RICE UNIVERSITY
PROVENANCE AND PALEOTECTONIC SETTING OF CONGLOMERATES IN THE VIRGILIAN HOLDER FORMATION, NORTHERN SACRAMENTO MOUNTAINS, NEW MEXICO
BY
CHARLES PRESTON DUNNING
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF ARTS
Thesis Director's Signature: ABSTRACT
PROVENANCE AND PALEOTECTONIC SETTING OF CONGLOMERATES IN THE VIRGILIAN HOLDER FORMATION, NORTHERN SACRAMENTO MOUNTAINS, NEW MEXICO
Charles Preston Dunning
The Late Pennsylvanian (Virgilian) Holder Formation of the Sacramento Mountains, south central New Mexico, is composed of clastic and carbonate rocks cyclically deposited on the narrow Sacramento Shelf. Coarse channel fill con¬ glomerates are often found in the lower terrigenous member of the cycles. The cycle thicknesses, coarse terrigenous clastic thicknesses and the trend of the conglomerate chan¬ nels respond to both the pre-cycle topography and the growth of topographic structures during deposition of the cycle. The prominence of the tectonic structures appear to have increased during the early Virgilian to a maximum during cycle 6. The relatively thin and uniform thicknesses during cycle 7 indicates that growth of shelf structures was at a minimum. Growth was renewed during cycles 8 and 9, documented by the rejuvenation of the La Luz Anticline and the Dry Canyon Syncline. Paleotransport directions from the lower 9 cycles generally indicate southwesterly transport, away from the Pedernal Uplift and into the Orogrande Basin. Where channels are recognized, there is good correlation between channel trends and transport directions.
During the height of the transgressive phase of the cycle, the Paleozoic detritus derived from the Pedernal
Uplift was involved in high energy coastal processes at the base of the escarpment. It was in this environment that
Paleozoic chert and Cambro-Ordovician Bliss Formation quartz¬ ite clasts were rounded, and whatever feldspars and Pre-
Pennsylvanian limestones that has not been reduced insitu on the uplift or in transport, were destroyed. High energy runoff from the uplift spread the cherts, quartzites and quartz sand across the shelf as clastic flows. These flows cut the channels, incorporating shelf linestones of
Virgilian age into the Virgilian conglomerate deposits. Acknowledgments
I would like to gratefully acknowledge the New Mexico
Bureau of Mines and Mineral Resources and its director,
Frank Kottlowski, for funds supporting field and laboratory work, and the Sigma Xi organization for additional financial assistance.
Dr. James Lee Wilson deserves much credit for comple¬ tion of this thesis. He suggested the study, helped obtain funding, spent time in the field and was involved in all stages of lab work and writing. His support, encouragement and constructive criticism are certainly appreciated. Thanks also go to the other members of my committee, Drs. John B.
Anderson and John E. Warme, for their help in the field and for critically reading the thesis. Mr. Ben Donegan,
Leonard Resources, Albuquerque, New Mexico, was very helpful in sharing his knowledge of previous work in the area.
I benefited from the assistance of and discussions with many students in the field, particularly John Van Wagoner,
Dennis Kurtz, Joan Mussler Spaw, Ann Leavesley, Camille
Hueni, and Jerry Kennedy. Mr. and Mrs. John Anderson of
Roswell, New Mexico, and their daughter, Jenna, made the field season more enjoyable by opening their home to me on numerous occasions. I wish to acknowledge the help of many people at Rice
University: Bill Lambert for his ongoing encouragement;
Keith Shanley for drafting and other assistance; Ken Hahn for help with photography, for reading the text, and for his invaluable assistance in preparing for the thesis defense;
John Van Wagoner, Dennis Kurtz and Roy Adams for many hours discussing the various geologic aspects of the thesis and reading the text; and Mary Bogert for typing and for her constant encouragement.
Appreciation is expressed to Mrs. James Lee Wilson for the use of her office and typewriter in preparing the rough copy, and to Charlena Williams for her fast and expert typing of the final copy.
Appreciation is extended to Mr. James Bogert, Garland,
Texas, for my "home away from home" and to my own parents and family for their financial and moral support during the past three years. And a very special acknowledgment goes to two individuals who have been most important to me these diffi¬ cult past two years; Cane, my field assistant--and my dog— and of course Mary--the little things make all the difference. TABLE OF CONTENTS
Page
INTRODUCTION 1
LOCATION 2
REGIONAL MORPHOLOGY 5
STRUCTURAL SETTING 6 Pre-Pennsylvanian 6 Pennsylvanian-Permian 6 Post Permian 9
PENNSYLVANIAN HOLDER FORMATION 10
TOPOGRAPHIC AND TECTONIC CONTROLS ON DEPOSITION OF VIRGILIAN CYCLES 15 Previous Work . 16 Field Data 20 Transport Systems 23 Provenance 24
CHARACTER OF VIRGILIAN CYCLES 29 Cycle 3 32 Cycle 6 37 Cycle 7 53 Cycles 8 and 9 57
SUMMARY 63
DISCUSSION 65 Paleoclimatology 65 Pedernal Uplift 66 Coastal Processes 67 Depositional Setting 68 Clastic Deposition 69 Provenance 71
BIBLIOGRAPHY 76
APPENDIX I 79
APPENDIX II 92 APPENDIX III 109 ILLUSTRATIONS
Figure Page
1 Field Area Location 3
2 Field Area 4
3 Pennsylvania Physiography 7
4 Generalized Holder Formation Cycle 11
5 Lower 9 Cycles, Holder Formation 12
6 North Field Area 17
7 South Field Area 18
8 North Field Area Cycle 3 Isopach Map 33
9 South Field Area Cycle 3 Isopach Map 34
10 North Field Area Cycle 3 Clastic Isopach Map 35
11 North Field Area Cycle 6 Isopach Map 38
12 South Field Area Cycle 6 Isopach Map 39
13 North Field Area Cycle 6 Clastic Isopach Map 41
14 South Field Area Cycle 6 Clastic Isopach Map 42
15 South Field Area Cycle 6 Channels 43
16 North Field Area Cycle 6 Size Distribution Map 51 17 South Field Area Cycle 6 Size Distribution Map 52
18 North Field Area Cycle 7 Isopach Map 54
19 South Field Area Cycle 7 Isopach Map 55
20 South Field Area Cycle 7 Clastic Thickness Map 56
21 North Field Area Cycles 8 and 9 Isopach Map 58
22 South Field Area Cycles 8 and 9 Isopach Map 59
23 North Field Area Cycles 8 and 9 Clastic Isopach Map 61
24 Composite Paleotransport Directions—Lower 9 Holder Formation Cycles 64
25 The Origin of Feldspars in Sandstones .... 73
PLATES
Plate
1 Virgilian Triticites in limestone clast ... 25
2 Paleozoic shell debris 27
3 Late Paleozoic ramose Bryzoan 27
4 Bliss quartzite 28
5 Virgilian quartzite clast 28
6 Cycle 6 conglomerate cutting into the top of the AR limestone 46 FIELD MAP In Pocket
BEEMAN CANYON CROSS SECTION In Pocket
DRY CANYON CROSS SECTION In Pocket 1
Introduction
The Late Pennsylvanian Holder Formation of the Sacra¬ mento Mountains, south central New Mexico, is composed of clastic and carbonate rocks cyclically deposited on the narrow
Sacramento Shelf. The carbonate units of each cycle are shallow, marine deposits, while the terrigenous clastic units range in character from shales to coarse silts and sandstones derived from the Permo-Pennsylvanian Pedernal Uplift to the east and northeast. In places, the terrigenous clastic portion of each cycle contains very coarse-grained conglom¬ erates. The areal distribution of these conglomerates and in particular their occurrence in down-cutting channels, afford significant insight into the Late Pennsylvanian paleoslope and drainage patterns of the Sacramento Shelf as related to local and regional tectonics. In addition, the distribution of the conglomerates and their channels docu¬ ment the direction from which the coarse elastics were de¬ rived. The sedimentologic character of the terrigenous elastics is a key to understanding the processes and environ¬ ment of their deposition. The mineralogy of the conglomerate clasts indicates that they are genetically related to the
Paleozoic sediments overlying the Pedernal Uplift to the northeast. 2
Location
The Sacramento Mountains, located in south central New
Mexico, trend north-south for almost 100 miles (Figure 1).
The field area is in the northern part of the mountain range lying almost totally within the Lincoln National Forest, and the majority of work was done along the steep west and south facing escarpments. The field area is roughly separated into northern and southern halves by New Mexico Highway 83 running east-west through Dry Canyon, connecting the city of Alamogordo with the town of Cloudcroft (Figure 2).
The topographic map for the field area is the U.S.G.S.
15 minute series, Alamogordo Quadrangle, New Mexico, Otero
County. Figure 1 Field Area Location
Figure 2 Field Area
Boundary fault is shown by
hatchered line
Biohermal buildup shown by
stippling 4 5
Regional Morphology
The Sacramento Mountains separate the Great Plains to
the east from the Basin and Range topography to the west.
The Tularosa Basin lying to the west of the Sacramento Moun¬
tains is a bolson created by Basin and Range tectonics which
is filled by Quaternary alluvium, a vast gypsum sand deposit
(the White Sands) and basalt flows associated with the rift¬
ing of the Rio Grande trough. The Sacramento Mountains form a steep western escarpment along the north-south trending
boundary fault (Figure 2). The mountains rise from 4500
feet in elevation in the basin to over 9000 feet at the crest,
7-13 miles to the east. The gentle (1°) eastern slope of the
Sacramento Mountains extends 80 miles to the Pecos River.
Four major west-northwest draining canyons cut the field
area creating exposures normal to those created by the north trending boundary fault. These canyons are, from north to
south, Fresnal Canyon, Dry Canyon, Beeman Canyon, and Indian
Wells Canyon (Figure 2). Access roads are severely restrict¬
ed by terrain and essentially all work was done on foot from
points accessed from New Mexico Highway 83. 6
Structural Setting
Pre-Pennsylvanian
Prior to Pennsylvanian time south central New Mexico was characterized by a broad, flat shelf across which a sea entering from the south deposited clastic and carbonate sediments from Cambrian through Mississippian time. These sediments were generally deposited as southward thickening wedges (Meyer, 1966).
Pennsylvanian-Permian
Post-Chesterian uplift in northern New Mexico resulted in southward tilting and erosion of pre-Pennsylvanian rocks.
Permo-Pennsylvanian strata were deposited on top of progress¬ ively older rocks toward the north, where Precambrian is exposed under the onlapping Late Paleozoic strata. As a result of the Early Pennsylvanian tectonics a large, north- south trending positive area was exposed throughout the
Pennsylvanian and Early Permian in east central New Mexico, approximately midway between the present day Rio Grande and
Pecos rivers (Figure 3). This feature, named the Pedernal
Uplift,has been recognized for many years (Willis, 1929;
Ver Wiebe, 1930; Thompson, 1942; Galley, 1958).
Major negative elements associated with the Pedernal
Uplift are the Permian Basin to the east and the Orogrande Figure 3 Pennsylvanian Physiography
After Meyer (1966) and Wilson (1977) 7 8
Basin to the west (Pray, 1959) (Figure 3). The Orogrande
Basin joined the Estancia Basin to the north and continued southward into Mexico to join the Pedregosa Basin. West of
the Orogrande Basin lay the Robledo Shelf in the vicinity of the present San Andres Mountains. The Sacramento Shelf lay in the vicinity of the present day northern Sacramento
Mountains between the Orogrande Basin and the Pedernal Uplift to the east (Figure 3).
Uplift of the Pedernal was pre-Pennsylvanian but maximum relief was most likely developed during Morrow-Atokan time
(Meyer, 1966). By Desmoinesian the landmass had been reduced to a few islands rising out of the Pennsylvanian ocean
(Meyer, 1966), but rejuvenation occurred during the Missourian and the Pedernal landmass remained exposed until the Wolf-
campian sediments buried it. Wells drilled near Cloudcroft
demonstrate that terrigenous sediments and limestones of
Permian age rest in places directly on Precambrian igneous and metamorphic rocks or on Cambro-Ordovician Bliss quartzite
(Foster, 1959).
Gentle anticlinal folding occurred along the Sacramento
Shelf throughout the Pennsylvanian and early Permian time.
Notable examples are the north-northwest trending, north- northwest plunging La Luz Anticline and the north trending
Dry Canyon Syncline (Figure 2). Late Pennsylvanian-Permian 9 movement of the Pedernal Uplift is suggested by the presence of the Fresnal growth fault, located just east of the field area (Pray, 1961). The downthrown block west of this normal fault has a complete Permo-Pennsylvanian section, but the early Permian Abo Formation rests unconformably on Des- moinesian-Missourian Gobbler Formation on the eastern up- thrown side (Pray, 1961). Development of the anticline and syncline played a major role in influencing deposition during the Virgilian-Wolfcampian time.
Post-Permian
Movement along the faults and growth of the folds may have resumed during Laramide deformation (Pray, 1961), and during Late Cenozoic time the Sacramento Shelf deposits were uplifted along the north-south trending boundary fault to the west of the escarpment (Pray, 1961) (Figure 2). Drilling
In the Tularosa Basin indicates a minimum of 7000 feet of displacement along the fault (Pray, 1959). Offset in recent alluvial gravels at the base of the Sacramento escarpment indicates ongoing movement along the Late Cenozoic fault. 10
Pennsylvanian Holder Formation
The Virgilian Holder Formation in the Sacramento Moun¬ tains is latest Pennsylvanian and consists of a maximum of
900 feet of sediments deposited in cycles (Pray, 1961).
Much work has been completed on these cycles by Pray (1959,
1961), Otte (1959a, 1969b), Oppel (1959), Cline (1959),
Wilson (1963, 1967), and others. Graduate theses by several of Clinefe students have contributed to the understanding of the stratigraphic and faunal character of these deposits.
A generalized Holder Formation shelf cycle as described by Wilson is illustrated in Figure 4. These shelf cycles and their relation to the Orogrande Basin are fully discussed by Wilson (1967). Several workers have studied in detail nine such cycles in particular in the lower part of the
Holder Formation. Traditionally, these cycles have been identified and named by their massive capping beds (member
3), but as new cycles were identified or as cycles disappear¬ ed or split, the terminology became confusing. The names for the capping limestones used in this thesis are taken from
Wilson (1967) and are illustrated in Figure 5. For this thesis, the names have no other meaning than to identify the capping beds. Because of limited outcrop and generally poor information, cycles 1, 2, 4, and 5 will not be discussed. Figure 4 Generalized Holder Formation Cycle
After Wilson (1967) 11 Figure 5 Lower 9 Cycles, Holder Formation
After Wilson (1967) 12
Cycle Cycle Capping Bed Number
9
8
7
6
5
4
3
2
1
Adapted from Wilson (1967) 13
All the cycles are capped by massive shoal water lime¬ stones except for cycles 7 and 8, which are capped by less conspicuous beds. Cycle 8 is commonly capped by a thin yellow-weathering algal plate limestone, forming a distinctive
"leopard rock." This ferruginous-stained limestone has a blotchy spotted upper surface, and is an altered algal-tubular foraminiferal boundstone (Toomey et al, 1977). Cycle 7 is capped by a black pellet and pisolite (BP) bed one to three feet thick, which commonly comprises all of the cycle (Wilson,
1967).
The lower parts of the Virgilian cycles have been des¬ cribed as transgressive (Wilson, 1967), because the lower terrigeneous member (member 1) represents elastics spread onto the Sacramento Shelf following subarial exposure of the underlying carbonate capping bed. However, the elastics may actually represent a regressive sequence built out across the shelf from the east before or during a general rise in sea level. As the transgression continues, these sediments grade upwards into marine shales and nodular limestones, finally becoming open marine limestones (member 2). The capping bed (member 3) is considered to be the beginning of the regressive phase apparently deposited during widespread sea
level drops. The massive carbonate capping beds of each
cycle are quite resistant and can usually be traced very 14 easily; however, the underlying sands, shales, and thinner bedded limestones are often weathered, and covered by soil and scrubby vegetation. For this reason, the variability of members 1 and 2 is not well observed. 15
Topographie and Tectonic Controls on
Deposition of Virgilian Cycles
Wilson (1967) has demonstrated that specific topographic and tectonic features of the Sacramento Shelf have played an important role in the depositional history of the Virgilian cycles. They are the Main Mound carbonate buildup, the
Yucca Mound, the West Dry Creek carbonate buildups, the La
Luz Anticline, and the Dry Canyon Syncline (Figure 2). The
Main Mound carbonate buildup is a basal Virgilian feature underlying all the cycles, and the Dry Creek mounds and
Yucca Mound are roughly contemporaneous with some of the lower cycles. The La Luz Anticline and Dry Canyon Syncline are primarily Permo-Pennsylvanian structures, present with varying
importance throughout the deposition of the Virgilian cycles.
The effect of these features on the thickness of the elastics and the distribution of channels is the focus of this study. 16
Previous Work
The effort involved in this thesis is based on the work of many people, but particularly that of J. L. Wilson and
Gordon Dolton, while both were with Shell Oil Company. One significant contribution of Wilson's work is the measurements of 41 sections throughout the field area, 32 of which I used in this thesis (Figures 6 and 7). The sections are distri¬ buted quite evenly between the north and south areas.
Wilson's sections contain very detailed thickness measure¬ ments and stratigraphic descriptions which prove indispensable in distinguishing cycle-capping beds which were often similar in appearance. Dolton's six weeks of field work resulted in
96 measured sections concentrated primarily in the northern part of the field area (Figures 6 and 7). These closely spaced sections allow accurate documentation of the changing thicknesses of the individual units, but are not as strati- graphically detailed as Wilson's. Wilson and Dolton measured
some sections in the same places, providing a comparison of
the two sets of sections. In general I found the sections
to be satisfactorily similar.
On the maps, Wilson's sections are identified by the
section number in a square, and Dolton's by the section number in a circle. In the text, Wilson's sections are Figure 6 North Field Area
Wilson's sections in squares
Dolton's sections in circles
Figure 7 South Field Area
Wilson's sections in squares
Dolton's sections in circles 18 19 identified by the section number preceded by "W", and
Dolton's by the section number preceded by a "D". New localities that were sampled or described were arbitrarily assigned a number (>100) preceded by a "C". 20
Field Data
The majority of field work was completed during 4 weeks
in June and July, 1976. This time was spent mapping the
outcrop of the conglomerates and the capping limestones and making detailed descriptions of the conglomerates and their
associated channels. Appendix I contains descriptions of
the variations of the lower terrigenous member (member 1)
of cycles 3, 6, 7, 8, and 9 over the field area. It is
compiled from Wilson's and Dolton's measured sections and
from my own field observations. My field descriptions of
the coarse terrigenous clastic units at the different
localities are presented in Appendix II. A summary of their
characteristics follows.
The coarse terrigenous clastic units are found in the
lowermost member (member 1) of Wilson's generalized cycle
(Figure 4), and include all the strata from the top of the
highest sandstone or conglomerate to the bottom of the lowest
sandstone or conglomerate. This often includes thin shale
and limestone units and covered intervals, but the entire
unit is treated as the interval of clastic deposition. The
thickness of the unit of clastic deposition for any cycle
ranges from a few inches to 69 feet. The thickest single
conglomerate unit measured is 40 feet. The coarse terrige- 21 nous elastics can occur anywhere within member 1, lying directly on or channeling into underlying cycles, lying in the middle of the member, or lying directly beneath member 2.
The conglomerates are characterized by two distinct size modes, consisting of limestone, chert and quartzite clasts in a fine to coarse sand matrix. The clasts are often in contact with each other. The limestone clasts were generally found to be angular to rounded, ranging in size from sand to 25 cm. Limestone clasts are light olive gray in color and chertification was observed in some places. The chert clasts are usually sub-angular to rounded and are brown, red, yellow, orange, green, gray, black and any hue in between. The clasts range from sand size to a maximum of
10 cm. Quartzite clasts were most often rounded to well rounded, although in some instances the clasts have fractured during transport or during exposure, creating a sharp corner bounding a flat side. The clasts range from sand size to a maximum of 18 cm. In general, at any location the limestone clasts were the largest, the quartzite clasts intermediate, and the chert clasts were the smallest.
The matrix consists of generally fine to coarse grained quartz sand with varying amounts of limestone fragments and fossils. Sand-sized chert and quartzite are also common.
All the matrix is somewhat calcareous. Sand units similar in 22 composition to the matrix may occur above, below or within conglomerate units or may comprise the entire clastic unit.
The base of the channel cuts are concave upwards and the conglomeratic units filling the channels have flat tops. 23
Transport Systems
Most of the Virgilian conglomerates can be seen to grade laterally and upwards into sandstone, the pebble and cobble fraction decreasing in maximum size, mean size and abundance. The conglomerates exhibit planar bedding and in places cut and fill structures, and the associated sand¬ stones exhibit trough and planar cross bedding. Simons and
Richardson (1961) have classified planar bed, standing wave and antidune bed-forms as an upper rapid-turbulent or shoot¬ ing flow regime, and assign ripple, dune and wave bed forms to a tranquil-turbulent flow regime. The transition from coarser bed load to finer sands and their respective sedi¬ mentary structures represents a change from the upper to lower flow regime by decreasing discharge, decreasing depth, and decreasing velocity (Gwinn, 1964). Because many
Virgilian conglomerates (most often those in channels) exhibit more than one upward fining sequence, it is likely that numerous high energy clastic flows contributed to their formation. 24
Provenance
The limestone clasts are the one type that is not derived from the uplift. The large size, angularity
(Appendix II), and Late Pennsylvanian fauna (Appendix III,
Plate 1) of the limestone clasts indicate they are largely intraformational, that is, derived from the older Holder
Formation rocks of the Sacramento Shelf. Because the Peder- nal Uplift was a positive feature from Morrow-Atokan time through the Wolfcampian, these Late Pennsylvanian rocks could not have been deposited on the uplift. A very few crinoidal stems and fragments have been seen in clasts or free in the matrix, which may indicate minor Mississippian exposure; however, these also could have been derived from the Pennsyl¬ vanian Sacramento Shelf rocks rather than the Pedernal Uplift.
The limestones are generally large, more angular and more elongate than the chert and quartzite clasts. If all of the clasts had been transported the same distance (i.e., from
the Pedernal Uplift), the less resistant limestone clasts
should be smaller and more rounded. In addition, in some of
the channel conglomerates chunks of limestone are found that appear to have simply fallen or slumped from a limestone bed
truncated by the channel. All the above indicates a very
local, probably a shelf source for limestone clasts. Plate 1 Virgilian Trlticites in limestone clast 25
i- H o I mm 26
In the Virgilian conglomerates, chert clasts exhibit a wider range of colors than the older Paleozoic cherts seen in the outcrops of the Sacramento Mountains (Appendix II).
One late Paleozoic Bryozoa was found in a red chert, and some clasts contain Late Paleozoic shell debris (Appendix III,
Plates 2 and 3). The source of these Late Paleozoic cherts found in the Virgilian conglomerates is the strata overlying the Pedernal Uplift to the east.
The petrography of the Virgilian quartzite clasts is similar to that of the Cambro-Ordovician Bliss quartzite from Franklin Mountains, El Paso, Texas (Appendix III, Plates
4 and 5). The Bliss Formation crops out in the southern
Sacramento Mountains and is known by drilling to be present over the Pedernal Uplift (Foster, 1959); however, the
Franklin Mountain samples were the only ones available for this study. Work by Foster (1969) documents the existence of Bliss Formation or Precambrian core in places directly underlying the early Permian Abo sediments over the Pedernal
Uplift. This indicates that the uplift had been exposed to these stratigraphic depths at least by Virgilian-Wolfcampian time and probably earlier, and was a logical source for the
Virgilian conglomeratic clasts. Plate 2 Paleozoic shell debris
Plate 3 Late Paleozoic ramose Bryzoan 27
i 1 0 I mm Plate 4 Bliss quartzite
Plate 5 Virgilian quartzite clast 28
H 0 mm 29
Character of Virgilian Cycles
The following parameters were investigated for each cycle in an effort to document the influence of topographic and tectonic elements of the shelf on deposition.
Cycle Isopach Maps
The cycle thickness documents to what extent each cycle was controlled by topographic and tectonic elements. The cycle isopach maps were constructed using thicknesses of the cycles determined from measured sections. Cycle thickness is considered to be the vertical distance from the top of the cycle capping limestone to the top of the capping lime¬ stone of the cycle underlying it (Figure 5). If a channel is present cutting into the underlying cycle, the cycle thickness would extend to the base of the channel cut.
Coarse Terrigenous Clastic Isopach Maps
The thickness of the coarse terrigenous clastic units document to what extent they were controlled by topographic and tectonic elements. The coarse terrigenous clastic isopach maps were constructed using thicknesses determined
from measured sections and field observations. Terrigenous clastic thickness is considered to be the vertical distance
from the bottom of the lowest sand or conglomerate bed to the
top of the highest sand or conglomerate bed. This unit 30 commonly contains covered intervals, shales, silts and some thin limestone beds.
Paleotransport Directions
Paleotransport directions indicate the direction in which the sediments were being transported. This would show a general direction of source and demonstrate how topographic and tectonic features affected transportation. The conglom¬ erates themselves generally exhibit very poorly developed directional features. No pebble imbrication was apparent in the field, and cobble-granule grain size does not permit development of cross stratification. Trough cross bedding does occur as the percentage of coarse sand matrix in the conglomerates increases. The vast majority of the trough crossbeds occur in the fine- to coarse-grained sandstones that lie above, below and within the conglomerates. Although the
direction of transport of the sands is not necessarily the
same as that of the coarser terrigenous elastics of the
channel trends, comparison shows that sand transport direc¬
tions observed in the Virgilian cycles indicate a positive
correlation with channel trends.
Size Distribution
Decreasing size and increasing roundness with dis¬
tance from a source has been observed and measured by 31
Plumley (1948) and Unrug (1957). In the Virgilian conglom¬ erates, roundness variations of the clasts were not found to be significant, but the size distribution gives an indication of direction of course and the area of high energy channel flow. The size distribution maps are constructed using the maximum size of any chert or quartzite clast observed within the terrigenous clastic unit. 32
Cycle 3 Isopach Map
Cycle 3 is confined largely to the western part of the field area, thinning to the east over and around the La Luz
Anticline (Figures 8 and 9). The thickest accumulations trend southwest to the point of greatest thickness at W 1 and then continues in a more southerly direction. A vast majority of the 88 feet found at W 1 is due to a thick sequence of channel fill sandstones and conglomerates that cut down into the underlying cycle. The southerly trend of the cycle thickness continues through W 13 where it is
33 feet thick, and a more easterly south trending branch continues through W 17, W 21, and W 32 where cycle 3 is
30-35 feet thick.
W 35 is an isolated section where the cycle is 63 feet thick. This section is located on the eastern side of the La Luz Anticline and is believed to have been influenced by the subsiding Dry Canyon Syncline, but cannot be confidently connected to the other sections because of the lack of control.
Cycle 3 Terrigenous Clastic Isopach Map
The thicknesses of the coarse terrigenous elastics show a similar but more confined trend (Figure 10).
Generally south and east of W 3,and W 13 and west of W 29, Figure 8 North Field Area
Cycle 3 Isopach Map 33 Figure 9 South Field Area
Cycle 3 Isopach Map 34 Figure 10 North Field Area
Cycle 3 Clastic Isopach Map
Arrows indicate paleotransport
direction 35 36 the elastics are not found. The primary clastic channel,
60 feet thick, trends slightly west of south, through W 1 to the vicinity of W 30. Also, a 13 foot accumulation is located at W 26. The position of the channel cut may be controlled to some extent by the carbonate buildups at W 29,
W 1 and D 30. Work by Wilson (1977) and Wray (1977) shows that the carbonate mounds had ceased their construction, the intervening low areas had filled with lagoonal shales and limestones, and that the Sub AR capping bed was deposit¬ ed almost horizontally over the entire unit, before cycle
3 deposition began. The cutting of the cycle 3 main channel appears to be controlled by some residual influence of the main mound mass, because the channel cuts down through the softer shales and thin limestones between the Dry Canyon
Mounds and West Dry Creek buildups.
Cycle 3 Paleotransport Directions
Very few transport directions were observed in the cycle 3 terrigenous member. At W 1, the sandstones within the channel cut exhibit variable directions, but trend primarily west to southwest (Figure 10). At C 151 the one directional feature indicates transport to the northeast, which is not consistent with the other indications for a southerly trending channel. 37
Cycle 3 Size Distribution
With data from only two points, little can be stated beyond the fact that the largest chert or quartzite clasts found were 11 cm at W 1 and 10 cm at C 151.
Cycle 6 Isopach Map
An isopach map of cycle 6 is presented in Figures 11 and 12. This map is somewhat similar to one constructed by Wilson (1967, p. 109) using 21 data points.
This cycle is the best representation of the effect of underlying structure on the thickness and character of an individual Virgilian cycle. The structures evidently exerting depositional control are the basal Virgilian Main
Mound buildups, the developing La Luz Anticline and the Dry
Canyon Syncline.
The slight unconformity between cycle 6 and the on- lapping cycle 7 is not significantly erosional (Wilson, 1967), and therefore the thickness of the cycle accurately reflects the topography of the shelf area at the onset of cycle deposition and growth of structural features during the cycle.
The cycle thins over and around the La Luz Anticline. This is visible in the thicknesses along Indian Wells Canyon (section
W 23 through W 5). There are several trends of major thick¬ ness of cycle 6. On the east side of La Luz Anticline Figure 11 North Field Area
Cycle 6 Isopach Hap 38 Figure 12 South Field Area
Cycle 6 Isopach Map 39 40 a southeast trending accumulation reaches a thickness of
84 feet at W 38. On the west side of La Luz Anticline a thickness of just over 50 feet trends southwest through
W 16 and D 71 and splits into an eastern accumulation of approximately 60 feet at W 21, D 85 and a western 50 foot thickness at W 14. These two trends are separated by a somewhat questionable high at W 17 and D 50.
The cycle thins over the carbonate mound complex at sections D 63 through W 1, and a westward thickening trend is recognized at sections D 62, W 18, D 61 and D 60.
In the far northern part of the field area the sedi¬ mentation is less confined, with the thicknesses indicating a terrigenous clastic lobe thickening to the west, into the
Orogrande Basin. This lobe appears to be broadly confined between the Main Mounds and a small carbonate buildup in cycle 6 at sections D 6 through D 12. The area of thin deposition at D 11 and D 12 over this carbonate buildup is evident in some later Virgilian cycles.
Cycle 6 Clastic Isopach Map
Figures 13 and 14 are isopach maps of the coarse clastic fraction of cycle 6. The trends seen in clastic thickness are very similar to those seen in the isopach map of the total cycle, but show that the coarse elastics Figure 13 North Field Area
Cycle 6 Clastic Isopach Map 41 Figure 14 South Field Area
Cycle 6 Clastic Isopach Map 42 Figure 15 South Field Area
Cycle 6 Channels
Arrows indicate paleotransport
directions
Trace of channels are stippled 43 44 follow well-defined, narrow channelways. There appear to be two major channelways distributing coarse elastics in cycle 6. One, informally named the Beeman Canyon Channel, extends southwestward along northeastern Beeman Canyon and into Indian Wells Canyon, and the other trending approxi¬ mately southwest from D 42, swings progressively west until it trends southwest through W 13 and W 14 (Figure 15). The latter channel is informally named the Dry Canyon Channel.
These two terrigenous clastic conduits appear to be separated by a northeast-southwest trending area of relatively thin clastic deposition. The possibility of a third major thick¬ ness of coarse elastics trending southeast from D 73 is suggested by data to be discussed later in this section.
As in the total thickness isopach, the coarse terrige¬ nous elastics thin noticeably over the carbonate buildups, and over and around the La Luz Anticline. The coarse elastics of the Beeman Canyon Channel, instead of smoothly wrapping around the nose of the La Luz Anticline, appear to turn sharply incising a very steep-walled channel which intersects the present hillside at sections W 20 and D 84.
At this point the channel appears to divide into two channels; one branch swings farther south, continuing to curve roughly around the outline of the plunging La Luz
Anticline, and the other swings sharply to the west to cut 45 the nose of the ridge at section W 19. The path of the latter channel is reflected in its outcrop pattern.
The deepest cut of the channel and the thickest accumulations of coarse terrigenous elastics in this vicinity are at D 84 on the southeast side of the W 19 ridge and D 85 on the west side of the ridge. D 84 and D 85 have clastic thick¬ nesses of 26 and 16 feet respectively. In both cases the channel has cut out much of the underlying cycle 4 (cycle
5 is not present in the area), and at D 84 has cut down into the top of cycle 3. The outline of the channel is shown in Plate 6. This photo, taken at D 84, shows cycle 6 conglomerate cutting down into the top of the AR limestone, the cycle 3 capping bed. Just off the photo to the right
(northeast) is the nearly vertical eastern wall of the channel estimated to be approximately 20 to 25 feet high.
At W 19, the conglomerate and coarse elastics are significantly thinner ( 6 feet) and are found to be totally within the confines of cycle 6, not having cut down into the underlying cycles. Because the deepest part of the channel cut trends west from W 20 toward D 85 and southwest from W 20 to D 84, W 19 appears to be relatively higher on the interfluve between the fork of the channels.
The southern channel continues south across upper
Beeman Canyon, where it appears to split again with a south Plate 6 Cycle 6 conglomerate cutting into the top of the
AR limestone 46 47 trending channel that is projected into Indian Wells Canyon approximately through section D 94. At this point the terrigenous elastics are 6 feet thick. The other limb trends southwesterly to westerly through W 32, D 98 and
W 23 (9 feet thick), but the general configuration is of the channel flattening out to become a lobe of terrigenous elastics, thinner to the west, south and northwest.
The Dry Canyon Channel is separated from the Beeman
Canyon Channels by a long narrow area of thin deposition
( 5 feet) at sections W 17, D 50, D 45 and D 89. The
Dry Canyon Channel trends southwestward at approximately
25 feet thick through W 14. The Dry Canyon Channel also has very steep walls, having relief of up to 25 feet, visible west of D 53, but the channel cut does not crop out as spectacularly as does the Beeman Canyon Channel at
W 20.
In the northern part of the field area the trends of the coarse clastic thicknesses are very similar to the total cycle thickness trends. A west building lobe occurs at D 20,W27and the persistent northern positive area is apparently in the thinning of the elastics in the vicinity of D 11 and D 12. 48
Relation of Cycle 6 Terrigenous Clastic Isopach Map to Cycle
6 Isopach Map
Comparison of the isopach of the total thickness and that of the terrigenous elastics reveal that they have simi¬ lar trends and are controlled by the same topographic and tectonic features. One notable exception is that of location
W 19. Total cycle thickness here is one of the thickest of the Beeman Canyon Channel, yet the coarse elastics are relative¬ ly thin. In fact, W 19 appears as a relative high; coarse elastics at W 19 are 6 feet thick among 16, 17 and 26 foot thicknesses in adjacent sections of coarse elastics. The ex¬ planation is that W 19 lies just southwest of the northern point of bifurcation of the Beeman Canyon Channel. Trending west, just to the north of W 19 the aforementioned channel cut is exposed at W 20, D 84 and D 85. Trending south, just to the east of W 19 is that part of the Beeman Canyon Channel that heads into Indian Wells Canyon. W 19 is a relative high between deeper channels on either side. Otherwise the isopach maps are very similar. The thickness of the total cycle and the terrigenous coarse clastic fraction are controlled by the same topographic features. As expected, the coarse fraction exhibits a more confined pattern of thickness as a result of channelization. 49
Relation of Cycle 6 Channels to Cycle 6 Transport Directions
In general, the directions are consistent with channel trends, but a few localities are especially noteworthy. At
D 152 there are nearly opposing transport directions (Figure
15). The westerly transport direction follows the trend of the main Beeman Canyon Channel, whereas the direction to the southwest suggests transport into the Dry Canyon Syncline.
The syncline is evident from the isopachs and may have a branch of the Beeman Canyon Channel entering it from approxi¬ mately C 152. However, the only indication of elastics in the syncline in cycle 6 is 11 feet of conglomerate at D 74,
3 feet of foreset bedded sand at W 35 and 3 feet of sandy con¬ glomerate at W 38, so neither clastic deposition nor channel¬ ization appear to be as significant here as in the main channel to the west of C 152 (Figure 15).
At sections W 20 and D 47 the transport directions show a range from southwest to northwest. This reflects the splitting of the main channel into one westerly trending branch and another trending southwesterly. Westerly transport at W 21 is consistent with the west branch heading into the
Orogrande Basin. Transport directions at 19 are south-south- west to south-southeast and indicate a general southerly spread of coarse elastics, and the varied directions at D 97 may reflect the fact that the elastics are no longer channelized 50 but are deposited in a northwesterly to southwesterly building lobe.
Cycle 6 Size Distribution Map
In general, the size distribution of the chert and quartzite clasts follow closely the occurrence of channels within cycle 6 (Figures 16 and 17). The largest clasts
(up to 15 cm) are almost always found within the deepest parts of the channels. The Dry Canyon Channel has a maximum pebble size of 12 cm at W 14. The Beeman Canyon Channel has
13 and 15 cm clasts at C 152 and 15 cm clasts at W 20, both of which are channel cuts, and also has 15 cm clasts at C 109.
In the Beeman Canyon Channel there is some indication that the clasts are becoming generally smaller away from the source
(smaller to the southwest). Sizes decrease from 15 cm at W 19 and 10 cm at C 114 to 5 cm at W 32, 4 cm at C 108 and C 110, and 2 cm at C 112. However, sizes also would be expected to decrease in general as one approached the fringes of coarse clastic deposition where the energy is necessarily lower.
Therefore, unless one is reasonably sure of the position of the outcrop within the channel, any interpretation of de¬ creasing clast size and its relation to distance from the source is risky at best. Figure 16 North Field Area
Cycle 6 Size Distribution Map
Maximum clast size in cm is contoured 51 Figure 17 South Field Area
Cycle 6 Size Distribution Map
Maximum clast size in cm is contoured 52 53
Total Cycle 7 Isopach Map
Little reliable data are available for cycle 7 due to the limited outcrops and lithologic variations of the Black
Pellet Bed capping cycle 7. The trend that is apparent, how¬ ever, is one of general increase in thickness of the cycle toward an accumulation of 24 feet at W 14 and 23 feet at W 21
(Figures 18 and 19). These thicknesses may represent a point where the shelf borders the subsiding Orogrande Basin to the west. Wilson (1967) believes that cycle 7 represents a period of near equilibrium between sea level fluctuations and the tectonic movements of the shelf. As a result the growth of the tectonic elements such as the La Luz Anticline are at a minimum and are not as important a control on deposition as in earlier cycles. Nevertheless, cycle 7 appears to be thicken¬ ing gently off the west flank of the anticline.
Cycle 7 Terrigenous Clastic Map
The occurrence of coarse terrigenous elastics is generally restricted to the southern part of the field area, present locally up to 9 feet in thickness (Figure 20).
Cycle 7 Paleotransport Directions
The few directions observed in cycle 7 (Figure 19) are consistently westerly, ranging from S 50 W to N 70 W. This suggests that the clastic sediments have been spread uni- Figure 18 North Field Area
Cycle 7 Isopach Map 54 Figure 19 South Field Area
Cycle 7 Isopach Map 55 Figure 20 South Field Area
Cycle 7 Clastic Thickness Map
Arrows indicate paleotransport
directions
5? formly across the Sacramento Shelf to the west.
Cycle 7 Size Distribution
Sizes of the chert and quartzite clasts, inferred from the little data available, decrease to the west. The largest clasts (5-6 cm) are found at C 100, D 88, D 26,
W 32 and D 91.
Due to difficulties in distinguishing the boundary between cycles 8 and 9 in much of the field area, the cycles are considered together. They are the youngest Virgilian cycles studied.
Cycles 8 and 9 Isopach Map
In the southern area the thickness of the cycles increases east and west somewhat symmetrically off a north-south trending high (Figures 21 and 22). This high is a remnant of the La Luz Anticline after previous cycle carbonate growth broadened and flattened its structural effect. The thinnest outcrop of the cycle occurs around section D 37 which has a thickness of 16 feet. The thick¬ ness of cycles 8 and 9 increases in both directions, 56 feet to the east in the Dry Canyon Syncline and to much greater thicknesses to the west and northwest. The character of the high is unknown farther to the north. West and north of the Figure 21 North Field Area
Cycles 8 and 9 Isopach Map 58 Figure 22 South Field Area
Cycles 8 and 9 Isopach Hap 59 60 high the cycle thickens drastically to 136 feet. Some of the thickest accumulations are found at sections D 63, D 64,
D 6, D 5, and D 4B. These thick accumulations of cycles 8 and 9 marginal to the shelf probably extend through the western edge of the southern field area but have been removed by recent erosion.
Cycles 8 and 9 Terrigenous Clastic Isopach Map
The coarse elastics of cycles 8 and 9 are not generally found in the southern part of the field area. The elastics are found to be absent over the axis of the La Luz Anticline in sections on the north side of Dry Canyon (Figure 23). At
D 34 and west, coarse elastics are present, forming a small south-southwest building lobe. Occasional lobes of elastics are found to build to the west, but generally avoid the D 11 and D 12 positive area. At the very north end of the field area, channels filled with elastics irregularly reach 69 feet in thickness in spectacular outcrop. The overall
impression of clastic occurrence in cycles 8 and 9 is that of a relatively thin, broad blanket covering the northern part of the field area with a major northwest trending channel at the very northwest edge of the field area. Figure 23 North Field Area
Cycles 8 and 9 Clastic Isopach
Map 61 62
Cycles 8 and 9 Paleotransport Directions
The only paleotransport directions seen are in the
far north part of the field area. At D 14 sand trough
beds indicated transport south 25 west, and approximately
60 m north of D 14, cross beds indicate transport was due
south. In the sections D 1 to D 5 there are very well
developed trough cross beds in the conglomerate-sand units which generally indicate northerly transport despite
occasional south and southeast directions.
Cycles 8 and 9 Size Distribution
The size of chert and quartzite clasts decrease to
the west and south of the area of largest clasts (15-18 cm), which occurs at D 3 and D 4. This again indicates that the
source for the clasts is probably east-northeast of the northern part of the field area. Within single foreset
beds some workers have observed decreasing clast sizes to
the north-northwest, further supporting transport in that
direction (Wilson, personal communication). 63
Summary
The cycle thicknesses, the terrigenous clastic thick¬ nesses and the trend of their associated channels respond to both the pre-cycle topography and the growth of topo¬ graphic structures during the cycle. The growth of these structures is believed to be intimately associated with orogenic pulses of the Pedernal Uplift. The prominence of the tectonic structures appear to have increased during
the early Virgilian to a maximum during cycle 6. The relatively thin and uniform thicknesses indicate that growth of shelf structures was at a minimum during cycle 7.
Growth was renewed during cycles 8 and 9, documented by the rejuvenation of the La Luz Anticline and the Dry Canyon
Syncline.
Paleotransport directions in all cycles studied generally
indicate southwesterly transport (Figure 24) away from the
suspected point of source, and where channels are present
there is good correlation between channel trends and
transport directions. Figure 24 Composite Paleotransport Directions—Lower 9
Holder Formation Cycles. Number of
readings in parenthesis. 64
m in
m CM 00 CM 65
Discussion
Paleoclimatology
The climatic model envisaged for the Virgilian Sacra¬ mento Shelf is one of warm, clear seas, westerly winds with heavy periodic rainfall on the Pedernal Uplift. Paleo- magnetic data indicate that during Late Paleozoic time the mean pole position was located on the Korean Peninsula
(Strangway, 1970). This would position southcentral New
Mexico at approximately 10-15 S latitude (Dott and Batten,
1971). At present, these latitudes are dominated by tropi¬ cal air masses.
The Pennsylvanian Orogrande Basin (Pray, 1959) formed a seaway approximately 100 miles wide between the Sacramento and Robledo shelves. Detailed studies of algal plate mounds on the Sacramento Shelf suggest that wave energy was greatest on the west side as a result of westerly winds (Wilson, 1975,
Toomey, et al., 1977). Carbonate growth indicates that seas were warm and at least periodically clear. The abundance of
fossil logs up to 2 feet in diameter in the Virgilian con¬
glomerates indicates the presence of large forests on the
Pedernal Uplift which would have required significant precipitation.
Rainy tropical climate prevails in lowland areas on
and near the equator and along tropical coasts which are 66 exposed to trade winds but backed by interior highlands
(Critchfield, 1966). The average precipitation of the rainy tropics ( 60 inches/year) is greatly exceeded where mountain ranges lie across trade winds which have traveled across a warm sea. In general the leeward sides of the mountain show small annual totals in contrast to heavy rainfall on the windward side. Most rainfall is associated with thunder¬ storms which are usually concentrated in small areas and are of short duration. The climate of the central New Mexico region may have been significantly more arid during the times of greatest regression of the upper Virgilian sea as indicated by the presence of evaporites in the Panther Seep
Formation found in the Orogrande Basin, and the absence of significant karsting on the exposed Sacramento Shelf. If such dramatic climatic fluctuations existed, they certainly had a considerable effect on sedimentary processes and rates of weathering.
Pedernal Uplift
The exact character of the escarpment of the uplift east and north-east of the Sacramento Shelf cannot be esti¬ mated, but fairly high cliffs may have been present following renewed uplift during Missourian time until late Wolfcampian sediments buried the entire uplift (Meyer, 1966). At least 67 moderate relief to the east is required for the rainy climate and the high energy runoff necessary for transport of the coarse elastics and the cutting of the channels. At
the mouths of the steep cut mountain valleys were alluvial
fans composed of debris from the entire Pre-Pennsylvanian
section.
Coastal Processes
During the height of the transgressions, while open marine limestones (member 2, Figure 4) were being formed on
the Sacramento Shelf, the strand line of the Pennsylvanian
Sea was east of the shelf lapping up on the Pedernal high¬
lands. The alluvial debris at the base of the uplift was
subjected to mechanical weathering due to surf action and
long shore transport. It was in this environment that
significant rounding of quartzite and chert took place, and
the removal of the feldspars and limestones occurred. This
is discussed in greater detail following the section on
Provenance. The clastic sand fraction was probably con¬
centrated in slightly deeper water along the shore. As the
seas regressed, the quartzite and chert cobble and sand
beach was left exposed to be entrained and spread onto the
Sacramento Shelf by high energy streams flowing off the
uplift. 68
Depositional Setting
The deposition of the coarse terrigenous elastics
responded to the topographic and tectonic features of the
Shelf as did most of the channels. The most pervasive
features in area and time are the La Luz Anticline and the
Dry Canyon Syncline which had a major influence on thick¬ nesses of cycles as well as the terrigenous elastics. Less
significant features influencing deposition were the carbonate
buildups in the vicinity of Yucca Mound and the persistent
high in the vicinity of W 17 and D 50.
After the deposition of the capping bed (member 3,
Figure 4) of each cycle, the shelf was very flat, but in most cases significant growth of tectonic elements occurred
during the time of shelf exposure. Cycle member 1 was
deposited during the height of the regression or during a
relative sea level rise following greatest exposure. Thick¬
ness variations indicate that the deposition of the member
was influenced by shelf topography. Growth of topographic
elements did not necessarily terminate with the onset of
member 1 deposition and usually continued throughout the
cycle as evidenced by variations in thicknesses of the upper
cycle members 2 and 3. The influence of the tectonic
elements increased during the early cycles to a maximum
during cycle 6. Cycle 7 was a time of relative stability 69 represented by near equilibrium between sea level and shelf tectonics. This equilibrium resulted in relatively thin and uniform deposition on the shelf. During cycles 8 and 9 the tectonic elements of the shelf experienced renewed growth.
The tectonic activity of the shelf reflects the movements of the Pedernal Uplift to the east and thickness evidence indicates that deposition of sediments usually kept pace with the growth of the shelf structures.
Clastic Deposition
The deposition of the coarse elastics are believed to be the result of catastrophic flow or a series of cata¬ strophic flows. The planar beds and cut and fill structures are classified as rapid flow by Simmons and Richardson (1961).
The size of the clasts require a fast moving transport system; however, the gradient of the shelf itself is not believed to have been great enough to cause these flows.
Instead, the elastics were introduced to the shelf by high energy streams off the highlands triggered by tropical rain¬ storms. These flows may have been seasonally controlled but any one flow was probably very short lived. The presence of channels suggest highly competent flow and coarse bed
load. Rounding of the clasts also occurred during their
transport from the highlands to the shelf, but what percent¬ age of weathering is due to coastal processes and what per- 70 centage is related to movement across the shelf cannot be estimated. Rounding of resistant chert and quartzite cobbles is so good that multicycle deposition is suggested.
The presence of channel fill conglomerates in lower
Pennsylvanian rocks (Van Wagoner, 1977), and the location of the Morrowan strand line on or near the Sacramento Shelf
(Meyer, 1966) suggests that the erosion of the uplift and coarse clastic deposition on the shelf began in the earliest
Pennsylvanian. A composite rosette of all reliable
Virgilian paleo ransport directions observed shows a major southwest trend (Figure 24). The general trend is the result of transport from uplift to basin while local vari¬ ations are probably due to topographic features on the shelf.
Because of the apparent lack of tectonic or topographic controls on sedimentation in the northern part of the field area during cycles 8 and 9, coarse elastics deposited during that time may be the best indication of the general configuration of deposition. A channel is present represent¬
ing the most rapid and most competent flow of the coarse elastics. The channel is bordered by a relatively thin blanket of elastics to the south and perhaps also to the north. This configuration suggests a broad fan of elastics deposited at the terminus of a channel as flow and competence decreased with distance from the uplift. The thin elastics 71 present on the shelf are a fan distal to clastic flow through a channel that terminated some unknown distance to the east. The channel present in the middle of the clastic fan in cycles 8 and 9 probably reached the Orogrande
Basin-Sacramento shelf-break, spilling elastics directly into the basin to the west. The thin elastics on either side of the channel may also represent to some extent channel bank deposits. Significant reworking of the coarse elastics probably occurred as the different pulses of elastics cut through existing channel deposits and/or fan deposits. The location and configuration of coarse elastics and their associated channels are largely controlled by slope of the shelf and the point source of the high energy clastic flow. When significant topographic or tectonic elements are present on the shelf, the processes are pre¬ cisely the same except that the thickness of the elastics and the presence of the channels respond to the depressions and positive areas of the shelf.
Provenance
The time required for material to be eroded off the highlands, deposited in alluvial fans, worked along the coast, and then spread in pulses onto the shelf cannot be estimated. Material was probably being eroded off the 72 uplift continuously, and that which was being eroded during one cycle most likely was not worked onto the shelf until a much later cycle. By latest Pennsylvanian a large percentage of Hississippian sediments had probably been removed from the uplift (Meyer, 1963). The presence of Mississippian age cherts in the Virgilian conglomerates indicates that some cherts probably had been involved in the alluvial fans and/or beach deposits since earlier Pennsylvanian. The presence of cobbles of Bliss quartzite in the Virgilian conglomerates is consistent with information that indicates that at least much of the uplift east and north of the field area was eroded down to or below the Bliss Formation by Wolfcampian time (Foster, 1959).
The absence of feldspar and any recognizable pre-
Pennsylvanian limestones in the Virgilian conglomerates presents a problem due to the proximity of the uplift to the shelf. Unfortunately this situation became apparent late in the study so that neither sampling in the field nor choice of thin sections had been conducted with this problem in mind. The abundance of feldspar (and limestone) in the shelf deposits is related to (1) source rock composition,
(2) chemical weathering in the source area, (3) abrasion and solution during transportation and (4) solution during dia¬ genesis (Pettijohn, et al., 1972) (Figure 25). Figure 25 The Origin of Feldspars in Sandstones
After Pettijohn et al. (1972) 73
Feldspars in Source Rock
V
Chemical Decay at Source
V
Abrasion ^Solution in Transport
•u
Solution during Diogenes is
> t
Feldspar in Sandstone 74
Because igneous rocks and limestones are known'ty drill¬ ing to have been exposed on the uplift during Late Pennsylvani¬ an, their absence in the shelf deposits must be a result of a combination of processes 2, 3, and 4. The survival of unstable minerals in the source environment and the possi¬ bility of transportation to the depocenter is a function of chemical'and mechanical weathering (Pettijohn, et al., 1972).
For reasons stated before, the uplift is believed to have had at least moderate relief during the Virgilian and to have had a rainy tropical climate. These conditions would promote degradation or "rotting" of feldspars and carbonates, insitu on the uplift. Eventually the feldspars and carbonates, as well as the other debris from the uplift were eroded and transported to the west through very deep cut mountain valleys.
Weathering of these unstable rock fragments continued during their movement to the base of the escarpment. The solubility, softness and cleavage of the unstable rock fragments decrease the probability of their survival during such stream transport
(Pettijohn, et al., 1972).
The same warm, high energy coastal environment that was present during the height of the transgression and was responsible for rounding of quartzite and chert clasts, further reduced the feldspars and limestones. All the rock fragments were involved in coastal processes for an indeter- 75 minate length of time during the Virgilian and may have been moved tens of miles up and down the coastline. As a result, by the time the elastics were shed westward during lower sea level stands, the ancient shoreline deposits were composed primarily of the stable components—quartzite, chert and quartz sand--which are found along with Virgilian shelf limestones in the Sacramento Shelf conglomerates.
The role of solution of the unstable rock fragments during diagenesis is questionable, and further thin section study would be required to address this question. The reduction of softer rock fragments through abrasion does exist but is not thought to be alone a major cause of change in compo¬ sition (Pettijohn, 1957). It is a combination of abrasion and chemical reduction of feldspars and carbonates on the uplift and in the coastal environment that results in their being insignificant components of the clastic sediments that reached the Sacramento Shelf. 76
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Appendix I
Refer to the general field map (in pocket) for outcrop
occurrence of the cycle capping limestone and course
elastics. Cycles are reviewed in this appendix in ascend¬
ing order, the discussion concentrating on areal distri¬ bution a.nd description of the terrigenous clastic interval
(member 1). Cross sections through Dry and Beeman canyons are in the pocket. 80
Cycle 3
Cycle 3 is that cycle which is capped by the AR lime¬ stone. West and north of W 18, cycle 3 is composed of basinal shale, offshore of the highest mounds at sections
W 26 and D 42. Typical cyclic development occurs in the shelf area east of the shelf margin mounds. At W 18 a shaley covered interval is the AR equivalent. The capping bed appears at W 26 east of the western most mounds, indicating that the AR bed, a shelf grainstone, is formed behind the marine shelf mounds. From W 26, the AR bed is found continuously to the south and to the east where it meets the highway. At W 26, the terrigenous member of the cycle is found to consist of shales, foreset conglomerate sands and siltstones. The cycle thins over the Main Mounds at W 29, as do those cycles underlying it. Here the terrige¬ nous member is covered. To the east of W 29, cycle 3 terrige¬ nous member is present very locally as a dark conglomerate sand containing quartzite, chert and limestone which rests directly on the upper surfaces of Sub AR. These are present approximately 100 meters west of W 1, at which point the terrigenous member cuts out the underlying Sub AR limestone and into the lower cycle 2 (Figure 5). Here the cycle 3 terrigenous clastic unit is a channel filled by a thick sec¬ tion of conglomerate, shale and sandstone. At the base of 81 the unit, a thin (less than one meter) dark green sandy con¬ glomerate is found, containing chert and quartzite pebbles, overlain by a 15-20 meter thick unit of sand and shale. The sand is in turn capped by another conglomerate that appears to be the same as the one directly overlying Sub AR to the west, and represents the top of the terrigenous unit. Wilson has described here approximately 60 vertical feet of channel sandstone cut through shale. The top of Wilson’s unit is the same conglomerate that has been described to the west of W 1. Wilson documents in section W 1 that Sub AR and cycle 2 have been cut out by the channelized sands and conglomerates of cycle 3. Cycle 2 continues to the east of W 1 as the channel abruptly ends. Between W 1 and W 15 the cycle 3 terrigenous member is shale with some covered
intervals. At W 15 however, the basal shale is cut by a six foot sand and conglomeratic unit that can be traced a
few meters to the west. The conglomerate is very similar
to that seen at W 1 being composed of a dark green sand¬
stone containing chert and quartzite.
To the south of New Mexico highway 83 the cycle 3
conglomerate and terrigenous channel fill of W 1 is believed
to be present in the vicinity of C 151 in a four foot out¬
crop. Support for this is found in their position relative
to the most massive buildup of the underlying mounds. The 82 conglomerate is also found in the float to the east of C 151 along Dry Canyon for about 100-200 meters. The sand of this unit is found only to the south of W 30 for approximately
30 meters and is lost in the float.
Along the north of Beeman Canyon, the terrigenous member of cycle 3 is represented by a covered interval through W 17. Some shale becomes exposed in the covered interval between W 17 and W 20 and continues around the nose of the ridge at W 19 to the east. A small outcrop of conglomerate, only a few feet square, grades upwards into interbedded sands at C 106. Throughout Beeman Canyon, cycle
3, where it is recognized, is shale or covered, except close to W 22 near the mouth of canyon. Approximately 100 meters to the north-east of W 22 a sandy conglomerate is present.
Another conglomerate, believed to be the same, is found approximately 300 meters to the south-west of W 22.
Little is known about the terrigenous member of cycle 3 along Indian Wells Canyon except that at W 5 it is represent¬ ed by 2 feet of worn bioclastic conglomerate and at W 9 by
1/2 foot of sand resting directly on, and possible cutting into, Sub AR. The cycle 3 terrigenous member is not confidently found further to the east, and in fact the AR capping bed is lost just west of W 11. 83
Cycle 6
Cycle 6 is that cycle capped by the D 2 limestone bed.
At W 40, cycle 6 clastic member contains one foot of sand
with conglomerate at the top. Above this is a 6 foot covered
interval with 5 feet of gravelly sand on top. A one foot
limestone bed separates this sand from a 3-1/2 foot green
micaceous sand. The coarse elastics have disappeared at
D 56. Sections D 57 and D 59 are characterized by shale
and covered sequences, but at D 59 a thin (2 foot) silty
shale layer appears, and becomes a slightly thicker 4 foot
sand unit. At D 61 this sand unit has reached 8 feet in
thickness but it thins and is lost abruptly to the east.
The coarse elastics reappear at D 22 as a conglomerate high
in member 1 and a greenish medium sand. Neither persist very
far but a similar sand is seen at D 24. Through sections
D 63 and D 64, the member is primarily limestone with about
4 feet of covered section. At W 25, however, a 5 foot brown
sand unit rests on top of ED 2. Again at W 1, cycle 6
member 1 is primarily limestone with about 4 feet of covered
section. At D 66 the underlying cycle has pinched out and
cycle 6 has increased in thickness. Here it is characterized
by thick covered intervals with some limestones and shales.
Between D 66 and W 16, the terrigenous interval is generally
covered. At W 16, however, on top of 46 feet of covered 84 strata is a twelve foot thick brown conglomeratic sand and at D 71, the conglomeratic sand has increased to 28 feet in thickness. Cycle 6 terrigenous elastics continue to the east at least as far as D 68, appearing as sands and conglomeratic> sands. There is a large channel conglomerate found at D 38, and at D 40 there are approximately 2 meters of conglomerate exposed grading upwards into medium to coarse, cross-bedded sands. At D 68, cycle 6 terrigenous elastics are represented by two sand and silt layers and a 3 foot conglomerate, located high in the cycle a few feet below D 2. The cycle is pri¬ marily covered or shaley at W 15.
In the southern part of the field area, cycle 6 is found at W 30 to be made up of 5 feet of sand and conglomer¬ atic sand resting directly on ED 2. A limey sand is also present higher in the cycle, but both the units are lost in a covered interval between W 30 and W 13. The cycle thick¬ ness decreased from 44 feet at W 30 to 11 feet at D 87.
Between D 87 and D 53, the cycle 5 capping bed, ED 2, continues to be found closer to the overlying D 2 bed until they appear to join. In fact, cycle 6 lies with a slight angular unconformity over cycle 5. The ED 2 limestone appears to have been truncated to the southeast by a cycle 6 clastic channel which is seen in sections W 14 and D 53. The conglomerate of W 14 is seen, in places, between W 14 85 and D 51.
At D 51, cycle 6 is represented by a basal 5 foot argilaceous sandstone, 5 feet of covered section, 1-1/2 feet of conglomerate, and 1-1/2 feet of sandstone. Just a short distance east, W 17 has 11 feet of sandstone, 1 foot of conglomerate and 4 feet of similar sand overlying it.
Cycle 6 at D 50 has four feet of sand and conglomerate on top of 15 feet of shale. A conglomerate is found approxi¬ mately 200 meters west of D 50 which is very similar to that found at D 50.
Between D 50 and W 21 the cycle is poorly exposed.
Some conglomeratic sand is found in the float at cycle 6 elevation, but cannot be confidently assigned to the cycle.
D 2 and the clastic interval become thicker near W 21 as cycle 6 cuts out the underlying cycle 4 and is found resting on top of AR. This section contains 8 feet of conglomerate and overlying cross-bedded sandstone with another foot of sandstone higher in the cycle. D 85 terrigenous member contains two feet of laminated sandstone, green sandstone and shale. The conglomeratic unit is well-exposed through
W 19, which contains 6 feet, and D 48. Between W 21 and just east of D 48 the conglomerate appears not to cut as deeply, being found above AR rather than resting directly on top of it. Just east of D 48 the cycle 6 conglomerate comes to 86 rest directly on top of AR. D 84 has 5 feet of conglomerate resting on AR overlain by 5 feet of sand. Covered inter¬ vals and some sand units are above. The conglomerate continues to the northeast to W 20 where it is seen cutting down into the AR limestone in a definite channel configura¬ tion. At this locality it has made its deepest cut into the underlying cycles and immediately to the east of a very steep channel wall, AR and Sub D 2 are again fully exposed. The conglomerate continues east of W 20 above
Sub D 2, in places cutting into the capping limestone bed.
At D 47 the conglomerate measures 5 feet in thickness and further to the east, at D 46A, 7 feet of conglomerate and 5 feet of sandstone are present. Although less well- exposed, the coarse terrigenous elastics continue to D 73, where 12 feet of conglomerate is channelized into 3 feet of basal shale. Above this, shales, sands and conglomerates are . ihterbedded for another 11 feet. Just to the east of
D 73 at section C 152 is a 39 foot thick section of con¬ glomerate that is approximately 30 feet in width, and has a definite channel configuration, cutting down into Sub D 2.
On the south side of Dry Canyon, D 89 has 5 feet of cycle 6 conglomerate that can be traced as far as D 45, where 5 feet of conglomerate is present. Between D 45 and
W 25, cycle 6 is largely covered but at W 25, Sub D 2 is 87 overlain by 19 feet of shaley covered strata, 6 feet of cross-bedded sand and 13 feet of shaley covered strata with a conglomeratic sand at the top. The clastic member thins to D 43 where a 2 foot pebbly sandstone rests on Sub D 2.
The sandstone persists approximately 100 meters to the west.
The south side of Beeman Canyon has an impressive and persistent conglomerate in cycle 6. At W 22, 5 feet of conglomerate are present which persist to the north and east, being found through sections C 115, C 114 and C 113.
At D 98, cycle 6 terrigenous unit is represented by 4 feet of shale channeled into by 17 feet of conglomerate. W 32 shows a thickening of cycle 6 with shale and covered inter¬ vals, sandstones and limestones. This unit becomes a conglomerate approximately 50 meters to the southeast. D 97 has 2 feet of sand resting on a covered interval 1-1/2 feet of shale and 2 feet of pebbly sandstone. In the cuts and valleys between D 97 and D 96, W 39, cycle 6 terrigenous clastic unit is poorly exposed if present, but can be seen at D 96, W 39. Here, cycle 6 terrigenous unit is 2 feet of pebbly sandstone.
D 90, W 23 has 5-1/2 feet of conglomerate and sand present. The terrigenous elastics thin and are poorly ex¬ posed to the southeast and at D 92 the cycle is totally limestone with a few thin covered intervals. 88
Sections along Indian Wells Canyon contain a cloritic sandstone within cycle 6. This is documented at W 5, D 93,
D 94, and D 95. At D 95, 5 feet of very sandy limestone is separated by 2-1/2 feet of covered strata from 4-1/2 feet of sandy conglomerate. Cycle 6 thins over the La Luz anticline to the east and disappears.
Cycle 7
Cycle 7 is that cycle capped by the black pellet bed
(BP) (Wilson, 1967). Wilson has observed the BP bed throughout the field area; unfortunately I did not find it as often. I found this marker bed to crop out in the northern part of the field area intermittently between D 42 and W 16, and again at W 18. Throughout this distance it usually lies less than 5 feet above the cycle 6, D 2 capping bed. The BP bed is documented in sections D 68 and W 18.
To the west of W 16 it has not been seen. The cycle 7 terrigenous interval is composed entirely of thin shale units in this area.
Across New Mexico highway 83 to the south, cycle 7 is recognized in section W 25 where the lower terrigenous member is composed of pebbly sandstones. At W 13, D 87 and
W 14, the sandstone rests directly on top of D 2. Although in the latter sections BP is not documented, the character 89 and position of the elastics indicate they are cycle 7 also.
W 14 cycle 7 terrigenous interval contains a basal conglomer¬ ate. W 17 has the BP bed with an underlying sandstone and conglomeratic unit separated from D 2 by a covered interval.
This sandstone and conglomeratic unit can be traced east to
D 50. Cycle 7 clastic member is found to be composed of a few feet of conglomerate all along the northeast branch of
Beeman Canyon, sections W 19 and D 73.
Cycle 7 is also believed to be represented at C 116 and 50-60 meters south of W 32 as sandy conglomerates cutting into D 2. Southwest of C 116 it is seen at W 23 and D 91 as a conglomerate cutting into D 2. W 23 has a BP bed to distinguish the top of the cycle. Cycle 7 thins toward the La Luz anticline but is seen at W 9 with a clastic interval consisting of a few feet of conglomerate separated from Sub D 2 by a small covered interval.
Cycle 8
Cycle 8 is that cycle typically capped by the "leopard" rock bed (Wilson, 1967). In the north it is documented intermittently from W 40 to D 13, D 63 to D 36 and at D 68.
The cycle 8 terrigenous clastic interval is primarily shale but a sandstone does crop out at D 42 and persists to D 68.
A five foot conglomerate is found at D 36, and a one foot 90 sandy conglomerate at W 1.
Along south Dry Canyon at D 43 a conglomerate and overlying sandstone are found. This is believed to be similar to the sand found at the same interval at W 30 and the conglomerates, sandstones, and limestone conglomer¬ ates of D 53 and W 14. These terrigenous sediments are lost to the northeast.
Cycle 9
Cycle 9 is that cycle capped by D 3 limestone bed.
This cycle can be seen throughout all of the north area and much of the south. In sections D 40 and D 41 the clastic interval is represented by a thin local conglomerate, and at
D 36 it is found as a 3 foot sandstone. West of this point the cycle gradually increases in size, due largely to covered intervals representing thick basal shales. Cycle 9 lower member at D 34 is a cherty conglomerate that becomes sand¬ stone to the west that is seen in sections D 33 and D 32.
This becomes a conglomerate at D 31 which grades to sand and again to conglomerate at D 30. Cycle 9 terrigenous interval at W 1 is composed of a thick basal covered interval and a thin sandy conglomerate that also is covered to the west.
At D 26, and to the west and north through sections D 25,
D 24, D 22, D 21 and D 20 is characterized by sandstones 91 and conglomerates not more than 5 feet thick. D 17 and D 18 have silt and conglomeratic sands. D 15 and west the cycle continues to increase in thickness and is characterized by a very thick, covered, occasionally shaley interval at the bottom with one or two conglomerates in the upper half of the cycle. These conglomerates become thicker and very prominent in the sections at the north end of the field area, in sections D 9 through D 2.
In the southern part of the field area, cycle 9 clastic member is made up of shale, thin limestones, and covered intervals.
The upper part of Cycle 9 is a persistent limestone capping bed named D 3 (Figure 5). This limestone is thin to the north but thickens into phylloid algae biostrome and small mounds over the central part of the area, forming an easily recognizable key bed to mark the top of the cycles studied in detail. 92
Appendix II
Descriptions of coarse terrigenous elastics at specific localities.
Key for roundness
a - angular
sa - sub angular
sr - sub rounded
r - rounded
wr - well rounded
Key for colors
g - gray
w - white
b - brown
bl - black
r - red
o - orange
y - yellow
t - tan
gr - green 93
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Overlain by brownish £ S z. 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U o cd 0 CO r-4 s-/ CO -C CO & CO u 1 1 •» l i n CO r-4 CM * « • «a ,o a r © c o /■> *4 /—N i/l o CM o CO u 0» vO vO^ 1 50 ON ON ^ 50 E 1 1 i CM cd m • CO • cd E 3 6 E V» U x v» 4M © o ~c © M w m E c ® 7 0 •Si J2 0 N 1 Q C O u "O SEE c 3 w o2 u _o E ÛC < C Î5 i ro CN o c5 CO © i a.2 r-* CM CM r—l i !3 CM *3 & Q P 106 Comments o a grained sandstone. lenses in brown medium Brown conglomeratic sandstone. Local conglomeratic £ c z. O O z _a> a> □ ± a. Q » >» o rH O m o vO i—1 o u N vO vO %- *» CM ^ 60 O ^ cd 60 m ' 60 u H iH U 1 O 1 1 o i 3 m m c m cd o • • CD • CO 60 ✓-s u *. cn O CO U i-l CO v> m m CN o CM CM cn & CM _c vO iH «S m • & U 1 i • ^ i •V CM <* CM CM cd i—i • • • a & w CO rO «> c m U o H CO «A CM O CO m CM o CO ^ 60 CM VO 60 CM m ^ 1 60 6 1 1 CO 1 i 3 m CO ^ CO cd • • cd • CO E 3 6 I 1A '— O x M o 0 <*"* Q> KJ c o ■§ o Size C las •o mean •*» m - D ^ 1O <5 0c M E < c cî i CO c5 d> i c * a-2 CM O r-4 o £ '3 r-l t—1 i—1 £ ° o Q & 107 "1 w I C0 E E o KJ w O a is 2 o o r 0> 0 o a. O s. o /-\ /- O ■ m 5-i m 00 O m U N 00 in 60 ON 00 w vO N*- 60 H 1 i-HI 5 00 1 0 1 1 #* i m u m m 5-1 o • CO • 50 • CO a u 8^ Ë v£) 5-i m CO M So m O «% O CO CO k. CO r-l a CO & w u N-' 1 4i 1 rH Xi 73 rû ^ • c <0 O Ë o 4J ü Ü m O «0 cO 4> r-l r-i a) m 60 E O 60 r-l 1 M! CM M 00 CO 5-1 CO •H (d ^ CO TH E 3 S E M 3 U K VI « O ^ a> M u C O 1 ■go O .22 f ®5 C “O o \J I» co E E 3 ^ o -Û E oc < *c ^ E CO cs co <5 CO 0 ! c a-» E o o a> M o CO i too r-4 i—I ÊO o g a CO to £> iH o w 0 0 "o £ a, O o o o Cd i-4 0 CO CO rû -C C *•o U V» O CM 0 CO I W) E I cd X M0 c 0 C * jo o N 1 -§ -2 "Ü » 5 0 z c ^ EE u 9 3 w § -O E Ctf < c c5 E CO CM CO CM CO il CO P 109 Appendix III Thin section study of limestone, chert and quartzite clasts. 11Q Thin Section Study Limestone Limestone clasts were studied in thin section only with the intention to date them using fusulinids. Fusulinids were found to be exclusively Triticites, a diagnostically Virgilian species (Plate 1). Chert Chert clasts were studied in thin section only with the intention to date them using any diagnostic fauna. Un¬ fortunately, fossil debris was only rarely preserved. Paleo¬ zoic Brachiopod shell fragments and rare Crinoid pieces comprized the majority of the fossil remains (Plate 2). In addition, one late Paleozoic ramose Bryzoan was found in a red chert (Identities by J. L. Wilson) (Plate 3). Quartzite The quartzite clasts were compared to Bliss quartzite obtained from the southern Franklin Mountains near El Paso, Texas. The samples were found to be very simple petrographic- ly. Both the Virgilian clasts and the Bliss quartzite were essentially orthoquartzites with the grains ranging in size from .05 to .3 millimeters. Grain contacts are sutured. Less than 5% opaque minerals were observed. Bliss and Holder Formation quartzites are shown in Plates 4 and 5.