the Salitral Shale) but its causal mechanisms are unknown. The gradually shifting palaeoslope and source area are a passive response of depositional patterns to the erosion and lowering of the highlands. In the latest Triassic, sediments were deposited by sluggish streams FIGURE 28: East-west cross-section of Chinle Formation, north-central New Mexico - Illustrating facies relationships between Upper Triassic units; see Table 12 for explanation 105 FIGURE 29: North-south cross-section of Chinle Formation, north-central New Mexico - Illustrating facies relationships between Upper Triassic units; see Table 12 for explanation North South 106 ENVIRONMENT of RELATIVE ENERGY Ss j Sh LEGEND CO fi w w fil S Q U 04 O H H H O £5 > H O H I!I II II !j I ,C m H * ...?i 00 S fil 3 m P ü en ? •• 1 P -H P c 0 •H 'O (0 •H «P H fH 4J G -P H W (0 CM H ^ fi Q 5 H ^ P (0 -H Q Table 12 : Explanation for Figures 28 and 29 108 flowing westward through marshy lowlands. These streams were part of a mature continental drainage system that was influenced only in a minor way by small, local uplifts. Palaeoclimatology Throughout the late Triassic, southwestern North America was situated at approximately 15° to 20° N latitude (Dubois, 1964; Irving, 1964). Thus, the climate was probably tropical, without distinct seasons (Lamb, 1961). Palaeowinds were to the southeast and southwest (Poole, 1962). The Mesocordilleran Geanticline (Seyfert and Sirkin, 1973) to the west and the remnants to the Ouachita Orogenic Belt to the southeast (Asquith and Cramer, 1975) influenced these wind directions and their moisture content. Isolated highlands, such as the Uncompahgre Uplift, and large lakes (eg. the Rock Point Lagoon of O'Sullivan, 1970, and perhaps the Salitral Lake) may have moderated local weather. A great deal of paléontologie evidence from the Chinle Formation exists but considerable doubt still remains as to whether the climate was humid subtropical (Ash, 1972) or tropical and semi-arid (Gottesfeld, 1972). Daugherty (1941) maintains that the climate was tropical but with wet and dry seasons. Such conclusions concerning late Triassic palaeoclimate are based largely on plant fossil character¬ istics (eg. leaf thicknesses, position and number of stomata, existence or absence of thick-walled palynomorphs, etc.). However, as Gottesfeld (1972) points out, plants are 109 representative of discrete communities and need not reflect the ecosystem as a whole. He feels that most of the upper Triassic fossil plants which were preserved grew in relatively localized, wet lowlands, near through-flowing streams. Gottesfeld also notes that much of the land area may actually have been covered by a sparse, semi-arid upland community which is only poorly preserved in the fossil record.. It is this upland community that would reflect regional climatic conditions. Geologic and paléontologie data suggest that climate during the late Triassic probably became more arid with time (McKee and others, 1959). The increasing prominence of the Mesocordilleran Geanticline (Seyfert and Sirkin, 1973), which could have altered circulation patterns and air moisture content, coupled with the northward drift of North America (Irving, 1964) into the desert latitudes were prob¬ ably major factors influencing this climatic change. Widespread late Triassic - early Jurassic desert dune deposits such as the Wingate Sandstone (Harshbarger and others 1957; Poole, 1962) were deposited. Unconformities formed at the top of marshland sequences in many areas (eg. Petrified Forest Member r Entrada Sandstone disconformity in the Nacimiento Mountains), perhaps due to the shrinking of wet lowlands with increasing aridity. Daugherty (1941) observes that reptile and plant remains become more scarce with time and he attributes this to climatic deterioration. Further¬ more, the vast area including most of the central United no States and Canada, where no record of late Triassic earth history remains, may also have been arid throughout this time. Air masses moving across this region would have been depleted of moisture long before reaching southwestern North America. Possible climatic causes for the Salitral transgression suggested in pages 101-102 (eg. increasing precipitation or decreasing evaporation) are therefore unlikely in view of the evidence at hand. More detailed studies of sediment and lithofacies distributions, sediment petrography and clay mineralogy, and palaeoecological variations are needed to understand this change in climate. Ill ECONOMIC GEOLOGY Copper Economically important Triassic sedimentary copper deposits in the Nacimiento Uplift are known only from the Agua Zarca Sandstone. These deposits have been mined in the Arroyo de Cobre, at the Nacimiento Mine, on Eureka Mesa, in San Miguel Canyon, and at the San Miguel Mine. Other much smaller prospects are present throughout this area. Sedimentary copper deposit's occur throughout the* south¬ western United States and have been discussed by many authors (Newberry, 1876; Emmons, 1905;.Lindgren, 1907, 1911; Tarr , 1910; Fath, 1915; Rogers, 1916; Finch, 1928, 1933; Fischer, 1937; Holmquist, 1947; Davidson, 1965; Kaufman, 1971; Woodward and others, 1974; LaPoint, 1975; Rose, 1976). They are interesting, not because of their economic importance, (few localities contain mineable ore bodies), but because of their unusual mode of occurrence. In southwestern North America two varieties of strata-bound copper deposits exist. The first variety contains large amounts of primary copper minerals formed under reducing chemical conditions (chalcocite, bornite, chalcopyrite, covellite, and native copper) (Haworth and Bennett, 1900; Kaufman, 1971) with associated secondary copper minerals formed under oxidizing conditions (malachite, azurite, and chrysocolla) (Woodward and others, 1974). These secondary copper minerals commonly form haloes around pockets of primary minerals (Kaufman, 1971). 112 The other variety of sedimentary copper deposits consists predominantly of secondary minerals with little or no primary mineralization. These deposits are small, with copper car¬ bonates present in the sediment matrix and as concentrations between layers. They have virtually no economic importance. The terms "primary" and "secondary" used in reference to groups of copper minerals pertain to any cycle of deposition wherein primary minerals are initially deposited and secondary minerals are later derived from them. Primary Copper Deposits Copper deposits along the eastern margin of the San Juan Basin and throughout the southern Rocky Mountains have several features in common. 1. Though these sedimentary copper deposits have often been referred to as "redbed" deposits (Emmons, 1905) mineralization zones rarely occur in redbeds (Lindgren and others, 1910). White or gray-white host rocks commonly overlie or are interbedded with red or reddish-brown units. The contrast is so striking that the lighter units appear bleached (Finch, 1928). Light-colored, copper-bearing sandstones of the Auga Zarca Sandstone on Eureka Mesa, which overlie Permian redbeds, are a good example of this. Iron oxides which are quite abundant in redbeds, constitute an insignificant portion of the diagenetic material in copper¬ bearing strata. 2. The most important concentrations of copper ores are found in porous, medium- to coarse-grained, arkosic, 113 sandstone and pebbly sandstone channel fill deposits (Emmons, 1905; Lindgren and others, 1910; Lindgren, 1911; Kaufman, 1971). However, minor secondary mineralization also occurs in limestones and shales (Schultz, 1896; Bastin, 1933; Fisher, 1937). Zones of mineralization in sandstones are discontinuous and commonly lensoid (Lindgren and others, 1910; Finch, 1928) (also Eureka Mine, San Miguel Canyon and las Minas Jimmie, this study). Porous, mineralized units overlie less porous, more impermeable, unmineralized strata. 3. Ore bodies are found in rocks ranging in age from Pennsylvanian to Cretaceous but they are restricted to a few stratigraphic levels. Almost all ore bodies are closely associated with erosional unconformities. In the field area, copper deposits occur just above the unconformable Permian-Triassic contact. Elsewhere on the Colorado Plateau, copper deposits are associated with the mid-Triassic uncon¬ formity (Finch, 1959)'. 4. Primary copper minerals in many instances replace organic material (Lindgren and others, 1910; Soule, 1956; Kaufman, 1971; Rose, 1976). Small pieces of cuprified fossil wood and bark are common. Petrified logs 6 to 8 feet (2-2.5 meters) long, almost completely replaced by copper minerals, can still be seen in the ceilings of the mine shafts on Eureka Mesa. Wood fossils are relatively undeformed and fine details of the plant cell structure still remain (Fath, 1915). Nodules of chalcocite with an oxidized crust are also found. These nodules are not entirely 114 composed of copper minerals, as organic matter remains (Rogers, 1916; Bastin, 1933). Some primary mineralization associated with clay lenses has been reported (Soule, 1956). Possibly these clays were organic-rich, and complexing with metalliferous ground waters occurred. In the study area, clay lenses served as substrates for secondary minerals; no primary minerals were observed in clayey layers. 5. Copper deposits almost everywhere occur in continen¬ tal, fluvial or fluvio-deltaic-deposits. Continental redbed strata, and in some cases evaporites, are associated with mineralized "units. In many places deposits occur in fossil log jams (Kaufman, 1971) or at the bases of fluvial channels where organic material was concentrated. 6. Ore bodies are found in faulted and folded strata. These structures apparently have little control over primary mineralization, however, except that they serve to expose the ore bodies. 7. Associated metals include silver, uranium, vanadium, and gold (Finch, 1959; Kaufman, 1971). Silver and gold occur in small quantities but uranium and vanadium are in some deposits the dominant metal. 8. In almost all deposits no intrusive rocks or other obvious sources for the copper are present. Also, no hydrothermal gangue minerals are found. Barite, however, is present in some locales (Lindgren, 1907, 1911). 115 Secondary Copper Deposits Major deposits of secondary minerals are, in most places, associated with one or more substantial primary deposit.. However, small deposits of oxidized copper minerals having no apparent association with primary deposits do occur throughout the stratigraphic section. Copper minerals in most of these minor deposits occupy an insignificant volume of the host rock. Host rocks include both redbeds and light-colored strata — sandstones, silt- stones, shales,'and limestones'"(Lindgren and others, 1910; Soule-*, 1956) . The copper carbonates are finely disseminated throughout clastic layers, and are present as concentrations between beds. Genesis of Copper Deposits It has long been recognized from field studies (Lindgren and others, 1910; Lindgren, 1911), petrographic evidence (Rogers, 1916), and geochemical principles that the primary sedimentary copper deposits containing chalcocite, bornite, etc. were deposited under reducing chemical conditions. These reducing conditions existed in the presence of organic matter and iron sulfides. Reduced minerals were later oxidized, remobilized, and redeposited principally as malachite and azurite (Rogers, 1916; Woodward and others, 1974). The exact order of mineralization, from primary to secondary minerals, is (1) pyrite to (2) bornite to (3) chalcocite and corvellite to (4) malachite and azurite (Rogers, 1916; Woodward and others, 1974). In this sequence, the position of chalcopyrite 116 is unclear (Woodward and others, 1974). Since hematite is not stable under reducing conditions, copper bearing strata are in many places light-colored. Rose (1976) has calculated an approximate temperature of formation for these copper minerals of 75°C indicating at least shallow burial. The relatively undeformed character of cuprified fossil wood fragments, however, indicates replacement before very deep burial occurred (Fisher, 1937; Kaufman, 1971). The association of redbeds~and evaporites has been shown by Rose (1976) to be critically important for the genesis of these copper deposits. This is one of the few geologic situations where migrating ground water can contain sufficient chloride ions under the proper Eh-pH conditions to form copper chloride complexes (Rose, 1976). Ground water carrying these complexes travels along fractures and through permeable sandstone channel fill deposits near erosional unconformities. Where reducing conditions are encountered in these porous rocks, copper minerals are deposited (Wood¬ ward and others, 1974) . Thus, copper deposits at different stratigraphic levels do not necessarily indicate different episodes of mineralization. Deposition could have occurred continually for long periods of time at optimum sites. Rogers (1916) has pointed out that replacement of pyrite by chalcopyrite probably took a long time and Lindgren (1911) has suggested that mineralization could be occurring at depth today. 117 Depositionally controlled physical characteristics of the host rock will influence subsequent copper mineralization only insofar as they affect the movements of ore-bearing fluids. Porous fluvial channel fill deposits may serve as conduits for ground water but these fluids may move in either direction through the channel. The regional palaeo- slope at the time of mineralization controls the regional hydraulic gradient; the palaeoslope which influenced the deposition of the host rock will not necessarily affect this gradient. Copper ions need not have been derived directly from Precambrian crystalline rocks, though they perhaps ultimately originated from them. Migrating fluids could have leached the copper from hematite and trace copper oxides in sedimentary rocks. Enough trace copper is associated with diagenetic hematite to be concentrated into an ore forming solution by chloride-rich fluids (Rose and Suhr, 1971). Given sufficient time, copper from a large volume rock could be concentrated into strata-bound ore deposits. Re-oxidation of these deposits could have occurred much later to redisperse copper throughout the host rock as copper carbonates and oxides. The deposition of malachite in sandstones after the formation of quartz overgrowths (Woodward and others, 1974) suggests that deep burial occurred before secondary oxidation. These secondary minerals might then be leached again by chloride-rich solutions and redeposited as primary reduced copper minerals. As long as chloride ions are 118 available under the proper chemical conditions, many cycles of primary copper deposition, oxidation, leaching, and re-deposition of primary minerals are possible. An interesting observation concerning many of the major ore bodies is that they occur in arkosic sedimentary rocks. This association between arkosic sediments and strata-bound copper deposits has not been explored, and its significance, if any, is unknown. Economic Potential Though many strata-bound copper deposits are known, only a few have any economic significance. As of 1956 approximately 21,000,000 pounds of copper had been produced from sedimentary copper deposits in New Mexico (Soule, 1956). The bulk of this came from mines in two areas (Table 13). All mines from which copper has been produced in Rio Arriba and Sandoval counties are within the field area. The most productive mines in Sandoval and Guadalupe counties are in Triassic strata (Soule, 1956). Discovering economically important sedimentary copper deposits remains a possibility. Previously unknown prospects may be found by more sophisticated scientific prospecting methods, and new mining techniques may convert once low grade or inaccessible deposits into producing ore bodies. Prospecting Copper deposits can be located by examining Pennsylvanian through Cretaceous, light-colored, porous, medium- to 119 New Mexico County Pounds of Cu produced (as of 1956) Guadalupe 12,004,437 Rio Arriba 2,898 Sandoval 6,560,546 Others 2,141,963 TOTAL 20,709,854 TABLE 13: PRODUCTION OF STRATA-BOUND COPPER IN NEW MEXICO AS OF 1956 (from Soulé, 1956) 120 coarse-grained, arkosic, fluvial sandstone channel sequences. Units containing carbonized wood and associated with an erosional unconformity are more likely to contain valuable copper deposits than those without these features. Since the paths that migrating fluids will follow are influenced by faults and folds, primary ore bodies may be found in association with structures only slightly younger than potential host rocks. Secondary oxidation of many deposits may have occurred during Laramide time, as the rocks were uplifted and re-exposed to oxidizing conditons. Fracturing and faulting occurring at that time would have provided conduits for fluid movement. The Bluebird prospect (W H sec. 12, T. 20 N., R. 1 W.) might have been formed in this manner. Though the secondary deposits are rarely economically important, studying them may enable prospectors to locate the more valuable primary deposits from which they were derived. In some cases, however, the original ore body may have been completely oxidized and no longer exist. No copper deposits are known from the fluvio-deltaic Poleo Sandstone lentil along the eastern margin of the San Juan Basin. This lack of copper minerals is probably due to the presence of a carbonate cement and to the fine grain size of the Poleo Sandstone, which reduced the porosity and permeability of this unit. Since this calcareous cement formed mainly in the deltaic environment, and insomuch as the unit does contain abundant organic material and little hematite, coarse upstream equivalents of the Poleo Sandstone 121 may well contain copper. One would look to the northeast, in the Chama Basin, for upstream deposits of the lower Poleo, and to the south and southeast for upstream equivalents of the upper Poleo Sandstone. Copper is more likely to be present in the lower Poleo Sandstone since these rocks are slightly coarser than the upper unit and since it uncon- formably overlies redbeds in many places. In basins like the Chama Basin and in areas where ore bodies might be deeply buried, geochemical tests of ground water may prove useful for locating copper. Where mesas are capped . by potentially ore-bearing rocks seismic refraction methods (Black and others, 1962) or gravity surveys may be designed to locate dense bodies of chalcocite. Uranium Triassic strata in the field area contain small uranium deposits of subeconomic importance. Such deposits are known from the Agua Zarca Sandstone, the Salitral Shale tongue, and the Poleo Sandstone lentil. Only slightly more than 600 tons of ore had been processed from this region as of 1974 (Chenoweth, 1974). Uranium occurs as oxides in lime¬ stones, coarse sandstones, and conglomerates, and is associated with organic material. Copper carbonates are closely associated in some deposits. Chenoweth (1974) believes that the uranium oxides were derived from devitrification of Tertiary tuffaceous rocks and were 122 deposited long after the copper minerals; but this need not have been the case. Copper-bearing fluids derived from Mesozoic sedimentary copper deposits could have been present in the Tertiary since mineralized zones were probably being re-oxidized. These fluids would have been affected by the same hydraulic parameters as uraniferous solutions and thus, copper might have been deposited with uranium at some sites. Petroleum Only small amounts of organic material were present initially in Triassic sandstones since the warm, dry climatic conditions probably precluded the presence of large tracts of vegetation (Daugherty, 1941). Most of the organic material that was present may have been immediately oxidized or replaced by copper minerals. Thus, no source for petroleum existed within Triassic strata. Porous, permeable Triassic sandstones may, however, serve as reservoir rocks for petroleum found in Pennsylvanian or Jurassic strata. Structural traps involving these rocks could exist along the eastern edge of the San Juna Basin, west of the border fault. 123 REFERENCES Allen, J.R.L., 1966, On Bed Forms and Paleocurrents: Sedimentology, v. 6, pp. 153-190. Anderson, J.B.;, 1970, Structure and Stratigraphy of the Western Margin of the Nacimiento Uplift, New Mexico Unpub. M.S. Thesis, Univ. New Mexico, 44 pp. Ash, S.R., 1972, Plant Megafossils of the Chinle Formation: in Breed, C.S. and Breed, W.J. (eds.), Investigations in the Triassic Chinle Formation, Mus. Northern Ariz. Bull., v. 47, pp.23-43. Asquith, G.B., and Cramer, S.L., 1975, Transverse Braid Bars in the Upper Triassic Trujillo Sandstone of the Texas Panhandle: J. Geol., v. 83, pp. 657-661. Baars, D.L., 196^2, Permian System of Colorado Plateau: Amer. Assoc. Petrol. Geol. Bull., v. 46, pp. 149-218. Bastin, E. S., 1933, The Chalcocite and Native Copper Types of Ore Deposits: Econ. Geol., v. 28, pp. 407-446. 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Map 1-300, 33 pp. Miall, A.D., 1974, Paleocurrent Analysis of Alluvial Sedi¬ ments: A Discussion of Directional Variance and Vector Magnitude: Jour. Sed. Petrology, v. 44, pp. 1174-1185. Muehlberger, W.R., 1967, Geology of Chama Quadrangle, New Mexico: New Mexico Bur. Mines and Min. Res. Bull. 89, 114 pp. , Adams, G.E., Longgood, T.E., Jr., and St. Johns, B.E., 1960, Stratigraphy of the Chama Quadrangle -Northern Rio Arriba County, New Mexico: New Mexico Geol. Soc. Guidebook - Eleventh Field Conference: Rio Chama Country, pp. 93-102. Newberry, J.S., 1876 , Geological Report: In Macomb, I.N. . Report of the Exploring Expedition from Santa Fe, New Mexico,to the Junction of the Grand and Green Rivers of the Great Colorado of the West in 1859, U. S. Army Eng. Dept., pp. 9-118. Ore, H.T., 1963, Some Criteria for the Recognition of Braided Stream Deposits: Contributions to Geology, v. 3, pp. 1-14. O'Sullivan, R.B., 1970, The Upper Part of the Upper Triassic Chinle Formation and Related Rocks, Southwestern Utah and Adjacent Areas: U.S. Geol. Survey Prof. Paper 644-E, 22 pp. 128 Parker, J.W., 1957, Nacimiento Mountains - History and Relation to the San Juan Basin: Four Corners Geol. Soc. Guidebook - Second Field Conference: Geology of South¬ western San Juan Basin, pp. 73-76. Passega, R., 1964, Grain Size Representation by CM Patterns as a Geologic Tool: Jour. Sed. Petrology, v. 34, pp. 830-847. Pelletier, B.R., 1958, Ponoco Paleocurrents in Pennsylvanian and Maryland: Geol. Soc. American Bull., v. 69, pp. 1033-1064. Peterson, J.A., 1972, Jurassic System: in Geologic Atlas of the Rocky Mountain Region, Rocky Mountain Association of Geologists, pp. 177-189. Pettijohn, F.J., Potter, P.E., and Siever, R., 1972, Sand and Sandstone: Springer-Verlag. 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U.S. Dept, of Commerce, 1974, Climatological DATA, National Summary, N.O.A.A. - Environmental DATA Service, v. 25:13, 115 pp. Wengerd, S.A., 1950, Triassic Rocks of Northwest New Mexico and Southwest Colorado: New Mexico Geol. Soc. Guidebook - First Field Conference: The San Juan Basin, New Mexico and Colorado, pp. 67-75. Wheeler, H.E., 1952, Permo(?)- Triasic Géosynclinal Facies in Northeastern Nevada and Northwestern Utah (abs.): Geol. Soc. America Bull., v. 63, p. 1311. Willis, R., 1929, Structural Development and Oil Accumulation in the Texas Permian: Amer. Assoc. Petrol. Geol. Bull., v. 13, pp. 1033-1043. Wood, G.H., and Northrup, S.A., 1946, Geology of the NacimientO-and San Pedro Mountains and Adjacent Plateaus in parts of Sandoval and Rio Arriba Counties, New Mexico: U.S. Geol. Survey Oil and Gas Inv. Prelim. Map 57. Woodward, L.A., 1974, Tectonics of Central Northern New Mexico: New Mexico Geol. Soc. Guidebook - Twenty- fifth Field Conference: Ghost Ranch, pp. 123-129. 131 Woodward, L.A., Kaufman, W.H., and Anderson, J.B., 1972, Nacimiento Fault and Related Structures, Northern New Mexico: Geol. Soc. America Bull., v. 83, pp. 2383-2396. , Kaufman, W.H., Schumacher, O.L., and Talbott, L.W., 1974, Strata Bound Copper Deposits in Triassic Sandstone of Sierra Nacimiento, New Mexico: Econ. Geol., v. 69, pp. 108-120. APPENDIX I: Locations and Descriptions of Measured Sections Upper Triassic Chinle Formation, North-central New Mexico 133 SECTION 1: SAN MIGUEL CANYON NW. Sect. 12, T. 19'.N,,R. 1 W. Measured with Jacob Staff and Brunton Compass UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 8 Triassic Chinle Fm. ? Upper Shale Member Sequences of reddish- yellow (5YR 7/6) to reddish-brown (2.5YR 5/4) friable shales with green and white string¬ ers of very fine silic¬ eous sandstone; poor exposures, largely cov¬ ered; lower contact is gradational 7 Triassic Chinle Fm. 3.0 Poleo Sandstone Brown (10YR 5/3) friable immature micaceous shale ; out¬ crop fair; poorly exposed small-scale cross laminations; upper contact grada¬ tional, top of this unit defined by first appearance of micaceous shale; lower contact also gradational; mus¬ covite is distributed within the rock between shaley layers 3.0 6 Triassic Chinle Fm. 2.5 Poleo Sandstone Transitional unit between units 7 and 5 Sequence of interbedded gray-green very fine siliceous, micaceous sandstones and purple micaceous shales; fair to good exposure; upper and lower contacts grad¬ ational ; shales and sandstones present in about equal amounts 5.5 134 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 5 Triassic Chinle Fm. 1.5 Poleo Sandstone Green (6Y 6/1) very fine siliceous, mica¬ ceous sandstone; good exposure; small-scale cross-laminations; upper and lower contacts gradational; top of this unit defined as the point where sand¬ stone becomes the dominant lithology 7.0 4 Triassic Chinle Fm. 2.0 Poleo Sandstone Same as unit 7; upper and lower con¬ tacts gradational, but no transitional unit such as unit 6 between units 4 and 5; last presence of mica¬ ceous shales marks lower contact of this unit 9.0 Triassic Chinle Fm. -100.0 Salitral Shale Sequence of green, red, purple, and orange friable shales with purple pellet limestone layers; poor partially covered ex¬ posures; upper contact gradational, lower con¬ tact covered but pro¬ bably sharp; lower con¬ tact defined by presence of first sandstone; due to poor exposures and the Nacimiento fault zone the thickness of this unit is only approxmate 109.0 135 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 2 Triassic Chinle Fm. ~60.0 Agua Zarca Sandstone White to buff colored indurated mature med¬ ium siliceous sand¬ stone with whitish- green indurated medium siliceous micaceous sandstone beds at and near the top of the unit, and purple indur¬ ated pebbly siliceous conglomerate beds to¬ wards the base of the unit; conglomerate clasts are quartzite; outcrop fair to good; some current structures but these are poorly exposed due to exten¬ sive fracturing; Fe oxide stains, Cu carbon¬ ates in float; upper and lower contact not seen, thickness is thus approximate; ridge- former -169.0 1 Permian Cutler Group 136 SECTION 2: SAN MIGUELITO CANYON Center Sect. 36, T. 20-N., R. 1 W. Measured with 100 foot Steel Tape UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 6 Triassic Chinle Fm. ? Upper Shale Member Sequence of red and maroon shales contain¬ ing a 3-4 m thick pebbly fine siliceous sand¬ stone, pebbles may represent lag deposits; poor outcrop - badland_ topography; cut and fill structures and oscill¬ ation ripples present in sandstone 5 Triassic Chinle Fm. 6-12 Poleo Sandstone approximate An upper siltstone(?) or very fine clayey siliceous sandstone underlain by an indur¬ ated fine siliceous mic¬ aceous sandstone con¬ taining scattered clay lenses and siltstone layers Upper part very poorly exposed, wea¬ thering to a vegetated slope with occasional ridges of more resistant sandstone; sandstones contain oscillation ripples and laminar bedding; small scale ripples and foresets show a wide variation in current directions; contacts not seen but upper contact may be gradational ~9 137 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 4 Triassic Chinle Fm. 0.30 Poleo Sandstone Sequence of indur¬ ated fine siliceous micaceous sandstone; many crosscutting cut and fill structures; upper and lower con¬ tacts sharp ~9.3 3 Triassic Chinle Fm. 7.5-13.5 Poleo Sandstone approximate Sequence of sand¬ stones and shales very similar‘to the lower part of Unit 5, Section 2; at least one ridge¬ forming massive sand¬ stone is present; upper contact covered but apparently gradational; lower contact of an indurated fine siliceous sandstone, containing oscillation ripples, with unit 2 is sharp and erosional -23.0 2 Triassic Chinle Fm. 3.0-4.0 Poleo Sandstone Pebbly Calcareous (?) limestone conglomerate channel with clay shards, fossil wood, bark, and plant fragments ; good outcrop - ridge former; abundant iron oxide staining; both contacts sharp and irregular; upper contact erosional, lower contact loaded; unit has a lensoid shape ~28.0 138 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 1 Triassic Chinle Fm. ? Salitral Shale Sequence of red and green clays forming poor exposures on a sparsely vegetated slope; root casts present - some apparently in place; upper contact sharp, loaded, with oxidized zones 139 SECTION 3: ARROYO DE LOS PINOS NW. Sect. 13, T. 19 N., R. 1 W. Measured with 100 foot Steel Tape UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 4 Triassic Chinle Fm. Upper Shale Member Sequence of red and purple clays weathering to a poorly exposed, alluvium covered slope; contact with underlying unit covered but appar¬ ently sharp 3 Triassic Chinle Fm. 3.0 Poleo Sandstone Sequence of purple- green, indurated fine siliceous micaceous sandstones; good exposures; sandstones form a low ridge; small scale ripples with a definite current bias present; pyro- lusite dendrites; upper contact sharp 3.0 2 Triassic Chinle Fm. 2.5 Poleo Sandstone Pebbly calcareous limestone conglomerate varying from indurated at the base to semi- friable at the top; good outcrop, weathers to form a low ridge; 5-7.5 cm. foresets present 5.5 1 Triassic Chinle Fm. ? Salitral Shale Sequence of reddish- purple clays weathering to form a poor exposure; upper contact (with unit 2) covered 140 SECTION 4: SENORITO CANYON SW. Sect. 1, T. 20 N., R. 1 W. Along New Mexico Highway 126. Measures with 100 Foot Steel Tape; and Jacob Staff and Brunton Compass UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 9 Triassic Chinle Fm. ? Upper Shale Member Sequence of inter- bedded orange, red, purple, and blue shales and well-indurated clays weathering to form low sparsely vege¬ tated hills; lower con- tact gradational 8 Triassic Chinle Fm. 9.5 Poleo Sandstone Partially covered sequence of interbedded dark purple shales, silt- stones, and purple or gray indurated very fine calcareous micaceous well- sorted quartz sandstones; outcrop fair—a roadcut; some oscillation ripples and 5-7^5 cm. foresets; these structures not always evident due to changes in cement or degree of induration; some apparently massive units can be traced lat¬ erally into beds contain¬ ing oscillation ripples; middle section of this unit has less calcite and mica than other portions; upper contact gradational lower contact sharp and irregular; many intra¬ unit sharp, erosional contacts 9.5 7 Triassic Chinle Fm. 24.5 Poleo Sandstone Sequence of gray- green micaceous quartz 141 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 7 sandstones with inter- (cont.) bedded brown and gray, indurated clay lenses and purple pebbly calcareous limestone conglomerates, sorting varies from poor to well, improving upwards ; grain size varies from medium sand to very fine sand (discounting pebbly lag deposits), becoming finer upwards; induration varies from well-indurated to semi-friable; a prominent ledge; units largely massive with some laminar bedding and oscillation ripples and trough sets; beds have a lensoid geo¬ metry and commonly are normally graded; iron oxide staining and organic matter present; upper contact sharp and placed at top of first massive channel; lower contact sharp 31.0 6 Triassic Chinle Fm. 12.0 Poleo Sandstone A single gray-white massive fine to medium calcareous micaceous quartz sandstone overlain by 60 cm. of gray silty clay; sandstone is well- indurated at the base to semi-friable towards the top; a blasted road- cut; upper contact sharp and marked by silty clay, lower contact sharp and irregular 43.0 142 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 5 Triassic Chinle Fm. 51.0 Salitral Shale Sequence of red, pur¬ ple, and green shales and well-indurated clays with some light green very fine siliceous micaceous sandstones and siltstones; out¬ crop poor-weathers to form a valley ; sand¬ stones and siltstones contain oscillation ripples; septarian con¬ cretions and root ~~ casts (some in place), frothy weathering sur¬ face; upper contact sharp, lower contact poorly ex¬ posed but apparently grad¬ ational 94.0 4 Triassic Chinle Fm. 9.0 Agua Aarca Sandstone Sequence of green, purple, and gray well- indurated, very fine, well-sorted siliceous micaceous (some) sand¬ stones with interbedded purple and brown shale lenses; blasted road- cut; sandstones massive or contain laminar bed¬ ding; fossil wood and iron oxide staining; upper con¬ tact gradational, lower contact sharp, arbitrary at a relatively persistent shale unit 103.0 3 Triassic Chinle Fm. 24.5 Agua Zarca Sandstone Channel sequence of white-gray siliceous sandstones interbedded with pebbly siliceous conglomerates and shale lenses; conglomerate unit at the base; individual •7' 143 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 3 channels fine upwards (cont. (grain size ranges from pebbles to fine sand size) and are more well-sorted towards the top, more friable towards the top, and contain clay shards at the base; some sand¬ stones micaceous; blasted roadcut; some laminar bedding and trough bed¬ ding but most units appar¬ ently massive; sharp intra¬ unit contacts between channels ; abundant iron oxide staining; hematized and carbonized fossil wood, sharp upper and lower contacts 127.5 2 Triassic Chinle Fm. 3.5 Ague Zarca Sandstone Sequence of light brown very fine indurated siliceous sandstone chan¬ nels with interbedded coarse sandstones, gray indurated clay lenses, and chert-bearing conglom¬ erate layers towards the top; blasted roadcut; channel cut and fill and laminar bedding ; upper and lower contacts sharp; upper contact at base of conglomerate, lower con¬ tact at top of unit 1 131.0 Permian Cutler Group ? Red-purple medium to fine siliceous indurated sandstones; blasted road¬ cut; upper contact marked by a distinct color change and erosional surface 144 SECTION 5: NACIMIENTO MINE NW. Sect. 1, T. 20 N. R. 1 W. On the Northern Face of the Mine Pit Measured with 100 foot Steel Tape UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 4 Triassic Chinle Fm. ? Upper Shale Member Sequence of poorly exposed red,purple, and gray shales and indurated clays ; tectonically thinned and pulverized apparently gradational lower contact 3 Triassic Chinle Fm. 12.0 Poleo Sandstone Sequence of interbed- ded very fine calcareous micaceous quartz sand¬ stones and red-brown in¬ durated clays—some of which are gradational downward into siltstones; exposure good, a blasted roadcut; small scale oscillation ripples and laminar bedding in sand¬ stones; clay shards at the bases of some units; mica and fossil plant fragments between layers; iron oxide staining; lower contact sharp at the base of a shale sequence; intra¬ unit contacts sharp and straight 12.0 2 Triassic Chinle Fm. 8.0 Poleo Sandstone Sequence of inter- bedded green indurated poorly sorted calcareous micaceous quartz sandstones and poorly sorted pebbly calcareous limestone- and quartzite-bearing con¬ glomerates; outcrop good, 145 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 2 a blasted cut; sand- (cont.) stones contain laminar bedding, trough sets, and 5-7.5 cm. foresets; conglomerates contain laminar structures; discrete organic and gypsum-rich layers; lower contact sharp and tec¬ tonically irregular; intra-unit contacts both sharp and gradational 20.0 1 Triassiq Chinle Fm. ? Salitral Shale Sequence of poorly exposed red, green, gray, and yellow shales and indurated calys with some medium to fine friable siliceous mica¬ ceous and clay-rich sandstone lenses; much tectonic deformation 146 SECTION 6: FRENCH MESA (Composite) A Composite of Two Sections 1: W. Sect. 11 and E. Sect 10, T. 23 N., R. 1 E. In the Gallina, New Mexico Dump 2: NW. Sect. 3, T. 23 N., R. 1 E. Measured with 100 foot Steel Tape, Jacob Staff and Brunton Compass UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 11 Triassic Chinle Fm. ? Upper Shale Member Interbedded red- purple and green shales, light green very fine indurated siliceous mica¬ ceous sandstones, and well-indurated pebbly calcareous limestone con¬ glomerate layers; weathers to a poorly exposed slope; sandstones contain oscil¬ lation ripples, clay shards in some and pyrolusite; contact with lower unit very gradational and drawn at the base of the last persistent shale layer 10 Triassic Chinle Fm. 12.0 Poleo Sandstone Sequence of light buff to light green fine indurated siliceous sand¬ stones; forms a talus covered slope; laminar bedding or very low angle trough bedding present; apparently several dis¬ crete channels; upper contact gradational and arbitrary, lower contact sharp 12.0 9 Triassic Chinle Fm. 3.0 Poleo Sandstone variable Light-green slightly friable fine siliceous micaceous sandstone chan¬ nel sequence; weathers 147 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 9 to form a well-exposed (cont. ridge of apparently massive sandstone; some clay shards present; upper and lower contact sharp; intra¬ unit contacts sharp -15.0 8 Triassic Chinle Fm. >23.0 Poleo Sandstone Light-green fri¬ able to well-indurated fine siliceous sand¬ stone unit, micaceous and pebble (quartzite, chert, jasper) towards the base; cliff-form¬ ing unit but only par¬ tially exposed; appar¬ ently massive but actu¬ ally contains lamina¬ tions; several channel sequences with thickest at the base; iron and manganese oxide stain¬ ing; upper contact sharp, lower contact covered -38.0 Triassic Chinle Fm. 6.0-7.5 Poleo Sandstone variable Light indurated fine to medium siliceous sandstone channel sequence cliff-forming unit weath¬ ering into large blocks and a vegetated slope at the base; 10-15 cm. foresets, trough bedding, and convolute bedding, present; upper contact not seen—unknown distance between top of unit 7 and base of unit 8, lower contact sharp and irregu¬ lar, intra-unit contacts sharp -44.4 148 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 6 Triassic Chinle Fm. 0.0-3.0 Poleo Sandstone variable Pebbly friable to in¬ durated calcareous lime¬ stone conglomerate chan¬ nel; poor and limited ex¬ posures; no structures evident; upper contact sharp, lower contact cov¬ ered but probably sharp -46.0 5 Triassic Chinle Fm. 14.0 Poleo Sandstone Light indurated med¬ ium siliceous sandstone channel sequence with a few pebble siliceous con¬ glomerate layers and a friable siliceous sand¬ stone at the base; wea¬ thers to a massive cliff (except) lowermost unit); 5-7.5 cm. foresets, chan¬ nel cut and fill, trough sets, iron concertions and iron oxide staining; upper contact not seen, lower contact sharp and arbitrary at poorly wea¬ thered layer ~60.0 Triassic Chinle Fm. 23.0 Poleo Sandstone variable Sequence of light friable to indurated fine to medium siliceous micaceous sandstone chan¬ nels locally chert-bear¬ ing and scattered pebbly conglomerate layers; sand¬ stones possibly contain a clay matrix giving rise to a dirty appearance; expo¬ sures poor to good; weath¬ ering to low cliffs and talus slopes; foreset and trough bedding present; fossil wood at base of some channels; channels 149 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 4 fine upwards; upper (cont. lower and intra-unit contacts sharp ~83.0 3 Triassic Chinle Fm. Poleo Sandstone Light indurated fine siliceous sandstone channel; cliff former; some chert fragments; apparently massive; upper and lower con¬ tacts sharp -87.0 Triassic Chinle Fm. 2-7.5 Poleo Sandstone variable Light gray-green well-rounded to sub- angular pebbly cal¬ careous chert and quartzite bearing con¬ glomerate channel with some clay clasts; poor exposures; no discernable structures ; in some places contains a light- brown fine siliceous sandstone layer with laminations; upper and lower contacts sharp and erosional -92.0 1 Permian Cutler Group Red and purple sand¬ stones and shales with an erosional upper con¬ tact weathering to a steep, sparsely vegetated slope 150 SECTION 7: SALITRAL CREEK (Composite) Composite of Several Subsections in N. Sect. 36, T. 23 N., R. 2 E. Measured With 100 Foot Steel Tape, Jacob Staff, and Brunton Compass UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 12 Triassic Chinle Fm. ? Upper Shale Member Badly weathered red and purple shales; lower contact transitional, drawn at top of first persistent sandstone 11 Triassic Chinle Fm. 4 Poleo Sandstone Sequence of inter- bedded pebbly calcareous micaceous limestone con¬ glomerates and very fine indurated calcareous quartz sandstones and siltstones; exposed as a knoll on top of a poorly weathered hill; sandstones contain foresets; conglomerates have laminar bedding; some fossil wood present; upper and lower contacts covered by appar¬ ently gradational; intra¬ unit contacts sharp, erosional 4 10 Triassic Chinle Fm. 7 Poleo Sandstone Poorly exposed sequence of brown shales and clays with suspected sandstone layers; a talus slope; no discoverable structures; contacts covered 11 Triassic Chinle Fm. 7.5 Poleo Sandstone Sequence of light brown fine indurated siliceous sandstone channels ; fair to good exposures; weathers to 151 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 9 low ridges and (cont. partially covered slopes; abundant trough and foreset bedding; upper contact covered; lower contact sharp 18.5 8 Triassic Chinle Fm. Poleo Sandstone Sequence of very fine indurated siliceous mica¬ ceous sandstones and silt- stones; good exposures, weathers to form a cliff; laminar bedding, foreset- bedding (8"-10") (15-25 cm) , oscillation pebbles (in siltstones), convolute bedding, and massive channels are present; frequence of foresets and amount of mica decrease upward; sharp upper and lower contacts 28 Triassic Chinle Fm. 15 Poleo Sandstone Sequence of very fine indurated siliceous sand¬ stone channels underlain by a single medium to coarse indurated siliceous sandstone channel; cliff¬ forming unit, well exposed; lower channel varies in thickness from 25'-40' (7.5-12m) and contains faint laminae and occasional pebbly layers; lower channel limey at the base; upper sequence contains some laminar bedding, oscillation ripples, and siltstone units; upper contact sharp but poorly ex¬ posed, lower contact sharp, erosional; intra-unit contacts sharp 43 152 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 6 Triassic Chinle Fm. 0-3 Poleo Sandstone variable Sequence remnant of units partially eroded by lower channel in unit 7; only observed in one locality; uppermost is a 1.5*(50cm) gray silt- stone which grades down¬ ward (possibly) into a gray very fine siliceous micaceous sandstone 1' (30cm) thick, below this is a 6' (180 cm) thick fine siliceous quartz sandstone overlying a pebbly calcareous lime¬ stone conglomerate channel, below the conglomerate channel and in places below the fine grained sandstone are red and yellow indurated clays; exposures are good - at the base of a cliff formed by unit 7; uppermost silt- stones- v. fine sand¬ stone unit contains foresets and small scale pebbles; the fine sandstone is massive with some laminar bedding at the base and contains car¬ bonized and ironized fossil wood, clay shards, and a few quartz pebbles; other units show no discernable structures; upper contact sharp and irregular, lower contact not seen; intra¬ unit contacts sharp in general. 44 5 Triassic Chinle Fm. 3-3.5 Poleo Sandstone Fine, well-sorted indur¬ ated calcareous quartz sand¬ stone channel sequence with some chert and quartz pebbles; fair exposure - weathers to form steep partially covered slope; laminar bedding and channel cut and fill present, 153 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 5 fossil wood, iron oxide (cont. staining; upper contact sharp, lower contact covered 47 4 Triassic Chinle Fm. Poleo Sandstone Fine to medium poorly sorted indurated siliceous sandstone underlain by a cobble calcareous lime¬ stone-quart zits chert con¬ glomerate, red and blue shale and clay lenses are present and a sandstone unit is locally present beneath the conglomerate; fair exposures on a steep weathered face; upper sandstone unit has a few chert pebbles, faint laminar bedding; conglomerate contains poorly developed foresets and trough bedding; upper contact covered, lower contact sharp and irregular 50 Triassic Chinle Fm. 14.5 Salitral Shale Interbedded purple- green, gray, red, green and blue indurated clays and shales, and purple in¬ durated pebbly calcareous limestone conglomerate; weathers to a steep poorly exposed slope; no discernable structures ; upper and lower contacts sharp and erosional 64.5 154 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 2 Triassic Chinle Fm. 1 Agua Zarca Sandstone Sequence of white fine to medium poorly sorted semi-friable siliceous quartz sand¬ stone beds; forms a low poorly exposed ledge; contains laminar bedding and foresets; some quartz pebbles; upper and lower contacts sharp and irregular 65.5 Permian“Cutler Group ? Sequence of purple coarse to medium poorly sorted pebbly siliceous sandstones and red shales; forms steep well-exposed to poorly-exposed slopes 155 SECTION 8: .ABIQUIU DAM Center Sect. 8, T. 23 N., R. 5 E. Measured with 100 foot Steel Tape UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 12 Triassic Chinle Fm.. ? Upper Shale Member Sequence of light green and red-purple shales and silty shales; weathers to low, vegetated hills and poorly exposed cuts; possible laminar bedding in silty shales; tran¬ sitional lower contact 11 Triassic Chinle Fm. 3 Upper Shale-Poleo Sandstone Transition Sequence of red and green micaceous silty shales and green and purple fine siliceous micaceous sandstones; poorly exposed,weathers - to low hills and ledges; fine sandstones contain oscillatory ripples; upper contact at a per¬ sistent sandstone, lower contact at top of section where lithologies of unit 10 predominate; contacts sharp and arbitrary 10 Triassic Chinle Fm. 2 Poleo Sandstone Light purple fine well-indurated siliceous sandstone overlying a purple indurated fine siliceous micaceous sand¬ stones; excellent road cut exposures; upper unit is massive but may represent a sequence of several channels; lower unit has laminar bedding; upper 156 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 10 and lower contacts (cont.) sharp and straight; intra-unit contact sharp and very irregular 5 9 Triassic Chinle Fm. Poleo Sandstone Sequence of purplish- yellow fine well-indur¬ ated siliceous micaceous sandstone channels; cliff- former, excellent expo¬ sures ; well developed (6-14", 15-35cm) straight to tangential foresets and trough sets; more massive and micaceous towards the middle of the unit; upper, lower and intra-unit con¬ tacts sharp and erosional load structures at the base of the unit 14 S Triassic Chinle Fm. 5 Poleo Sandstone Interfingering, purplish-yellow semi- friable to well-indur¬ ated fine siliceous sand¬ stone channels with clay shards and gypsiferous red and green shales; excellent exposure on a blasted cut; some poorly developed trough sets in sandstones; upper contact is drawn at top of a semi- friable sandstone and is sharp and irregular, lower contact is at the base of a reddish-green shale; intra-unit contacts sharp, irregular with shale units varying in thickness; gypsum in shales probably secondarily crystallized 19 157 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 7 Triassic Chinle Fm. 17 Poleo Sandstone Sequence of many cross¬ cutting light purple well-indurated to semi- firable fine to medium siliceous micaceous pebbly sandstone channels, ex¬ cellent exposures on a blasted face, cliff former; channels largely massive with some faintly developed trough sets; channels range in thickr_. ness from 10" (25cm) to 15' (450cm) becoming more calcareous towards the base and finer and less indurated upwards; quartz pebbles and clay shards at the base of most channels; fossil wood at uppermost contact with overlying shales; iron oxide staining; upper contact sharp and straight, lower contact sharp and irregular with load structures; intra-unit contacts sharp 36 6 Triassic Chinle Fm. 6 Poleo Sandstone Interbedded reddish- purple indurated clays, light brown well-indur¬ ated fine siliceous sand¬ stones, and well-indur¬ ated quartz and chert pebble conglomerates with a fine sand matrix; ex¬ cellent exposures on a cliff and blasted face; abundant fossil wood and organic matter; some poorly developed trough (?) bedding in the conglom¬ erates; no imbricate 158 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 6 pebbles noted; upper (cont. contact at top of a shale very sharp and irregular; lower contact very sharp and irregular below pebble conglom¬ erate; contacts between sub-units sharp and regular 42 5 Triassic Chinle Fm. 4.5 Poleo Sandstone Light brown fine well- indurated siliceous sand- stones overlying a light brown poorly sorted sil¬ iceous, gypsiferous quartz and chert pebble conglom¬ erate with a medium sand matrix; excellent exposure; unit as a whole has a len¬ ticular geometry; some clayshards and abundant fossil wood in conglomerate; manganese oxide and iron oxide staining; upper con¬ tact sharp and irregular, lower contact gradational into underlying conglomerate; intra-unit contacts sharp 46.5 Triassic Chinle Fm. 16 Poleo Sandstone Sequence of poorly sorted indurated quartz, chert, and limestone pebble con¬ glomerate locally overlying a light brown fine siliceous sandstone (channel?) ero- sional remnant; excellent exposures; cliff former; some large (3-4', 100- 130cm) foresets and trough sets scattered throughout the conglomerate, possibly some poorly developed imbricate structures; upper contact transitional, lower contact sharp and irregular 159 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 4 with load structures; (cont. intra-unit contact sharp and erosional 62.5 3 Triassic Chinle Fm. 11 Salitral Shale Reddish-purple in¬ durated clays with inter- bedded light brown fri¬ able fine sandstones overlying yellow-gray shales; weathers to a talus slope, poorly ex¬ posed; sandstone layers 6-10" (Ï5-24cm) thick; no discernable structures; some malachite; sharp, ir¬ regular upper and lower contacts 73.5 Triassic Chinle Fm. 1.2 Agua Zarca Sandstone Sequence of several light medium semi-friable siliceous sandstone layers; fair exposure; some clay balls but no dis¬ cernable structures; upper and lower contact sharp and wavy 74.7 1 Permian Cutler Group ? Light purple friable medium siliceous sand¬ stone overlying red and reddish-purple sandstones and silty shales; weathers to form poor to good expo¬ sures on cliffs or steep talus-covered slopes; trough bedding, foresets, and cross' cutting channel sequences present 160 SECTION 9: LAS MINAS JIMMIE At U.S.G.S. Fossil Plate Site 10144 Approximately 106° 20'W., 36° 17'N. Area Unsurveyed. Measured with Jacob Staff and Brunton Compass UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 4 Triassic Chinle Fm. ? Poleo Sandstone Basal conglomerate similar to Unit 4- Section 8 underlain by a thinly bedded (4", 10 cm) light green siltstone 3 Triassic Chinle Fm. 3.5-4.5 Salitral Shale Sequence of purplish- blue and green medium friable siliceous sand¬ stones interbedded with shales (?) ; poor expo¬ sures - a talus covered slope; shales not seen but presence suspected; no discernable structures, sharp upper and lower contacts *“4.5 2 Triassic Chinle Fm. ~6 Agua Zarca Sandstone variable Sequence of inter¬ bedded light buff coarse friable siliceous sand¬ stone channels with scattered pebble con¬ glomerate lenses over- lying a basal quartz pebble conglomerate; good exposure, cliff former; clay lenses and clay shards throughout the section; carbonized and malachized fossil wood and plant fragments; upper contact sharp and regular, lower contact covered -10.5 161 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 1 Permian Cutler Group ? Red and purple shales and sandstones; weather to a steep talus-covered slope; abundant foresets; upper contact covered 162 SECTION 10: ARROYO DEL COBRE Approximately 2 miles South of Section 9 Measured with 100 foot Steel Tape UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 5 Jurassic Entrada ? Sandstone Buff medium well-sorted to a steep talus-covered slope 4 Triassic Chinle Fm. ~3 Poleo Sandstone Purple friable very fine siliceous micaceous sandstone; weathers to a poorly exposed, steep talus covered slope; massive with faint cross beds, fossil wood; upper contact covered, lower contact sharp ~3 3 Triassic Chinle Fm. .3 Poleo Sandstone Red and green clay layer; very poorly exposed; sharp upper and lower contact; no discernable structures ~3.3 2 Triassic Chinle Fm. 4 Poleo Sandstone Sequence very similar to Unit 4 - Section 10 with a possible clay shard conglomerate at the base; upper contact sharp, lower contact sharp, angularly uncon- formable; exposures fair ~7.3 163 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 1 Permian Cutler Group Red and reddish- purple sandstones and shales weathering to a steep talue covered slope 164 SECTION 11: RED MESA West Sect. 28, T. 16 N., R. 1 E. Measured with 100 foot Steel Tape UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 9 Triassic Chinle Fm. 10 Red Mesa Sandstone Sequence of light very fine to fine quartz sandstone layers with northward palaeocurrent structures; a quartzite and chert pebble channel lag deposit-at the base of-this unit just above the lower contact; upper contact unknown, lower contact sharp and erosional; this unit forms the top of the mesa at this point; an unknown thickness of strata from this same Triassic unit has been eroded from above this sequence 10 8 Triassic Chinle Fm. 6 Red Mesa Sandstone Sequence of light very fine to fine quartz sandstone units with south- westwardly dipping palaeo¬ current indicators; at the top of this unit is a clay layer which has been partially eroded to the overlying unit; the upper contact is sharp and ero¬ sional; the lower contact is sharp 16 7 Triassic Chinle Fm. 4.5 Red Mesa Sandstone Fe-stained conglomerate channel fill deposit with some interbedded light very fine to fine quartz sandstones; pebbles are chiefly quartzite, agate and some chert; upper and 165 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 7 lower contacts (cont. both sharp 20.5 6 Triassic Chinle Fm. 4 Red Mesa Sandstone Thick light very fine to fine quartz sandstone units with interbedded conglomerates ; pebbly lag deposits are present in some sandstone layers; quartzite, agate, and chert pebbles are pre¬ sent; palaeotransport directions in this unit were northwest to west; upper and lower contacts are sharp and erosional 24.5 5 Triassic Chinle Fm. 6.0 Red Mesa Sandstone A sequence very similar to Unit 5, but with south- westwardly palaeotransport; upper and lower contacts are sharp and irregular 30.5 4 Triassic Chinle Fm. 4.2 Red Mesa Sandstone A single massive light colored very fine to fine quartz sandstone channel fill deposit; overall geometry is lenticular; upper and lower contacts covered 34.7 3 Triassic Chinle Fm. 3.3 Red Mesa Sandstone Covered-probably sandstones as above 38.0 2 Triassic Chinle Fm. 8.0 Red Mesa Sandstone Sequence of light colored very fine to fine quartz sandstones with generally south- westwardly dipping 166 UNIT DESCRIPTION UNIT TOTAL THICKNESS THICKNESS (METERS) (METERS) 2 palaeotransport indi¬ (cont. cators; : upper contact covered, lower contact sharp and erosional 46.0 1 Permian Cutler Group ? Red shales and medium grained sandstones; large scale (~lm) crossbedding dipping towards the southwest in sandstones; upper contact irregular, sharp, and erosional 167 APPENDIX II Locations and lithologic descriptions of samples used in this study. Sample numbers associated with an asterisk (*) indicate those samples used to study clast lithology (Table 11). Most of the remainder are plotted by stratigraphic unit on triangle diagrams (Figures 22 and 23). In addition, the sample numbers which were disaggregated and analyzed using the RUASA are underlined. All measured sections are described in Appendix I. 168 N g P eu G P eu P N •H fd P >1 1 G P TJ G •H rH eu *0 p P eu cr G P G G eu fd eu g eu P eu •H (d (U g G G i eu eu eu p eu G p co S P 0 D1 O eu P G P fd 04 o G *H rH P G -H •H -H P •H G On S co •H G P G N N CO co P TJ eu G G T p eu eu P P Tl G rH eu P 0 •H C p p p p G O -H g O P 0 O O LO o LO un o o •H • • • • • • • • P P rH CN O rH O r- r-* m ? CO co CN CN CN Pen üa) •H CO eu te P O =#= eu eu eu (U £ £ £ -P P •H •P i 03 N N P p p ob N P -P -P CO N £ N P (U P P >1 >1 p p >1 03 P P Ë 03 G p eu p eu p p P CO P fO 0 £ £ eu -P eu P 03 03 eu 03 £ H O1 G > -H > -H £ £ > >i £ O1 G £ £ G1 G1 P G1 £ ** (U •%. Q) i—i a> 0 co eu co eu CO P CO P eu eu CO P eu £ ü £ £ £ £ £ 03 £ 03 £ £ £ CO £ P t £ P «j -P 03 -P 03 P 03 P £ >i £ 03 P £ >1 £ P H 03 fU g en g co Ë D1 Ë G1 > 03 > 03 Ë G1 to > 03 P P O CO eu co CO CO CO co CO CO 'O co co £ £ £ £ £ £ £ £ £ 0 O 0 >i 0 O 0 0 o O 0 (U eu eu P eu eu eu eu P eu eu p p p P P p p p G ü P - o 03 G - 03 r£ 03 03 03 03 O P p P ü o ü G ü ü ü ü P P eu P P P P •H P P P P 0 P p P P ■03 03 03 P 03 03 03 03 P co ü P co O ü ü co o ü O ü P -P •H £ P (U p 1 1 1 1 P P 03 P 'G P 03 P 'G p p P 03 p 03 eu G G G eu G 03 eu 03 fa) 03 eu G eu P £ P 03 P O P 03 P 'G p P P 03 p £ 0 £ £ £ £ £ £ £ £ £ £ £ £ £ G P G* £ G» £ G» £ G £ en G G £ G £ co £ 0 £ 0 £ O £ 0 £ £ £ 0 £ 03 03 P 03 P 03 P G P G G G P G 1 £ 1 1 1 1 1 1 1 1 1 1 1 1 1 P 03 P P P P P p P P P P P P P £ CO £ £ £ £ £ £ £ £ £ £ £ £ £ co co co CO co CO co co co CO CO CO co co 0 03 eu eu P £ 0 £ £4 P . P P £ eu co O CU eu p ü Q4 co eu D 03 P P P tu P Ë P Ë w £ .O CU P £ p P 0 0 0 0 0 0 0 0 0 P eu • • • • • • • • • P P £ CN en r* r* V0 LD co 00 CN P ü 0 P P vo 10 C0 CO V0 in m en G eu P p co co eu 03 ffi p £ 0 G CO 0 eu p =#= 0 eu eu O P p en 0 p O P eu eu P £4 m KD r- 00 co (Ti Ë £ 1 1 l 1 1 1 1 1 1 G 0 û Q Q Q a 0 Û Q P CO P < c <2 C < < < <3 * 170 eu eu eu eu 1£ £ •P -P •1 r| •H •H •H m MH >, £ £ -P N eu eu eu eu 1>1 >1*H •P eu £ eu P p £ eu p eu P -rl P £ eu •H ■P ed ed •H -P N (U -P eu CQ ed •H • p 1 £ MH -i rl MH * H MH • H «P > -H > 1 £ N N £ P £ N O £ •«* (U -P P •* eu ed O eu rH 0 =#= 04 in CO (U P ed .0 O no eu rH rH rH .H (U 0 0 O 0 0 1 1 1 O 04 rH rH rH rH H CO H H H g 04 I | | I 1 1 H H H ed D Q q q Q q p S S S W 2 2 2 < 2 < (p (p Height from base Sample # of section (meters) Lithologic description (Lower Poleo Sandstone lentil continued) •H 1 •H CD iH 4-i -P -Q X rH *0 «P vo O 00 (p S H H G nS 0 CO > nS CD P P (D >i G G nJ P N ü o ns P CD G CD G CO nJ P G 0 G ns CD P 'G G G I 1 1 • OT- •H 1 •H P •H •H •H 4-4 nJ r*4 *G Xî iH -P in PM S H H 0 CO CD G CD g O ns P N CO ü CD G G CD g -P G G CO G G nS G ns G P 1 *H 1 « i TT- 1 •H -H -H *H •H 4-1 G 4-4 N i—4 XI rH m S H H CD (D CD O CO CD G P nS >i P G nS (D P G -P CO CO ü G > G G nS G G ns p G ! (D -P *FMII-po 0.5 chert pebble conglomerate 171 Height from base Sample # of section (meters) Lithologic description Upper Poleo Sandstone lentil *H 0) 4-4 4-» •P -P *H 4-4 3 A rH O O in 0) G cd p N G G fa «J CQ ü CD G CQ G cd P cd G* G l • •P G •H CD 4-1 4-> •P rP •H CD 4-> rH (d X! cd rH 3 A 4-4 P o en en G p N V0 > <0 CD P >i G G CD G to 57 CD fa S3 >i 0 P CQ O G» ü CQ cd G G» G cd • •H CQ G •H CD 4-1 -P •P rH 4J iH cd A cd H CD A iH 4-4 P rH o en G r> > cd CD cd P N CD P P CD >i G G G fa k >1 0 CQ O cd P G* O CQ cd G G» G G 1 • •P CQ G 1 •P >i •H 0 4-4 N-P 4-1 4-4 iH rH XI CM G o CD 00 G 03 CD-P CD G CQ CD Gcd P flJG >1 P'G P CD O > O03 fa S ü cd ü cd G cd G G cd P CQ G 1 rH G l l • 4J CQ —cd 1 •H -P •H G 4-4 *H «H iH T3 A CM 00 CD ro 0) P G CD G O -P CD N 0 G «d 0 CQ Ë O o (d cd P o P CD G fa k cd CD P ü cd CQ G cd P G O G G 1 • CD G 1 •H NM-3a 6.0 sub-angular calcareous micaceous; very fine quartz clayey 174 eu g G G ■H N •H MH -P eu TJ P G eu >1 fd •H g 0) p eu G eu eu MH eu -P (U -P O1 G *P -p •«* *H >*H •H *H co G G g MH G P G G eu •* a) G i eu eu eu O P CO P •H (U P > P p Cd G fd TJ G G fd eu O eu eu •H 4-t N eu N g 4J 4-1 N >1 N G -H -P ü -P 1 -H -P ■H 4-> O CO P fd P eu G >i P .Q 5-1 •H 04 fd ü fd G eu p cd 42 (0 •H P eu G <1) 3 -P >i G •H G 1 04 tn O1 g O1 MH fd > O* ûi O •H P O CO CU co co co 'O CO G G G CO G 0 0 >i O G O 0 eu eu rH (U O •H eu p p -P P (U Cn o fd fd *G~fd— O O •H rH ü ü rH en ü •H rH rH •H H rH •H •H «H •H O =«= -H -p fd fd -P rH fd •H •G CO G o ü G CO ü G CO -P G eu (U 0 ■H 0 G TJ CT» G en TJ tr» rd Cn G CO G 0 G G 0 G G G S co G O « G P fd fd P fd fd fd O fd P w «J 1 1 co 1 1 i co i S* fd I t w o X XI X X X o Xi ,0 « P G G 0 G G G 0 ■3 § p G G u fd co CO eu co co co eu co U fd co co td rH rH (SI P5 0 0 t-5 04 04 w fd S3 D G EH XJi p p ü CT» H < eu eu H < i-5 S Oi H C Q 04 CO h5 51 0) CO CO fd *Q S O P G tn o o MH O •H o LD en in CN -P -P cn X! ü tn O) •H CO eu ÎG 4-1 O =#= eu 10 TJ fd CN in en g rH rH rH l fd fd fd fd fd S co CO co co co co 175 0) G d) •H 1 P CD N 4H •H G >1 >1 P G >1 >1 G •H N N P P P >i G »G >i CD IP P 0) P CD CD rH G P CO G G P P CD P G P G >*H G Q) G fd CD fd >iP fd O fd O CO 01 > >i CO CO > P -H G P G P •%» P N CD G G4 co O1 CO CO N •*. •** rH •** •** p > CD 'G G P Ü o CO -H CO CD CO CD CO p P CD G CD G 0 P CD *H G G CO G G G G G fd td G fd G fd p fd G P 0 0 0 0 0 0 0 G CD *«H CO *H CO CD G O P 4J CD N CD P CD P CD O1 P N 4H *P ip trp G •H CO Ü P CD 0 CO Ü CO U •H P >i •rH CO 0 rH -P (0 P G cd -P fd P fd CD P P >iP >iP CO CD 'G •H P iH ü (U o ü H Ü rH U G 0 fd P »H P rH a G G -P G -H •H G*P •H -H •H »H •H -H •H G CD -H CD -H G 1 CH CO CO g G1 CO g CO e to g m P Ü > CO > CO Cji'P CO CD CO co co co co co co 'G G G co co G G G G G O O G G O >1 O O O O O CD CD O 0 CD rH CD CD CD CD ■H CM P P CD CD P P P p P P G* 4*= G G U ü G P G G G G O O ü •H *r| ü CP O ü O O rH G rH rH H rH rH rH •H rH rH rH rH 0 0 -H G G •H ■H G rH G G G G p •H P Ü ü CO CO ü C0 U ü ü ü p P G •H U CD P CD «H CO 1 , , CD P 'O P *Ü P P p P P *G p 'G 1 G G CD G CD G G G G G CD G G O rH *G rH *G rH rH rH rH «H 'O rH 'G E5 P G G G G G G G G G G G G O CO CP G P G G* P Cn CP CP G CP G *G G O G O G G G G G O G O Z G G P G P G G G G G P G P < G 1 1 1 1 1 1 1 1 1 1 1 1 U CO P P P P P P P P P P P P G G G G G G G G G G G G O 0 co co co co CO CO CO co CO co CO CO EH CD H rH P O W PH P ü P H CD a S o co P 8 c O in in w o o r- fO CO o in o o 4-» -P CM rH rH rH rH rH p o CP O •H CO =tt= 0) «H en CM rH O CH rH rH rH |H m rH CP 00 g 1 1 1 1 i 1 1 1 i G O O O 0 0 O O 0 O CO PH CH CH PH PH PH CH PH PH 176 i i i eu eu (U (U eu fi fi fi fi -P •H •H •H 1 •H 4-1 44 >i 44 44 >i TJ g fi TJ 4-> fi 3 >i eu >i >i fi >i >üH >-H > > *H > 44 ►i g *H N fi N fi N 4J g fi r—t +) «* (U •H 4J •«. 0) •* 4J •* rH 3 eu ~43 (0 P CO P co P eu co p CO P eu CO •H •H p CO fi fi fi fi fi fi cd fi fi (0 fi fi fi fi CO TJ fi fi 10 O fi1 0 0 fi 0 O 0 fi 0 0 eu o eu eu o eu N (U tf-p eu N i 0 > CQ CD -H ü m CP Ü P «H >iP £ td a) £ rd o P £ N i 3 P 1 4a • CQ £ -P rd 1 no CN *H rH cn O P *H «H rH (d rH 0) rH P P P p o CQ rd P £ O O £ Cn £ £ rd 0) P ^ CQ O >i 0 N CP Ü £ rd P CQ CO 0 a) 1 £ 1 • £ CQ CN rH o cn •H rH P rH T3 rH rd P rd rH i 0 > CQ CD *H P iH >iP cr* to N 1 £ rd 0) £ rd O P £ £ P 1 1 • 'O CQ >1 £ rH •rH rH rH W 1-3 ft P P 0 o a) 0 CD p £ CQ 0 a) CU rd £ £ 1 CP rH O cn P rH •H rH rH P P P ft 0 rd CQ o £ rd £ £ referred to in this study 179 • • • • s M * • *» w M S • rH H iH S CO TT rH • • • CM PS • PS PS • • CM « PS PS * *> d) • • % * • % Gï EH Z • 2 S • • • c Z Z Z w rd *> ON CD Z Z Z 2 Z Z Z Z Z Z 2 Z Z Z Z Z 'G G -P LO O CO LD CM rH «H 00 TT o o o CO •H rH iH iH rH m H O o O rH rH CM CM iH o CO G> G 0 0 0 0 0 0 0 0 0 0 0 0 C 0 * 0 0 Q vo vo vo vo LO vo vo vo vo vo vo vo vo vo vo m ü CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO CO *G H«H rd 2 £ £ £ 2 s £ £ £ £ £ £ £ £ £ £ 0) *G O m CM O in r* 00 CM CM vb rH r^ CO LO CM O G CM CM CM in CO LO in LO *?• in CM LO m -P •H O 0 0 0 O 0 0 0 0 0 0 0 0 0 0 0 -P VO vo vo vo vo vo vo vo VO vo vo vo vo vo VO vo rd O O O O o o o o O o o o o o o o hH iH rH rH rH rH «H «H iH rH rH rH iH «H rH rH rH 0 CO 0 ü 0 0 ü •H *G o ■H X • « s s % rH r—! a * • • O • P$ CM 53 * % • CO a% • • H tn iH a SS G * m • c* O CM tn EH rH CM rH * 'G * • • rH G CM EH EH rH fd * % * * CM Û4 r-H «a* ■o* •H • CM CM rH • .G • S & CO CO • • • c •P «H -P -P -P H O ü ü ü 0 CU • eu 0) fl) • EH CO CO CO CO • • • • • • • CU S a a a 53 a a •0 G * » » *» » * » +J in m r- CM O r* r- •H in in in co O rH r-H Cn G 0 0 0 0 0 0 0 0 m m in m V0 V0 V0 H) co en co co co co co 'O G * • • • • • • G S & s S a a & 0) * » m » » « 'O rH rH rH r> CM O O G in m m LO CM CM HJ •H 0 0 0 0 0 0 0 HJ V0 ce V0 KO V0 KD V0 fd O o O O O O O PI rH r-H rH rH H rH rH •• CO eu 0 HJ U •H eu nd CO •H eu 0 ë PU ü eu g •H HJ •H eu X fd 'O eu rH G S PH CO co O G fd fd >i eu 0 G rH G G G G >i eu 0 •H •H •H fd •ri G a >i CO S S U S (d G CO u * fd 0 co co CU rH rH 0 U PH fd fd g eu eu 0 U PI PI G G G rH 'O 0 • a G* Di .0 •H HJ CO i I •H •rH fd CQ •H • eu S S CH X ÎH a TJ* in O 0 • rr ns G G G G *G co rH rH rH fd (d fd fd (U • O O PH CO co co co CO O rH rH 181 APPENDIX IV: Summary of Characteristics of Stratigraphic Units 132 AGUA ZARCA SANDSTONE Contact relationships lower contact sharp, unconformable, and angular on a regional scale; upper contact gradational and conformable Lithology predominantly sandstone but cobble and pebble conglomerates prominent near the base; some dispersed claystone lenses; unit becomes finer up-section Palaeocurrent directions southwestwardly Environment of deposition high to moderate energy braided streams and associated sub-environments, has some meandering stream characteristics near the top Provenance predominantly a crystalline and metasedimentary terrain to the northeast, probably a southern Uncompahgre Highlands; Paleozoic sediments also contributed minor amounts of material 183 SALITRAL SHALE TONGUE Contact relationships lower contact gradational and conformable; upper contact with Upper Shale Member unknown but probably conformable, contact with lower Poleo Sandstone sharp and unconformable, contact with upper Poleo Sandstone gradational and unconformab1e Lithology predominantly shale with some limestone and sandstone beds Palaeocurrent directions none - no structures Environment of deposition lacustrine and marshland - Provenance a northeasterly terrigenous source with a con¬ siderable influx of volcaniclastic material LOWER POLEO SANDSTONE Contact relationships lower contact sharp and unconformable; upper contact both sharp and gradational, but conformable 184 Lithology predominantly sandstone with prominent pebble con¬ glomerates and some shale lenses, particle size decreases upwards Palaeocurrent directions s outhwe s twardly Environment of deposition- high to moderate energy meandering streams (in some places entrenched), and.associated sub¬ environments; has some braided stream features Provenance a northeasterly sedimentary source with minor amounts of metasedimentary and crystalline rocks UPPER POLEO SANDSTONE Contact relationships lower contact conformable, both sharp and grada¬ tional with the lower Poleo Sandstone, and gra¬ dational with the Salitral Shale; upper contact gradational and conformable Lithology predominantly sandstone with small conglomerate lenses near the base and prominent claystone and siltstone beds near the top; particle size decreases upwards Palaeocurrent directions northwestwardly 185 Environment of deposition moderate to low energy meandering streams and associated sub-environments; delatic Provenance A sedimentary source to the south and southeast UPPER SHALE - PETRIFIED FOREST MEMBER Contact relationships lower contact with upper Poleo Sandstone grada¬ tional- and conformable-?- contact with Salitral Shale unknown but probably conformable; upper contact sharp and disconformable Lithology predominantly shale with some siltstone and sandstone beds Palaeocurrent directions westwardly Environment of deposition marshland and very low energy deltaic and fluvial; possibly partly lacustrine Provenance a sedimentary source to the east, northeast, and southeast with an important influx of volcanic material