Petrology, Diagenesis, and Genetic Stratigraphy of the Eocene Baca Formation, Alamo Navajo Reservation and Vicinity, Socorro County, New Mexico

OPEN FILE REPORT 125

PETROLOGY, DIAGENESIS, AND GENETIC STRATIGRAPHY OF THE EOCENE BACA FORMATION, ALAMO NAVAJO RESERVATION AND VICINITY, SOCORRO COUNTY, NEW MEXICO

APPROVED : PETROLOGY, DIAGENESIS, AND GENETIC STRATIGRAPHYOF THE EOCENE BACA FORMATION, ALAMO NAVAJO RESERVATION AND VICINITY, SOCORRO COUNTY, NEW MEXICO

STEVEN MARTIN CATHER,B.S.

THESIS Presented to the Facultyof the Graduate School of The Universityof Texas at Austin in Partial Fulfillment of the Requirements for the Degreeof

MASTER OF ARTS

THE UNIVERSITYOF TEXAS AT AUSTIN August 1980 ACKNOWLEDGMENTS

I wish to sincerelythank Drs. R. L. Folkand

C. E. Chapin fortheir enthusiasmtoward this study and

theirpatience in tutoring a novicegeologist in their

respectivefields of expertise. Dr. A. J. Scott provided

many helpful comments concerning lacustrinesedimentation

and thesisillustrations. Discussions with BruceJohnson

contributedgreatly to my understanding of thedistribution

of facies and paleoenvironments within the Baca-Eagar basin.

I would also like to thank my colleaguesin Austin and e Socorrofor their helpful comments and criticisms. Bob Blodgettserved as studenteditor. Finally, I would like to

acknowledge JerryGarcia, who providedunending inspiration

and motivation throughout the course of this study.

Financialsupport for field work and the writing

of this manuscript wa-s generously provided by the New Mexico

Bureau of Mines andMineral Resources. The University of

TexasGeology Foundationalso provided funds for travel

expenses and field work.

This thesis was submitted tothe committee in

June, 1980.

iii PETROLOGY, DIAGENESIS, AND GENETIC STRATIGRAPHY OF THE

EOCENE BACA FORMATION, AIAMO NAVAJO RESERVATION

AND VICINITY, SOCORRO COUNTY, NEW MEXICO

Steven M. Cather

ABSTRACT

The Eocene Baca Formation of New Mexico and. correlative EagarFormation and Mogollon Rim gravels of

Arizonacomprise a redbedsequence conglomerate,of sandstone, mudstone,and claystone which cropsout from near

Socorro, New Mexico, to the Mogollon Rim of Arizona. These sediments were deposited in a largeintermontane basin present in western New Mexico and easternArizona during Laramide time. The Baca-Eagar basin is bounded tothe north by theDefiance, Zuni, andLucero uplifts,to the southwest by the Mogollon Highland, to theeast by theSierra-Sandia uplift, and tothe southeast by the Morenci uplift. The studyarea consists of anapproximately 12 mi’(31kmL) exposureof the Baca Formationlocated along the north end of theGallinas Mountains about 20 mi (32 km) northwestof

Magdalena, New Mexico. In the studyarea, the Baca is about

940 ft (289 m) thick and can besubdivided into three

iv V

informal members (lowerred, middle sandstone, and upper red units) which aregenetically related to large-scale lake-levelfluctuations in a large,closed lacustrine system presentin the Bear-Gallinas Mountains vicinity during Baca time. Three component facies were recognizedwithin the

Baca in thestudy area. These arethe distal braided alluvial-plain,fine-grained lacustrine delta, and lacustrinebasinal facies. The regionalpaleocurrent directionin the Gallinas Mountains vicinityappears to be toward thenortheast. Baca sediments in thestudy area were derivedpredominantly from the Morenci uplift and from the westernflank of thatportion of theSierra-Sandia uplift I which lies to the south of the area.

Three episodes of structural developmenttook place in thestudy area: (a)formation of a Laramide (?) monoclinalflexure in the southeastern portion of thearea,

(b) Neogene extensionalfaulting, drag folding, and dike intrusion, and (c) uplift of the Colorado Plateau in Neogene time, which resulted in theshallow southward dip of the strata in the area.

Lithologiespresent within Baca conglomerateclasts includemetaquartzite, limestone, chert, granite, sandstone, siltstone,schist, mudstone, and volcanics. The majority of thesandstones in thestudy area are lithic arkoses, feldspathiclitharenites, and sublitharenites. Two genetic vi

quartztypes, common and metamorphic quartz,are present in

subequalproportions. . Orthoclase is the dominant feldspar,

followed by microcline and plagioclase. Rock fragment

varietiesinclude metamorphic, chert, mudstone, granite/

gneiss,volcanic, carbonate, siltstone and sandstone rock

fragments.Upsection changes in sandstonemineralogy

suggest the progressive"unroofing" of Precambrian source terranesduring basal Baca sedimentation. Heavy-mineral

suites fromnon-red Baca sandstones show evidence of

alteration by negative-Ehgroundwaters associated with

Miocene dikeintrusion and dolomitecementation. Illite is e the dominant detrital clay mineral in the study area. Four typesof cements (calcite, kaolinite, quartz,

and dolomite)are present thein Baca sediments. The

environment of deposition strongly controlled the diagenetic

historyof the sediments in the study area. The widespread

redcoloration of the.Baca was produced diagenetically,by

intrastrataldissolution of unstableiron-bearing heavy

minerals,precipitation of limonite, and subsequent

dehydration of this limonite to form the hematitecolorant.

Yellow-gray coloration is restricted to permeablesandstones

locatednear mafic dikes, and occurred by oxidationduring

recentoutcrop weathering of reducediron which was

primarilyincorporated within dolomite cements that formed 0 synchronously with dike intrusion in Miocene time. . TABLE OF CONTENTS

TEXT Page INTRODUCTION ...... 1 Study Area ...... 1 Purpose and Methodsof Study ...... 3

GENERAL STRATIGRAPHY ...... ~ 5 Late Cretaceous Crevasse Canyon Formation..... 6 Eocene Baca Formation ...... 7 Correlation and previous work ...... 7 Lower Tertiary equivalents ...... 9 Age and fossils ...... 12 Contacts ...... 15 General description and informalunits ..... 17 Lower red unit ...... 20 Middle sandstone unit...... 21 Upper red unit ...... 21 Oligocene Spears Formation...... 22 . Miocene Dikes ...... 23 Piocene (?) to Holocene PiedmontGravels ..... 24 Quaternary Alluvium...... 25 LARAMIDE TECTONICS ...... 26 Styles of Deformation...... 26 Cordilleran foldbelt ...... 26 f basement uplifts ofthe Laramide Rocky Mountains ...... 29 Colorado Plateau monoclinal flexures...... 33 Baca-Eagar Basin Tectonic Framework...... 35 Mogollon Highland ...... 37 Lucero uplift ...... 38 Zuni uplift ...... 38 Defiance uplift ...... 39 Sierra-Sandia uplift...... 40 Morenci uplift ...... 43 Plate Tectonics andthe Development of the Laramide Foreland ...... 47 STRUCTURAL GEOLOGY ...... 53 Laramide Folding ...... 53 Early Rift Faults. Drag Folds. and Dikes ..... 55 Colorado Plateau Uplift ...... 57 vii viii

Page DEPOSITIONAL SYSTEMS ...... 58 Braided Alluvial-Plain System...... 58 Distal braided alluvial-plain facies...... 59

Facies characteristics ...... ~ 59 Depositional processes ...... 62 Braided alluvial plain model ...... 65 Lacustrine System ...... 68 Lacustrine fine-grained delta facies...... 69 Facies characteristics ...... 69 Depositional processes . : ...... 78 Basinal facies ...... 80 . Faciescharacteristics ...... 80 Depositional processes ...... 80 General characteristicsof the 1acustrine.system ...... 82 Lacustrine model ...... 87 DEPOSITIONAL RECONSTRUCTION ...... 91 Sediment Dispersal Patterns...... 91 Depositional History ...... 95 PETROLOGY ...... 102 Conglomerate Petrology ...... 102 Clast composition ...... 102 Clast morphology...... 106 Sandstone Petrology ...... 110 Quartz ...... 111 Common quartz ...... 113 Metamorphic quartz ...... 114 Feldspar ...... 120 Orthoclase ...... 128 Microcline ...... 128 Plagioclase ...... 128 Rock fragments...... 129 Metamorphic rock fragments ...... 129 Chert ...... 134 Granite and gneiss rock fragments...... 137 Volcanic rock fragments...... 137 Mudstone rock fragments...... 140 Sandstone and siltstone rock fragments... 141 Carbonate rock fragments...... 142 Heavy minerals ...... 144 Mudstone Petrology ...... 150 PROVENANCE ...... 156 e .ix DIAGENESIS ...... 160 Cements ...... 160 Calcite ...... 160 Quartz ...... 171 Kaolinite ...... 178 Dolomite ...... 182 Source of Coloration ...... 187 Red coloration ...... 187 Soil-derived red coloration ...... 187 Diagenetic red coloration ...... 189 Source of red coloration in the Baca Formation ...... 190 Yellow-gray coloration ...... 198 Porosity ...... 199 Primary porosity ...... 199 Secondary porosity ...... 200 Diagenesis Summary ...... 201 PALEOCLIMATE ...... 205 ECONOMIC GEOLOGY ...... 207 * Uranium Occurrences ..: ...... 207 Ore Genesis ...... 208 SUMMARY OF BACA-EAGAR BASIN DEPOSITIONAL SYSTEMS . . 210 SUMMARY AND CONCLUSIONS ...... 212 APPENDIX A: Baca Formation Measured Section..... 218. APPENDIX B: Summary of Point-Count Data for . Study-Area Sandstone Thin Sections ... 229 REFERENCES CITED ...... 231

VITA ...... '. .... 244 X LIST OF TABLES

Table -Page 1 Summary of Previous Studies ...... " 10 2 Summary of Fossil Age Determ'inations.... 13 3 Distal Braided Alluvial-Plain Characteristics ...... 60 4 Lacustrine Fine-Grained Delta Facies

Characteristics ...... ~ 71 Lacustrine Basinal Facies Characteristics. . 81 Summary of Pebble Composition Data..... 103 Distribution of Coloration, Cements, Porosity, and Heavy Minerals in Relation to the Various Baca Facies... 203

LIST OF FIGURES Figure Page 1 Location of study area andBaca, Eagar, and Mogollon Rim gravels ...... 2 2 Map showing outcrop areas of lower Tertiary units in New Mexico and adjacent areas...... li Cloven hoof prints found in a Baca delta- plain sandstone. Lens cap is approximately seven cm in diameter...... 14 Hematitic mudstone-filled ostracod test in sample 49. Field width=0.8 mm, crossed nicols...... 16 Comparison of generalized Baca stratigraphic sections measured inthe Gallinas and Bear Mountains, showing similarity of lithofacies and probable correlationof informal units. Sections are separated by about10 mi (16 km). Bear Mountains section modified from Massingill (1979)...... 19 xi e Figure 6 Map showing distribution and structural styles of Laramide uplifts in western North America, and the location of the Baca-Eagar basin. Modified from Eardley (1962) and Drewes (1978)...... 27 7 Section across Spring Mountains, southern Nevada, showing typical Cordilleran- foldbelt style of deformation. Note imbricate, east-verging thrust plates.... 28 8 Two hypothetical styles of deformation in Laramide Rocky mountain uplifts. (a) Uplift caused by vertical forces acting along range-bounding faults which steepen with depth. Section through Owl Creek .uplift,-.Wyoming. -From Couples and Stearns (1978). (b) Uplift created by horizontal stresses acting along thrust faults. Cross section through theWind River Mountains, 31 Wyoming. Modified from Sales (1968). .... 9 Experimental deformational model showing probable style of Colorado Plateau e monoclinal flexures, using "strata" of kaolinite and modeling clay resting on rigid "basement" of pine board. Davis (1978)...... 34 10 Laramide tectonic framework in eastern Arizona and western New Mexico. Heavy barbed line is 'northeastward.limit. of Cordilleran foldbelt (according to Drewes,1978). Baca- Eagar-Mogollon Rim gravel outcrop areas are also shown. Reference points: S=Socorro, M=Magdalena, D=Datil, Sp=Springerville,SL= Show Low, SC=SilverCity, T=Tucson. Modified from Eardley (1962)...... 36 11 Location of Morenci lineament and distribution of Laramide plutons adjacent to the Baca- Eagar basin.. Presumably the majority of these plutons (circled) were associated with coeval volcanic centers. Reference points same as Figure10, except MoSMorenci. Lineament and plutondata from Chapin and others (1978)...... 44 xii

Figure -Page 12 Distribution of lineaments (stippled) in the Four Corners area which controlled the development of monoclines (heavy lines). The coalescingCow Springs (CS), Organ Rock (OR), and Comb Ridge (CR) monoclines are about the same length and trend the same direction asthe proposed Morenci uplift. Modified from Davis (1978)...... 46 13 Map showing plate positions and relative motion vectors prior to Laramide orogeny. F=Farallon plate, K=Kula plate, P=Pacific plate, NA=North- American plate. Vectors show motionof Farallon (Far) and North American (NA) plates relative to hot-spot frame of reference. Resultant shows direction and magnitude of plate convergence. Spreading centers shown by stippled areas between parallel lines, subduction zonesby heavy barbed lines, thrust belts bylight barbed lines, metamorphic core complexes by fine wavey lines, and arc terranes by short, dashed lines. Modified from Coney (1978)...... 49 14 Map showing plate positions and relative motion vectors during Laramide orogeny.Uplifts shown by small, elongate, closedareas. Other symbols sameas Figure 13. Modified from Coney (1978)...... 50 15 Map showing plate positions and relative motion vectors during post-Laramide, middle-Tertiary ignimbrite flare-up. CP=Colorado Plateau. Dotted areas are large caldera complexes. Other symbols sameas Figure11. From Coney (1978)...... 52 16 Structure index map of study area...... 54 17 Typical vertical section of distal braided alluvial-plain facies. Modified from Johnson (1978)...... 61 xiii

Figure __Page 18 Map showing distribution of bar types and sedimentary structures in proximal and mid- fan facies of Scott glacial outwash fan, Alas ka. From JohnsonFrom Alaska. 67 (1978)...... 19 Typical vertical section of lacustrine fine- grained delta facies, showing upward- coarsening cycles. Modified from Johnson (1978)...... 70 20 Branching horizontal burrows (feeding traces?) in a red, lacustrine sandstone...... 73 21 Baca fine-grained delta deposit exhibiting unusually steeply inclined prodelta foreset beds...... 15 22 Gently inclined prodelta foreset beds overlain by quasi-spheroidally weathering delta front deposits. Hammer handle points in direction of delta progradation...... 76 0 23 Exceptionally large deltaic distributary-channel deposit. Note the low width-to-depth ratio (less than 1O:l) and the highly symmetrical nature, which are characteristic of distributary channel deposits in marine deltas...... 71 24 Map showing crude, concentric zonation of facies in Eocene Lake.Gosiute,Wyoming, during a high stand-(Tipton..Shale Lime).. - Marginal facies is composed of calcareous sandstones and siltstones, somewhat similiar to the lacustrine system rocks in the study area. Surdam and Wolfbauer (1975)...... 89 25 Well-developed parting-step lineation ina fine- grained Baca sandstone. Paleoflow was parallel to trend of mechanical pencil...... 92 26 Comparison of paleocurrent data in western (a,b) and eastern (c,d) portions of study area. The significant portions of the bidirectional rose diagrams (shaded) were determined by comparison with associated unidirectional data. The area was arbitrarily divided along the line between Indian Spring Canyon and Indian Mesa quadrangles...... 94 xiv

Figure Page __ 27 Generalized Baca stratigraphic section in the study area showing relationships between shoreline migrations, depositional systems, and informal units...... 97 28 Generalized stratigraphy of the Green River Formation in Bridger and Washakie basins of Wyoming,. showing alternation of high stands (Tipton and Laney Shales) and low stands (Luman Shale, Wilkins Peak)in Eocene Lake Gosiute. From Surdam and Wolfbauer (1975). - 101 29 Ternary plot of Baca pebble compositions. Data from 15 pebble counts within the study area.. 105 30 Sphericity-form diagram showing shapes of 71 Baca pebbles. Average effective settling sphericity is 0.754. L=long diameter, I=intermediate diameter, S=short diameter. Sphericity-form diagram from Folk (1974)... 107 3 1. Classification of Baca sandstones based on data e from 26 thinsections. Cross shows average composition. After Folk (1974)...... 112 32 Stretched metamorphic quartz grain. Sample 68, crossed nicols, field width=0.75mm. .... 115 33 Schistose metamorphic quartz grain. Note minor amounts of parallel oriented muscovite and :.elongated. quartz cry~stallites. .-SampleC, crossed nicols, field width=0.75 mm. ....116 34Recrystallized metamorphic quartz grain. Note equant shapeof and simple boundaries between crystallites. Sample 32, crossed nicols, €ield width=0.75 mm...... 117 35 Plot of grain size vs. metaquartz/common quartz for 27 Bacasandscnes, showing tendency towards-increasing abundances of metaquartz with increasing grain size...... 119 xv

Figure -Page 36 Plot of metaquartz/total quartz ratios as a function of stratigraphic position. Mean grain size ranges between 1.6 and 2.96.... 121 37 Cavernous dissolution inanorthoclase grain from a fluvial sandstone (sample 48). Second- ary porosity within the grain appears blue, due to impregnation of sample with colored epoxy. Plane light, field width=0.75 mm. ..122 38 Orthoclase grain being replaced by Falcite. Sample 34, crossed nicols, field width= 0.75 mm...... 123 39 Plot of feldspar abundance as a function of stratigraphic position. Mean-grain size of . samples ranges between 1.6 and 2.96. .... 127 40 Quartzose metamorphic rock fragment. Note abundant inclusions of muscovite. Sample 48, crossed nicols, field width=0.75 mm. .... 130 a 41 Argillaceous metamorphic rock fragment which has been ductilely deformed by adjacent grains during compaction. Sample C, crossed nicols, field width=0.75 mm...... 132 42 Plot of abundanceof metamorphic rock fragments as a function of stratigraphic position. Mean grain size ranges between 1.6 and .....2.96 133 43 Chert grain. Sample A, crossed nicols, field width=0.75 mm...... 135 44 Plot of chert abundance as a functionof stratigraphic position. Mean grain size ranges between 1.6 and 2.96...... 136 45 Volcanic rock fragment showing lath-like plagioclase crystals in dark, aphanitic groundmass. Sample 45, crossed nicols, field width= 0.75 mm...... 139 46 Well-developed oolite in an upper-delta sandstone. Note both concentric and radial structures. Sample 73, plane.light, field width=0.75 mm, ...... 143 xvi

Figure -Page 41 Typical heavy-mineral grain mount from a red Baca sandstone (sample 48). Note dominance of magnetite/ilmenite/specular hematite. White grains are leucoxene. Field width= 0.8 mm...... 146 48 Typical heavy-mineral suite from a yellow Baca sandstone (sample 56), showing dominance of limonite-stained leucoxene grains. Note subhedral zircon grain. Field width=0.8 mm . . 147 49 Hexagonal biotite observed ina heavy-mineral grain mount from sample 48.Width of field =0.75 mm...... 149 50 Churned, disrupted texture typical of Baca lacustrine mudstones, whichwas probably produced by intense bioturbation. sample F, crossed nicols, field width=3.0 mm. . - . 152 51 Large, spar-filled void spaces in a sandy lacustrine mudstone (sample 8), which are indicative a very early, pre-compactional cementation. Crossed nicols, field width= 3.0 mm...... -1.54 52 Plot of metaquartz/total quartzratios and abundances of feldspar, chert', and metamorphic rock fragments within Baca sandstones as a function. of stratigraphic position. . Samples were taken froma section measured by the writer in the study area.Mean grain size of the sandstones rangesbetween 1.6 and2.9#...... 158 53 Plot of metaquartz/total ratios and abundances of feldspar, chert, and metamorphic rock fragments within Baca sandstonesas a function of stratigraphic position. Samples were taken from Massingill's (1979) measured section in the Bear Mountains vicinity. Mean grain size varies widely, therefore caution is :advised in the interpretationof this data...... 159 xvi i

0- Figures Page- 54 Well-developed calichified paleosol showing numerous tube-shaped rhizoconcretions. ... 161 55 Baca caliche as seen in thin section. Note wide range in calcite crystal sizes and the presence of numerous grains which exhibit hematitic clay cutans. Sample 34, crossed nicols, field width=0.8 mm...... 163 56 Coarsely crystalline spar cement in an upper- delta sandstone (sample C). Crossed nicols, field width=0.8 mm ...... 165 57 Coarsely crystalline sparry calcite cement in cigar-shaped sandstone concretion (sample 49). Loosely packed fabricis indicative of very early cementation (prior to compaction). Crossednicols, field width=0.8 mm...... 166 58 Hematite-stained fine-grained sparry calcite in a frontal-splay sandstone (sample13). Note poorly compacted nature of this early- cemented lacustrine sandstone. Crossed nicols, field width=0.75 mm...... 169 59 Syntaxial overgrowth on detrital quartz grain Sample 45, crossed nicols, field width= 0.75 mm...... 172 60 Flamboyant megaquartz cement. Sample M50, crossed nicols, field width=0.8mm...... 173 61 Inhibition of syntaxial quartz overgrowthby coarsely crystalline sparry calcite cement. Sample B, crossed nicols, width of field= 0.75 mm...... 174 62 Plot of authigenic quartz content in studyarea sandstones as a function of stratigraphic position...... 176 63 Plot of authigenic quartz content in Baca sandstones as a functionof stratigraphic position. Data from point counts made by the writer on thin sections from Massingill's (1979) measured section in the Bear Mountains area...... 177 xviii

Figure Page- 64 Authigenic kaolinite. Bluish tint is due to impregnation of kaolinite microporosity with blue epoxy. Sample 48, crossed nicols, field width=0.15 mm...... 179 65 Scanning electron photomicrograph of vemicular stacks of kaolinite crystallites on a quartz sand grain. Sample 56, 5,OOOX...... 180 66 Dolomite cement showing alternating iron-rich and iron-poor zones. Sample B, crossed nicols, field width=0.75 nun...... 183 67 Dolomite rhombohedron replacing adjacent quartz grain. Sample B, crossed nicols, field width= 0.75 mm. ... : ...... 184 68 Generalized map ofthe study area showing coincidence of occurrence of dikes and areas in which Baca sandstones are yellow-gray in color (stippled areas)...... 186 e 69 Distribution of coloration for 52 Baca sandstone samples fromthe study area. Values were determined by comparison with MunsellSoil Color Chart. Color terminology after Folk (1969)...... 188 70 Hematite flecks uniformly distributed on Baca sand grains, except at contact points between grains. Sample P, plane Light, field width= 0.15 mm...... 192

71 Scanning electron photomicrographof euhedrally terminated hematite crystals onquartz sand grain. Sample P, 7,OOOX...... 193 72 Change of color with geologic are shown on a plot of hue vs. saturation. Third color dimension oflightness shown by the numbers 4/ etc. and degree of darkening of the squares. C, initial unweatheredsand; A, Saharan sands, data from Alimen;Sf, Simpson Desert, surface fluff of Holoceneage; Sm, xix

Figures Page - Simpson Desert, modal color en masseof fine sands; Sp, Simpson Desert,czor of hematite- clay cutans in pits, formed during Pleistocene lateritization; P, other Pleistocene sediments Hm, color of powdered pure hematite; +Py M, color of Mesozoic and younger Paleozoic red rocks; Po, Older Paleozoic and Precambrian rocks. Processes that create these changes shown by the "evolutionary" arrows. (Modified from Folk, 1976)...... 197 73 Diagenesis flowchart for the Baca sediments within the study area...... 202 74 Plot of grain size vs. percent cement plus primary porosityzowing tendency towards looser packing in the fine-grained lower- delta and lacustrine basinal sediments. Note the presence of yellow-gray coloration only in the coarser grained, non-lacustrine sandstones...... 204 a 75Hypothetical model for the distribution of facies and paleocurrents in the Baca-Eager basin during early Baca time (high lake stand). Letters in parentheses indicate paleocurrent data sources. J=Johnson (1978), P=Pierce (19791, S=Snyder (19711, C=Cather (this study). Reference points same as Figure 10...... 211 INTRODUCTION

Study Area

The Eocene Baca Formationof New Mexico and

,correlativeEagar Formation and Mogollon Rim gravels Of

Arizona .comprise a redbedsequence conglomerate,of

sandstone,mudstone, and claystone which crops outin

discontinuous exposures -along a west-trending belt from near

Socorro, New Mexico to the Mogollon Rim ofArizona. The

studyarea consists ofan approximately 12 mi2 (31.1 km 2 )

exposure.of the Baca Formation atthe north endof the

GallinasMountains about 20 mi (32 km) northwest of

Magdalena, New Mexico (Fig. 1). The easterntwo-thirds of

thestudy area is located on the southernportion of the

AlamoBand Navajo Reservation.Quality of exposureranges from excellently exposed stream channelincisions to very

poorlyexposed badlands topography. The best exposuresare

in the eastern portion of the studyarea. Maximum relief in

thearea is about 400 ft (123 m), and theaverage elevation

is approximately 6700 ft (2061 m). The area lies within a

semiaridclimatic zone and receives most of its annual

precipitation during July andAugust.

1 2

0 Baca Formation n Eagar Formation

Mogollon Rim gravels

Figure 1. .Location of study area and Baca, Eagar, and Mogollon Rim gravels. 3

Purpose and Methods of Study

The primaryobjectives of this study were: a) to

describethe petrology and provenance of the Baca Formation

in thestudy area; b) to determinethe diagenetic history

of the Baca sediments; c) todefine the environments of

deposition and dispersal patterns of the Baca.sediments in

thestudy area, and; d) todelineate and summarize the

:tectolric.-framework-and paleoenvironments forthe entire

Baca-Eagar basin.

Pebble counts,petrographic examination of

sandstones,and petrographic and x-raymineralogic study of

mudstones and claystones were utilizedto describe the

petrology and determinetheprovenance of the Baca

sediments. Data for reconstruction of thediagenetic

history of ,the sediments was provided by optical and

scanning electron microscopy.

Detailed mapping at a scale of 1:24,000 (Plate l), paleocurrentmeasurement, and the measurementoneof

complete and several partial stratigraphic sections provided

thebasis forreconstruction of paleoenvironments and

sediment dispersalpatterns. Depositional environments were

interpreted'using thefollowing criteria: litho'facies'

geometry, lateral and verticalvariation in grain size and . sedimentary . structures,nature of contacts between e 4 lithofacies,petrographic data, and fossils. I Interpretation of the tectonic paleo-and

environmental history of the Baca-Eagar basin was based both

on my own data and that.ofother workers. I have examined

ona reconnaissancelevel the majorityof the exposures

alongthe Baca-Eagar outcrop belt. I' have reliedheavily

upon Johnson (1978) for his highly accurate facies analyses,

especiallyin the ,westernportion of .the outcrop belt.

Addi.tiona1. pal.eorurrent~~.data was drawn from Snyder(19711,

Pierce and others(19791, and Johnson(1978). e GENERAL STRATIGRAPHY

Upper Cretaceousthrough Quaternary strata are

exposed along the northernflank of theGallinas Mountains.

The oldestrocks.present are the sandstones and shales of

theLate Cretaceous Crevasse Canyon Formation. These are

overlain by the Eocene Baca Formationwhich, in the study

area,consists of a redbed sequencesandstones,of

.. -mudstones; "claystones,and minor conglomerates. The Baca is

overlain by thevolcaniclastic sedimentary rocks, ash-flow

tuffs, and lavastheOligoceneof Spears Formation. a Numerous north-trendingmafic dikes were emplacedduring Miocene time in the easternportion of the studyarea.

Extensivepiedmont gravels of late Pliocene(?) to Holocene

agerepresent coalesced alluvial-fan deposits resulting from

the southward erosional retreat of the steep northernflank

of theGallinas Mountains. Quaternary sands and gravels are

present in andalong modern ephemeral stream channels and as

valley fill. Only the Baca Formation was studied in detail.

5 e 6 LateCretaceous

Crevasse Canyon Formation

The Crevasse Canyon Formation wasnamed by Allen and

Balk (1954) for exposures on th'e west side ofthe San Juan

Basin. It is equivalentto the Dilco and Gibson Members Of

the Mesa Verde Formation and occupies the stratigraphic

" .. . . interaal.between .the..Gallup..and .. Point lookout Formations.

Tonking (1957) was the first to use the term Crevasse Canyon

inthe Bear-Gallinas Mountains vicinity. Tonkingmeasured a 0 section1,052 ft (324 m) thickin the Crevasse Canyon Formation directlynorth of the studyarea. Winchester

(1920) collectedfloral remains from theCrevasse Canyon

which were identified by Knowlton as being of Montanan age.

Gardener (1910)tentatively dated the formation as either upperColoradan or lower Montanan.

The Crevasse Canyon Formationconsists of a sequence

of yellowish-gray,fine- to medium-grained sandstones,

medium-gray to blackcarbonaceous shales, and a few thin

coalbeds. The formation was desositedin littoral and

paludalenvironments as theLate Cretaceous epicontinental

sea withdrew northeastward from west-central New Mexico for

thelast time (Tonking,1957). 7

Eocene

Baca Formation

Inthe study area, the Baca Formation overliesthe

Crevasse Canyon Formationand, inturn, is overlain by the volcaniclastic Spears Formation. TheBaca records the first

Tertiary sedimentat.ion in the area.

Correlation and Previous Work

Winchester (1920) publishedone of the first geologic maps of west-central New Mexico. In this bulletin, he proposed the name "DatilFormation" for an approximately

2000-ft(615 m)-thick sequence of Tertiaryvolcanic and non-volcanic,rocks exposed in westernSocorro County.

Wilpolt and others (1946) definedthe Baca Formation asthe basal non-volcanicportion of Winchester's Datil Formation.

However, the Baca section which they measuredand described was in the Joyita Hills, and not wherethey defined the type section.' The Baca type section is 684 ft (210.5 m) thick and is locatedin the upper part of BacaCanyon on the eastern,flank of the Bear Mountains inwestern Socorro

County. Wilpolt and others(1946) also extended the term

Baca toequivalent rocks exposed in the Joyita Hills 8 vicinity about 30 mi (48 km) east of the type locality. Wilpolt and Wanek(1951) further extended the Baca southeast of the type locality into the Carthage area. Within New Mexico, the Baca Formation has been tracedwestward along a discontinuous outcrop belt by Tonking (1957), Givens (1957), Willard and Givens (19581, 'Willard and Weber (1958), Willard (1959), and Foster(1964). Baca-equivalent rocks have also been described in east-centralArizona. The Mogollon Rim gravels were interpreted by Hunt (1956) to be Late Cretaceousto Eocene in age. Pierce and others (1979) state that these gravels were derived from Precambrian sources to the southwest duringEocene-Oligocene time. The Baca-equivalent sandstones and conglomerates of the Eagar Formationwere named and described by Sirrine (1956). In his regional Baca-Eagar study, Johnson(1978) was the first to correlate the Baca, Eagar, and Mogollon Rim gravels. Based on the continuity ' of gravel compositions, paleocurrent indicators, lateral facies variations, and the configuration of the early Tertiary tectonic framework in the southwestern United States, I strongly support this correlation. Very few previous investigations within the study area have been carried out. Tonking (1957) studied the geologyof the Puertecito quadrangle, an area which 9

encompasses theeastern two-thirds of thestudy area. Givens (1957) studied the western one-third of thestudy

areain his work on the Dog Springsquadrangle. Both of these studies were reconnaissance in nature. .. Recently, much detailed work hasbeen performed in

areasadjacent to the study ' area. Mayerson (1979) and

Jackson (inprogress) have studiedand. mapped in detail

areas to the east and northeast of thestudy area,

respectiv~ely.Laroche (.in progress)has studied and mapped

a portion of thevolcanic rocks exposed inthe Gallinas

Mountains. Snyder(1971) studied the Baca exposedbetween

the Rio Grandeand the Arizona-New Mexico state line,

includingthe Baca exposednorth of theGallinas Mountains.

A summary of previous work is presented in Table 1.

Lower Tertiary Equivalents

Several lower Tertiary formations are present in New

Mexico (Fig. 2). Theseinclude theRaton, San Jose,

Nacimiento, Galisteo, Cub Mountain, McRae, and Love'Ranch

Formations.These units were deposited in isolated basins

which were created by the Late Cretaceous to Eocene Laramide orogeny. 10

Table 1

Summary of Previous Studies

Location ofWorkers Study

Carthage Gardener (1910) Wilpolt and Wanek (1951) j_ ! Joyita Hills Wilpolt and others (1946) Bear Mountains Winchester (19201 Wilpolt and .others (1946) Tonking (1957) Potter (1970) Massingill (1979) Gallinas Mountains Tonking (1957) Givens (1957) Mayerson (1979) Datil Mountains Willard and Givens (1958) Schiebout, J.A. (ongoing research) Quemado Willard and Weber (1958) Schiebout, J.A. (ongoing research) Schrodt, K. (in progress) Red Hill O’Brien (1956) Springerville Sirrine (1956) Mogollon Rim Hunt (1956) Pierce and others (1979) Regional Studies Willard (1959) Snyder (1971) Johnson (1978) 11

Figure 2. Map showing outcrop areas of lower Tertiary units in New Mexico and adjacent areas. . 12

Age and Fossils

The Baca andEagar Formations and the Mogollon Rim gravelshave been assigned ages ranging from LateCretaceous toearly Oligocene. Most agedeterminations, however, give an Eocene ageforthese units. The Laramide orogeny occurredbetween about 80 and 40 m.y. ago(Coney, 1971). The sediments of the Baca and equivalentunits apparently were derived from Laramide uplifts, thus lending support for a Late Cretaceous to Eocene agefor these sediments. In the

Joyita Hills, a 37.1 m.y. radiometric date has been obtained from a volcanic clast in the Spears Formation which overlies the Baca(Weber, 1971). Therefore, in that area, the Baca must be olderthan earliest Oligocene. Age determinationsfor the Baca andEagar Formations and the

Mogollon Rim gravels are summarized in Table 2.

No age determinations for the Baca havebeen made in thestudy area. The writer discoveredseveral fossil cloven-hoof printsin the lower red member of the Baca at the Nw 1/4, SE 1/4, SE 1/4, sec. 31, T.2N., R.6W. (Fig.

3). Photographs of theseprints were shown to several vertebrate.paleontologists, including Dr. Wann Langstonof theUniversity of Texas at Austin and Dr. Beryl E. Taylor of the American Museum of Natural History, New York. As yet, no one has been able toidentify or date these trace Table 2

Summary of Fossil Age Determinations

Author Unit Location Unit Author Age How Dated

Pierce and MogollonMogollonRimradiomekric Rim Eocene- others (.1979) gravels Oligocene Sirrine (1956) SpringervilleEagar pollen,spores L. Cretaceous Belcher, R. SpringervilleEagar rhinocerosEocene tooth (pers. comm, , Johnson, 1978)

Gardener (.1910) Baca Carthaye Palaeosyops,middleEocene rhinoceros- like mammal Schiebout, J'.A. Baca Quemado,Datil, titwothere, Eocene-early (pers. comm.1 Mountainstuetle,croc- Oligocene odile, lizard

Snyder (1971 Baca DatilMountains Protoreondon ~- lateEocene pumu'lus, sheep- like mammal Chaiffetz (1979) Baca BearMountains pollen, spores Eocene Figure 3. Cloven hoof prints found in a Baca delta-plain sandstone. Lens cap is approximately seven cm in diameter. 15

fossils. Many ostracods were observedwhile examining thin

Sections from thestudy area (Fig. 4). As I will discuss

later, theseostracods provide additional evidence of a

largelylacustrine environment of depositionfor the Baca,

however, I havenot been able to use them fordating the

unit.

The onlyother fossil remains observed consist of

carbonaceousand siliceousplant and wood fragments. The

Galisteo Formation,which is exposed south of Santa Fe, has

beendated as Paleocene or Eocene on thebasis of fossil

plantremains (Lee and Knowlton,1917). Such datinghas not

yet been attempted in the Baca.

Contacts

The contact of the Baca withthe subjacent Crevasse

Canyon Formation is generallypoorly exposed in thestudy

area. Where visible, it is a low-relieferosfonal

unconformity of little or no angularity. The lowercontact

of the Baca wasmapped at the base of thefirst reddish

sandstoneor mudstone. Rare paleocalichehorizons were

observed to bedeveloped inthe uppermost portion of the

Crevasse Canyon directly below thecontact with the Baca

Formation.These paleocaliches were produced by soils e formed in arid orsemi-arid climatic conditions during the 16

Figure 4. Hematitic mudstone-filled ostracod test in sample 49. Field width=0.8 mm, crossed nicols. 17

hiatus between final deposition of Crevasse Canyon sediments and the inception of Baca sedimentation. In some areas, a poorly exposed pebble lag appearsto be present along the Baca-CrevasseCanyon contact, perhaps representing a paleo-deflationsurface which developed on theuncon- solidatedCrevasse Canyon sediments prior toBaca deposition. In areas outside the study area, the lower contact is reported to beunconformable to apparently

conformable '(Willard, 1959; Tonking, 1957; Potter, 1970). The upper contact of the Baca with the Spears Formationgenerally is gradational, the contactbeing e defined at the first occurrence of megascopically visible volcanicmaterial. The gradation between Baca- and Spears-type lithologies takes place within a few meters.

General Description and Informal Units

In the study area, the Baca Formation consists of a redbed sequence of sandstone, mudstone, claystone, and minor conglomerate.One complete stratigraphic section was measured in the eastern portion of the study area. It has a total thickness of 941.5ft (289.7 m), and is presented in Appendix A. e 18

In the Bear Mountains area, Potter (1970) divided the Baca into three informal members which he termed, in ascending order, the lower red unit, the middle sandstone unit, and the upper red unit. I found this terminology applicable in the study area, but I must emphasize that these informal units are genetically related to lake-level fluctuations in a large Eocene lacustrine system presentin the Bear-Gallinas Mountains area and are not applicableto

ga the preaominantly fluvial Baca-Eagar-rocksto the west or to the Baca exposed east of the Kio Grande. This subject will be discussed in detail in later sections. On the basis of a thickness,stratigraphic position, and similarity of lithofacies,the units defined in the study area are probably directly correlative with the units of Potter (1970) in theBear Mountains (Fig. 5), although the paleo-continuity of these units has yet to be demonstrated due to.late Tertiary structural complications and cover. In his work in the Bear Mountains area, Massingill (1979) recognized the same three informal unitsin the Baca asdid Potter. However, Massingill chose to abandon Potter's terminology in favor of, in ascending order, the

conglomeratic, the gray sandstone, and the red mudstone members. I favor retention of Potter's terminology onthe basis of the following criteria: a) the lower red unit in

e thestudy area (equivalent to Massingill'sconglomeratic 19

ChLLIrnS mUNPAINS SECTION

Figure 5. Comparison of generalized Baca stratigraphic sections measured in the Gallinas and Bear Mountains, showing similarity of lithofacies and probable correlation of informal units. Sections are separated by about 10 mi (16 km). Bear Mountains sectionmodified from Massingill (1979). 20

member)contains very little conglomerate, as willbe discussed in a later section. The conglomeratesin the Bear

Mountain area are present dueto the relative proximity of the source area, thus the term lower red unit is more descriptively viable on a regional level; b)the coloration of! Massingill's gray sandstone member reflects only the diagenetic history of the sediment. Indeed, the middle sandstone unitin the study area is often red. The lower red, middle,sandstone, and upper red unit terminology will be utilized only for easeof reference and to facilitate generalized lithologic descriptions,as below. e For the more detailed descriptions and interpretations in later sections, the principlesof genetic stratigraphy will be employed to subdividethe formation into component

depositional systeq as defined by Fisher and Brown(1972).

Lower Red Unit. The lower red unitis 323 ft (99.4 " m) thick at the measured section and is composed of red, gray, and yellow sandstones intercalated with red mudstones and claystones. The ratio of sandstoneto mudstone is about 1:l. Minor amounts of conglomerate are present as thin lenses within sandstone units. Sandstone beds in the lower red unit average about18 ft (5.5 m) thick andrange between a few inches to 25 ft (7.7 m) in thickness. Some of the 0 thicker beds canbe traced for as much as a mile (1.6 km) 21

along strike. Mudstone and claystone beds averageabout 10

ft (3.1 m) thick and rangeup to 50 . ft (15.4 m) in

thickness. Mudstone unitsare calcareous and effervesce in

cold,dilute hydrochloric acid.

Middle SandstoneUnit. The middle sandstoneunit is

246, ft (75.7 m) thickat the measured section and consists almostentirely of fine- to coarse-grainedyellow, gray, or

red sandstone. The s-andstones are most oftenhorizontally

laminated,but frequently display well-developed, medium- to

large-scaletrough crossbedding and rare tabularcross- e bedding.Mudstones and claystonesare very rare in the middle sandstoneunit, occurring primarily thin,as

discontinuous clay drapes within the sandstones.

Upper ".Red Unit. The upper red unit is very similar in appearance to the lower red unit. At the measured

section, it consists of a 372.5 ft (114.6 m) sequenceof red,yellow, and graysandstones intercalated with red

mudstonesand claystones. Conglomerates are rare and occur

asthin lenses within sandstone units. The upper red unit

contains less conglomeratethan the lower. This 'has also

beennoted in the Bear Mountains area(Potter, 1970;

Johnson,1978; Massingill, 1979). The sandstone to e mudstone ratio is about 1:l. Mudstones areusually 22

calcareous. Both sandstone and mudstone beds tend to be thicker in the upper red unit than in the lower. Sandstones

range in thickness to as much as about 100 ft (30.8 m): mudstones can be as thick as80 ft (36.9.m).

Oligocene '

Spears Formation

Tonking(1957) divided all but the basal Baca portion of Winchester's (1920) Datil Formation into' three a members. These members are, in ascending order, the Spears, HellsMesa, and La Jara Peak. The Spears Member was upgraded to. formational status by Chapin and others(1978).

A section of Spears measured by Tonking (1957)in the Bear Mountains, about 12 mi (19.4 km) east of the study area, indicates a thickness of 1,340 ft (412.3 m). The Spears Formation has been dated radiometrically with dates ranging between 37 and 33 m.y.B.P. (Chapin and others,1978). The Spears is composed of a sequence of grayish- purple volcaniclastic sandstones, conglomerates, and mudflow deposits, with interbedded lavas and ash-flow tuffs in the upperpart. The formation is latitic to andesiticin compositionandrepresents theearly intermediate- a composition phaseof the Datil-Mogollon volcanic field 23

(Chapinand Seager,1975). The Spearscontains more

volcanicflows tothe south, indicatingeruptive centers in

thatdirection. Trends of mudflow-filledpaleovalleys in

the basal Spears directly south of the study area indicate a ,. generally northward transport'direction.

Miocene

Dikes

I Numerous north-trendingdikes are exposed inthe 0 easternportion of thestudy area. These.dikes range in width up to about fivefeet (1.5 m), dipnearly vertically,

and in some cases may be traced foras much as 1.5 mi (2.4

km). The dikes,were emplaced alongfaults and fractures

associated with the openingof the Rio Grande rift. Potassium/argondating of two dikesin the Bear Mountains vicinity yielded dates of 24.3-kO.8- and 24.W-0.6- m.y.B.P.

(C. E. Chapin,unpublished dates).

Dike rocksare generally greenish-black and range

texturally from aphaniticvery-fine-grainedto holo-

crystalline.In the Bear Mountains area, these dikerocks

havebeen classifiedasbasaltic andesites (Massingill,

1979). The dikes commonly exhibit fine-grained chilled

margins. Baked zones are common in the country rock 24 adjacentto the dikes, but alteration is usuallylimited to azone less than a foot (0.3 m) thick.

Pliocene (?) to Holocene

L PiedmontGravels

Extensive piedmont gravelsare present throughout thecentral and westernportions of thestudy area. Two units of differingages were mapped.The older unit is topographicallyhigher than the youngerand forms isolated

depositscapping the higher areas north of ' theGallinas

Mountainsescarpment. The older piedmont gravelranges up toabout 200 ft (61.5 m) in thickness. The younger gravel occupies a large,continuous area in thecentral portion of thestudy area. The difference in elevation of thebases of the two units is as much asabout 100 ft (30.7 m). However, thiselevation difference appears to decrease westward. The thickness of the younger unit is lessthan that of the older, and ranges up toabout 50 ft (15.4 m). Both units locally overlie the Baca with angular unconformity.

Grain size in both units ranges from coarsesand to cobbles.Clast composition of both units is identical, consistingpredominantly of subequal proportions of Spears, aells Mesa, and A-L Peak Tuff lithologies.Pebble 25 imbricationindicates flow was generallynorthward. Tnese piedmont gravelsrepresent.coa1esced alluvial-fan deposits related to the erosional retreat of the steep northern flank of theGallinas Mountains. Similar piedmont gravelsin westernSocorro county range in age from late Pliocene(?) to

Holocene (J. Hawley,,1979, oral commun.).The difference inelevation of the two gravelunits indicates a periodof erosion prior to deposition of the younger unit.

Quaternary

Alluvium

Locally derived sands and gravels are present in and nearpresently active ephemeral stream channels and as valley fill in the northern portions of the study area. LARAMIDE TECTONICS

Styles of Deformation

The Baca-Eagar basin is oneof a multitude of basins

and' upligts whichformed during theLate Cretaceous to late

EoceneLaramide orogeny in western North America (Fig. 6). Threegeneral structural provinces and associatedstyles of

deformationcan be delineated. These are the 6, thin-

skinned",low-angle thrusts and folds of theCordilleran ! i foldbelt,the asymmetric basement-cored anticlinaluplifts 0 of theclassic Laramide Rocky Mountains, and themonoclinal flexures of the Colorado Plateau.

Cordilleran Foldbelt

The Cordilleranfoldbelt of NorthAmerica extends

fromGuatemala to Alaska(Drewes, 19781,and was active

predominantlyduring Late Jurassic to late Eocene time. The

dominant style of deformation is one of imbricateeast-

verging,low-angle thrustingwith associated folding within

allochthonousplates (Fig. 7). With thenotable ,exception

of those in southernArizona and New Mexico, the thrust

sheets generally did not involve Precambrianbasement rocks.

Amounts of supracrustal transport throughout the Cordilleran

26 27

Asymmetricalbasement- ColoradoaD Plateau cored anticlinal monoclinal flexures Cordilleran foldbelt uplifts uplifts

.... 7 ...... Colorado Plateau Eaatward limit of boundary Cordilleran foldbelt

Figure 6. Map showing distribution and structuralstyles of Laramide uplifts in western North America, and the location of the Baca-Eagar basin. Modified from Eardley(1962) and Drewes (1978). 28

W E

Figure 7. . Section across Spring Mountains, southern Nevada, showing-typical,Cordilleran-foldbelt style of deformation. Note. imbricate, east- verging thrust plates. 29

foldbeltprobably averaged between 30 and 60 mi (50 to 100

km), and may be as much as 120 mi (200 km) in some regions

(Drewes, 1978).Structures within foldbeltthe are

indicative of compressionaldeformation, although the

mechanisms by which thestresses were transmitted and the

dynamics 'of thrust plate movements are highly controversial.

Thrustingin Nevada andUtah, west of theclassic Laramide

Rocky Mountains, ceased in LateCretaceous time (Armstrong,

19681,. but major thrust movements inthe Cordillera of

Mexico and Canada persisted until late Eocene time(Coney, 1976).

Basement Uplifts of the Laramide Rocky Mountains

The geographicdistribution, as well as the style of

deformation,of the classic Laramidebasement uplifts is

distinctlydifferent from Chat of theCordilleran foldbelt;

the former was favoredon the thincratonic shelf while the

latter was favored inthe Paleozoic miogeosyncline (Coney,

1978). The asymmetricalbasement-cored anticlinaluplifts

of the Laramide Rocky Mountains form a belt which extends

from southern Montana to southern New Mexico. Inthe

northernportion of thebelt, the uplifts trend generally

northwestward,while inthe central and southernportions e they trend northward. The upliftsusually contain large 30

exposures of Precambrian basement and are often bounded on one or more sides by thrust faults. Structural relief between uplifts and adjacent basins may be as much as 49,000

ft (15,000 m) (Coney, 1976). Theoriesconcerning the origin of the basement uplifts can be broadly divided into two schools of thought: vertical and horizontal tectonism (Fig. 8a, b). Proponents of vertical tectonism suggest that the uplifts were produced bydifferential vertical movements of high-angle-fault- bounded basement blocks driven by phase changes or plastic flowage in the lower crustor upper mantle, with resultant drape folds and subsidiary thrusts in the overlying strata (Stearns,1978; Palmquist, 1978; Woodward, 1976). Arguments in favor of vertical tectonism are largely based onevidence against the alternativehorizontal-stress- generatedbasement-upthrust model, such as: (a) space problemsresulting from relative thrust-block movement

(Stearns,1978; Palmquist, 1978); (b) necessarymajor wrench faults bounding lateral edges of thrust blocks have notbeen observed (Stearns, 1978); (c) non-uniform direction of thrust-block motions throughout the Laramide Rocky Mountains suggeststhat the uplifts were not formedby unidirectionalhorizontal compressive stresses (Stearns, 1978). Proponents of vertical tectonism believe the thrust .- faults in the Laramide Rocky Mountains,steepen with depth.

I 31

S N'

.. E /--\ a

0 10 km I ,.

.Figcrre.:.8L.. ..Two hy.po.thetica1 styles of deformation in Laramide Rocky Mountain basement uplifts. (a) Uplift caused by vertical forces acting along range-bounding faults whichsteepen with depth.Section through Owl Creek uplift, Wyoming. From Couples and Stearns (1978). (b) Uplift created by horizontal stresses acting along thrust faults. Cross section through Wind River.Mountain.5,Wyo-' ming. Modified from Sales (1968). 32

The contrasting point of view is the that uplifts of the classic Laramide foreland were created by.horizonta1 stresses, either horizontally directed compression (e.g. Coney, 1976; Thom, 1955) or a combination of horizontally directed compression and left-lateral shear (Sales, 1968). These horizontal stresses resulted in uplifted basement- cored blocks which were upthrust along moderate-angle thrust faults or were uplifted due to underthrusting by another thrust block. Evidence for creation of basement uplifts by horizontal stresses includes: (a) rapid oblique subduction

Of the Farallon plate under the North American plate resulted in strong southwest-northeast-directed compressive stresses throughout the foreland during Late Cretaceous to lateEocene time (Coney, 1976); (b) COCORPseismic profiiing indicates that the southwestern boundary.. of the Wind River uplift, Wyoming,-is a thrust fault which extends at about a .30.-to 35:.degree-angle to a depth at of least 24 km, which suggests horizontal motions were more important than vertical in thedevelopment of the uplift (Smithson and others, 1978); (c) thrust faults are widespread throughout theLaramide Rocky Mountains structural province; (dl superincumbent. folding commonly associated with basement upliftsis easily explained by large-scale horizontal movements of the basement. 33

Deformation in the Laramide Rocky Mountains foreland

was initiatedin Late Cretaceous time andended by late

Eocene time (Tweto, 1975), and may accountfor as much as 60

mi (100 km) crustalof shortening (Coney, 1976), but generally more modest amounts are visualized.

i i

ColoradoPlateau Monoclinal F lexures

The generallyflat-lying strata of theColorado

Plateau are locally interrupted by largemonoclinal flexures

of Laramideof age (Kelley,1955) (Fig. 9). The monoclines e arecurvilinear, often branching inshape and may be hundredsof kilometers long. Trends are variable, but Davis

(1978)recognizes two dominanttrend directions of N20°W and

N55'E. Structuralrelief is generally less thanthat in the

classic Laramide Rocky Mountains to the north and east.

.Precambrianbasement. is usuallynot exposedalong the monoclines.Monoclines of the ColoradoPlateau owe their

originto draping of strata over basement reliefcreated by

high-anglereverse faults formed inthe northeast-southwest

Laramide compressive stress field (Davis,1978). Davis also

statesthat these monoclines .can be fit to a systematic,

interdependent network of regionallineaments which are

inferredto represent basement-fracture zones which were e re-activatedasreverse faults duringLaramide time. a 34

Figure 9. Experimental deformational model showing probable style ofColorado Plateau monoclinal flexures, using "strata"of kaolinite and modeling clay resting on rigid "basement" of pine board. Davis (1978).

..: 35

The reasons for the differences in sty,le and degree of deformation between Che Colorado Plateau and the Laramide

Rocky Mountains is not well understood. The greaterdegree ofdeformation in the Laramide Rocky Mountains may have occurred due to the presence of previously deformed basement producedduring uplifting-of the Late Paleozoic ancestral

Rocky Nountains in that area (Coney, 1978). . Basement uplifts may initially develop as monoclines which are later cut by faultsas the upliftcontinues to rise (Woodward and others, 1972).

Baca-Eagar Basin Tectonic Framework

The Baca-Eagar basin was a west-trendingstructural depression which ' formed in the southwestern portion of the

ColoradoPlateau during Paleogene time. The sediments depositedin the Baca-Eagar basin. were derivedfrom surrounding Laramide uplifts(Fig. 10). Examples of all threestyles of Laramide uplifts (Cordilleranfoldbelt uplifts,asymmetrical basement-cored anticlinal uplifts, and

ColoradoPlateau monoclinal flexures) are'presentadjacent to the basin. 36

Figure 10. Laramide tectonic framework in eastern Ari'zona and western New Mexico. .Heavy barbed line is northeastward limit of,Cordiller'anfolabelt (according to Drewes, 1978). Baca-Eagar- . . Mogollon Rim gravel outcropareas are also. shown. Reference points: S=Socorro, M=Magda- lena, D=Datil, Sp=Springerville, SL=Show Low, SC=Silver City, T=Tucson. Modified from Eardley (1962). 37

Mogollon Highland

The MogollonHighland southernof Arizona and southwestern New Mexico forms thesouthwestern boundary Of the basin and was the dominant contributor of detritusto the basin. The highlandcoincides'with the segment of the

Cordilleranfoldbelt present in southern Krizona and New

Mexico (Drewes, 1978).Early Tertiary thrusting probably createdthe relief in the highlands during Baca time. In the Little Hatchet Mountainsof southwestern New Mexico,

Loring and Loring(in prep.) have shown thatthe thrusting inthat portion of the Mogollon Highland is Paleocenein age.According to Drewes (1978),the dominant style Of deformation in the highland was oneof imbricate thin- skinned thrusting andfolding similar tothat in those portions of the Cordilleran foldbelt present in southwestern

Canada, near Salt Lake City andLas Vegas in the United

States, and northern Mexico. Davis(1979), however, states thatthe early Tertiary deformation in southeastern Arizona is related to the formation of large basement upliftsin thatarea, similar tothose of theclassic Laramide Rocky

Mountains to the north. The MogollonHighland was often a positivearea during Mesozoic time. It was animportant sourceterrane for boththe Triassic Chinleand Upper

JurassicMorrison Formations (Cooley andDavidson, 1963; e 38 Dodge, 1973).Lithologies present in thehighland include

Precambrian quartzites,granites, and schists, and Paleozoic

quartzites,sandstones and . carbonaterocks (Dodge, 1973;

Silver, 1978; Hayes, 1978).

Lucero Uplift

The northern boundaryof the Baca-Eagar basin is

defined by theLucero, Zuni, and Defiance uplifts which are

Laramide monoclinalflexures typical ofthe Colorado Plateau

(Kelly,1955). The Lucero uplift lies on theeastern margin e ofthe Colorado Plateau and consists of broadlyarched, westward-dipping strata except along its eastern flank which

is marked by a complexly faulted andfolded section of

steeplyeast-dipping strata (Callender and Zilinski, 1976).

Sedimentaryrocks ranging in age from Pennsylvanian to

Triassic comprise the majorityof the exposureson the

uplift. The structurally low area between the Lucero and

Zuni uplifts is termed the Acoma sag(Kelly, 1951).

Zuni Uplift ..

The Zuni uplift trends northwesterly and is markedly

asymmetric; the northeasternflank dips gentlyinto the San

Juanbasin, while thesouthwestern flank (the Nutria 39 monocline) dipssteeply toward the Baca-Eagar basin.

Precambrian gneisses and schistsare exposed in .thecenter of theuplift. The majority of theremainder of the exposures on theuplift consist of sedimentaryrocks of Permian toTriassic age. The structurally low area between the Zuni and Defianceuplifts is termed theGallup sag. More- than 8,000 ft (2462 m) of structuralrelief exists between thehighest part of the Zuni uplift and theGallup sag. The structuralrelief between theuplift and the deepest part of the San Juan basin to the north is more than

13,000 ft (4,000 m) (Kelly,1955)-

Defiance Uplift

The north-trendingDefiance uplift is asymmetrical with its steep eastern flank,facing the Gallup sag and the SanJuan bas~in. The steepeastern boundary o€ the uplift is defined by the sinuousDefiance monocline. Structural relief between the highest part of the uplift and the Gallup sag is atleast 8,000 ft (2462 m) (Kelley and Clinton,

1960). With theexception of a few smallexposures Of

Precambrian quartzite,the majority of the.rocks whichcrop

Out in theDefiance uplift are Permian to Triassic sedimentary rocks. 40

Sierra-Sandia Uplift

Eardley(1962) proposed theexistence of two

Laramide upliftsadjacent to the eastern boundary of the

Colorado Plateau in New Mexico and termed them the Sandia

and Sierrauplifts. The term Sierra-Sandiauplift will

hereinbe used to denoteboth the Sierra and Sandia uplifts

Of Eardley(1962) and theintervening area between them.

Data from Johnson(1978), Snyder (1971), and this study

suggest that the area between the Sierra and Sandia uplifts

in theSocorro vicinity was a Laramide positivearea, as e will bediscussed in later sectionsin more detail. Evidence forthis conclusion includes: (a)paleocurrent and

paleoenvironmentaldata from the Baca outcrops east of the

Rio Grande (between theJoyita Eills and Carthage)indicate

thepresence ofsoutheastward to northwardflow in an

alluvial-fanenvironment (Johnson, 1978), thus suggesting a

sourcearea nearby tothe west; (b) paleocurrentsare

generally southwest-directed in the Baca exposed in the Bear

Mountains vicinity(Johnson, 1978;Snyder, 1971) and

presumably representflow off the western flank of the

uplift; (c) thepresence of a large,persistent, closed

lacustrinesystem in theBear-Gallinas Mountains area

indicates the presence of a damming element to the east. 41

The generallywest-dipping Paleozoic strata exposed in the Ladron,Lemitar, and Magdalena Mountains and Socorro

Peak probablyrepresent the western flank of the Sierra-

Sandia uplift, although anabpreciable component of this dip issurely related torotation of faultblocks during rifting. The geometry of thenorthern portion of the uplift is problematical.Eardley (1962) depicts the Sandia uplift as encompassingboth the Nacimiento uplift and thepresent- day Sandiauplift. Paleocurrent analysis of the Galisteo

Formation,however, indicatesthat the vicinity of the present-daySandia uplift may nothave been a Laramide positive area (Gorham and Ingersoll,1979). Drill-hole data in the western portion of the Albuquerque-Belen basin shows thepresence of earlyTertiary sediments (Foster, 19781, thusrequiring the western boundary of the Sierra-Sandia upliftto lie to

The uplift configurations depicted in Figure 10 in that area should be regarded as highly tentative.

Baca sedimentsderived from the Sierra-Sandia uplift are arkosic and contain abundantPrecambrian granite and schist clasts. Only a few isolatedexposures of Precambrian rocks are presentalong the RioGrande rift nearSocorro.

Thus, the Precambrian detritus in the Baca was probably 42

largely derived from sourceterranes which havebeen

subsequentlydownfaulted in the Rio Grande rift. This is

analogous tothe Laramide Tularosa uplift in southern New

Mexico (Eardley,1962; Kottlowski, 1967), in which the

central portion of the uplift has collapsed to form a graben

during late Tertiary regional extension.

The Sierra-Sandiauplift implies a Laramide

compression$lphase prior to late Tertiaryextension along

the Rio Grande rift. This earlystage of compressive

tectonismhas been noted by Kelly and Clinton (1960,p.

55). They state:

. ..there is much evidencealong the Rio Grande troughof an early tectonic compressional history. The structures of theCaballos Mountains(Kelly and Silver, 1952,p.136-146) areclearly early Tertiary and compressional. Low-angle (15' ) faults in which gneiss rests on Cambrian limestone are clearlydisplayed within a few hundredyards of the Rio Grande troughin the Fra Cristobal Range &:Sierra County, New Mexico (Jacobs, 1957, p.257). Thrusts and overturns have been mapped in many placestheSandiain and Manzano uplifts ...

Thrusts ofLaramide age have alsobeen reported along the

eastern margin of the rift in the -vicinity of Socorro

(Wilpolt and Wanek, 1951).

On the basis of trend,geographic location, the

presence of thrust faults along its east flank, and evidence e forlarge exposures of Precambrian rocksalong its crest, 43

theSierra-Sandia uplift probably represents a basement-

coredanticlinal uplift similar ,in structural styleto the I classic Laramide Front Range uplifts Coloradoof and Wyoming.

Morenci Uplift

The probableexistence of a Laramidedrainage divide

alongthe southeastern margin of the Baca-Eagar basin which

was subsequentlyobscured'by thick accumulations of

mid-TertiaryDatil-Mogollon volcanic rocks was pointed out

to the writer by C. E. Chapin(1980, oral commun.). The I' presenceof large amountsof Laramide volcanic and plutonic rocks in southwestern New Mexico and adjacentArizona (Fig.

11) and thenearly complete lack volcanic-derivedof

detritusin the eastern portion of the Baca outcrop belt

(seevolcanic rock fragment section) implies thepresence of

an intervening uplift which prohibited the input of volcanic

materialsinto the eastern part of the Baca-Eagar basin.

The Morencilineament (Chapin and others,1978) provides a

likely site along which a drainagedivide may have

developed. I

Mpnoclinalflexures represent the dominant style of

I deformationtheColoradoon Plateau during theLaramide, and Davis(1978) statesthat these monoclineswere produced by 44

F'igure 11. Location of Morenci lineament and distrir: bution of Laramide plutons adjacent to the Baca-Eagar basin. Presumably the majority of these plutons (circles) were associated with coeval volcanic centers. Reference points same as Figure 10, except Mo=Morenci. Lineament and pluton data from Chapin and others (1978). 45 reactivationalong basement fracture zones (lineaments) duringearly Tertiary compression. Thus; if a drainage divide was present in the vicinity of the Morenci lineament during Baca time, this so-called "Morenci uplift" was possiblymonoclinal innature. Davis (1978)recognized two sets ofdominant trenddirections (N 2OoW 'and N .55O E) Of lineaments which controlledthe developmentof.Colorado-

Plateau monoclinalflexures (Fig. 12). The orientation of the Morenci Lineament is closelyparallel to that of the northeast-trending set. Coalescingseries of northeast- trendingmonoclines (e.g. Cow Springs, Organ Rock, and Comb

Ridgemonoclines) may be as much as about175 km in length, which is approximately the lengthinferred for the Morenci uplift. The existence of a Morenci uplift would also explainthe increased abundanceof volcanicclasts in the

Baca-Eagar gravelsin the westernportion of theoutcrop belt .. (Johnson,1978, p. 87-88);-.-the~-Laramidevolcanics in the vicinity of Norenci,Arizona, would nothave been isolated by the drainagedivide and could haveeasily suppliedvolcanic detritus to thewestern portion of the

Baca-Eagar basin.

On theother hand, additional data indicating the existence of an uplift inthe vicinity of the Morenci lineament is lacking.Residual Bouguer gravity maps of the

Datil-Mogollon volcanic province do not suggest the presence 47

Of a Laramide upliftin the area, but thegravimetric manifestationofsuch an upliftcould easily havebeen masked bysubsequent mid-Tertiary volcanism and plutonism.

Colorado Plateaumonoclinal flexures generally do not containlarge exposures of Precambrianrocks. However, the occurrenceof large amounts of metamorphic detritus in the study area (see provenancesection) and elsewhere throughout the Baca-Eagar outcropbelt indicates that the Morenci uplift musthavecontained significant exposures of

Precambrian terranes.

Plate Tectonics and the Development

of the Laramide Foreland

Numerous theoriesconcerning originthe and mechanics of Laramide forelanddeformation have been proposed. In order to understand therelationship of the

Baca-Eagar basin and associated uplifts to the overall Early

Tertiary tectonic framework in westernNorth America, I will attempt to, brieflydescribe the evolution of the Laramide foreland.

In Late Jurassic to Late Cretaceous time, priorto theinception of the Laramideorogeny, thelocus of deformation inthe for'eland was alongthe Cordilleran foldbelt which extendedwithout major interruption from 48

Alaska to Guatemala (Drewes, 1978)(Fig. 13). During this

time, motionof the Farallonplate relative to the North

American plate was east-northeastward at' about 8 cm/yr

(Coney, 1978).Deformation along the Cordilleran foldbelt

continuedthroughout Laramide time in Mexico andCanada

(Coney, 1976), but ceased inLate Cretaceous time in Nevada

andUtah (Armstrong, 1968).

About 80 m.y. ago the relative plate motionbetween

the North American and Farallon plates changed, resulting in

northeastwardmotion of the Farallonplate with respect to

theNorth American plate. The magnitudeof relative plate 0 convergence rates also changed,increasing to about 14 cm/yr (Coney, 1978)(Pig. 14). Thesechanges created a strong

northeast-southwest-directedcompressive stress fieldin

westernNorth America and marked thebeginning ofthe

Laramideorogeny (Coney, 1976; 1978).Dickenson and Snyder

(1978) suggest that the upli€tsof the Laramide foreland

formed in response to compressiongenerated aby very

shallowlydipping to subhorizontal subducted platescraping

along the bottom of the continentallithosphere in the

westernUnited States. This model seems to explainboth how

Laramidedeformation took place so farinboard of the

subduction zoneand the largelymagmatic nature of the

Laramideorogen. In Late Cretaceous to Paleocene time, the 0 asymmetricbasement-cored anticlinaluplifts ofColorado- 49

Figure 13. Map showing plate positions and relative motion vectors prior to Laramide orogeny. F'Farallon plate, X=Xula plate, P=Pacific plate, NA=North American plate. Vectors show motion of Farallon (Far) and North American (NA) plates relative to hot-spot frame of reference. Resultant shows direction and magnitudeof plate convergence. Spreading centers shown by stippled areas between parallel lines, subduction zones by heavy barbed lines, thrust belts by light barbed lines, metamorphic corecorn- plexes by fine wavey lines, and arc terranes by short, dashed.1ine.s. Modified from Coney (1978). 50

.. .Figure 14. Map showing plate positions and relative motion vectors during Laramide orogeny. Uplifts shown by small, elongate, closed areas. Other symbols same as Figure 13. Modified from Coney (1978). 51

Wyoming and themonoclinal flexures of the ColoradoPlateau

began to form, creatinglarge, often closed, intermontane

basins betweenthem. The Baca-Eagar basinprobably also

began to forin during thisperiod, but differed from the

deeper,better-known Laramide basins tothe north in that

its southwesternboundary. and dominant'source. of detritus was comprised by a segmentof theCordilleran' foldbelt, the

Mogollon Highland.

About 40 m.y. ago tectonicactivity nearly

synchronouslyceased along the entire Cordillera, including the deformation in the Laramide Rocky Mountainsand Colorado

Plateaustructural provinces (Fig. 15). Thiscessation of

deformationcoincides chronologically with a decrease in

relativeplate convergence rates along the west coast of

North America (Coney, 1976). The tectonicallyquiescent

periodfollowing the end of the Laramideorogeny resulted in

the development of a widespread,late Eocene erosional

surface of low relief in some areas of the Rocky Mountain

foreland(Epis and Chapin, 1975). The next phase of the

Cenozoic tectonichistory of the western United States is

represented by thewidespread Oligocene calc-alkaline

ignimbrite flare-up. 52

I I 100'w

Figure 15. Map showing plate ,positions and relative motion vectors during post-Laramide,' middle- Tertiary~ignimbriteflare-up. CP= Colorado Plateau. Dotted areas are large caldera complexes. Other symbols same as Figure 11. From Coney (1978). STRUCTURAL GEOLOGY

The study area is locatedin the transitionzone

between the Colorado Plateau to the north and the RioGrande

rift system tothe southeast. Three episodes of structural

~ development havemodified the studyarea. A poorlydefined

monoclinalflexure of probable Laramide age is presentin

thesoutheastern portion of t'he area. North-trendingnormal

faults and associateddrag folds and maficdikes are common

in the eastern part of the area andwere produced by Neogene extension.Epeirogenic uplift theofColorado Plateau e during Neogene time resulted in the southward tilting of strata in the region.

Laramide Folding

Monoclinalflexures typify the Laramide style of

deformation of theColorado Plateau. The presence of a

poorlydefinedsoutheast-facing monocline in the

southeasternpart of the studyarea is indicated bya zone

of steepenedbedding and the abrupt termination of a

plunginganticline (Fig. 16). Maximum structural relief

alongthe monocline is about 250 ft (77 m). C. E. Chapin

(oral commun., 1978) suggests that the structure may have e resulted from drapingof sedimentary strata over a vertical

53 54

Figure 16. Structureindex map of study area. 55 displacement in the crystalline basement along a northeast- trendinglineament. A similar origin,for monoclineson the

ColoradoPlateau has beenproposed by Davis(1978).

Paleocurrentdata suggest that the monocline may havebeen activeduring Baca deposition,as will be discussed in a later section.

The monocline lies approximately on trend with the

Tijeraslineament, which is well exposed inTijeras Canyon near Albuquerque, New Mexico.Chapin and others(1979) describeseveral Tertiary features west of,the Rio Grande which may be related to reactivationalong the Tijeras lineament.

Early Rift Faults, Drag Folds,

andDikes

The RioGrande rift consists of a series of essentiallyparallel, north-trending horsts and grabens formed by Neogene regionalextension and extendsabout 600 mi (966 km) from near El Paso, Texas, Leadville,to

Colorado.

Numerous north-trending normal faults are exposed in theeastern part of. the study area (Fig. 15). In the

Magdalena Mountains, similar faults may be as old as 32 m.y.

(C. E. Chapin,1979, oral commun.). Mafic dikes dated at 56

24-25 m.y. (C. E. Chapin,unpubl. dates) have been intruded along many of these faults.

Most of thefaults dip steeply westward and are

downthrown thewest.to Fault-plane dip angles average

about 75 degrees, and range from verticalto about55

' degrees.Rare slickensides indicate slip was generally

verticalwith little or no horizontal component. Vertical

separationalong the faults is generally less than a few

tens of feet. Faults A, . €3, and C (Fig.14), however, exhibit large displacements of as much as 750 ft (231 m).

350 ft (108 m), and 830 ft (255 m), respectively. All of a thelarge-displacement faults dip steeply westwardand exhibit down-to-the-west movement.

A largesouth-plunging asymmetrical anticline is

present in theeastern part of thearea. The steep limb

facesIeastward and thefo,ld axis plunges southward at about

five degrees,terminating abruptly-against the southeast-

facingmonocline described previously. The configuration of

the anticline corresponds to that of a fold produced - by drag

due to down-to-the-west movement alongfault B. Local

small-scaledrag folds associated with normal faults are

common throughout the study area. 57

Colorado Plateau Uplift

The Colorado Plateau is a broad,stable, relatively

undeformed segment of the crust comprisingabout 150,000 mi2

(388,500 km2 ) of northwestern New Mexico, northeastern

Arizona,southeastern Utah, and southwesternColorado.

Locally,the generally flat-lying strata of the plateauare

interrupted by largemonoclinal flexures of Laramide age

(Kelly, 1955; Davis, 1978).

The study area is located in a west-trending belt of

gentlysouth-dipping sedimentary rocks along the southern e marginof the Colorado Plateau; thisbelt has beentermed the Mogollon slope(Fitzsimmons, 1959). ' Except where

modified by normal faulting anddrag folding, strata within

thestudy area dip from three to six degreessouthward.

Southward tiltingresulted from the epeirogenic uplift of

theColorado .Plateau ~ which began inearly Miocene time

(ChapinandSeager, 1975). This tiltingprobably also

produced the southwardplunge of theanticline described

previously. Similar southward tilting ofpre-Miocene strata

inthe Bear Mountains vicinity was reported by Massingill '. (1979). DEPOSITIONAL SYSTEMS

k depositional system is a genetically defined, three-dimensional, physical stratigraphic unit composed of a contiguousset of process-related sedimentary facies (Gal'loway, 1977). Depositionalsystems are the stratigraphic manifestation of major ancient geomorphic features, such as barrier islands, lakes, eolian dune fields, etc. Criteria utilized in discrimination of paleoenvironments include lithofacies geometry, lateral and vertical variation in grain size and sedimentary structures, nature of contacts between lithofacies, petrographic data, and fossils. Two ancient depositional systems are presentin the Baca Formation in the Gallinas Mountains vicinity. These are the lacustrine and braided alluvial-plain systems.

Braided Alluvial-Plain System

The importancebraided-stream-dominated of depositional systems within the Baca-Eagar basin has been demonstratedbyJohnson (1978), whodescribed the depositional framework of the entire basin. Inthis study I

Will use the term "braided alluvial plain" instead Of Johnson's "humid alluvial fan", although the sediments and depositional processes represented by both systems are

58 * 59 identical. I chose to abandonJohnson's terminology for the

followingreasons: (a)the adjective "humid", although

intended todistinguish this system from its aridalluvial

fancounterpart, connotes climatic conditions that were not

presentduring Baca time inwest-central New Mexico; (b)

there is no evidencefor a €an-shapedgeometry forthe

braided-stream-dominated Baca-Eagar system. In contrast,

this systemprobably consisted of a series of highly

coalescedfans forming an extensive, low-gradient, piedmont

slope or braided alluvial plain.

Johnson (1978) recognizedproximal-, mid-, and 0 distal-fanfacies within the Baca-Eagar outcropbelt. Only the distal facies is present in the study area.

Distal Braided Alluvial-Plain Facies

Facies.Characteristics. The distal braided alluvial

plainfacies comprises the entire middlesandstone unit and

thebasal 40 ft (13 m) ofthe lowerred unit. 'Facies

characteristicsare summarized in Table 3. A typical

verticalsection (Fig. 17) shows a dominance of horizon-

tallylaminated, fine to coarsesandstone with minor lenses

of conglomerate.of Mudstones and claystonesarerare,

occurringprimarily as thin,discontinuous layers within e sandstones.Large-scale vertical textural trends are Table 3

Distal Braided Alluvial-Plain Facies Characteristics

L ithology Sedimentary Structures Sedimentary Lithology

Moderate- to well-sorted, very coarse- to Dominantly horizontal laminations fine-grained sandstone with minor with subordinate trough crossbedding conglomerates and mudstones. Conglomerates and minor tabular crossbedding. occur as thin, discontinuous lenses within Vertical textural trends are generally sandstones. Mudstones Qccur as isolated, lacking, but upward-fining sequences thin layers within sand units. Sandstone/ are sometimes observed. Channels are mudstoneratios are very high. poorly developed and exhibit high width-to-depth ratios. Root mottling is locally abundant.

m 0 61

pVE SAND % nI.LlTH -Gr

= roots

Figure 17. Typical vertical section of distal braided alluvial-plain facies. Modified from Johnson (1978). e 62 generallylacking; however,upward-fining sequences as much

as 6 ft (2 m) thick were occasionallyobserved. Channels

arepoorly defined and range in width up to 150 ft (46 m).

Channel deposits exhibit highwidthldepth ratios (ranging

between about50:l to 1OO:l) andabundant medium- to

large-scaletrough-crossbeds as much as one meter thick.

Foresetcrossbedding is medium scale ,and occursrarely. Root mottlingand plant fragments are common. Contorted

.. . bedd-ing Land. dewakering. strnctures _are..sporadically abundant.

Coloration of sandstonesranges from red toyellowish gray

and is controlled by diagenesis,.as will be discussedin a e later section.

DepositionalProcesses. The prevalenthorizontal

laminationswithin distal braided alluvial-plainsandstones

representplane-bed transport and aggradationpredominantly

occurringduring overbank flow in broad, flatinterchannel

areasduring flood events. Similar horizontallaminations

havebeen reported by McKee and others (1967) in the recent

overbank flooddeposits of ephemeral Bijou Creek, Colorado.

Large-scale,upward-fining sequences were deposited

predominantly in overbankareas in a regimeof decreasing

flowvelocity during waning floodstages (McKee and others,

1967). a 63

Channel deposits arecharacterized by anabundance of troughcrossbeds, minor tabular crossbeds, and often

containthin conglomeratic lenses. Horizontal laminations,

similarto those describedabove, are common. The trough

crossbedding is the result of deposition by subaqueous

sinuous-cresteddunes which migrated down the channel during

high-dischargeperiods. Similar intrachanneltrough

crossbeddinghas been described in the recent flood deposits

ofBijou Creek, Colorado (McKee and others,1967). Dunes

were the dominant bedform within channels in recent flooding

of sandyephemeral streams incentral Australia (Williams, e 1971). Rare tabular crossbedswere produced by lateral accretion on foreset surfaces of transverse or linguoid bars

during waning andlow-flow stages. Such bars are common in

channelareas ofbraided streams during low-flow stages

(Williams and Rust, 1969; Harms andFahnestock, 1965;

Williams, 1971). Thin,"discontinuous conglomerate lenses

are sometimes presentinthe basal portions oftrough

cross-sets.These conglomerates represent channel-lag

gravels which were deposited inscours on thelee side of

migrating dunes. Similar recentpebble-floored hollows

preceeding dune lobes havebeen described by Williams

(1971). t I 64

Mudstones arerare within thedistal braided alluvial-plainfacies. Occurrences arelimited to thin

(generally less than a few centimeters thick), discontinuous layerswithin sandstone units. These layers wereformed by settling of silt and claywithin ponds in bothchannel and floodplainareas following flood events. These claydrapes were rarelypreserved; they were usually reworkedby subsequenthigh-discharge events and occuras claystone- clastlag gravels in superjacentsediments. Thin clay deposits formed in post-floodpools and cut-offchannels havebeen described in central Australian braided streams by

Williams (1971) . Braided streams of the distal braided alluvial-plain faciesexhibited flashy discharge characteristics and may havebeen ephemeral. The abundanceoverbankof flood deposits and thescarcity of tabularcrossbedding in this faciesdiffers from perennialbraided stream models (Smith,

1970; Kessler, 1971; Rust, 1972a;Cant and Walker, 1976;

Costello and Walker, 1972). The ephemeralbraided streams

Of central Australia (Williams, 1971) and BijouCreek,

Colorado (McKee and others, 1967) provideuseful models for thebraided streams of thedistal braided alluvial-plain facies.Miall (1977) summarizes braidedstream characteristics. Lack of eolianfeatures and presence of common plantdebris and rootmottling may indicatevegetative 65

stabilization of sediments,although the nature of the

vegetation is not known.

Braided AlluvialPlain Model. Johnson(1978)

described an extensivebraided-stream-dominated humid

alluvial-fan system (renamed braidedalluvial-plain system

in this study) in thewestern portion of the Baca-Eagar

basin, of which onlythe distal facies is present in the

studyarea. The following discussion of thebraided

alluvial-plain system is drawn largely fromJohnson (1978).

Two alluvial-plain models are commonly cited in the e literature. The first is thefamiliar arid alluvial fan (Bull, 1972; Hooke, 1967), which is characterized by

relativelysmall size, steep gradients, lack of systematic

downstream decrease in grain size, and thepresence of

braided-stream, mudflow, sieve, and eoliandeposits. The

recentfans of thearid and semiaridsouthwestern United

Statesare stereotypical arid alluvial fans.

In contrast, humid or tropical alluvial fans exhibit gentle, smooth gradients,large areal extents, systematic

decreases in grainsize downstream,and the dominanceof

braided-streamprocesses tothe virtual. exclusion of all

others. Themost commonly citedrecent examples of these

fansare the Scott proglacial braided outwash fan (humid e alluvialfan) in Alaska(Boothroyd and Ashley,1975) (Fig. 66 la), and theKosi tropical fan in India(Gole and Chitale, 1966).Braided streams on thesefans are perennial and do notexhibit the flashy discharge characteristics of arid alluvialfan ephemeral braided streams. The dominanceof fluvialprocesses and the relatively uniformdischarge characteristicsof braided-outwash ana tropical-fan braided streamsdistinguish these fans from arid alluvialfans. The I sporadicsupply of coarse, unsorted sediments by mudflows to allsurfaces of arid,alluvial fans prevents the development of thesystematic downstream decrease in grainsize characteristicofbraided outwash and tropicalfans, on which mudflow processesarerare or absent (3. C.

Boothroyd,1979, oral commun.) . ' The relatively uniform flow conditionsbraided-outwashof and tropical-fan braided streams result in steady, long-term deposition, which favors the development of the large,low-gradient surfaces of these fans.Gradients on the Kosi tropicalfan range betweenone and fivefeet per mile, and averageabout three feet per mile.

The Scott braided outwash fan (Boothroyd and Ashley,

1975) and the Kosi tropicalfan (Gole and Chitale,1966) bothprovide useful models forth& Baca-Eagar braided alluvial-plainsystem, however, neither is entirely satisfactory(Johnson, 1978). Two important differences exist between the braided outwash model and the analogous 67

CHANNEL ANDBAR MORPHOLOGY

htlloa "--- %ORIZONTAL STPATIPIED SAND 0 CROSS STPATIFED SAND 0 Kllanatsrs 8

0%- CfiAyEL -..

~. Figure 18. Map showing distribution of bar types and sedimentary structures in proximaland mid- .. fan faczes of Scott glacial outwash fan, Alaska. From Johnson (1978), after Boothroyd e and Ashley (1975). 68

system in the Baca andEagar Formations. First, thelength

Of proglacial braided outwash fans is onlyabout five to ten

miles (8 to 16 km),compared to a length of about 100 mi

(160 km) forthe Baca-Eagar braidedalluvial-plain system.

This sizedifference can probably beexplained by thefact

that the Scott braided outwash fanhas only been active for

a relativelyshort'period (during Pleistocene and Recent time), whilethe Baca-Eagar braidedalluvial plain was

presumably activefor several millions of years during the

earlyTertiary. Secondly, the braided streams of theScott

fanare perennial, while the Baca-Eagar braidedstreams show .

evidenceof more f1ashy;perhaps evenephemeral, flow. The

Kosi tropicalfan is comparable in size withthe Baca-Eagar

braidedalluvial-plain system, but contains. more fine-

grainedsediments in its distal portions.

Lacustrine System

The lacustrine system is widespreadthroughout the

Baca-Eagar basin.Lacustrine sedimentation took place in

two generalsettings (Johnson, 1978): (a) in small,

semipermanent lakessituated on fluvial-systemfloodplains;

(b) in a large,shallow, persistent lake located in the * Bear-Gallinas Mountains area. 69

Johnson (1978) recognizedlacustrine fan-delta,

fine-grained delta, and basinal facies within the Baca-Eagar

lacustrine system. Only thelacustrine fine-grained delta

and basinalfacies are present in theGallinas Mountains

area. The lacustrine systemcomprises theentire upper red

member and all of the lower red unit in thestudy area

exceptthe basal 40 ft (13m). In contrastto the braided alluvial-plainsystem, mudstonesand claystonesare a

volumetricallyimportant constituent of the lacustrine

system; sandstone/mudstone aboutratiosare 1:l. Coloration is controlled by thediagenetic history of the e sediment and ranges from red to yellowish gray. I Lacustrine Fine-grained Delta Facies

FaciesCharacteristics. Cyclical upward-coarsening

sequences characterizevertical sequences in this facies

(Fig. 19). Faciescharacteristics are summarized in Table

4. Individualcycles ' range in thickness from about 20 to

120 ft (6 to 35 m), andaverage about 30 ft (9 m) thick.

The basalportion of anidealized cycle consists of

a laterallypersistent calcareous mudstone orclaystone

intercalatedwith thinly bedded (generallyless than 25 cm thick)very-fine to medium sandstones. Mudstone carbonate

content,determined by weighing samples before and after 70

.. GRAIN SIZE SAND % Gr 1 Sd lSiC

1 " t.

Lu U -E

Figurc 19. Typicalvertical section of lacustrinefine- graineddelta facies, showing upward- coarseningcycles. Modified from Johnson (1978) - Table 4

Lacustrine Fine-Grained Delta Facies Characteristics

L itholog y Sedimentary Structures Sedimentary Lithology

Upward-coarsening sequences of mudstones Mudstones: burrows and rare . and sandstones with minor conglomerates. laminations.Sandstones: horizontal Mudstones are generally highly calcareous. laminations, parting step lineations, Sandstones are very fine to coarse grained ripple cross-lamination, occasional and moderately to well sorted. Sandstone/ trough crossbeds, rare tabular mudstoneratios areabout 1:l. crossbeds.Conglomerates formrare thin, discontinuous lenses in the upper parts of delta cycles. Large- scale, gently inclined foresets are sometimes present near the basesof cycles. Burrows and root mottling are locally abundant. 72

acidizationwith cold, dilutehydrochloric acid, generally

rangesbetween 10 and 15percent byweight. Carbonate

contentdecreases upward within deltaic cycles, as will be

.discussed in more detail in thediagenesis section.

Mudstones rarelyexhibit laminations and are usually

structureless and homogeneous, withtheexception of

burrows.Burrowing inthe mudstones is pervasive. In

contrastto sandstones in the lower portions of deltaic

cycles, mudstonesdo not exhibitwell-defined burrows, but

rather show a churned,curdled texture bothmegascopically and microscopically, which gives rise tothe homogeneous

nature of the mudstones.

Structurespresent within thinlythe bedded

sandstonesintercalated with the above-describedmudstones

includehorizontal laminations, current-ripple laminations,

partingsteplineation (McBride Yeakel,and 1963). occasional normal-gradedbeds, and burrows. Burrows (Fig.

20) arevertical, horizontal, and oblique,range in diameter

from 1/4 to 2 in (1 to 5 cm), andsometimes exhibit knobby surfaceornamentation and scoop-shaped backfilllaminae

(Johnson,1978, p. 77). According to Johnson these burrows

are similar to Scoyena sp., which are common in non-marine

redbeds(Hantzschel, 1975), and are believedto have been

formedbypolychaete worms. Rare horizons of calcium- 0 carbonatenodules andmudcracks were observedwithin the 73

Figure 20. Branching horizontal burrows (feeding traces?) in a red, lacustrine sandstone. 74

mudstones. The thinly bedded sandstones.in the lower

portions of cycles are ofteninclined (Fig. 21), forming

large-scaleforesets with dip anglesranging up to 15

degrees inrare instances. Dip anglesare morecommonly

only a few degrees(Fig. 22), and areoften so gently

inclinedthat the angularity~ is notreadily apparent in a

solitary exposure.

The above-describedunits are transitionally

overlain by a horizontal and current-ripple-laminated,

laterallycontinuous, finecoarseto sandstone which

averagesabout 5 ft (1.5 m) inthickness. Orientation of a current-ripplecrosslaminations usually indicate a direction offlow at highangles to that shown by other paleocurrentindicators within the same deltaiccycle. The

well-sorted,nearly homogeneous nature of thesesandstones

gives rise to a ’ quasi-spheroidalweathering habit (Fig.

22). Colorationof this and superjacentsandstones within a

given cycle may be red or yellowish gray, whereas coloration

of previously described subjacent intercalated mudstonesand

sandstones is almost always red.

Moving upsection, the next and uppermost part of an

ideal cycle will sometimes exhibit large,symmetrical,

channel-shaped units whichhave erosional bases. Channels

may be as much as 50 ft (15 m) wide and 15 ft (5 m) deep

a (Fig.23), but aregenerally much smaller. Intrachannel 75

Figure 21. Baca fine-grained delta deposit exhibiting unusually steeply inclined prodelta foreset beds. Figure 22. Gently inclined prodelta foreset beds overlain by quasi-spheroidally weathering delta front deposits. .Hammer handle points in direction of delta progradation. 77

Figure '23. Exceptionally large deltaic distributary- channel deposit. Note the low width-to-depth ratio (less'than10:l) and the highly symmet-- rical nature, which are characteristic of distributary channel deposits in marine deltas. 78

sedimentarystructures include , medium-scale trough

crossbedding and planebeds. Where symmetricalchannel-

shaped units are notpresent, the base of the upper part of

eachcycle is represented by anirregular, low-relief,

erosionalsurface. The remainderof the upper portion of

thecycle is composed of fine- to coarse-grainedsandstones

with minorconglomerates and mudstones exactly similar to

the rocks of the distal braided alluvial-plain facies.

DepositionalProcesses. The cyclicalsediments of

thefine-grained delta facies record alternate deltaic

progradation and abandonment in a shallowlake. Geometry of e lithofacies and vertical and lateral sequences of textures

and sedimentarystructures are similar tothose found in

lobatehicjh-constructive marine deltas (Fisher and others,

1969).Depositional processes are inferredto be the same

as .in these marine deltas (Johnson, 1978).

The lower intercalatedmudstones and thinly bedded

sandstoneswere deposited in a prodelta and distal

delta-frontenvironment. Silt and clay were deposited by

settling from suspension. The thinsandstone beds are

frontal-splaydeposits, probably representing prodelta

turbidites. Normal graded beds, a typicalfeature of

turbidites, are occasionallyseen thesein sandstones. e Turbidites in lacustrine environmentshave been described by 79

many workers, including Norniark and Dickson (19761,

Theakstone (1976),and Grover and Howard (1938).

The laterallycontinuous, quasi-spheroidally

weathering,horizontally laminated and rippled sandstones

are delta-front deposits. Thesesandstones were deposited

in channel-mouth bars Lor in longshor'e-current-redistributed

bars. The primaryprocesses on thedelta front were

plane-bed aggradation and ripple migration. The large

divergencebetween paleocurrent directions shown by

delta-front ripples and that of otherindicators within a

given deltaic cyclesuggest that ripple migrationdirection e was predominantlycontrolled by longshore currents. The large,symmetrical, channel-shaped sandstones

sometimes presentin the upper partof deltaic cycles are

distributary-channeldeposits. Sedimentary structures

indicatethat plane beds and subaqueousdunes were the dominant bedforms within.distributary channels. Vegetative

stabilization of delta-platform braided stream channelsnear

thelake allowed for the developmentof large,symmetrical,

relatively low width-to-depth-ratiodistributary channels.

Deltaicdeposits whichlack distributary channels indicate

non-stabilization of delta-platformstream channels, as is

common in classic "Gilbert-type'' delta or fan-delta deposits

(Gilbert, 1885; Theakstone,1976). The remainder of the a upper deltaic cycle is composed of delta-platformsandstones 80

and minorandconglomerates deposited by braided-stream

processes identical thoseto of the distalbraided

alluvial-plainfacies. These sedimentsrepresent the

braided-stream-dominatedsubaeria1,portion of the delta, and

are analogous tothe topset beds of classicGilbert-type

deltas.

Basinal Facies

FaciesCharacteristics. When present,the

lacustrinebasinal facies occurs directly beneath the

above-describeddelta facies and consist;s of generally e structureless mudstones and claystones with sparse, thin

interbeds of veryfine sandstone and siltstone. Evidenceof

burrowing is abundant.Faciescharacteristics are

summarized in Table 5. The basinalfacies is identicalto the lower prodeltaportion of thefine-grained delta facies

withthe exception that the sand and silt interbeds of the

basinalfacies are never inclined and aregenerally thinner

and less abundantthan those of thedelta facies. The

boundarybetween the two facies is arbitrary.

DepositionalProcesses. Basinal depositional

processesare exactly the same as.those of the lower e prodeltaicportion of thefine-grained delta facies, and Table 5

Lacustrine Basinal Facies Characteristics

L ithology Sedimentary Structures Sedimentary Lithology

Dominantly mudstones with sparse, thin Mudstones: burrows and rare horizontal interbeds of sandstones and siltstones. laminations. Sandstones and siltstones: Mudstones are usually highly calcareous. horizontal laminations, ripple lami- . Sandstones and siltstones are poorlyto nations, burrows, and parting step well sorted and are usually calcareous. lineation. 82

includesettling of silt and clay from suspension,

deposition of silt andsand by turbiditycurrents, and

homogenizationof mudstones and claystones byburrowing.

The thin,sparse natur.e of the sand and silt interbeds

indicatesdeposition far from nearshoresources of coarse

sediments.

General Characteristics of the Lacustrine System

The laterallypersistent nature of thedelta front

and deltaplatform sandstones and thegeneral lack of 0 destructional-phase features indicate that Baca fine-grained deltas were mainlyhigh constructive lobate (Fisher and

others, 1969), which suggestsprogradation into relatively

shallowwater. The othervariety of high-constructive delta

is theelongate or birdsfootdelta, which is characterized by laterallydiscontinuous bar-finger sands which result

from progradationinto deeper water. Water depth, as

indicated by the thickness of prodelta mudstonesand

prodeltaforeset beds, was generallyless than 20 ft (6 m). During depositionof the upperred unit, however, water

depth may havebeen considerably deeper, as attested by the

large thicknesses of mudstones in that unit. 83

Baca deltaic and basinalsediments in theGallinas Mountains area show evidence of deposition in a closed

lacustrinebasin. Evidence includes: high (a)

concentrations of early(pre-compaction), phreatic,

authigenic calcite in basinal and lower-delta mudstonesand

sandstones(see diagenesis section): and (b) rarity of

lacustrine megafauna. Only a few scatteredostracods were

observed. The climate in west-central New Mexico during

Baca time was probablysemiarid (see paleoclimatesection).

Langbein (1961) statesthatclosed lakes are found

exclusively in arid and semiaridregions. Closed lakes e usuallyexhibit widely fluctuating water levels (Langbein, 1961). Rare caliches andmudcracked horizons in Baca

basinal and prodelta sediments areindicative of lake-level

low stands. The presence of thefluvially dominated middle

sandstone unit between thepredominantly lacustrine upper and lower red units indicates a large-scaleregression,

which was probablycaused by a drasticdrop in lake level

due to a climatic change (see depositional history section).

Steeplyinclined Gilbert-type foresets are rare in

thefine-grained de1ta.facie.s. Factors contributing to the

development of foresetsinclude: (a) homopycnal flow

(inflow,density approximately equal to lake-water density)

whichcauses three-dimensional mixing and an abrupt decrease * in current velocity,resulting rapidin deposition of 84

sediment (Bates, 1953); (b) deltaicprogradation into deep

water (McGowen, 1970;Axelsson, 1967; Hjulstrom, 1952);

and (c) transport of coarsebedload sediments, of which

Gilbert-typeforesets are 'composed, tothe distributary

mouth (Smith,1975; Axelsson, 1967). The conditionslisted

abovewere rarelypresent during deposition of the Baca

deltas inthe Gallinas Mountains.area. Water depthswere

shallow and Baca sediments in the study area were rarely

coarserthan coarse sand. The closednature of thelake

implies that lake waters were more dense (due to salinity) thaninflowing river water, producinghypopycnal flow and

plane jet formation(Bates, 1953). Lack of three- a dimensionalmixing. causes the plane jet tomaintain its

velocityover a relativelylongdistance basinward,

resultingin deposition ofsediments over a considerable

distance from thedistributary mouth. Thisleads to the development of a gentlysloping prodelta surface, which

producesthe typical shallowly inclined prodelta foresets

seen inthe fine-grained delta facies. The Gilbert-type

deltas in the Baca Canyon area(Johnson, 1978) probably

formed in response topossible deeper-water conditions and

theinput ofcoarse, conglomeratic sediments derived from

the nearbySierra-Sandia uplift. ' e 85

Thin,destructional-phase shoreface sequences producedby reworking of upper-delta sediments by waves and

longshorecurrents following deltaic abandonment and

subsidence(Fisher and others,1969) are rarelyseen. The

paucityof destructional-phase shoreface sequences suggests

relatively low-energy conditionswithin the lake basin.

Attenuation of wave energyresulting from shallowwater

depths may explaintherarity destructional-phase of

features.

The large,shallow lake systempresent during Baca

time in the Bear-Gallinas Mountains area would tend to be 0 polymictic(frequent overturn) since surface mixingdue to eddy diffusion would be expected topenetrate at least

severaltens of feet (Johnson, 1978). Evidence of abundant

burrowingof basinal and prodeltasediments indicates that

lake-bottomsediments wereoxygenated, supporting a polymicticregime. Preservation of laminations in lake-

bottomsediments is usually restricted to oligomictic and

meromictic(permanently stratified) lakes, in which lack of

oxygen inhibits the activities burrowingof organisms.

Johnson(1978, p. 85)states that the shallow,polymictic

nature of the Baca lacustrine system resulted.inoxygenated

lake-bottomwaters which may be responsiblefor the almost

completelack of reduced(gray and green) Baca lacustrine e sediments. Lake bottomsediments in shallow,frequent- 86 overturn, Lake Chad, Africa, are darkyellowish brown

(Mothersill, 1975), which probablyindicates the present of limoniteand, thus, positive-Eh conditions. Upon burial Of lessthan 60 cm, however, the bottomsediments of Lake Chad assume a darkgreenish-gray coloration, probably indicative of reducediron andnegative-Eh conditions.This change from anoxygenating to a reducingregime is probably due to removal,during burial, bottomof sediments from the presence of oxygenatedlake waters. Isolation of bottom sedimentsfrom oxygenated lake waters would allow for the uptakeof free oxygen inthe interstitial waters through decomposition of organic material by aerobic bacteria, which would lead to negative-Ehconditions. The majority of the coloration of the Baca sediments (inboth lacustrine and fluvialfacies) in the study area appears to be due to intrastratalsolution, ofiron-bearing minerals, precipitation of hydratediron oxides, and the subsequent dehydration of theseoxides, which results thein development of hematite pigment. Thus, the. Baca sediments did notnecessarily contain any iron-oxidecolorants upon deposition;in fact, there is some evidence tosuggest that thelacustrine sediments were reducingand contained iron sulfides which laterprovided iron for red coloration, as will be discussed in the diagenesis section. a7

Several other lines ofevidence exist which support

a lacustrineinterpretation for large parts of the Baca

Formation in the Bear-GallinasMountains area. These

include:(a) the presence of rareoolites and ostracods and

common intraclasts which were observed in thin sections from

many parts of the Baca inthe study area, as will be

discussed in the sandstonepetrology section. . (b) In his

workon thepalynology of the Baca Formation in the Bear

Mountains vicinity,Chaiffetz (1979) stated thatthe

predominantlypink-gray Baca sediments (i.e. the lower and

upper red units, which I interpretto be dominantly

lacustrine)yielded only a few poorlypreserved pollen

grains and the fresh-wateralga, Pediastrum. However, a

greenish-gray shale sample from a thin mudstone in the

'fluvially dominated middle sandstoneunit (I was present

duringsampling) produced spores and pollen from a wide variety uplandof flora, includingconifers. Chaiffetz

(1979)further states thatthe presence Pediastrumof

I, ...p erhaps requires some standing fresh-water bodies in the

region at that time".

. I.. 88

Lacustrine Model

The limitedexposures of the Baca lacustrinesystem make comparison to modern and ancientanalogues difficult.

Exposures in the study area and the Baca Canyon area consist

only of marginallacustrine deposits. Paleocurrent

directions in the studyarea (see sediment dispersal patternssection) generallyare northeast directed,

indicating that the location of the basin center was in that

direction. The nature of thebasin-center deposits are not known due toerosional stripping following late Tertiary

uplift of theColorado Plateau. Any model of the Baca

lacustrinesystem must takeinto account the following

characteristics: (a) shallowwater depth: (b) fluctuating

lakelevels: (c) rarity of lacustrine megafauna;and (d)

highconcentrations of early authigenic calcite cements in

marginallacustrine sediments.

Surdam andWolfbauer (1975) have proposed a closed-

basin,playa-lake model for Eocene Lake Gosiute. in Wyoming which adequately fits mostof thecharacteristics of the

Baca lacustrinesystem. Lake Gosiute was a large,shallow,

closedlake which exhibitedwidely fluctuating lake levels

and crude,concentric zonation of bothchemical and detrital

facies (Fig. 24). The marginalfacies of Lake Gosiute is

characterizedcalcareousby sandstones and siltstones 89

Figure 24.~ Map;-showing crude,concentric zonationof facies in Eocene Lake Gosiute, Wyoming, during a high stand (Tipton Shaletime). Marginal facies is composed of calcareous sandstones and siltstones, somewhat similar to the lacustrine system rocks in the study. area. Surdam and Wolfbauer (1975). 90

(Surdam andWolfbauer, 1975, p. 339) somewhat similar to

Baca marginallacustrine rocks. Megafaunal diversityin

Lake Gosiute was greaterthan that of the Baca Formation

lacustrinesystem, and includedostracods, molluscs, algal

reefs,and fish. The more restricted assemblageof the Baca

lacustrine system may be due to: (a) lackof exposure of

potentially more fossiliferousbasin-center deposits; (b)

highersalinity resulting from higherevaporation rates in

the more southerly Baca lacustrine system.

Deep Springs Lake in InyoCounty, California(Jones,

19651, exhibits quasi-concentriczonation of facies similar a' to that of ancient Lake Gosiute (Surdamand Wolfbauer, 19751, and may represent a modern 1 analogue of the Baca

lacustrinesystem. inthe Bear-Gallinas Mountains area.

Marginalsediments of Deep Springs Lake containhigh

concentrations of authigeniccarbonate minerals. Certain

aspects of Lake Chad, Africa,(Mothersill, 1975) are similar

tothose inferred for the Baca lacustrine system.These

include:(a) polymictic regime; (b) closednature of the

lake;and (c) shallow water depth (less than 5 m).

, DEPOSITIONALRECONSTRUCTION

Sediment Dispersal Patterns

Paleocurrentindicators were used to delineate

sediment dispersal patternsin the study area. Two general

types of paleocurrentindicators, unidirectional and

bidirectional, were utilized.Unidirectional ,indicators

-includeimbricated pebbles andtrough-crossbed axes. Rust

(197213) has documented the relationship betweenflow

direction and imbricated pebble orientationin modern e streams. The coincidence of the azimuths of trough-crossbed axes with flow direction has beendemonstrated by Dott,

(1973), Highand Picard, (1974),and Meckel (1967).

Unidirectionalpaleocurrent indicators occur in the

braided-stream-dominated delta-platform portion of the

fine-grainedlacustrine delta faciesand the distalbraided

alluvial-plain facies.

Parting step lineations (McBride andYeakel, 1963)

(Fig.25) constitute the bidirectionalindicators and, due

to their abundanceand ease of measurement, were the

dominant paleocurrentindicator utilized in this study.

Stokes(1947) has documented that parting-steplineation

trend is parallel to current flow. Due to its bidirectional e nature,,parting-step lineation data must be used in 91 92

..:_i

Figure 25. Well-developed parting-step lineation in a fine-grained Baca sandstone. Paleoflow was parallel to trend of mechanical pencil. 93 conjunction with unidirectionaldata in order to determine thetrue transport vector. Parting-step lineations are sometimes seenin braided-stream deposits in the delta- platform portion of the fine-grained lacustrine delta facies and thedistal braided alluvial-plain facies. They are very commonly observed ,. thin,in 'prodelta frontal-splay sandstones. As discussedpreviously, these sandstones probablyrepresent deltaic turbidity-currentdeposits.

Parting-steplineation within turbidites has been described byCrowell (1955).

Paleocurrentdirections varyin a somewhat systematic mannerfrom thewestern to eastern portions of thestudy area. Inthe western part of thestudy area, paleocurrents were generallynortheastward directed (Fig.

26 a, b), whereas, inthe eastern part of thearea, paleocurrent directions trendgenerally east-southeastward

(Fig. 26 c; d). Thisdivergence of paleoflow may havebeen caused by activity along the previously described northeast- trendingmonoclinal flexure present inthe southeastern portion of the area. Downwarping of thesoutheastern side of the monoclineduring Baca time may havecreated a topographically low area which may be .responsiblefor the deflection of paleocurrentsto the east-southeast in the easternportion of the study area. If thishypothesis is true,then the "regional". paleocurrent direction in the 94

bldireccional bidirectional

Figure 26. Comparison of paleocurrent data in western (a,b) and eastern (c,d) portions of study area. The significant portions of the bidirectional rose diagrams (shaded) were determined by comparison with associated unidirectional data. The study area was arbitrarily divided along the line between Indian Spring Canyonand Indian Mesa. quadrangles. 95

Gallinas Mountains area was probablynortheastward, as

represented by the paleocurrentindicators in thewestern

portion of thearea which were farther away from,and

presumablyunaffected by, the monocline. Baca sediments in

the study area were probably derived mainly from the Morenci

upliftto'the southwest, although detrital input 'by

tributariesdraining the western flank of thatportion of

the Sierra-Sandia uplift which lies to the south of the area

may have also been important.

DepositionalHistory

The rocks of the Baca Formation in the study area

recordthe alternate prevalence of distal braided alluvial

plain and lacustrine environmentsdeposition.of The

alternation' of these two environments reflectlarge-scale

fluctuations of waterlevel in a large,shallow lake present

during Baca time in theBear-Gallinas Mountains vicinity.

Baca sedimentation in thestudy area was initiated

bybraided-stream deposition of sands andminor gravels

predominantlyderived from thesouthwest on a low-relief

erosionalsurface developed on theLate Cretaceous Crevasse Canyon Formation.Braided-stream sedimentation in thelower

Baca is representedby the distal braided alluvial-plain e facies which comprises thebasal 40 ft (13 m) of the lower 96

red unit (Fig. 27). The initial intervalof braided-stream sedimentation was followed by a prolonged period of marginal lacustrine sedimentation, as isrepresented by the cyclical deposits of the fine-grained delta and lacustrine basinal facies, which comprise the remaining283 ft (86 m) of the lower red unit. These lacustrine deposits indicate a large-scale southwest- ward transgression of the shoreline of a lake whose center was to the northeast (as suggested by paleocurrent data), which resulted in onlap of lacustrine sediments over older braidedstream deposits, Local, small-scale regressive- e transgressive sequences within this initial transgressive phase of Baca sedimentation are recorded by cyclical deltaic progradation, abandonment, and subsidence. The initial period of lacustrine sedimentation was followed by a return to a braided-stream-dominated regime, as represented by the sediments of the distal braided alluvial-plainfacies which comprise the entire middle sandstone unit. The return to fluvially dominated processes indicates a large-scale regression of the lake shorelineto the northeast. The final phaseof Baca sedimentation in the study area is represented by the dominantly lacustrine sediments of the upper red unit. These sediments indicate a second .major, widespreadshoreline,transgression coupled with minor 99

thelocus of lacustrinesedimentation due totectonism

withinthe basin: (b) increasederosion caused by tectonic

activity in thesource area, with resultant large-scale

regression due to progradation of alluvial aprons basinward:

transgressive and regressivephases. The relative

importance of each of theabove-listed hypotheses is

difficulttodetermine. However, one line of evidence.

suggeststhat climatic changes weremajora cause of the

lake-levelfluctuations. The lacustrine mudstones in the

study area.,with the exception of the prodelta mudstone of e thebasal deltaic cycle in theupper red unit, arehighly calcareous and indicative of probablesaline, closed-lake

conditions. The essentiallynon-calcareous nature of the

basal mudstone of the upper red unit (2 percentcalcite by

weightas compared with 10-15 percent in other mudstones) implies temporarya change to more fresh-waterconditions

during thebeginning of the second transgressivephase. If

thetransgression wasdue to achange to a wetterclimate, thelow-saline characteristics could be easily explained by

theintroduction of large amounts of freshwater to the

lake.Neither of thetectonic alternatives canexplain the

non-calcareous.nature of the basal mudstone of the upperred

unit. Large-scalelake-level fluctuations in contempor- a aneousEocene Lake Gosiute, Wyoming, havebeen attributed to 97

Type of Shoreline Depositional Migration System Unit ‘

Upper Lacustrine Red I Unit

- transgression

100 30 Braided Middle ft m Alluvial Sandstone Plain Unit 0 i 0

-regression

Lacustrine Lower Red Unit

- transgression”---- Braided Alluvial Plain

Figure 27. Generalized Baca stratigraphicsection in the study area showing relationships between shorelinemigrations, depositional systems, and informalunits. 98

deltaicregressive sequences, similar tothose of the first

transgressivephase. Water depths during the second

transgressivephase may havebeen deeper than those of the

first, as shown by thelarge thicknesses of basinal and

prodeltamudstones inthe upperredunit. Lacustrine

conditionscontinued toprevail in some parts of thestudy

areaduring deposition of the lowermost Spears Formation, as

attested by theoccurrence of deltaic deposits in the basal

portionof that unitin some. areas(basal Spears deltaic

deposits are well exposed in SW 1/4 and NW 1/4, SW 1/4,sec. 24, T.lN., R.6W.). e Examinationby the writer of the Baca stratigraphic sectionin Baca Canyon revealed a similar, although

coarser-grained, vertical.sequence of facies to that seen in thestudy area, indicatingthat the transgressions and

regressionsrecorded in the study area were not just local

features,but were .manifestedthroughout the lake basin.

The Baca lacustrine system in'the Bear-Gallinas Mountains

vicinity may havebeen initiated in response to creation of

the LaramideSierra-Sandia uplift which acted as a damming

element across the generallyeastward-dipping regional

paleoslope.

Three possibleexplanations exist for the large- scaletransgressions and regressionsrecorded inthe Baca

sediments in the study area. These are: (a) shifting of 100 climatic changes by Surdam and Wolfbauer (1975). It is interesting to notethat the relative thicknesses, sequence of occurrence, and number of transgressive and regressive phases in EoceneLake Gosiute (Suxdam and Wolfbauer, 1975) is verysimilar to those in thelarge lacustrine system presentduring Baca time in theBear-Gallinas Mountains vicinity(Fig. 28). Although climaticfluctuations generallyaffect large regions, not enough data is available to determinepossible relationships between thethe transgressive and regressivephases of Lake Gosiute and the

Baca lacustrine system.

The existence of deltaicdeposits in thebasal

SpearsFormation in some parts of thestudy area indicates that tbe lakepersisted for a short time duringthe beginningof Oligocene volcanism. The cause of thefinal demise of thelake is not known. Likelypossibilities includeclimatic chanae and rapid infilling of thelake with volcaniclastic sediments. 101

Figure 28. Generalized stratigraphy of the Green River Formation in Bridger and Washakie basins of Wyoming, showing alternation of high stands (Tipton and Laney Shales) andlow stands (Luman Shale, Wilkins Peak) in Eocene Lake Gosiute. From Surdam and Wolfbauer (1975). PETROLOGY

Conglomerate Petrology

Conglomeratesare scarce but widespread throughout the study area. They occurprimarily as thin,discontinuous lenseswithin sandstones of thedistal .braided alluvial- plain facies and in the upper portions of lacustrine deltaic cycles. Clast size averagesabout 1.6. in (4 cm) , and ranges up to as much as 5 in (12.5 cm).

Clast Composition

Pebble counts from 15 localitieswithin the study area were utilized to determine the clast composition of the

Baca conglomerates.Lithologies present in the conglom- , erates are highlyvariable and consist predominantly of metaquartzite,limestone, and chert. Many other lithologies, however, arealso present andoften are importantconstituents in some areas. Silicified wood fragments are locally abundant inthe middle sandstone and upper red units.Pebble composition data is summarized in

I Table 6. A ternary plot of pebblecompositions is presented in Figure 29.

102 Table 6

Summary of Pebble Composition Data

LithologyRange ( % ) Average (%) ColorRemarks metaquartzite 0-90 black,red, 44.2 Usually well rounded and pink gray subequant. white, multi- I colored limestone 0-85 29.4dark tomedium Dominantly micrite. gray Sometimes contains a few crinoid stem fragments or septarian cracks. chert 0-65 10.4dark gray to black granite 0-75 grayish-pink 6.2 Contains abundant pink orthoclase.Usually present in small amounts. High concentrations (75%) only seen at one locality. sandstone 0-40 5.6 gray I pink I Some may be reworked Baca and siltstone red sediments. schist 0-20 1.6 mediumto Contains abundant biotite. dark gray P 0 W 104

W. m d 0

Ln 0 N d I I 0 0 105

Quartzite, schist

/ - Gran ite, Limestone, Granite, cher.t, volcanics sandstone, siltstone, mudstone

Figure 29. Ternary plot of Baca pebble compositions. Data from 15 pebble counts within the study area. 106

Baca conglomerate clasts arelithologically similar

to rocksexposed in that portion of the Sierra-Sandia uplift

now represented by the MagdalenaMountains. The variable

nature of theclast compositions in Baca conglomerates

suggestsderivation fromnearby sources. Some of the carbonateclasts exhibit fragile, platy shapes, whichimply . shortdistances of transport, and, thus, were ,probably

' eitherderived from a Paleozoiclimestone source in the

nearbySierra-Sandia or Morenci uplifts or from an

intraformationalsource. Limestones of lacustrineorigin

havebeen reported in the lowerred unit of the Baca e Formation in the BearMountains vicinity(Mawingill, 1979, p. 100).

Clast Morphology

Baca conglomerateclasts from'within the study area

areusually highly spherical. Thirty-eight approximately

equal-sized (2-3 cm in length)internally isotropic clasts

from ~ two localities within the study area weremeasured by

the writer and 33 clasts fromone location were measured by

members of R. L. Folk's Geo 391 Sediments and Sedimentary Rocks class and plotted ona sphericity-form diagram (Fig.

30). Most pointsplotted in thecompact'platy, compact a bladed, and compact elongatedfields, with a few points 107

COMPACT

Figure 30. Sphericity-form diagram showing shapes of 71 Baca~pebbles. Average-maximumprojection sphericity is 0.754. L=long diameter, I=intermediate diameter, S=short diameter. Sphericity-form diagram from Folk (1974). 10%

falling in the compact, bladed, and platyfields. Average 'A maximum projectionsphericity ( (S2/LI) ) (Sneedand Folk,

1958) for the samples was 0.754.According to Folk (1978,

oral commun.), the meanmaximum projectionsphericity is

usually less than 0.66 for beachpebbles, between 0.66 and

0.75,-fornon-braided-stream fluvial pebbles, and greater

than 0.75 braided-streamfor pebbles. As described

previously,the primary mode of transport of the Baca

sediments in thestudy area (and', presumably,upstream of

the study area) was by braided streams. The 0.754 value obtainedfor the Baca pebblesthus marginally supports

Folk's hypothesis.

Dobkinsand Folk (1970). statethat the low- sphericity, platy form of Tahiti-Nuibeach pebbles is caused

by sliding backand forth due tothe effects of swash and

backwash. The highsphericity of braided stream pebbles may

reflect the flashy discharge characteristics typical of most

braided streams. High-dischargeevents, common in braided

streams, would tend totransport pebbles in a turbulent,

chaotic manner. This would result in nearlyequal

distribution of abrasiveaction to a11surfaces of the

pebbles,thus favoring the development of nearlyequant clasts. Non-braided fluvialsystems, on theother hand,

typicallyexhibit more uniform dischargecharacteristics, e which would favortransport of pebblesby rolling along 109 theirshort and intermediateaxes. Such rolling would preferentially abrade the short and intermediate axes of the pebbles,resulting in a moderate-sphericity,bladed or elongate form typical ofnon-braided fluvial clasts.

According tothe visual-comparison roundness scale of Powers (1953), Baca pebblesare generally rounded to well rounded.Fine-grained, well-indurated pebble types (e.g. metaquartzite,chert, micrite) tend to be more rounded than the coarser-grainedor less well-induratedlithologies.

Althoughmetaquartzite clasts similar to those in the Baca are present in several Mesozoic units in west-central New

Mexico,such asthe Triassic Chinle Formation(Givens,

1957), and in several Precambrian units in southernArizona (J. MacMillan, 1980, oral commun.), the well-rounded nature of the Baca clastsdoes not necessarily imply thatthese pebblesare multicycle. In fact,the well-roundednature of theobviously first-cycle cobbles andboulders of the proximalbraided alluvial-plain facies exposedalong the

Mogollon Rim impliesthat many of the well-rounded Baca pebbles are probably first cycle.

Baca pebbles in thestudy area often exhibit small, cresenticpercussion marks on theirsurfaces. These surface

featuresare indicative of high-velocityimpacts (Folk,

1974, p. 12), whichprobably occurred during transport in braided streams.

.. 110

Sandstone Petrology

Twenty-six thin sections of Baca sandstones from the

study area were examined and 200 random points from each

thin section were counted.Thin section data is presented

I in Appendix B. Sieve-equivalentgrain size was calculated

from thin-sectiondata using Harrell and Eriksson's(1979)

linearregression formula, asfollows: (sieve grain size in

phi units) = 0.227 + 0.973(thin-sectiongrain size in phi units).Sieve-equivalent grain size for the sandstonethin

sections examinedranges from 0.6# (coarsesand) to 3.1+ e (veryfine sand), with most samples fallingin the medium and finesand categories. Sandstone sorting values ranged from verypoorly to well sorted, andaveraged moderately to

moderately well sorted. Most sandstones were texturally

mature or submature,however, two samples containedgreater

thanfive percent detrital clay matrix (illite) and were

classified as immature(Folk, 1951). According to Powers'

(1953)visual-comparison roundness scale, most grains

examined fell into the subangular andsubrounded categories.

A smallpopulation of rounded to well-roundedquartz grains were observed,however, and probably represent multicycle gr ains. 111

According to Folk's (1974)mineralogic sandstone classification, most of the Baca sandstones examined petrographically are sublitharenites,feldspathic litharenites,or lithic arkoses(Fig. 31). Only one sample plotted in either the subarkoseor litharenite portions of the trianglediagram. Contrary to the 'interpretations of previousworkers (Givens, 1957; Tonking, 19571, none of the

Baca sandstones from the studyarea which were examined in thin sectioncould be classifiedas "true" arkoses (as defined by Folk,1974). Average feldsparcontent of these sandstones is 14.3percent of the framework grains.In the

Baca Canyon area, however, Baca Formationsandstones are very feldspathic, probably due to derivation from the nearby

Sierra-Sandiauplift in whichPrecambrian granites .are common.

Quartz (47-84%, average 66% of framework grains)

Quartz is the volumetrically mostimportant detrital mineral of the- Baca sandstones within thestudy area. In thisstudy, I haveassigned quartz types to two genetic categories: common quartz and metamorphic quartz.These two geneticcategories reflect only the probable ultimate sourcerock type and are not meant to imply that no quartz has been reworked from older,sedimentary rocks. Within 112

Quartz

Feldspar plus Other Granite/Gneiss Rock Fragments Rock Fragments

Figure 31. Classification of Baca sandstones based on data from 26 thin sections. Cross shows average composition. After Folk (1974). 113 these two generalcategories, several empirical quartz types

(Folk, 1974) and geneticquartz types (Krynine,1946) can be delineated.

Common Quartz(12-59%, average 37%of framework grains;average, 55% of totalquartz). Monocrystalline quartzgrains which showed straightto slightly undulose extinction(extinction shadow sweeps acrossgrains with stagerotation of five degrees or less) were classifiedin thiscategory. Grains are usually subequant sind xeno- morphic. A smallpercentage of the common quartzgrains

(generally less than five percent) exhibit probable reworked quartzovergrowths or are anomalously weil rounded, which is most likelyindicative of derivation from oldersedimentary sources. Common quartzgrains often exhibit’a few vacuoles which are randomly distributedthroughout the grain and probably formed duringcrystallization. More rarely, vacuolesoccur inlinear zoneswithin grains and may representhealing incipientof fractures. Microlites

(mineralinclusions) were occasionallyobserved. Minute, hair-like rutile inclusions were present in a few grains.

Common quartz is most often derived from plutonic or gneissicsource rocks and, less commonly, from recrystal- lizedigneous veins andmetamorphic rocks (Folk, 1974). 114

Metamorphic Quartz (13-51%, average30% of framework

grains;average 45% of total quartz) Monoc,rystalline quartz

grains in which the extinction shadow sweeps the grain with

greater than fivedegrees rotation of the stage(strongly unduloseextinction) and polycrystalline(composite) quartz

grains were placed in this category.Individual crystals in

polycrystallinegrains usually exhibit strongly undulose

extinction, however, a smallpercentage of these grains are

composed of crystals with straightto slightly undulose

extinction. According to Krynine's(1946) genetic class-

ificationof quartz types, Baca metamorphicquartz is a predominantly composed ofstretched metamorphic quartz(Fig. 32), with minoramounts (generally less than a few percent)

Of schistose(Fig. 33) and recrystallized metamorphic

quartz(Fig. 34).

The relative abundance of thevarious metamorphic.

quartztypes is, in part, a function of grain size;

polycrystalline,highly undulose quartz is usually more

common in the coarsergrain sizes, whereas monocrystalline.

scronglyundulose grains tend to be more prevalentin the

finer-grainedsandstones. Thus, due to the effects of the

wide range ingrain size of the Baca sandstones examined

petrographically, no attempt wasmade totabulate exact

percentages of thevarious metamorphic quartztypes. .: Indeed,even therelative abundancesofmetamorphic and 115

Figure 32.. Stretched metamorphic quartz grain. Sample 68, crossed nicols, field width=O,.Y5 rmn. 116

F-,-re 33. -2histose metamorphic. ,lartz Note minor amounts of parallel oriented muscovite and elongated quartz crystallites. Sample C, crossed nicols, field width=0.75 mm. 117

Figure 34. Recrystallized metamorphic quartz grain. Note equant shape of and simple boundaries between crystallites. ‘Sample 32, crossed nicols, field width=O,.75 mm. common quartz

35 indicates a vague trend towardshigher metamorphic/total quartz ratios with increasinggrain size. The diminished

abundanceof metamorphic quartz in the finergrain sizes is

due to the factthat increasingly strained crystal lattices

are necessary to producestrongly undulose extinction with

decreasing grain size and that polycrystalline quartz grains \ are less common in the finer sandstones.

Metamorphic quartzgrains are generallysubequant andxenomorphic . They oftencontain a few randomly dispersedvacuoles and sometimes exhibit minuteinclusions e muscovite.of Quartz grains containing large, aligned muscovitecrystals were classified as metamorphicrock fragments.

According to Folk(19749, monocrystalline, highly

undulosequartz is most commonly derived from stretched metamorphicand gneissicsources, but also may occur in some

strainedgranites. Polycrystalline grains showing strongly \ unduloseextinction are probably the most reliablequartz-

type indicator of a metamorphicsource area. and are derived

primarily from gneisses and stretched metamorphicrocks.

Polycrystallinegrains exhibiting straight slightlyto

unduloseextinction are most commonly derived from schists

and recrystallized metamorphic rocks,,but may, to a small * extent, be derived from granites(Folk, 1974). Basedon his 119

. . . .-...... * . . . 0. I . . . 3- . . .. 6

4' 1 I I I 1 0.0 0.2 0.4 0.6 0.8 1.0

Metaquartz/Total Quartz

Figure 35. Plot of grain sizevs. metaquartz/common quartz for 27 Baca sandstones, showing tendency',. towards increasing abundancesof metaquartz with increasing grain size. 120

studyof quartz from sedimentswith known sourceareas,

Blatt(1967) states thatmonocrystalline, strongly undulose

quartzgrains and polycrystallinegrains composed of less

thanfive crystal units are not useful in distinguishing

between plutonic andmetamorphic sourceareas. In a similar

study,however, Basu and others(1975) found that both

degree .of polycrystallinity and type of extinction was usefulin discriminating source rock types. The abundance

of metaquartzite pebbles in thestudy area implies that a

largeportion of the quartz in the Baca sandstones was

probably derived from a similarlithology. The e metamorphic/totalquartz ratio appears to increase in the basalportion of thesection (Fig. 36).

Feldspar(6-308, average 14%of framework grains)

Threetypes of feldspars are present in the Baca

sandstones inthe study area. These are, inorder of

decreasingabundance, orthoclase, microcline, and plagioclase.Alteration typesinclude vacuolization,

sericitization.cavernous dissolution (Fig. 37), and minor

calcitization(Fig. 38). Vacuolization is the most common

tyse of alteration seen in Baca feldspars, and may be producedby surfaceweathering, hydrothermal alteration, or

as inclusionstrapped within the crystallizing feldspar. ! 121

Metaquartz/Total quartz 0 .2 .4 .6 .8 1.0

__. 30

_. J ' 150 , Middle Sandstone Unit loo1 e G feet o J- 0. meters

Lower E Red Unit C B 34

Sample Number

Figure 36. Plot of metaquartz/total. quartz ratios as a function of stratigraphic position. Mean grain size rangesbetween 1.6 and'2.9$1. .. 122

Figure 37. Cavernous dissolution in an orthoclase grain from a fluvial sandstone (sample 48). Second- ary porosity within the grain appears blue, due to impregnation of sample with colored epoxy. Plane light, field widtht.0.75 mm. L 124

Sericitization is most oftenthe product of hydrothermal alteration, although it may also formunder arid or semiarid weatheringconditions (Folk, 1974, p. 84). Calcitization takes place through replacement of feldspar by calcite in an

alkalineenvironment. Dissolution porosity within feldspars

can form in an acidic environment in soils (Begle,1978) or

in thesubsurface, or by decalcitization of calcitized feldspars(Jonas andMcBride, 1977). Extent of alteration

ranges from unaltered(fresh) extremely to altered,

occasionally making recognition of thefeldspar type

impossible. Degreeof alteration is, in part, a function of

feldsparcomposition. Plagioclase is generallythe most

altered,followed by orthoclase and microcline, which is the

leastaltered. This is consistentwith most reports on the

relativestabilities of these three feldspartypes (Folk,

1974, p. 84).

~~ Alteration ~0.ffeldspars cantake place in several differentsettings: (a) by deuteric or hydrothermal

processes in thesource area; (b) in source-areasoil

zones; (c) in soil zones in thedepositional basin; (d) by

reactionwith groundwaters after burial; and (e) in modern

Soilzones. One line of evidence suggeststhat, with the

exception of minor vacuolization and sericitization which

probablyoccurred within the sourcearea, the majority of

thealteration of Baca feldsparstookplace post- 125 depositionally, due interactionto withgroundwaters migratingthrough permeable sandstones. The thinSandstones which representfrontal-splay deposits in the lower portion of deltaiccycles exhibit noncompacted textures and are tightly cemented withfine-grained sparry calcite, and thus indicatecementation within the., lacustrine environment or followingvery shallow burial (see diagenesis section);

Thisvery early and completecementation enables us to view thefeldspars and other components of thesesandstones as theyappeared when they were deposited;the very low permeabilityinherent from thetightly cemented nature of these sandstones protected the feldspars from the effects of later-diageneticgroundwaters. Feldspars within these sandstonesare generally fresh, but sometimes exhibit minor vacuolizationand/or sericitization. The abundance of fresh feldspars in the early-cemented Baca sandstonescan be explained by erosion,transportation, and depositionin an arid orsemiarid climate and/orrapid erosion and burial which would protectfeldspars fromprolonged exposure to weatheringprocesses (Folk, 1974). However, theinter-

mittent natureoftransport implied, by ~ the possibly ephemeral Baca braided streams suggests lohg residence times for sedimentswithin the near-surface fluvial environment, and thusfavors the preservation of feldsparsby a dry climate. 126

On theother hand, the sandstones of the upper,

non-lacustrineportions of deltaiccycles and of the distal

braidedalluvial-plain facies wereincompletely cemented,

which resultedin moderate highto porosities and

permeabilitiesand, thus, favored the utilization of these

sandstones as aquifers.Feldspars within these permeable

sandstonesare almost invariably more alteredthan those in

thepreviously described tightly cemented sandstones.

Degreeof alterationranges from slightto extreme.

Alterationtypes include vacuolization, sericitization,

calcitization, cavernousand - dissolution. Cavernous e dissolution, whichproduces secondary porosity, is almost entirelyrestricted to feldspars in the permeableupper-

deltaic and fluvialsandstones (see diagenesissection).

Feldsparsexhibiting secondary porosity occur inthe

majority of these sandstones.

Feldsparsobserved in thinsection are elongate to

subequant, and range in roundness from angularto

subrounded. Feldsparcontent increases upwards inthe basal portion of the Baca stratigraphicsection, andremains

relativelyconstant throughout the remainder of thesection

(Fig. 39). 127

.Percent of Framework Grains 0 10 20 30 40 50 - 30

- J

Middle 'Sandstone Unit G -

Lower E Red Unit C B -34 Sample Number

Figure' 39. . Plot of feldspar abundance as a function of stratigraphic position. Mean grain size of . samples ranges between 1.6 and 2.9+.. 128

Orthoclase (4-238, average 11%of framework grains;

average 74% of total .feldspar). Orthoclase is the most

abundant feldspar in the Baca sandstones whichwere examined

in thin section. All untwinnedfeldspars were classified as

orthoclase,thus small amounts of untwinned plagioclase and

microcline may 'havebeen included inthis category. Due to

its availability,orthoclase is the most common feldspar in

terrigenoussandstones, despite thefact that microcline is

I .general.ly more resistantalterationto (Folk, 1974). Granites and gneisses are the primarysource rocks for

orthoclase.

Microcline(0-lo%, average 3% of framework grains;

average 21% of totalfeldspar). Microcline was presentin

all except oneof the sandstonethin sections examined.

Despite its relativelyhigh stability, microcline is always

less abundantthan orthoclase, even inhighly altered Baca

sandstones.Microcline is dominantlyderived from plutonic,

pegmatite, and gneissicsource rocks (Folk, 1974).

Plagioclase.(O-3%,average 1%of framework^ grains;

average 5% of total feldspar). Plagioclase is presentin about half of~the'sandstones which were examined petro-

graphically. It is evenlydistributed throughout the Baca e stratigraphicsection, which suggests that theplagioclase * 129 was notderived fromcontemporaneous, sporadic supply of airbornevolcanic ejecta by Eocene volcanic centers tothe,

south.Instead, it was probablyderived from oldervolcanic

and/orplutonic rocks exposed in thesource area. The widespreadoccurrence ofminor amounts of volcanicrock

~.fragments throughout the Baca stratigraphic section suggests

a largely volcanic source for the plagioclase.

Rock Fragments (8-29%, average 20% offramework grains)

Rock fragmentspresent in the Baca sandstones e include (in orcierdecreasingof abundance) metamorphic, chert, mudstone, granite/gneiss,volcanic, carbonate,

siltstone and sandstonerock fragments. Most rock-fragment

varietiesdecrease in abundancewith decreasing sandstone

grain size.

Metamorphic -Rock Fragments (2-20%, average 10% of framework grains). Two majortypes of metamorphicrock fragmentswere observed. The first, andmost common,

consists of stronglyundulose, often composite, quartz which , contains numerous alignedmuscovite crystals (Fig. 40). I Fragments of thistype were probablyderived from Precambrianmicaceous metaquartiites in thesource area. e The major othertype metamorphicof rock fragment is 130

Figure 40. Quartzose metamorphic rock fragment. Note abundant inclusions of muscovite. Sample 48, crossed nicols, field width=0.75 nun. 131 composed alignedof illite and fine-grainedmuscovite crystalsinterspersed with small amounts of silt-sized quartz(Fig. 41). These rock fragments often show evidence of ductiledeformation by adjacent framework grains dur'ing compaction.Fragments of this typewere derived from' low-rank,metamorphic rocks, such as argillites and phyllites.Argillaceous metamorphicrock fragments were distinguished from mudstonerock fragments bythe more coarsely crystalline and generally better-oriented nature of their argillaceous and micaceous components,and by the lack

Of redcoloration that is characteristicof most Baca mudstone fragments.

Metamorphic rockfragments from the Baca Formation in the study area were derivedpredominantly frommicaceous metaquartzitesand, to a lesser extent, from phyllites and argillites. However,.due to theeffects of abrasionduring transporton the soft, low-rankmetamorphic rock fragments, thecontribution of detritus by argillites and phyllites to the Baca-Eagar basin was probably much larger than would be assumed byexamination of the sandstones in the study area.

The abundanceof metamoxphic rock fragments increases upward through the basal portion of the Baca stratigraphic section, and remains relativelyconstant throughout the remainder of thesection (Fig. 42). 132

Figure 41. Argillaceous metamorphic rock' fragment which has been ductilely deformed by adjacent grains during compaction. Sample C, cros'sed nicols, field width=0.75 mm. 133

.Percent of Framework Grains 0 10 20 30 40 50 - 30

P’

Middle Sandstone Unit loo 1 G feet o -I-o meters

Lower E Red Unit C B -34 Sample Number

,Figure 42. Plot of. abundance of metamorphic rock fragments as’a Tunetion of stratigraphic position. Mean grain size ranges between 1.6 and 2.9+..: 134

Chert (1-17%, average4% of framework grains).

Chert.(Fig. 43) is a relativelyminor constituent of the

Baca sandstonesin the study area, but it was presentin

everythin section that was examined. Grainsconsisting of

microcrystallinequartz averaging less than 20 microns in

diameterwere classified chert:as finely crystalline'

quartzgrains whose averagecrystal diameter was greater

than 20 micronswere counted as metamorphicquartz. There

appears to be a gradation between chert and finely

crystalline metamorphic quartzin the sandstones studied,

which may indicatethat some of the metamorphicquartz was e derived from meta-cherts.Chert is always of sedimentary t origin(Folk, L974), but may beprimary (e.g. derived from

a silicified limestone)or reworkedfrom anolder sandstone.

Detrital chalcedony was also present in the Baca sandstones

in trace amounts.Approximately halfof this chalcedony is

length-slow, whi-ch is indicativeformationof anin

alkaline,sulfate-rich environment (Folk andPittman, 1971).

Chertabundance decreases upwards in the basal portion of

the Baca stratigraphicsection in the study area, and

remains relativelyconstant throughout the remainderof the

section(Fig. 44). 135

Figure 43. Chert grain. Sample A, crossed nicols, field width=0.75 mm. 136

.Percent of Framework Grains 0 10 20 30 40 50 - 8 I I I I 30

- J *O0 150 Middle' Sandstone Unit loo'1 G -

Lower E Red Unit C B 34

Sample Number

Figure 44. Plot of chert abundance as a functionof stratigraphic position. Mean.grain size ranges between 1.6 and 2.9$. 137

Granite -and Gneiss -Rock Fragments (0-7%, average 1% of framework grains).These rock fragmentsconsist of varyingproportions of feldspar and quartz.Feldspar types includeorthoclase and microcline.Quartz may exhibit straightto strongly undulose extinction. The majority of the granite/gneissrock fragments consist predominantly of one mineral,such as a graincontaining a largecrystal of orthoclase. anda smallerone of quartz. Typesand occurrence of alterationwithin compnent feldspar crystals areexactly the same asthose described previously for feldspar grains.

The relative importance of contributions from graniticversus gneissic sources is difficultto determine.

The occurrence of minor amounts of granitepebbles in the

Baca imply thatat least some of thegranite/gneiss rock fragments were derived from a graniticsource. Granite/ gneissrock fragments andthe remainder of therock fragmentswhich will besubsequently discussed are present in suchsmall quantities that I do not feel it would be meaningful to make any statementsconcerning vertical trends in their abundance.

Volcanic Rock Fragments (0-6%, average 1% of framework grains).Volcanic rockfragments are a widespread, but minor,component of the Baca sandstones in 138 thestudy area. They occurred in aboutone-half of the thin sections examined. Grainscounted volcanicas rock fragmentsrange in appearance from dark,aphanitic grains containinglath-shaped plagioclase crystals (Fig. 45) to grains which aresimilar to, and oftendifficult to distinguish from, chert (see Folk, 1974,' p. 81). The chert-like rockfragments were probablyderived from silicifiedvolcanic rocks (X. L. Folk, 1979, oral comhun.),however, theclassification of many of these grains is equivocal. The presenceofonly minoramounts of volcanic- derived detritus in the Baca sediments in the study area has some importantimplications concerning the geometry of the southeastern marginof the Baca-Eagar basin (C. E. Chapin,

1980, oral commun.). During the Laramide, thearea adjacent to this portionof the basin was thesite of widespread volcanism and plutonism. The factthat only small amounts of volcanicmaterial were incorporatedinto the Baca sediments in theeastern part of theoutcrop belt implies theexistence of a drainagedivide (herein termed the

Morenci uplift; see tectonic framework section)basinward of the Laramide volcanic centers in southwestern New Mexico. The non-peaked stratigraphic distribution of volcanicrock fragments in the Baca suggeststhat these fragments were derived from oldervolcanic rocks (Precambrian?) in the 139

Figure 45. Volcanic rock fragment showing lath-like plagioclase crystals in dark, aphanitic groundmass. Sample 45, crossed nicols, field width =0.75 mm. 140

sourcearea, and not by sporadic airborne supply to the

basin by Eocene volcaniccenters to the south. Cameron and

Blatt (1971) show thatfelsic-silicic volcanic rock

fragmentscan endure several hundred miles of braided-stream

transportwithout a significantreduction in their I .- abundance,which implies thatabrasion during transport

cannotaccount for the nearlycomplete lack of volcanic

detritusin the easternportion of the basin. Thus, the

smallpercentage of volcanicrock fragments in the Baca

sandstones in the study area suggests that volcanicrocks

were a relatively minor lithology in the source areaduring

Laramide time.

Mudstone Rock Fragments (0-5%, average 2% of

framework grains). Mudstonerock fragments occur inabout

one-half of thethin sections examined. . They are usually

red (due to hematite pigmentation) and may contain

significant amountsof quartz and feldspar silt. When not

.masked by red coloration, .the component clays generally show

yellowishbirefringence and an index higher than balsam,

which is characteristic of illite (Folk, 1974, p. 91).

Some mudstonerock fragments exhibit evidenceof ductile

squashing by adjacentgrains during compaction. 141

The redcoloration of the mudstonerock fragments

indicatesderivation from a. redbedsource, such as the

Permian Ab0 Formation or the Triassic ChinleFormation, or

penecontemporaneousreworking of Baca mudstones. The soft,

easily abraded nature of these fragmentssuggests short

distancesof transport land, thus, favorsderivation from

intraformationalsources.

Sandstone -and Siltstone Rock Fragments (0-2% of framework grains). Theserockfragments aregenerally

presentintrace amountsand were observed in about

. one-fourth of thethin sections examined. A complete

gradationinsize of component detrital grains exists

betweenboth types of theserock fragments. Sandstone and siltstone rockfragments are highly quartzose but may

containminor amountsof orthoclase.Grains within these

fragments are tightly cemented with syntaxialquartz; the

originaldetrital grains are sometimesvacuolized, which

facilitates distinguishing between grains andcement.

Sandstone and siltstone rockfragments were derived from

oldersedimentary rocks inthe source area. The highly

quartzose,silica-cemented nature of theserock fragments

precludes the possibility of an intraformational source. 142

Carbonate Rock Fragments (0-8%, average 1% of framework grains).Carbonate rockfragments were'present in aboutone-third of thesandstone thin sections examined.

They oftenare stained red by hematite and aredominantly composed of well-sorted,subequant sparry calcite crystals averagingabout 10-15 microns in diameter.;Carbonate rock fragments'are more abundant in lower-deltasandstones than elsewhere.This, in additionto the fact that the calcite in thesefragments closely resembles the early-diagenetic lacustrinecarbonate cements (seediagenesis section), suggeststhat many of thecarbonate rock fragments were penecontemporaneously derived from withinthe lacustrine basin, and thereforeare more properly termed "intraclasts"

(Folk, 1974, p. 160). The presence of probablePaleozoic limestonepebbles in the Baca, however', impliesthat these carbonate rock fragments were, in part,derived from older limestones exposed in the source area.

The presence of trace amountsof oolites(Fig. 46) was noted in several of thethin sections. The oolites averageabout 0.3 -mm in diameter. They areusually complete and show little or no effects of abrasion during transport, which indicatesthat the oolites were locallyderivee. The sporadicoccurrence of oolites in bothdeltaic and fluvial sandstones implies that the oolites were not formed in situ, but were probably derived intraformationally and transported 14 3

Figure 46. Well-developed oolite in an upper-delta sandstone. Note both concentric and radial structures. Sample 73, plane light, field width=0.75 mm. 144 a shortdistance prior todeposition. Oolites form in current- orwave-agitated, carbonate-rich environments, and havebeen noted in modern lakes,such as the Great Salt

Lake, Utah (Sandberg, 1975), ana in ancientlake deposits

(Swirydczukand others, 1979). The compositionof oolites can be initiallyeither calcite or aragonite, however, aragoniteinverts relatively rapidly to calcite. Assereto andFolk (1976) statethat square-endedrays in pisolites indicate an initialaragonitic composition. No square-ended rays were seen in the few oolites whichwere availablefor petrographicexamination, but, due to the lack of sufficient quantities of oolites which could be inspected, I donot feelthat an initialaragonitic composition can be ruled out.

Heavy Minerals (trace)

Heavy minerals were generallypresent in trace amounts (tl$) withinthe Baca sandstonethin sections, although abundancesof as much asseveral percent were occasionally ,.~observed within placers. Petrographic examination of six heavy-mineralgrain mounts revealedthe presence of the followingspecies (in order of decreasing abundance): magnetite/ilmenite/specular hematite,leucoxene,zircon, garnet, apatite, biotite, and muscovite. 145

Magnetite,ilmenite, andspecular hematite were not

differentiated. The majority of the heavy mineralsrange in size from 0.1 to 0.2 mm. The relative abundance of several of theheavy

mineralspecies (magnetite, ilmenite, specularhematite, and

leucoxene)appears to be a function of thedegree and type

of alteration of theirhost sandstone. Magnetite, ilmenite, and specularhematite are the mostabundant types of heavy

mineralspresent in therelatively little-altered, red Baca

sandstones,usually comprising about 90 percent of theheavy

mineralfraction (Fig. 47). However, the yellow to gray e Baca sandstones, which show evidencefor the passage of negative-Ehgroundwaters (see diagenesissection), contain

little or none of these minerals.Instead, the yellow to graysandstones contain abundant leucoxene (which is present

only in minoramounts in thered sandstones) (Fig. 481, which was presumablyproduced asan alteration product of

ilmenite. A heavy-mineral suite from one yellow-sandstone

sample (54) showed subequalproportions of magnetite/

ilmenite/specularhematite and leucoxene.This incomplete

removal of magnetite/ilmenite/specular hematiteindicates a lesserdegree of negative-Ehalteration of the host

sandstone, which probablytook place by eitherthe passage

of a smaller volume of reducinggroundwater through the e rock, or by interaction with more mildlyreducing 146

Figure 47. Typical heavy-mineral grain mount from a red Baca sandstone (sample 48). Note dominance of magnetite/ilmenite/specular hematite. Whitemm. grains are leucoxene. Field width=0.8 147

Figure 48. Typical hea'vy-mineral suite from' a yellow Baca sandstone (sample ,561, showing dominance of limonite-stained leucoxene 'grains. Note subhedral zircon grain. Field width=O,.% mm. 148 groundwaters. The common occurrence of abundantmagnetite and ilmenitein red sedimentsand the near or complete absenceof these mineralsin non-red areas within,or intercalated with, redbeds was notedby Miller and Folk

(1955), who conclude that the lack of magnetiteand ilmenite in the non-red portionsof redbed sequences is due to their removalby dissolutionin a reducingenvironment. In summary, the heavy-mineral suite of the red Baca sandstones is dominatedby magnetite/ilmenite/specular hematite, whereas that in the yellow to graysandstones are generally dominatedby leucoxene.

Compared with the above-describedminerals, the volumetricimportance of the remainingheavy minerals is minor.Zircon was present in all of the heavy-mineralgrain mounts. It exhibits extreme relief,usually clear to slightlypinkish color, andranges from well rounded to euhedral in shape. Mica typesinclude biotite and muscovite.Flakes of bothvarieties usually show ragged, abraded edges,but a few of the biotite grainsexhibited hexagonal outlines(Fig. 49). Euhedral biotite is most commonly derived from volcanicrocks (Folk, 1974, p. 88).

Apatitegrains are usually clear, exhibit low relief, and range from subhedral toeuhedral in shape. Garnets are clear topink in color, commonly exhibit well-developed conchoidal.fractures, and are most oftenanhedral to 14 9

Figure 49. Hexagonal biotite observed in a heavy-mineral grain mount from sample 48. Width of field =0.75 mm. 150 subhedral in shape.Garnets fromone of thekaolinite- cemented sandstonesamples (51) exhibit crystallographically controlled,etched surfaces. Both precipitation of authigenickaolinite and thedissolution of garnettake place in an acidicenvironment, as will be further discussed

Ln the diagenesis section.

Mudstone Petrology

Baca mudstones in the studyarea are generally red in color,devoid of sedimentarystructures except occasional poorlydeveloped burrows, and commonly containappreciable

amounts of silt. Mudstonesdominantly occur in thebasinal

lacustrinefacies and in the'lower prodelta portion of

fine-grainedlacustrine deltas; their occurrence in the

distalbraided alluvial plain facies and the upperportion of lacustrine deltas is rare and limited thin,to

discontinuous mud-drape deposits.Prodelta and basinal mudstones, exceptfor those in the lowermost portion of the upper red unit, are generallyhighly calcareous. Calcite

content,as determined by weighing crushed samples before

and afteracidization with cold, dilute hydrochloricacid,

ranges between 10 and 15 percent by weight,with the

exception of the basal mudstone in the upper red unit, which

containsabout two percentcalcite. The thin mudstones 151

which represent mud-drape deposits are usuallynoncal- careous. The coloration,red typical of most Baca

mudstones, was probablyproduced diagenetically, as will be

discussed in a later section.

Six thin sections of Baca mudstonesfrom thestudy

area were examined. Most dontainedappreciable (as much as

30 percent)quartz and feldspar silt. All lacustrine

mudstone thinsections exhibited, tovarying degrees, a ', churnedand disrupted texture (Fig. 50) which was probably

producedby bioturbation. This explainsthe general lack of

sedimentary structures in the Baca mudstones and suggests e thatthe lake-bottom muds wereoxygenated, which supports a polymicticregime for the Baca lacustrine system. .. Carbonateswithin lacustrine mudstones usuallyoccur as

homogeneously distributed,small (10-15 micron),subequant

crystals of calcitespar. This calcite is similar in appearance to that observed inthe loosely packed, tightly . cemented frontal-splaysandstones which areintercalated

with the-lacustrine mudstones. These cements formed very

early in thediagenetic history of the Baca lacustrine

sediments, and will be further discussed in the diagenesis

section.Another mode of occurrence of calcite was observed in one lacustrine-mudstonethin section (sample 8). This

sample containedlarge void spaces (as much as 1.5 mm in

diameter)which were subsequently filled with large crystals 152

Fiwre 50. Churned, disruptedtexture typical of Baca lacustrine mudstones,. which was probably produced by intensebioturbation. Sample F,crossed nicols, field width=3.0 nun. 153 of sparry calcite (Fig.51). Many of thesevoid spaces are filledwith a single,large crystal of spar. These features arereminiscent of "bird's-eyestructures" in limestones

(Folk,1959). The presenceoflarge, spar-filled voids impliesvery early cementation (prior to compaction) of the mudstone: cementation must haveoccurred in thelacustrine environment or upon veryshallow burial. Similar features . havebeen reported by Van Houten(1962) inthe red lacustrine mudstones of the Upper TriassicLockatong.

Formationofwest-central New Jersey and adjacent Penn- sylvania.

When not maskedby hematitestain, the clays in the

Baca mudstonesfrom thestudy area generallyexhibit yellowishbirefringence and haveanindex greater than balsam, which is characteristic of illite (Folk,1974, p.

91). X-ray diffractiondata for the clay-sizefraction of six Baca mudstonesamples indicatesthe presence of illite with' minoramounts of montmorillonite and kaolinite.

Similarcompositions for Baca claystheinGallinas

Mountains area havebeen reported by Johnson (1978, p. 99).

ArgillaceousPrecambrian rocks exposed in that portion of the Sierra-Sandiauplift now comprised by the Magdalena Mountains (Sumner, 1980)and were probably a major contributor of illite to the Baca lacustrine system inthe Bear-GallinasMountains vicinity. See Johnson (1978) for 154

Figure 51. Large., spar-filled void spaces in a sandy lacustrine mudstone (sample 81, which are indicative of very early, pre-compactional cementation. Crossed nicols, field width= 3.0 nun. 155 more informationconcerning the distribution of claytypes throughout the Baca-Eagar outcrop belt.

\ PROVENANCE

The occurrence of large amounts of metamorphic- and igneous-deriveddetritus in bothBaca conglomerates (metaquartzite, granite, schist') and sandstones (metamorphic quartz, orthoclase, microcline, metamorphic rock fragments, granite and gneiss rock fragments) indicates that a large portion, possibly the majority,of the Baca sedimentsin the study area were suppliedby a Precambrian source terrane, although some was undoubtedly derived from older sedimentary rocks containing Precambrian detritus. The red coloration e of the majority of the Baca sediments and the abundance of detrital ilmenite and magnetite in these sediments indicate that much of the Baca detritus was not recycled from older sediments but must havebeen derived from a metamorphic or igneous source (Miller and Folk, 1955). Minor occurrences

of plagioclase, volcanic ' rock fragments, and euhedral biotite imply that volcanic rocks made small contributions to the Baca sediments. The presence or significant amounts of detritus derived from sedimentary rocksin both the Baca conglomerates(chert, limestone, sandstone, siltstone, mudstone) and sandstones (chert, carbonate rock fragments, sandstone, siltstone, and mudstone rock fragments) indicate bothextraformational and intraformational sedimentary a . sources.

156 157

There is much evidence tosuggest the progressive

"unroofing" of a Precambrianp1utoni.c and metamorphic source

area during Baca time. The upward increase in feldspar,

metamorphicrock fragments, and metamorphic/total quartz

ratioswithin the basa1,portion of the Baca stratigraphic

section, and thecoincident decrease in the abundance of

chert upward throughthe same interval(Fig. 52), strongly suggeststhe diminishing importance ofolder sedimentary

sources during early Baca time. . 1 canenvision no

mechanism, whether pre- or post-depositional, which can

explainthe observed upsection changes in mineralogy of the a Baca sandstonesexcept forincreasing contributions by Precambrian.plutonic andmetamorphic terranes,atthe

expensesedimentary of sources, during basal Baca

sedimentation in thestudy area. Interestingly, point

counts made by, the writer on fivethin sections from G.L. Massingill's (1979) Baca Formation measured section near the BearMountains show similarupsection changes (Fig. 53).

However, due tothe greater variability in thin-section

grain size and theutilization ofonly, fivethin sections

from the BearMountains stratigraphicsection, I feelthat

the data in Figure 53 shouldbeinterpreted somewhat cautiously. 158

Percent of Framework Grains

J 2oo 150 Middle Sandstone Unit loo 1 e -G feet o L o meters

E Lower Red C Unit B 34 - I I. I 0 .2 .4 .6 .8 1.0 Metaquartzltotal quartz -FeldsparMetamorphic RockFragments - .' -.,------Chert - - "MetaquartzITotal Quartz

Figure 52. Plot of metaquartz/total quartz ratios and abundances of feldspar, chert, and metamorphic rock fragments within Baca sandstones aas function of stratigraphic position. Samples were taken froma section measured by the writerin the study area. Mean grain size of e the sandstones ranges between1.6 and 2.9 +. 159

Percent of Framework Grains 0 10 20 30 40 50

Middle 50 Sandstone Unit

0 meters Lower Red Unit

0 .2 .4 .6 .8 1.0 Metaquartzftotal quartz "- FeldsparMetamorphic RockFragments ""_ Chert "_ MetaquartzfTotal Quartz

Figure 53.' Plot of metaquartzftotal quartz ratios and abundances of feldspar, chert, and metamorphic rock fragments . .. within Baca sandstones as a function of stratigraphic. position. Samples were taken from Massingill's (1979) measured section in the Bear Mountains vicinity. Hean grain size varies widely, therefore caution is advised in the interpretation of this data. DIAGENESIS

Cements

Four types of cements occurwithin the Baca

sedimentary.rocks in thestudy area. These are (in order of

decreasingabundance) calcite, kaolinite, quartz, and

dolomite.

Calcite (0-48%, mean 18%, median.l2%of bulk rockvolume) e Authigenic calcite is the most abundant and earliest cementingagent in the Baca sediments in thestudy area.

There arethree modes of occurrence of calcite in thethin

sections:finely crystalline spar which is associatedwith

lacustrineprodelta and'basinal sediments, large, blocky, pore-fillingcalcite crystals dominantly occurring in

upper-delta and braidedalluvial-plain sandstones, .and

caliche.Caliches are scarce, but widespread,within the

Baca. They occurasthin, laterally persistent, often

nodularhorizons within both lacustrine and non-lacustrine

sediments. Calicheswithin lacustrine rocks presumably represent soils which developedduring lake-level low

stands. A laterallycontinuous zone of rhizoconcretions

0 (calichifiedroot tubes) (Fig. .54) is present in thebasal 160 161

Fi'gure 54. Well-developed calichified paleosol showing numerous tube-shaped rhizoconcretions. 162 portion of the lower red unit in the eastern portion of the study area. In thinsection, caliche characteristically exhibits a wide range of crystalsizes and morphologies.

Crystal size ranges from micrite ((4 microns) to blocky, equantcrystals of sparas much as 60 microns in diameter

(Fig. 55). Detritalgrains withinL calichesareoften surrounded by horrentlyoriented blades of spar. Clay- coatedgrains, typical of soilzones, are common. Caliches areproduced.in the B-horizonof alkaline soils in arid and

semiarid ' climates (Goudie,1973). The widelyvarying calcite crystal size in the caliches reflect the fluctuating vadose-zonechemical environment. Incipientcaliches have formed in thearchaeological site at Tel Yinam, Israel, withinthelast 1,300 years (R. L. Folk, 1979, oral commun.). Caliches produced in lacustrine mudstones during lake-level low stands, however, may have formed over a considerablyshorter period of time, due tothe initial highlycalcareous nature of thesesediments.

Large crystals of blocky,pore-filling spar are a common cement type in thesandstones of thedistal braided alluvial-plain facies and the upper, non-lacustrine portions of deltas.Occurrence of thistype of cement within lower-delta and lacustrine basinal sediments is limited to minor, spar-filled voidspaces within lacustrine mudstones, as has been previouslydescribed in the mudstone petrology 163

Figure 55. Baca caliche as seen in thin section. Note wide range in calcite crystal sizes and the presence oSnumerous grains whichexhibit hematitic clay cutans. Sample 34, crossed nicols , ,field width=O. 8 mm. 164

section. The sparcrystals are moderately well sortedwith

respectto size (Fig. 56); the mean grainsize ranges

,betweenabout 80 and 250 microns.Poikilotopic calcite cementswere occasionallyobserved. Coarsely crystalline

calcite sometimes partiallyreplaces feldspar in non-

lacustrinesandstones (see feldspar ,section). In thin

section, coarsely crystalline spar cements generally exhibit

patchydistribution. Sand grainswithin spar-cemented

patchesare typically loosely packed, indicating that these

areas were cemented early(prior tocompaction) in the diagenetichistory of thesediment. The patchy,incomplete 0 nature of theearly calcite cementation within upper-delta and distal braided alluvial-plainsandstones resulted in

relativelyhigh porosities and permeabilitieswithin these

sandstones and favoredtheir utilization as aquifers by

later-diageneticgroundwaters.

Severalparallel-oriented, brown, cigar-shaped

concretionsas much as 6 inches (15 cm) in diameter were observedwithin sandstones of the middlesandstone unit at

NW l/4, SW 1/4, NW 1/4, Sec. 6, T.lN., R.6W. Sand grains within these concretions exhibit very loose packing, andaxe

' tightly and completely,cementedwith coarse sparry calcite

which is morphologicallyidentical to that described above

(Fig. 57). The reasonsfor the parallel orientation, the 0 cigar-likeshape, and thetightly cemented nature of these 165

Figure 56. Coarsely crystalline spar cementin an upper- delta sandstone (sample C). Crossed nicols, field width=0.8 mm. 166

Figure 57. Coarsely crystalline sparry calcite cement in cigar-shaped sandstone concretion (sample 49). Loosely packed fabric is indicative of very early cementation (pr.iorto compaction). Crossed nicols, field width=0.8 nun. 167

concretions are not known. The moderatelywell-sorted nature of the coarsely crystallinespar implies cementation took place under

relatively uniform, stable geochemical conditions. This,

and the lack of characteristic vadose-cement indicators such

asgravitationally oriented and meniscate cements (Dunham,

1971), shows thatcementation occurred below thewater

table. The generallyloosely packed nature of thespar-

cemented grainsindicates cementation prior to burial of

more than a few tens of feet(Jonas and McBride, 1977, p.

73). The subequant,subhedral morphology ofthe calcite

crystalssuggests that the magnesium/calcium ratio of the

porewaters atthe time of crystallization was less than

about 2:l (Folk, 1974,p. 176).

The third variety of calcite cementwhich is present

in the,study area is fine-grained(10-15 micron), very

well-sorted,sparry calcite, which is abundant in lower-

delta andbasinal. sediments. The finegrain size and

well-sortednature of thiscalcite is similar tothat of microspar, which is produced by aggradational neomorphism of

micrite (Folk,1965). .However, due to the completeiack of

micrite in the Baca (with the exception of that in caliches)

and thefact that lower-delta and lacustrinebasinal

sedimentsnever contain more than 48 percentcalcite .. (greater abundancesof carbonate could not be .accommodated 168

as pore filling in veryloosely packed sandstones and would

necessitatethe formerpresence of carbonate mud matrix in

thesediments), I believethat the fine-grained lacustrine

calcite formed assimple pore fillings and notthrough

neomorphism. Thus, the term microspar is notapplicable to

thesecarbonates. Fine-grained calcite is commonly stained

red by hematite(Fig. 58) and occurs in bothlacustrine

mudstonesand thin-frontalsplay sandstones. Detrital

grains within thefrontal-splay sandstones are very loosely

packed, which indicates that cementation took place prior to compaction. The lacustrine sediments aretypically tightly e and completely cemented withearly-diagenetic spar, resulting in ve.ry low porosities and permeabilities which precluded alteration by later groundwaters. - The fine-grained,pore-filling sparry calcite

crystals are generally well sorted, indicating precipitation in a uniform,stable, alkaline geochemicalenvironment. The

lack of vadose-zonelackof cementation indicators such as

gravitationallyoriented and meniscatecements (Dunham,

1971) and thewell'-sorted nature of thecalcite strongly

suggest that the fine-grainedspar formed below thewater

tableorwithin the lacustrine environment. The loosely

packed nature of the spar-cemented,frontal-splay sandstones

impliesthat cementation took place before burial of more

0 than a few tens of feet(Jonas and McBride, 1977,p. 73). 169

Fit_ ! 58. Hematite-stai ne-_ ined sparry calcite in a f.ronta1-splay sandstone '(sample 13). Note poorly compacted nature of this early- cemented lacustrine sandstone. Crossed nicols, field width=0.75 mm. 170

Subequant calcite crystalmorphologies- indicate precipitation from pore waters withmagnesium/calcium ratios of lessthan about 2:l (Folk, 1974,p. 176). The boundarybetween upper delta sediments which are incompletelycemented with coarsely crystalline spar and lower-deltaand basinal sediments which are tightly cemented with fine-grained calcite is sharp and generallycorresponds tothe approximate position of theoriginal lake level during deltaic progradation. The contrast in extent of earlycalcite cementation betweenupper and lower delta sedimentssuggests that the lower-delta and basinal deposits were cemented withinthe lacustrine environment: if both upperand lower deltasediments werecemented below the water tablefollowing burial, one would expectboth to exhibit similar degreesof cementation. The .finer-grained nature of thelacustrine carbonates may be explained by:

(a) inhibition of crystal growthby ions,such as magnesium, providedby clays in the lacustrine environment(Folk, 1979,. oral commun.), or (b) rapidcrystallization rates caused by highconcentrations ofcarbonate within the closed-lake system. The presence of fine-grainedsparonly in lacustrinesandstones and notin non-lacustrine 'sandstones of- similar grainsize shows thatcarbonate crystal size was notstrongly controlled by thediameter of the interstitial spaces. The incompletecementation upper-deltaof and e 171 distalbraided alluvial-plain sandstones coarselyby

crystallinecalcite probably occurred below the water table

followingshallow burial. Slow crystallization from dilute

phreat'icwaters or the lack of substantial amounts of clay

which couldinhibit crystal growth may explain the

relatively large calcite crystal size in thesesandstones.

Quartz (0-9%, mean l%, median 0.5% of bulkrock volume)

Followingearly-diagenetic calcite, quartz Cements

i were the next to form in the Baca sandstones in thestudy

area. The majority of theauthigenic quartz in the Baca

sandstonestakes the form of simplesyntaxial overgrowths

(Fig. 59), but a small amount (less than 5%) occursas

radiallyoriented flamboyant megaquartz (K. Millikan, 1979,

oral commun. ) (Fig. 60). ,Quartz cements were onlyobserved in upper-delta and distal braided alluvial-plain Sandstones;

early,complete calcite cementation precluded the formation

of quartz subsequentand cements in the lacustrine sediments. Indeed, even in thenon-lacustrine sandstones

quartz and calcite occurantipathetically,. Detrital quartz

grains were oftenobserved which exhibitedsyqtaxial quartz

overgrowthsexcept along those portions of thegrain

boundarywhich are cemented withcalcite spar (Fig. 61).

Recognition of grainswith older, reworkedovergrowths was I 172

Figure 59. Syntaxialovergrowth on detritalquartz grain. Sample 45, crossednicols? field width=0.75 mm.

174

Figure 61. Inhibition of syntaxial quartz overgrowth by coarsely crystalline sparry calcite cement. Sample B, crossed nicols, width of field= 0.75 mm. 175 difficult. However, theoccurrence of rare,isolated quartz grainswith syntaxial overgrowths in theearly-cemented lacustrinesediments indicates that a smallpercentage of thequartz overgrowths observed in thenon-lacustrine rocks wereprobably reworked. The presence of minute hematite flecks(the primary source of red coloration in the Baca; see colorationsection) beneath quartz overgrowths suggests thatthe hematite, or its hydratediron-oxide precursor, formed prior to quartzcementation. The highest concentrationsof authigenic quartz wereobserved near the top of the Baca stratigraphicsection in thestudy area

(Fig. 62), which suggeststhat much of thesilica for these cements was derived from theoverlying volcanics and, thus, impliesthat quartz cementation in the Baca tookplace followingthe inception of volcanism in earlyOligocene time. P. similar upward increas,e in authigenicquartz was observed by the writer inthin sections from Massingill's

(1979) Baca measured section in the BearMountains vicinity

(Fig. 63). The solubility of quartzincreases with increasing pH and temperature(Jonas and McBride, 1977).

The onlyconclusions whichcan be drawn concerningthe geochemical natureof the interstitial waters during quartz precipitation is thattemperatures.were probably relatively low (< 200' C?) and the pH was notstrongly alkaline (<

10.5). 17 6

Percent of Bulk Rock Volume

*O0 l- 50 Middle Sandstone Unit

Lower Red Unit

S amp1 e Number

Figure 62. Plot of authigenic quartz content in studyarea sandstones as a function of stratigraphic position. 177

Percent of Bulk Rock Volume -0 5 10 15 20 25 50

200 Middle Sandstone 70, Unit 100 50

27 . 0 .o feet meters Lower Red 16. Unit

-3. Sample Number

Figure 63. Plot of authigenic quartz content in Baca sandstones' as function 0.f stratigraphic position. Data from point'counts madeby the writer'on . . thin sections from Massingillis (1979) measured section in the bear Mountains area. . 178

Followingquartz, kaolinite was thenext cement in

thediagenetic history of the Baca sediments inthe study

area. The distribution of aufhigenickaolinite is the same

as that of quartz; it occursonly inupper-delta and ,. fluvialsandstones. Kaolinite occurs as a porefilling between grains which havenot Seen previously cemented with

calciteorquartz. Precipitation of bothkaolinite and

quartz in interstitial areas which were not tightly cemented

by early,patckily dis-tribuked sparresults in the common e occurrence of both kaolinite and quartzwithin a givenpore space.In other words, quartz and kaoliniteoften occur

together,whereas calcite rarely occurs wiynin a givenpore

spacewith either of these cements. The more closelypacked

nature of thekaolinite-cemented areas within sandstones relative to adjacentcalcite-cemented areas indicates a

period of compaction srior to kaoliniteprecipitation. In

thin section, kaolinite appears as tiny flecks or vermicular

stacks(Fig. 64, 65) as much as 30 microns inlength and

exhibits very low birefringenca and low positivarelief. X-ray diffractiondata on theclay-sized fraction fro= one

clay-cementedsandstone (sample 56) confirmsthe presence of

kaol.inite. Tkie comnan vermicular'habit of thekaolinite * observed inthin section and its relativelylate occurrence 179

Figure 64. Authigenic kaolinite. Bluish tint is due to impregnation of kaolinite microporosity with blue epoxy. Sample 48, crossed nicols, field width=0.15 mm. 18 0

Figure 65. Scanning electron photomicrograph of vermicular stacks of kaolinite crystallites ona quartz sand grain. Sample 56, 5,OOOX. 181

in thediagenetic history of the Baca sedimentsindicate

that thekaolinite is authigenic.In contrast to the other

Baca sandstone cements, kaolinitecontains appreciable

microporosity,especially when arranged in vermicular

stacks.This microporosity results in permeabilitiesas

much as8.8 mD (Jonas andMcBride, 1977, p.81-82), and

thus may significantly enhance theaquifer characteristics

of kaolinite-cementedsandstones. Microporosity produces

the typicallight-blue tint of kaolinite when observed in

thin sections whichhave been impregnated with bl,ue epoxy.

Kaolinite formsunder slightlyacidic conditions e where there is anexcess of silica but few potassium and magnesium ionsin solution (Jonas andMcBride, 1977). The

common association of corrodedfeldspars and kaolinite

cement in thinsection, the instability of feldspar under

acidic conditions requisite for kaolinite precipitation, and

thepresence of a few feldspargrains which havebeen

replaced by kaolinite imply thatsecondary-porosity-

producingcavernous dissolution of Baca feldspars took place

synchronouslywith kaolinite cementation. The etching.of

garnetswithin kaolinite-cemented sandstones probably also

tookplace at this time. Feldspardissolution may have

provided some of thesilica necessary forkaolinite

precipitation, however, due tothe relatively small volumes

of feldspar which havebeen dissolved, I do notfeel that e 182 feldspar dissolution could have supplied all of the silica.

Dolomite (0-lo%, mean 1%,median 0% ofbulk rock volume)

Dolomite was thefinal cementing agentin the Baca

sandstones'in the study area. \ Its occurrence is restricted

tonon-lacustrine sandstones, as is the. case withquartz and

kaolinitecements. Unlike the earlier cements, however,

dolomiteonly occurs in the yellow-gray(non-red) Baca

sandstones. In fact,oxidation of dolomite tolimonite

awing modern outcropweathering appears to be the 0 predominant source of yellowcoloration in these sandstones (seecoloration section). Dolomite commonly contains small

amounts (as much as a few percent) of iron (R. L. Folk,

1980, oral commun.). Inthin section, dolomite generally

occurs as well-formed rhombohedrawhich oftenexhibit

alternatingiron-rich and iron-poorzones (Fig. 66). It usually is not a simplepore filler, but appears as a

replacementproduct of kaolinite,calcite, and, to a lesser

extent,quartz (Fig. 67). Dolomitemost oftenoccurs as

small,isolated rhombohedra withinsandstones. except where it replacesrelatively large calcite- or kaolinite-filled void spaces, in which case it appears as numerous, poorly

developed,intergrown rhombohedral crystals. 183

Figure 66. Dolomitecement showing alternatingiron-rich and iron-poorzones. Sample €3, crossednicols, field width=0.75 mm. 184

Figure 67 Dolomite rhombohedron replacing adjacent quartz grain. Sample B, crossed nicols,field width= 0.75 mm. 185

The formation of golomite within the Baca sandstones apparently is relatedboth spatially and temporallyto the intrusion of Miocene mafic dikes in the area. Figure 68 shows thecoincidence in occurrence yellow-gray,of dolomite-bearing sandstones withthe location of dikes in the study area. Dolomiteforms in alkalineenvironments.

The initial presence ofsmafl amounts of Fe8 .in thedolomite

(as shown by recent limoniticalteration) implies .reducing conditions at the time of precipitation. The following is a possible model €orthe formation of the Baca dolomite cements. During dike emplacement, oxidizing,carbonate-rich groundwaters were migratingthrough permeable Baca upper- delta and braidedalluvial-plain sandstones. As these watyrsneared theintrusions, they progressively became reduced,presumably due to the introduction of reductants (Hz

S?) by the dikes. These reducing groundwatersdissolved , hematite and magnetite and altered ilmenite to leucoxene in thehost sandstone (see heavy mineralsection). At the same time, thegroundwaters became heated by thenearby dikes, causingdolomite toprecipitate (carbonate solubilities decreasewith increasing temperatures). Much o€ theiron incorporated within thedolomite was probablyderived from the above-describeddissolution of heavyminerals. Thus, this model presents a geochemicallyviable, simple explanationbothfortheoccurrence of yellow-gray, 186

.. Figure 68. Generalized map of the study area showing coincidence of occurrence of dikes and areas in which Baca sandstones are yellow-gray in color (stippled areas). e 187 dolomite-bearing sandstones in proximity to dikes, and the anomalousheavy-mineral suite within these yellow-gray sandstones.

Source of Coloration ..

mere are two modes in the coloration of the Baca sediments in the study area (Fig.69). The majority of the

sediments average about 10R 6/2 (weak light red-orange). Upper-delta and distal braided alluvial-plain sandstones which are near mafic dikes are often non-red, averaging e about lOYR 7/3 (very light yellow-orange).

Red Coloration

In thegrossest sense, theories concerning the genesis of redbeds can be lumped intotwo basic schools of thought: soil-derived and diagenetic red coloration. The latter process is the most important in the red Baca sedimentary rocks in the study area.

Soil-derivedRed Coloration. - Soil-derived red coloration is oneof the oldest and most well-rooted theories of redbed genesis, and was popularized by Krynine 0 (1949,1950) and Van Houten (1948). According to this 188

V. uenk /I i-

2.5~ 5~ 7.5~ loa 2.5~~ SYR 7.5~~~OYR 2.5~ 5~ Red orange Yello" HUE

Figure 69. Distribution of coloration for 52 Baca sandstone samples from the study area. Values were determined by comparison with Munsell Soil Color Chart. Color terminology after Folk (1969). 189

theory,redbeds result fromsediments which were derived from lateritic soils which formed in igneousor metamorphic

terranes in tropicalareas with seasonally high rainfall.

In these areas, iron-bearing heavy minerals in the A horizon'

of lateritic soils become severelyaltered and leached by suchprocesses as hydrolysis and attack by carbonic and

humic acids.Destruction of theseheavy minerals yields

clayminerals (also derived from theweathering of

feldspars) and hydratediron oxides (limonite) which are

transported downward by percolatinggroundwaters. In the B

soil horizon illuviation takes place and these hydrated iron e oxides and clayminerals form cutans(clay coats) on grains. Upon erosion, these soilsprovide large amounts of iron, in

the form of grainscoated with ferrugenous clay, to the

basin of deposition. When deposited,the sediment is

yellowish brown in color due tothe hydrated nature of the

iron oxide (Van Houten, 1964). Post-depositional . dehyd-

ration results in hematite forming withinthe cutaris, giving

the redbeds their coloration.

Diagenetic Red Coloration. Walker (1963, 1967) ~ proposedthat redbeds can form diagenetically by

intrastrataldissolution of iron-bearing heavy minerals. By

this method,any heavy-mineral-bearingsediment, regardless

ofthe climate in the sourcearea, can later become red if 190

theright set of conditionsprevails inthe basin of

, deposition.Igneous or metamorphic sourcerocks are

necessaryto provide the heavy minerals (Miller and Folk,

1955).After burial, the iron-bearingheavy minerals

(particularlyiron silicates) areleached by intrastratal

solutions,yielding hydrated ferric oxides and clay

minerals. Dehydration results inthe formation of hematite

which providesthe red coloration.Dehydration can take

place below the water table ifthe water is alkaline

(Walker,1974).

Diagenetically produced redcoloration is primarily e dependent- upon conditionswithin the basin of deposition. According to Walker (1967,366),p. these conditions

include:(a) occurrence of iron-bearingdetrital grains,

(b) post-depositionalconditions that favor the intrastratal

dissolution of iron-bearinggrains, (c) an interstitial

%-pH environment thatfavors the formation of hydrated

ferricoxides, (d) absenceof subsequent reduction of ferric

oxides, (e) enough time fordehydration to ~ hematite to

occur, and (f)relatively warm temperatures(above about

'65-70 F).

Source of Red Coloration in the Baca Formation. " "- Through thepetrographic microscope, the predominant source e Of redcoloration in the Baca sandstonesin the study area e 191 appearsto besmall (0.5-3.0 micron) flecks of hematite

which areuniformly distributed on thesurfaces of sand

grains. These flecksare present on boththe corners and

embayments of grains, but do notoccur atcontact points

betweengrains (Fig. 70). Similar flecksare assumed to be

the source of coloration of the red Bacamudstones, however,

thebirefringence of theclays and carbonate in theserocks made determination of the nature of the hematite impossible.

Morphologically indistinct evenunder high power withthe

optical microscope,these flecks are usuallyseen with the

scanningelectron microscope to be plate-like in shapewith e ragged,irregular edges, although hematite crystals with euhedralterminations are not uncommon (Fig. 71). No clay cutans are associated with these flecks.

Three additional,relatively minor modes of occurrence of hematite were observed in the Baca sandstone

thinsections. A few magnetite/ilmenite/specular hematite grains exhibited dissolution features and were surrounded by

hematitehalos. In two thinsections, minor amounts (< 1%)

of pore-fillinghematite cement were observed. This appears

black in handspecimen,however, and thusdoes not contribute

tothe red coloration of the Baca Formation.Approximately

two percent of the sandgrains examined 'withthe

petrographic.microscope displayed~ hematitic clay cutans. * Thesecutans typicakly show theyellow birefringence of 192

Figure 70. Hematite flecks uniformly distributed on Baca sand grains, except at contact points between grains. Sample PI plane light, field width=0.15 mm. 193

Figure 71. Scanning electron photomicrograph of euhedrally terminated hematite crystals on quartz sand grain. Sample P, 7,000~. 194

clay:hematite occurs as a diffusestain within the clay

coats.Possible sources of theclay-coated grains include

soil zones in the source area or older Paleozoic or Mesozoic

redbedsequences.

I believe that the majority of the red coloration in

the Baca Formation was diagenetically produced.Evidence

forthis includes: (a) the presence of euhedralhematite crystalssimilar to those described in otherdiagenetically

produced redbeds (Walker, ,1967, 365):p. (b) uniform

distribution of hematiteflecks on bothcorners and

embayments of grains and lack of flecksat contact points e between grainsindicates that the hydrated iron-oxide precursorhematitethe to was precipitatedpost-

depositionally; (c) thepore-filling nature of thehematite

cement shows thatpost-depositional precipitation of iron

oxides did occur: (d) paucity of augite and hornblende in

heavy-mineral suites from,sandstones derived from

predominantlymetamorphic and igneoussource areas (as is thecase with the Baca) indicatesthat intrastratal

dissolution of theseminerals took place (Walker, 1967, p.

,365) (thegenerally fresh nature of. the feldspars in

early-cemented Baca sandstones shows thatthese heavy

minerals were probably not destroyed within source-area soil

zones):(e) general lack of clay-coatedgrains indicates e thatthe red coloration did not form pedogenically; (f) e 195 hematitehalos around iron-bearingheavy-mineral grains

indicate that intrastratal dissolution has occurred.

Intrastrataldissolution of iron-bearingheavy

mineralsprovided the iron for the hematite colorant. A

largeportion of this iron, however, may have been supplied

by authigeniciron sulfides .. whichformed' withinthe

lake-bottomsediments. These ironsulfides are produced by anaerobic,sulfate-reducing bacteria that may havelived in

thelacustrine sediments whichprobably became anoxic

followingshallow burial. Sulfate-reducing bacteria

typically producesmall (0.03-1.0 mm) pyriteframboids e (Love, 1971). No such framboid-shapedhematite was observed,however, which indicatesthat if authigenic iron

sulfides existed in thelacustrine sediments, they were not 1 pseudomorphously replacedby hematite or its precursor. The

lacustrine sediments are,generally redder thanassociated non-lacustrinerocks, which implies higher concentrations of

hematite in thelacustrine rocks and, thus, may indicate the

formerpresence of authigenic iron sulfides in theserocks. The presence of hematiteflecks on thesurfaces of

sand grainswithin calcite-cemented portions of red,

non-lacustrinesandstones and the occurrence of these flecks

both onsand grains and interspersedwithin spar cement in

lacustrine sediments indicatesthat iron mobilization took

placevery early (prior to and during calcitecementation) e 196 in thediagenetic history of the Baca sediments. Dehyd-

ration of thehydrated-iron-oxide hematite precursor may

haveoccurred during lake-level low standsor below the

watertable in an alkalineenvironment (Walker, 1974). The

presence of caliches and otherearly-authigenic carbonates

in the Baca impliesthat groundwaters were alkaline during

theearly portion of thediagenetic history of the

sediments.

Folk (1976) statesthat redbedsundergo an

evolutionary change in colorthrough time (Fig. 72). Although the yellow-gray Baca sandstones fit well into this e evolutionarysequence, the pale-red sandstones do not. Red Baca sandstonesexhibit lower saturationvalues (average /2)

than would be expectedfor an Eocene redbedsequence:

according to Folk (1976) thissaturation value is .more typical'of an Early Paleozoic sandstone. Moreover, the

averagevalue (lightness) of the red Baca sediments (6/) is somewhat lightfor an earlyTertiary redbedsequence. The

relativelycoarsely crystalline nature of the Baca hematite - is probablythecause for the largediscrepancy between

predicted andobserved saturationvalues. The reasonfor

thiscrystallinity is not known, but it may be relatedto

the fact that the hematite formed. diagenetically and is not

soil-derived (R. L'. Folk,1980, oral commun.). 197

RED RED-ORWGE' ORANGE YELLOW-OR- SR ZSR.. IOR , 25YR . 5YR ZSYR IOYR

Fig. 7 2 Change of mlor with gmlogic axe shown on a pldt of hue m. raturatioe Third color dimasion of lightness shova by the numbers 4/ etc and degree of dackening of the squires. C, initial unweathered quartzose sand; A, Saharan sandr. data fmm Alimm; SI.Slmpson Deserc surface fluff of Holocene age; S.. Sipson Desert, modal wlor nl mnrrc of fine sands; S,. Simpron Desert, mbr of hematite-clay cutan$ in pits. formed duringPleistocene latcritiration; P, otherPleistocene sediments; Hm.color of powdered pure hematite; F'y 4- hl, wlor of Mcramic md younger Palmzoic red rocks: Po, Older Palmzoic and Pr-bnan rocks. Procases that create thue Fhvlges shown by the "~olutionuy"armx~

(Modified from Folk, 1976) 198

Yellow-gray Coloration

Permeable upper-deltaand distal braided alluvial-

plainsandstones which are located near mafic dikes are

oftenyellowish-gray to orange in color (average lOYR 7/3).

. The majorityof this coloration is cue toalteration of

iron-bearingcarbonate cements to limoniteduring recent

outcropweathering, as was previouslydiscussed in the

dolomitesection. Additionally, a significantportion of

the non-red colorationin the Baca sandstonesoccurs as a

diffuse limonitic stain within calcite and kaolinite cements

and on detrital grains.In summary, the eventsleading to a the developmentof yellowish toorange coloration in the

Baca were: (a) oxidizing,alkaline groundwaters which

migratedthrough permeable non-lacustrine sandstones became

reduced due to interaction with reductants (HZS?) associated

with dike emplacement in Miocene time (Thesenegative-Eh

groundwatersdissolved hematite and magnetite and altered

ilmeniteto leucoxene in the .host sandstones.Dissolved . iron from these heavyminerals was incorporatedinto dolomitecements and formed diffusestains within the

sandstones); (b) Heating of groundwater by the dikes resultedin precipitation of dolomite; (c) Following.late

Tertiary uplift anderosional stripping, these reduced-iron- 0 ‘bearingsandstones were exposed to near-surfaceweathering 199

conditions,resulting in oxidation of thereduced iron to limonite, which gives rise to the present-daycoloration of

thesesandstones.

Porosity ,

Porositydistributions played a veryimportant role

inthe determination of the sequence of diageneticevents

which affectedthe Baca sediments. Knowledge theof

porositydistribution within the sediments may proveuseful

toexplorationists in delineatingprobable pathways of 0 migrationuranium-bearing of groundwaters. Porosity determinations were made usingpoint-count data from

blue-epoxy-impregnated thinsections. Two types of porosity,primary andsecondary, existwithin ,the Baca

sediments in the studyarea. Average overallporosity in

the. Baca (primaryplus secondary porosity) is about 3.6%,

andranges from essentially zero to ten percent.

Primary Porosity

Primary porosityoccurs as intergranular areas in

Baca sandstonesthat have not been filled with' cement.

Lower-delta and lacustrine basinalsediments exhibited e virtually no primaryporosity, due toearly andcomplete 200

cementationwith fine-grained calcite. Priniary porosity

within non-lacustrine sandstones averages about 3.2%, and rangesfrom practically zero to ten percent. Of the non-lacustrinesandstones, the yellowish-gray sandstones contain greater primary porosity thando the red sandstones, the !average values for these sandstones being about 5.1 and 1.5%, respectively. Microporosity in kaolinite cements may contribute slightly to the overall porosity of the Baca Formation.

Secondary Porosity

Secondary porosity is diagenetically produced and predominantlyoccurs as cavernous dissolution features within feldspars. Skeletalized magnetite/ilmenite/specular hematite grains within yellow-gray sandstones are a minor sourceof secondary porosity. Early-cemented lacustrine sediments exhibit virtually no secondary porosity. Within upper-delta and distal braided alluvial-plain sandstones, , the average secondary porosity value is about 0.4%, and ranges from essentially zero to two percent. Secondary porositywithin red, non-lacustrine sandstones averages about 0.2%' whereas that.- within yellow-gray sandstones averages about 0.7%. Cavernous dissolution of feldspars 0 took place intrastratally, due to interaction with acidic 201

groundwaters(see feldspar and kaolinite sections). Skeletalizationof magnetite/ilmenite/specular hematite grainswithin yellow-gray sandstones probably occurred during dolomite precipitation in an alkaline, reducing environment.

Diagenesis Summary

A flow chart depicting the diagenetic sequence of events for the Baca sediments in the study area is presented in Figure 73. Thepresent-day characteristics of the sediments are highly dependent on their environment of deposition. Table 7 summarizes the distribution of cements, heavy minerals, porosity, and coloration among the various Baca,facies. The lower-delta and lacustrine basinal sediments tend to be finer grained and less compacted than the non-lacustrine sediments. A plot of percent cement plus primaryporosity vs. grain - size (Fig. 74) indicates a trend towards increasingly looser packing in the fine- grainedlacustrine sediments, and reflects the early, pre-compactional cementation of these sediments. Figure 74 also shows the presence of yellow coloration only in the coarse-grained, non-lacustrine sediments. 202 DBFJSITION

Figure 73. Diagenesis flowchart for the Baca sediments within the study area. WILINM wnosnr HFAW HIHEPALS " "

UPPER-DELTA AYD DISIN. mmw ALLWW " PLAIN swmms

LOUER-DELIA AND LACUSTRINE red BASINAL swmws

Table 7. Distribution of coloration, cements,porosity, andheavy minerals in relation to the various Bacat facies.

N 0 W 204

0 r COLORATION

1 GRAIN SIZE

I. I I I I 1 10 20 30 40 50 PERCENT CEMENT PLUS PRIMARY POROSITY

. Figure 74. Plot of grain size s.percent cement plus primary porosity showing tendency towards looserpacking in the fine-grained lower-delta and lacustrine basinal sediments. Note the presence of yellow-gray coloration only in the coarser grained, non-lacustrine sandstones. PALEOCLLMATE

. Interpretations of thepaleoclimate in west-central

New Mexico during Baca time range from aridto semiarid

(Snyder, 1971; Potter, 1970) to subhumid (Massingill, 1979;

Johnson, 1978). Evidencefrom thisstudy points to a hot,

semiarid,possibly savanna-.like climate in the study area

during the Eocene. The probableclosed nature of the Baca

lacustrine system in theBear-Gallinas Mountains vicinity is

indicative of semiaridor arid conditions (Langbein, 1961).

Rare paleocalicheswithin the Baca were pedogenically a produced in an aridto semiarid environment. Goudie (1973) statesthat caliche formation is favored in semiarid

climates where rainfall is seasonal(as i’n a savanna) and

less thanabout 20 in (50 cm) annually. The presence of

titanothere, turtle, and crocodilefossils in the Baca

exposuresnear Quemado and Datil (J,, A. Schiebout, in progress)suggest a savanna-typeclimate. The common

occurrence of rootmottling and the lack of eolianfeatures

indicatesvegetative stabilization of sediments and,thus,

! impliesthat arid conditions didnot prevail, since

sufficient water must have been availableto support this

vegetation. Modern ephemeral braided streams which are

similar to the flashy-discharge Baca braided streams usually

e occur in arid and semiaridclimates. However, the 205 206

pre-Miocene lackof abundant upland grasses may havecaused high sediment yields, large amounts of runoff, .and flashy

riverdischarge characteristics even in wet climates

(Schumm, 1968). The initial steps inthe development of

diagenetic red pigmentation are favored by high temperatures

andan alkalinegroundwater chemistry (Walker, 1967), which are commonly associated with semiaridclimatic conditions.

Feldsparswithin Baca sandstoneswere generally fresh upon depositionand suggest preservation by arid or semiarid climatic conditions.

, ECONOMIC GEOLOGY

Uranium Occurrences

Uranium is theonly known economicallyimportant

resource which occurs;inthe Baca Formation. Potter (1970,

p. 34-35) describedthree minoruranium occurrences

associatedwith carbonaceous materialswithin the lower red

and middlesandstone units in BacaCanyon. Uranium

mineralizationoccurring near the contact between the Baca

Formation and Cretaceousrocks in the Red Basinarea north

of Datilhas been described by Griggs(1954) and Anonymous

e (1959). TheRed Basindeposits may be economicand are

currently beinginvestigated by Gulf Minerals, Inc.

Bachman, Baltz, and Griggs (1957) reportseveral uranium

occurrencesalong Jaralosa Creek, west of the Bear .. Mountains. One of these,the Hook. Ranch prospect, lies

withinthe study area andwas examined by the writer. It

consists of severalexploration pits excavated into

delta-front and lower delta-plainsandstones of thebasal

deltaiccycle of the upperred unit. The uranium-bearing

. sandstones are yellowish gray to gray, medium to verycoarse

grained,conglomeratic, and contain much carbonaceous

materials. Carbonaceous debris ranges toas much asseveral e centimeters in length and is more abundant in theprospect I 207. 208

sandstonesthan innon-mineralized Baca sandstones. One

sandstonethin section (#73) .from the Hook prospect was

examined. It exhibitedrelatively high porosity (both

primary and secondary) and contained calcite, kaolinite, and

dolomitecements. Leucoxene was the predominantopaque

heavymineral. Alteration ofilmenite toleucoxene by

oxygen-deficientgroundwaters has been reported by Adams and

others(1974) in the uraniferoussandstones of the Jurassic

MorrisonFormation.

Ore Genesis

Baca uranium deposits,such as the one at Hook Ranch, probablyoriginated by concentration of uraniumfrom

verydilute, oxidizing groundwater solutions by the reducing

action of carbonaceous debris and,possibly, by reaction

with negative-Ehwaters which were associated with the

1 emplacementofmafic .dikes.Reduction of uranium-bearing solutionsby sulfide-rich groundwaters leaked upward along

faultsin south Texas has beenreported by Gallowayand

Kaiser (1979). Uranium is highlysoluble inoxidizing

aqueoussolutions, but precipitatesunderreducing conditions,usually to form complex organic compounds (Douros,1967). The thicksequence volcanicof rocks e overlying the Baca Formationprobably were the source of the 209 uranium, although intrastratal dissolutionof heavy minerals may also have beena source (Walker, 1975). Future uranium exploration efforts in the Baca in the Gallinas Mountains vicinityshould be restricted to permeable, non-red upper-delta and braided alluvial-plain sandstones which have been altered by reducing groundwater solutions, and areas which contain anomalously high concentrations of carbonaceous materials. SUMMARY OF BACA-EAGAR BASIN DEPOSITIONAL SYSTEMS

Johnson(1978) recognized humid alluvial-fan (renamed braided alluvial plain in this study), meanderbelt, . and lacustrine depositional systems within the Baca-Eagar outcrop belt in west-central New Mexico and eastern Arizona. Withineach ofthese depositional systems, Johnson delineatedtwo or more component facies. Based.on the facies distribution of Johnson (1978), the paleocurrent data fromSnyder, (1971), Johnson (1978), Pierce and others (1979),and this study, and the Baca-Eagar tectonic framework proposed in this study,a hypothetical model for the basin-wide distribution of facies and paleocurrents is presented in Figure 75.

210 211

Figure 75. Hypothetical model for the distribution of facies and paleocurrents in the Baca-Eagar basin during early Baca time (high lake stand). Letters in parentheses’indicate paleocurrent data sources. J=Johnson (1978), P=Pierce (19791, S=Snyder (19711, C=Cather (this study). Reference points same as Figure 10. SUMMARY AND CONCLUSIONS

The sediments of the EoceneBaca Formationand

correlative Eagar Formation andMogollon Rim gravels of

Arizona representthebasin-fill deposits of a large

intermontane basin present in western New Mexicoand eastern

Arizona during Laramide time. The Baca-Eagar basin is

bounded tothe north by the Defiance, Zuni, and Lucero

uplifts,to the southwest by the Mogollon Highland, to the

southeast by the Morenci uplift, and tothe east by the

Sierra-Sandiauplift. These Laramide uplifts,particularily

the Mogollon Highland, were the dominant contributors of

0 detritusto the basin. All threestyles of Laramide uplifts

(Cordilleranfoldbelt uplifts, asymmetricalbasement-cored

anticlinaluplifts, and ColoradoPlateau monoclinal flex-

ures) are present adjacent to the basin.

Within the study area, the Baca is about 940 ft (289

m) thick and can be subdividedinto three informal members

(lowerred, middle sandstone, and .upper red units) which

reflect large-scalelake-level fluctuations in a large,

closedlacustrine system which was present during Baca time in theBear-Gallinas Mountains vicinity. These lake-level

fluctuations were probablycaused by climatic changes. A

semiarid, savanna-type climate is proposed forwest-central

New Mexico during Baca time. Facies which were recognized

212 e 213 in the study areainclude distal braided alluvial-plain, fine-grainedlacustrine delta, and lacustrinebasinal

facies. The distalbraided alluvial-plain facies is

dominantlycomprised of fine tocoarse sandstones and minor

conglomerates which were deposited by low-gradient,

flashy-discharge(ephemeral?) braided streams.i Similar

streams supplied sediment tothe fine-grained lacustrine

d'eltas, whose depositsarecharacterized by cyclical

upward-coarseningsequences which representalternate

deltaicprogradation and abandonment in a shallowlake.

Lacustrinebasinal sediments are similar to those of the

lower prodeltaportion of deltaicsequences and consist Of e red,calcareous mudstones intercalated with thinfine-

sandstone and siltstone beds. The hishlycalcareous nature

of thelacustrine sediments indicates a closedlake system.

The characteristics of the lacustrine basin-center deposits,

if any were present', is not known, 'aserosional stripping

followinglate Tertiary uplift of the ColoradoPlateau has

removed all but the marginal lacustrinedeposits. The

"regional"paleocurrent direction in thestudy area appears

to be towards northeast;the eastward-directedthe

paleocurrents in theeastern portion of thestudy area

probablyresulted from flow deflection by a Laramide

monocline in that area. 214

The studyarea is locatedin the transition zone between the Colorado Plateau to the north and the Rio Grande rift system to the southeast.Three episodes of structural developmenthave modified thearea. A poorlydefined monoclinal flexure of.probableLaramide age is present in the southeasternportion of the study area. North-trending normal faults andassociated drag folds and mafic dikes are common in the eastern part of the area andwere produced by

Neogene extension.Epeirogenic uplift of the Colorado

Plateauduring Neogene time resulted inthe southward tilting of strata in the region.

Baca conglomerates inthe study area exhibit a wide rangeof clastlithologies, including (inorder of decreasingabundance) metaquartzite, limestone, chert, granite,sandstone and, siltstone, schist,mudstone, and volcanicrocks. Conglomerate clasts are highly spherical andusually well rounded. The majorityof the sandstones in the studyarea are lithic arkoses,feldspathic litharenites, . and sublitharenites. Two geneticquartz types, common and metamorphic quartz,are present insubequal proportions.

Orthoclase is the most common feldspartype, followed by microcline and plagioclase. Rock fragmentvarieties, listed

in orderdecreasingof abundance, include metamorphic,

chert, mudstone, granite/gneiss,volcanic, carbonate,

sandstoneand siltstone rock fragments.Upsection changes 215

in sandstone mineralogy indicate increasing contributions by Precamhrianplutonic and metamorphic terranes, at the

expenseof ' sedimentarysources, during basal Baca sedimentation.The dominant source areas for the Baca Formation in the study area were the Morenci uplift and the western flank of that portion of the Sierra-Sandia uplift whichlies to the southof the area.Baca sandstones exhibit two distinct heavy-mineral suites. The first is associated with red sandstones and consists predominantly of magnetite/ilmenite/specular hematite. Non-red (yellow-gray) Baca sandstones, on the otherhand, contain a heavy-mineral suite which is dominated by leucoxene. Subordinate amounts e of zircon, garnet, apatite, 'biotite, and muscovite are present in both of these suites. The anomalous heavy- mineral suite in the non-red sandstones was produced through interaction with negative-Eh groundwaters associated with dike emplacement and dolomite cementation during Miocene time. Illite is the dominant detrital clay mineral in the study area. Fourtypes of cement are present in the Baca sedimentsin the studyarea. These are, in order of decreasingabundance, calcite, kaolinite, quartz, and . dolomite. Calcite cements, in the form of caliche, fine- and coarse-grained spar, were the first to precipitate. 0 These formed during very shallow burial or within the 216

lacustrine environment.Quartz cements were the next to form and, as is thecase. with subsequent cements, are

restrictedto permeable upper-delta and braidedalluvial-

plainsandstones. Authigenic quartz content increases at

the top of the Baca stratigraphicsection, indicating that

muchof thesilica was probably i derived from overlying

volcanic units. Kaolinite cements followedquartz

precipitation, and areindicative of acidic groundwater

~ conditions.Dolomite was thefinal cement, usually

occurringas a replacementproduct of kaolinite and calcite.

It onlyoccurs in permeablesandstones which arenear mafic dikes, andwas precipitated from alkaline groundwaters ' e'during dike intrusion in Miocene time.Recent oxidation of

dolomite to limonite is the predominant source ofyellow

coloration in the Baca. The widespreadred coloration in

the Baca is produced by early-diageneticintrastratal

dissolution of unstable heavy minerals,precipitation of

limonite, and subsequentdehydration of limoniteto produce

the hematite colorant.

Uranium is theonly known economicallyimportant

resource in the Baca. .The Hook Ranch uranium prospect'is

present in a deltaic sandstone unit in theeastern portion

of studythe area. The mineralizedsandstones are

dominantlygray incolor and contain abundantcarbonaceous a debris. Uranium explorationefforts within the Baca in the 217

Gallinas Mountains vicinity should be restricted to non-red sandstones which are indicative of the former presence of reducing groundwaters, and areas which contain anomalously high concentrations of carbonaceous materials. APPENDIX A

Baca Formation Measured Section

EGplanation of Symbols

B ioturb ation Sedimentary Structures Sedimentary Bioturbation h 4 roots JJ11/1 laminationcrossripple TI- burrows /////// tabularcrossbedding u troughcrossbedding

Lithology

0 OoOe conglomerate .. * ..a...... * . sandstone

c -- mudstone , " 219

SEDIMENTARY STRUCTURES BASIC AND TEXTURES ROCK TYPE GRAIN SIZE zran css fss silt clav

I i \I iii

I I It 220

SEDIMENTARY STRUCTURES BASIC ROCK cl REMARKS 0 TYPE U GRAIN SIZE eran css fss silt clav

"" "_ - 5R% "- "" " " ".- ..... I

L I! ! I !\I

~ ...... ,.-. . .. * ._ IDYR% . *. ,. ~:.'..*

:" I 221

SEDIMENTARY STRUCTURES BASIC ROCK REMARKS TYPE GRAIN SIZE 222

SEDIMENTARY STRUCTURES AND TEXTURES BASIC ROCK REMARKS TYPE GRAIN SIZE gran css fss silt clay I -...

...... ,_. . .-.:

. .I. . . * .. ~ ...... *-- ...... e. e .-.... .-. -...- . . ...(.~

...... -- .. ... I I Ii i-”I I

. ”” I ! I I \I ” - ”- -” ”-...... -...... e ... 223

SEDIMENTARY STRUCTURES BASIC AND TEXTURES ROCK REMARKS TYPE GRAIN SIZE gran css fss silt clay

i SEDIMENTARY STRUCTURES

"I I I -L

I i 225

SEDIMENTARY STRUCTURES BASIC ROCK REMARKS TYPE GRAIN SIZE c 226

SEDIMENTARY STRUCTURES BASIC ROCK cf 0 TYPE V GRAIN SIZE gran css fss silt clav

1 e 75

I i 227

SEDIMENTARY STRUCTURES BASIC AND TEXTURES ROCK REMARKS TYPE GRAIN SIZE

a 850

a00 228

I SEDIMENTARY I STRUCTURES . AND TEXTURES APPENDIX E

Summary of Point-count Data for Study-area Sandstone Thin Sections

N N W I

APPENDIX B (continued)

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\ VITA

StevenMartin Cather was bornin Van Nuys,

California on July 25, 1953, the son ofFranklin Eugeneand

BettyJoel Cather. Aftergraduating from El Camino Real

HighSchool in Woodland Hills, California, he entered

CaliforniaState University at Northridge andbegan his

studiesin geology.InJune,' 1974, with the intent of

escaping the overpopulatedmadness of Los Angeles, he

migratedto the hinterlands of New Mexico, where he was

enrolledas a transferstudent at New Mexico Institute of

Miningand Technology inSocorro. There he graduated with a

e Bachelorof Science degree in geology in May, 1976. For the

majorityof the yearfollowing his graduation, he was

employed in two short-termuranium geology jobs with

Kerr-McGee. Corporationand Houston Oil And Minerals,Inc.

In September,1977, heentered the graduateschool at the

University of Texas at Austin. While workingtowards the

Master of Arts degreein geology, he was employed as a

teaching assistant by the Departmentof Geological Sciences.

He is presently employed by the New MexicoBureau of Mines

andMineral Resources.

I Permanent address: 111 Clancy, Truth or Conseqences, 'N. M. * Thisthesis was typedby Steve Cather on the DECsystem-20 computer at NMIMT.