PGJ/F-012(82)
National Uranium Resource Evaluation
AZTEC QUADRANGLE
NEW MEXICO AND COLORADO
U.S. Geological Survey Golden, Colorado
Issue Date
September 1982
Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed in this report, or represents that its use would not infringe privately owned rights. Reference therein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
This report is a result of work performed by the U.S. Geological Survey, through an interagency agreement with the U.S. Department of Energy, as part of the National Uranium Resource Evaluation. NURE was a program of the U.S. Department of Energy's Grand Junction, Colorado, Office to acquire and compile geologic and other information with which to assess the magnitude and distribution of uranium resources and to determine areas favorable for the occurrence of uranium in the United States.
Available from: Technical Library
Bendix Field Engineering Corporation P.O. Box 1569
Grand Junction, CO 81502-1569
Telephone: (303) 242-8621, Ext. 278 Price per Microfiche Copy: $8.00
NATIONAL URJU~IUM RESOURCE EVALUATION AZTEC QUADRANGLE
NEW MEXICO AND COLORADO
Morris W. Green, Principal Investigator Virginia P. Byers Steven M. Condon A. Curtis Huffman, Jr. Russell F. Dubiel Allan R. Kirk Robert D. Lupe Charles T. Pierson Jennie 1. Ridgley Jacques F. Robert son
Keith Robinson Pamela G. L. Sikkink J:.Iark R. Stanton Robert E. Thaden Robert S. Zech September 1982
PREPARED FOR THE U.S. DEPARTMENT OF ENERGY GRAND JUNCTION AREA OFFICE
UNDER CONTRACT NO. DE-All3-78GJ01686
This report has not been edited for conformance with U.S. Geological Survey stratigraphic
This is the final version of the subject-quadrangle
evaluation report to be placed on open file. This report
has not been edited. In some instances, reductions in the
size of favorable areas on Plate 1 are not reflected in the text.
CONTENTS
Abstract • • • Introduction •
Purpose and scope (by M. W. Green). Acknowledgments •
Procedures (by P.G.L. Sikkink, M. W. Green, and Keith Robinson) • • • • • • • • • • • • •
Geologic setting. (by J. L. Ridgley, M. W. Green, and
P.G.L. Sikkink) • • • • • • • • • • • • • •
Rocks of Precambrian age
.
.
. .
. . . .
. . . .
Rocks of Paleozoic age
.
.
.
. .
. . .
.
.
Rocks of Mesozoic age.
.
•
.
. .
.
. .
. . . .
Rocks of Cenozoic age.
Rocks of Quaternary age.
.
. . .
.
Definition of favorability and criteria used for evaluation • • Environments favorable for uranium deposits.
Area favorable for uranium deposits in the Burro Canyon(?)
Formation (by J. L. Ridgley) • • • • • . . • • • . • • • • •
Stratigraphy and environments of deposition.
Uranium mineralization and host rock characteristics • Favorable area •
Summary. • • •
Area favorable for uranium deposits in the Ojo Alamo Sandstone (by J. L. Ridgley) • • • • • • • • • • • •
Stratigraphy and environments of deposition.
Uranium mineralization and host rock characteristics . Favorable area • Summary. • • • • 1 1 1 2 2 4 6
. . .
.
10. . . .
10. . . .
12 13 13 15 15 16 18 20 21 21 22 23 24 25CONTENTS (Continued)
Area favorable for uranium deposits in the Morrison
Formation (by M. W. Green). • • • • • •
Stratigraphy, structure, and lithology • Environments of deposition •
Uranium mineralization and area favorable • • Summary. • • •
Environments unfavorable for uranium deposits. Precambrian igneous rocks (by P.G.L. Sikkink) Pennsylvanian formations (by J. L. Ridgley) •
Permian formations (by J. L. Ridgley)
Cutler Formation • •
Triassic formations (by R. E. Thaden) •
Jurassic formations (by M. W. Green).
Cretaceous formations (by R. S. Zech) • Dakota Sands tone (by M. i>l. Green). •
Tertiary formations (by V. P. Byers and P.G.L. Sikkink)
Nacimiento Formation (by J. L. Ridgley) • • San Jose Formation (by J. L. Ridgley) • • • Quaternary volcanic rocks (by P.G.L. Sikkink) ••
Unevaluated environments (by M. W. Green) • • • • .
Aerial radiometric data (by P.G.L. Sikkink) • •
Interpretation of hydrogeochemical and stream sediment reconnaissance data (by Keith Robinson, R. F. Dubiel, M. R. Stanton, and P.G.L. Sikkink) • • • • • • • • • •
Interpretation of U.S. Geological Survey
stream-sediment and data • • •
Geochemistry of stream sediments •
25 25 27 28 30 31 31 31 32 32 35 36 37 39 40 41 42 45 45 45 46 51 52
CONTENTS (Continued)
Geochemistry of ground and surface waters. 55
Interpretation of results • • 57
Geochemical data (by M. W. Green) • • 59
Recommendations to ·improve evaluations (by M. W. Green
and J. L. Ridgley) . • • . • • • • • • • • • 59
Selected bibliography (compiled by J. F. Robertson and others) •• 61
Appendix A. Uranium occurrence table of the Aztec Quadrangle
(compiled by A. C. Huffman, Jr., A. R. Kirk, W. Aubrey, and P.G.L. Sikkink) •
Appendix Bl. Table of chemical analysis • .
Appendix B2. USGS field data and chemical analysis of
stream-sediment samples (compiled by Keith Robinson,
• In pocket • • In pocket
J. J. Irwin, and Deborah Archelger). • • • • • • In pocket
Appendix B3. USGS field data and chemical analysis of water
samples (compiled by Keith Robinson, J. J. Irwin,
and Debora Archelger). • • • • • • • . • • • • • In pocket
Appendix B4. USGS statistical summary of geochemical data from
stream-sediment, ground- and surface-water samples
(compiled by Keith Robinson and C. Adkisson) • • In pocket
Appendix C. Uranium-occurrence reports . In pocket
Appendix D. Partial list of test wells which penetrate the
Morrison Formation (compiled by P.G.L. Sikkink) . • • • In pocket
Appendix E. Partial list of measured stratigraphic sections
(compiled by S. M. Condon) • • • • . • • • • • • • • • In pocket
Figure
ILLUSTRATIONS
1. Location of Aztec Quadrangle • • • • • •
2. Generalized stratigraphic column (upper part) of the
Aztec l 0x 2° Quadrangle • . . • • • . •
2a. Generalized stratigraphic column (middle part) of
the Aztec 1° x 2° Quadrangle • • • • • . • • • • •
5
7
Table
Plate
ILLUSTRATIONS (Continued)
2b. Generalized stratigraphic column (lower part) of
the Aztec 1° x 2 ° Quadrangle. • • • • • • • • •
3. Index map of the San Juan Basin, Grants mineral belt,
the Aztec l0
x 2° Quadrangle, and adjacent areas
1. Summary of HSSR data (sediment samples)
la. Summary of HSSR data (water samples) • • •
2. Summary of element concentrations in less-than 170
(88-micron) stream sediment samples collected from outcrop areas of Tertiary sedimentary units,
Aztec Quadrangle • • • • • • • • • • • • •
3. Summary of chemical analyses and physical
parameters in ground- and surface-water samples,
mesh 9 29 47 49 53 Aztec Quadrangle. • • • • • • • • • • • • • • • • • • • • • 56
1. Areas favorable for uranium deposits in the Burro Canyon
Formation, Ojo Alamo Sandstone, and the Brushy Basin and Westwater Canyon Members of the Morrison Formation
2. Uranium occurrence map
3. Interpretative map of aerial radiometric uranium anomalies
4. Summary of anomalous sediment and water samples, hydrogeochemical
and stream-sediment reconnaissance
4a. Uranium/conductivity in water samples
4b. Distribution and concentration of uranium (ppm) in
stream-sediment samples
4c. Distribution and concentration of uranium (ppb) in ground- and
surface-water samples
4d. Distribution and concentration of uranium (ppb) x 1000/
conductivity in ground- and surface-water samples
4e. Interpretive map of USGS hydrogeochemical and stream-sediment
data
5. Rock sample locality map
Sa. Location map of oil and gas test wells and measured stratigraphic
ILLUSTIL~TIONS (Continued)
5b. Location of stream-sediment samples (partial)
5c. Location of stream-sediment samples (partial)
5d. Location map of Ground- and Surface- water samples
6. Drainage map
7. NONE
8. North-south stratigraphic sections in the San Juan Basin area
8a. East-west stratigraphic sections in the San Juan Basin area
9. Index map to location of stratigraphic sections in the Aztec and
some adjoining NURE quadrangles
10. Preliminary geologic map of the Aztec 1° x 2° Quadrangle,
northwestern New Mexico
*
lOa. Major structural and tectonic features
lOb. Schematic distribution of Precambrian rocks in northwestern New
Mexico, northeastern Arizona, southwestern Colorado, and southeastern Utah
lOc. Schematic distribution of Tertiary igneous rocks in northwestern
New Mexico, northeastern Arizona, southwestern Colorado, and southeastern Utah
11. Geologic-map index
12. Generalized land status map
13. Culture map
Plates in accompanying packet
*
This map is available as U.S. Geological Survey Open-File Report 78-466and may be obtained from the Open-File Services Section, Branch of Distribution, U.S. Geological Survey, Box 25425 Federal Center, Denver,
ABSTRACT
Areas and formations within the Aztec 1°x2° Quadrangle, New Mexico and Colorado considered favorable for uranium endowment of specified minimum grade
and tonnage include, in decreasing order of favorability: (1) the Early
Cretaceous Burro Canyon(?) Formation in the southeastern part of the Chama Basin; (2) the Tertiary Ojo Alamo Sandstone in the east-central part of the San Juan Basin; and (3) the Jurassic Westwater Canyon and Brushy Basin Members of the Morrison Formation in the southwestern part of the quadrangle.
Favorability of the Burro Canyon(?) is based on the presence of favorable host-rock facies, carbonaceous material and pyrite to act as a reductant for
uranium, and the presence of mineralized ground in the subsurface of the Chama
Basin. The Ojo Alamo Sandstone is considered favorable because of favorable
host-rock facies, the presence of carbonaceous material and pyrite to act as a reductant for uranium, and the presence of a relatively large subsurface area in which low-grade mineralization has been encountered in exploration
activity. The Morrison Formation, located within the San Juan Basin adjacent
to the northern edge of the Grants ~ineral belt, is considered favorable
because of mineralization in several drill holes at depths near 1500 m (5000 ft) and because of favorable facies relationships extending into the Aztec Quadrangle from the Grants mineral belt which lies in the adjacent Albuquerque and Gallup _Quadrangles.
Formations considered unfavorable for uranium deposits of specified tonnage and grade include the remainder of sedimentary and igneous formations
ranging from Precambrian to Quaternary in age. Included under the unfavorable
category are the Cutler Formation of Permian age, and Dakota Sandstone of Late Cretaceous age, and the Nacimiento and San Jose Formations of Tertiary age. The Nacimiento and San Jose Formations contain isolated uranium occurrences and locally favorable facies characteristics, but no evidence of specified minimum grade and tonnage is present to indicate a favorable classification
for these formations. Other formations, particularly those which were
deposited in marine and arid eolian depositional environments, do not show favorable facies, radioactiviby, or geochemical characteristics.
INTRODUCTION PURPOSE AND SCOPE
The purpose of the National Uranium Resource Evaluation (NURE) program of the
u.s.
Department of Energy (DOE) is to identify and delineate areas and environments within the Aztec 1°X2° Quadrangle, New Mexico and Colorado, that exhibit characteristics favorable for the occurrence of uranium deposits that have the potential to contain at least 100 metric tons U303 total endowmentwith a minimum grade of 100 ppm (0.01 percent)
u
3o
8 • Evaluation is based ongeochemical, mineralogic, hydrologic, aerial radiometric, sedimentologic, and general geologic characteristics of the formations in the quadrangle.
Formations of both volcanic and sedimentary or~g~n have been evaluated by surface investigations and subsurface data analysis to a depth of 1500 m (5000
ft). The evaluation involved a comprehensive review and synthesis of the
geologic literature, knowledge of on-going research, fieldwork, and the work of several geologists with uranium-related experience and knowledge of the rocks in the quadrangle.
The NURE program was conducted from January, 1978, to March, 1980, by
geologists and support personnel of the USGS (USGS). This study was managed
by the Grand Junction Office of the DOE, and quadrangle evaluation has involved approximately 4.5 man-years of literature search, fieldwork,
laboratory work, data evaluation and final report writing by USGS personnel.
ACKNOWLEDGMENTS
Evaluation of the Aztec Quadrangle has required the support of many other
professional and technical personnel in the USGS. Compilation of the
land-status map was coordinated by Phil Mudgett, computer support was made
available by Cheryl Adkisson and Ron Wahl, and tables and cross sections were
compiled, in part, by D. P. Bauer and W.M. Aubrey, all of the USGS. We also
gratefully acknowledge the clerical support within the USGS provided by M. L. Frisken and staff, and the drafting support provided by Mary Durrett. We owe special thanks to Glenn R. Scott, Kim Manley, and Reinhard Wobus of the USGS
for providing the geoloic map (Pl. 10) of the Aztec Quadrangle. Rock, water,
and sediment samples were analyzed by personnel of the USGS analytical
laboratories in both Menlo Park, California, and Denver, Colorado. Discussion
of geology and resource assessment with Fritz Loomis of Bendix Field
Engineering Corporation (BFEC) and H. K. Holen of the Albuquerque office of the DOE have also contributed significantly to the evaluation of the Aztec Quadrangle.
We gratefully acknowledge the release of data on mineralization in the
Ojo Alamo Sandstone by the Triple S Development Company. Permission granted
by the Tennessee Valley Authority {TVA) for USGS geologists to examine logs of
uranium mineralization in the Burro Canyon is also acknowledged. (M.
w.
G.)PROCEDURES
In the Aztec Quadrangle, a representative number of the uranium
occurrences listed in Appendix A and shown on Plate 2 were visited, except
where access was either prohibited by terrain or denied by landowner. It was
impossible to fielu check all occurrences due to the time constraints of the NURE program. The locations of these uranium occurrences were determined from
Preliminary Reconnaissance Reports of the
u.s.
Atomic Energy Commission andfrom published literature.
Of the occurrences visited, only those considered significant were
"anomalous occurrence" was arbitrarily defined as one containing a minimum of
two times the background radioactivity at each location. Information on
general geology, lithology, mineralogy, structure, m~n~ng activity, and
radiometric values was recorded at each occurrence on the uranium occurrence forms (App. C).
Rock samples were also collected at most of the anomalous areas (Pl.
5). All rock samples were grab samples from the mineralized zone and not
systematically collected from the geologic unit(s) at each occurrence. The
rock samples were analyzed at USGS laboratories in Menlo Park, California, and
Denver, Colorado. Samples were analyzed by delayed-neutron activation for
uranium and thorium content; and by semiquantitative emission spectrometry for
45 other elements. Results of these analyses are shown in Appendix B1.
Raw data for the Aztec Quadrangle from the National Hydrogeochemical Stream and Sediment Reconnaissance (HSSR) program conducted by Los Alamos Scientific Laboratories (Bolivar, 1978) were analyzed by USGS personnel to
determine areas anomalous in uranium concentration. The interpretation of the
HSSR data involved: (1) dividing the quadrangle into areas by distinct
drainage systems; (2) dividing the drainage areas on the basis of geologic
units of similar age~ the presence of igneous and metamorphic rocks; and (3)
performing a statistical analysis on the raw data for each of these areas. For each area sho\vn in Plate 4, the uranium concentration of both water and stream-sediment samples was analyzed for mean, standard deviation, and
variance. Extremely high uranium concentrations were normally left out of the
statistical calculations but are plotted on the interpretation map (Pl. 4). Anomalous values are defined as values greater than two standard deviations
above the mean. Results of the statistical analysis on these areas and
samples are shown in Tables 1 and 1A. Raw data from water samples were also
standardized using the uranium-to-conductivity ratio rather than the uranium
concentration. Results of these calculations are plotted in Plate 4A. A
total of 338 water and 1744 stream-sediment samples were analyzed according to the previously mentioned procedures.
A total of 1003 stream-sediment and 186 water samples were collected in the Aztec Quadrangle by USGS personnel under a program directed by Keith
Robinson in a followup study to the National HSSR program. Sampling was
confined to selected areas of Paleocene and Eocene sediments in the western half of the quadrangle and to a small area of Miocene and Pliocene sediments
in the southeast corner of the quadrangle. The USGS followup HSSR program in
terrain underlain by Tertiary rock units within the quadrangle was designed to enhance information on resource potential and add information concerning the potential of Tertiary volcanic rocks as a possible source of uranium contained
in adjacent sedimentary sequences. Sample sites were located to afford
optimum density coverage, access, and integrity of the resultant geochemical
data. Replicate or duplicate stream-sediment and water samples were collected
at several localities in order to test the variance of analytical results and
sampling error. The sediments were obtained from both dry and actively
flowing stream beds. Approximately 10 pounds (4.5 kg) of sediment was
several points along the active drainage channel.
All the stream-sediment samples were oven dried at a temperature of less than 100°F and sieved to obtain a less-than-170-mesh (88 micron) fraction. The less-than-170-mesh fraction was then routinely analyzed, without further preparation, for 34 elements, based on a 6-step, DC-arc, semiquantitative
emission spectrographic method. Uranium was determined by a modified
extraction fluorimetry method, described by the American Society of Testing
Materials (ASTM), in the Annual Book of ASTM Standards (1975). A weighed
subsample of the fine fraction was also ignited at 550°C for 10 minutes and the loss of weight, in percent, reported.
The water samples were collected from wells, springs, streams, and
ponds. At each site three unique water samples were obtained. A 1000-mL
sample was collected, filtered through a 0.45-mm membrane filter into an acid rinsed polyethylene bottle and then acidified with an ultrex-grade
concentrated nitric acid to a pH of <2. This sample was analyzed for uranium
by an extraction fluorimetry method. An untreated and unfiltered 250-mL water
sample was obtained for analysis of the degree of alkalinity and a 125-mL filtered but untreated sample was collected for sulfate, phosphate, and
nitrate analysis. The latter sample was kept at a near-freezing temperature
at all times. In addition, at each site, water temperature, specific
conductance, pH, and dissolved oxygen were also measured using untreated water.
Subsurface analysis of Jurassic through Cretaceous rocks included a systematic study by R. D. Lupe, D. P. Bauer, and others of more than 300 oil
and gas well logs located throughout the San Juan Basin area. Using virtually
all usable well logs of holes that penetrate the entire Jurassic section (App. D), outcrop stratigraphy was projected into the subsurface, gamma-ray
anomalies were located and noted, and some knowledge of the distribution of
potential uranium host rocks in the subsurface was gained. Surface control
was maintained using measured sections from outcrops in the area (Pl. SA and
App. E). The location of selected borehole logs used in the final analysis
and the cross-section diagrams are shown in Plates 8 and 8A, and indexed in
Plate 9. (P. G. L. S., M. W. G., and K. R.)
GEOLOGIC SETTING
The Aztec Quadrangle is located between latitude 36° N and 37° N and longitude 106° Wand 108° W (Fig. 1). The western two-thirds of the Aztec Quadrangle includes the San Juan Basin and the Chama Basin of the southeastern
part of the Colorado Plateau physiographic province. The Colorado Plateau
portion of the quadrangle is underlain by generally flat-lying sedimentary rocks of continental and marine origin which range in age from Paleozoic through Cenozoic, and by Precambrian rocks of intrusive and metasedimentary origin. The sedimentary rocks reach a maximum total thickness in the San Juan Basin of approximately 3600 m (11,880 ft) and are intruded locally by volcanic
-
I 10" 1os• I~ 102" -100" 48°'t__
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i NEW MIEXICO I ' I . i I -\ l T '! i I \ ARIZONA I i -,J I ' I I CA.! ) I I I iparts of the Colorado Plateau, folding and faulting of the sedimentary sequence in the Aztec Quadrangle is subtle and restricted to the marginal areas of broad uplifts such as the Nacimiento Uplift, the Brazos Uplift, and
the Archuleta Anticlinorium (Pl. lOA). The dip of formations along these
uplifts may be moderate to steep; however, the dip of formations in the Chama and San Juan Basins, in areas away from these marginal uplifts, is generally low (1° to 5°).
The eastern one-third of the Aztec Quadrangle is located within the
southern part of the Rocky Mountain physiographic province. It is
characterized by generally north-south-trending anticlinal structures separated from each other by intermontane down-folded and down-faulted
basins. The basins are floored by Tertiary sedimentary and volcanic rocks
several hundred meters thick. Erosion of anticlinal mountains has exposed
cores of Precambrian granite and metamorphic rocks in the northern part of the Nacimiento and Tusas Mountains.
A preliminary geologic map of the Aztec Quadrangle was compiled by Kim Manley and others in 1978 (Pl. 10) using smaller geologic maps indexed in
Plate 11 and doing extensive fieldwork. The present stratigraphic
nomenclature and generalized lithologic descriptions of the area are
summarized in Figures 2 through 2b. Major structural and tectonic elements
are compiled on Plate lOA. Plate lOA includes the important faults,
anticlines, synclines, and monoclines within each major tectonic division of this quadrangle.
Rocks of Precambrian Age
In the northern part of the Nacimiento Mountains Woodard and others (1977) have recognized 14 Precambrian rock units of metasedimentary,
metavolcanic, and plutonic igneous origin. The metasedimentary rocks include
meta-quartz wacke, meta-arkose, metaquartzite, metagraywacke, and silty argillite. The metavolcanic rocks consist predominantly of metaquartz latite. The rocks were derived by regional metamorphism of siliceous volcanics and were later locally metasomatized near the contacts with a
tonalite intrusion (Woodard and others, 1977). Amphibolite derived from basic
igneous rocks also is locally present. The plutonic igneous rocks include
ultramafic rocks, gabbro, diorite, tonalite, leucogranodiorite, quartz
monzonite, muscovite-biotite granite, biotite granite, leucogranite, and mafic dikes.
In the Tusas Mountains, Bingler (1974) noted the occurrence of quartzite, quartz-muscovite-biotite schist, hornblende-chlorite schist, leptite, granitic
gneiss, quartz diorite gneiss, and granite porphyry. Bingler (1965)
summarized the major Precambrian events in this area. Plate lOB shows the
>-c:: Santa Fe Group ' ~ ~!
: !l.ll
f - '---:-"":"-':~-:-':"'---!
~ 0:: f-j _ _ _ _ ln_t_r_u_si_v_e_ro_c_k_• _ _ _ _,i,_,·"":..:.·,...:_,·_:,·_:,_·_ ·."'1-1
I
V··:.· ....
~: 1 Los Piiios Formation2
l
~ri ---~~:_-:~:~.~-:~·>7>~>~'-
l f' A A A A A1 . Abiquiu Tuff 3 k yAy A yAy A y;;
I :vvvvv. ~ " u 0 uJ ! ~ A A A A
4
Tr~::asure Mountain Tuff"
Conejos Formation
R itit.O Conglomerates
El R:to Format10n
San Josfi Formation •
~·-~
~··· ·:.:
:· ... '
...
'.. . .
.' • •• •, ~ • • • • I
.:Favorable for uran1um deoos1ts based primar·!y on tne presence of uranium occurrences. Uran1um Deoostts
· Sources: Manley, Kim, and Scott. GR .. 1978, Barker, F, 1958; and Smnh, H.T., 1938.
7 Rests disconformably on Treasure Mountam Tuff m north. Further soutr the basal contact ts graoattonal or intenonguing With the Rmto Conglomerate.
l Of Smi.th (19381; overlies E! Rno Formation unconformably. Uoper part orobably equivalent to upper part of
Los Pu1os Formanon. Present m east-centra~ oart of the mao.
4
D1sConformabiy overl1es R:tno Cong1cmerate or ConeJOS Format1on.
'Of Barker (1958); oresent onlv 10 eastern part of map. Rests disconformably on El Rita Formation; locally contact between the two is gradational.
5 Present from Colorado-New Mexico border north of Chamas southward to Chaves Creek near Brazos.
1 Of Smith (1938); present only in eastern parts of map. Rests unconformably on rocks of Precambrian to Early Cretaceous age .
..._....-unconformity
Figure 2. Generalized stratigraphic column (upper part) of the Aztec
w ....
"'
>-"'
)o a: <( t= a: w 1-"' :J 0 u.J u < .... u.J a: u u v; "' <( a: :J .., "' w a: w"'
" c:~
~ STRATIGRAPHIC UNIT Nacimiento FormationOjo Alamo Sandstone • • 6
Animas Formation
Picture Cliffs Sandstone
Lewts Shale La Ventana Tongue Po1nt lookout Sanastone 19 20 21 22
GENERALIZED ROCK DESCRIPTION'
13. Shale, gray co olive gray, and minor interbedded sandstone; as much as 1900 ft
14. Sandstone, brown, crossbedded; conglomerate near base; contains minor lenses of gray shale; abundant petrified wood; 50-200 ft
15. Conglomerate, sandstone, and shale, greenish gray and tan, andesitic; unconformably overlies I
Fruitland Fonnation or Lewis shale in north-central
i
part of mapped area; grades southward into the 1 ::laci;nic:nto Fm.; lJ00-3300.±. fc16. Shale, greenish gray with int~rb~dded
yellowish-brown, crossbedded sandstone: 0-lUOU! it
17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27 2S. 29. JO.
Dark shale and yellowish-brown crossbedded ~and
stone; coal; 50-150! ft
Sandstone, yellowish gray to grayish orange; interbedded with brown sandstone and gray shale; 0-250+ ft
Sandy-shale, olive gray (upper part); shaly sand-stone, gradational (lower part); 300-2100:!: ft
Sandstone, tan, fin~-to medium-grain~d, ~ith some
interbedded thin gray shale; 50-lJSOj: ft Sandstone, tan and brown; gray and bra·~ shale and coal; layers of large ironstone and limestone concretions; 350-2300:!: ft
SdtHL.-'>tone, tan and brown; some ,;r...1y .:;h:J.le; tl5-215 f t
Shdlt:, gr:1y and dark olive gr:.:1/, ...:~l'i_,:,lr~u~ . ..; 1n,j
sandy shale; some limestone cor.t.:r':'.'tLvns in up;_Jt->r
pdrt
Sl1ala, hard to soft, calcareou~; ~ome llilrd ~r3y li:ne~tone oeds
Shale, dark, r..;ith he~s of platy, _;;and'! li!!1t-'...,tOne and septarian concr~tions
Li::::12~conc, lL~ht -sr..:1y, jensa, .Jnd ·i.-trk:er gr3.y calcar2ous ~ilAL0
3halt:, gray ~~J jlack, evenly ; ~!lCire
Mancos unit; ljQrJ-2550± ft
Sandstone, gr2y to 3rayish oran~0, ~ell cem~nted,
massive to cros~b~dded, locally con~Lomeratic;
dark-gray carbonaceous shale and coal; 70-250:!: ft
Sand~tone and conglomeratic sandstone, ~hite to buff
~udsc0ne, ~r~eni~h ~ray and orange, int2rheJded with olive-tan to rusty-tan, fine- to ~o~r52-~rain2d ~Jnd~:o~e
11. S:ln~._L;tone and cLl::st·)r>,•..!, interbe,J:..!-_·:i, l:;:-:c to dark reddish-~rc~n. very fine-to fin~-~rqin~d;
toc1l ~or!':"i~vn '!Dit .:lbout 900 ft
32. Limes tone, gray, :ned itim-gralned; lvc:.1lly ,5r.1des
upward into very pure gypsum; some platy shale;
0-lOO ft
33. Sandstone, white and reddish oran~e, fine-to
medium-~rain~d, well sorted, massiv~, cro~~b~dd~d;
100-350:!: ft
•Favorable for uranium deposits based pnmarily on the presence of uranium occurrences. ••uranium deposits
-....rtJnconformity
1 Source>: Manley, Kim, and Scott, G.R .. 1978; Barker, F., 1958; and Smith, H.T., 1938.
• Unconformity between Ojo Alamo Formation and Kirtland Shale. Also unconformable on lower part of Animas Formation in the northwestern part of map area.
1 Upper conglomeratic sequence is possibly equivalent to Burro Canyon (7) Formation of Early Crf!taceous aqe.
Figure 2a. Generalized stratigraphic column (middle part) of the Aztec
ai ~
1- ii: STRATIGRAPHIC UNIT LITHOLOGY' GENERALIZED ROCK DESCRIPTION
:~ w
~~~:=~----~~~~---~
I ' ·
'
Uppershale f-=--::::::.-_-_-_-=:;34 34. Shale, red, green, and maroon, interbedded with1 u member r~:::=-::::j___ lenticular sandstone and siltstone; 500-650± ft
i ;:;:; Poleo Sandstone r-,·· . ,-;:-:;::J 3
-l "' !J. Chinle Lentil• ~_:-=i ) 35. Sandstone, micaceous, conglomeratic, fine-to 1 ~ 5 Formation•• Salitral Shale ;-_-..=-_-_-:_-:
3, coarse-grained; some subordinate green and maroon 1- Member f=-:::=-:::=-:;:-.::::-::::: 0 shale; as much as 17 5 f t
Aqua Zarca Sand-L.. 13~ 36. Shale, maroon and minor green; 0-115 ft
1 stone Member ! 1
~~ ~~ 37. Sandstone and conglomerate, white to buff,
fine-d: z! ~ 1 • ~~ • • ;,&: to coarse-grained, quartzose; 0-132 ft
1 w ::; I 0;; Cutler Formation 38 38
I
~ ~I ~ 1 • Sandstone and siltstone, crossbedded, arkosic;=~~~ also purple-orange mudstone; 1650+ ft
~~~· F~/k 39. Limestone, light gray, fossiliferO"us; also gray to
I
I~z
Madera Formation !~·.,·
.~• 39 buff orthoquartzite, pink arkose, minor reddishZ<: ' ' ' ' ' and light gray shale, and arkosic limestone.
I~> i =---=;
~~:::~
''·
illil:~;~i;l;::;~;;;;i;:;~;iiii;;;;;::iii;;i:,::,
south-central areas; thickness varies *Favorable for uranium deposits based primarily on the presence of uranium occurrences.
••uranium Deposits.
~Unconformity
' Sources: Manley, Kim, and Scott, G.R., 1978; Barker. F.; 1958; and Smith, H.T., 1938.
Figure 2b. Generalized stratigraphic column (lower part) of the Aztec
Rocks of Paleozoic Age
By the close of the Precambrian, the surface in the Aztec area was eroded
to a peneplain (Hilpert, 1969). Throughout most of the Paleozoic era this
area remained a stable shelf. A warm, humid to subtropical climate prevailed
and seas invaded the area numerous times. Cambrian through Mississippian age
rocks are absent in the quadrangle, although this is probably due to uplift and erosion of the marine sediments rather than to nondeposition (Ridgley and
others, 1978). Evidence of Cambrian, Devonian, and Mississippian seas is
present, but only in the subsurface to the west and south of the Aztec Quadrangle.
During early and mid-Pennsylvanian time the relative crustal stability of
the area ended. Several structural highs formed that affected later
Pennsylvanian and Permian sedimentation. The highs included the southern
Colorado Uncompaghre Uplift, Zuni-Defiance Platform, Sierra Grande Uplift,
Pedernal Uplift, and Penasco Uplift in New Mexico and Arizona. The birth of
the Ancestral Rocky Mountains also began at this time in Colorado, Utah, and
Wyoming. Concurrent with development of these structural highs a large trough
formed between the Zuni-Defiance Platform and the Uncompaghre Uplift. This
trough extended from southeast Utah to northwest New Mexico. Shallow seas
transgressed the area depositing sediment into the trough in a sequence of interbedded fossiliferous limestone, arkosic limestone, arkosic sandstone, and
shale which is known as the Madera Limestone in the Aztec Quadrangle. The
basal part of the Madera Limestone is absent owing to nondeposition in the vicinity of the Penasco Uplift (Ridgley and others, 1978).
Yne Pennsylvanian period was a time of extensive erosion as well as
vigorous mountain building. Structural highs during this time were
continually being eroded, adding vast quantities of sediment to the system.
In the Aztec area, only the Madera Limestone was deposited. By the end of
Pennsylvanian time, only the Uncompaghre Uplift remained as an important structural high and source of sediments during the Permian.
During the Permian Period continental deposition resumed. Coarse fluvial
sediments from the Uncompaghre Uplift make up the Cutler Formation in the
Aztec area. The sediments were transported primarily by a system of streams
which flowed south-southwest from uplifted areas toward the sea which lay to
the southwest of the Aztec area. The shoreline fluctuated widely, leaving
shoreline and shallow-marine sandstones intertongued with continental red
beds. By mid-Permian time, the structural highs were diminished and clastic
sediment was greatly reduced. By the close of the Permian Period, erosion of
Permian rocks resulted in a widespread unconformity between Permian and Triassic strata.
Rocks of Mesozoic Age
During Triassic time, the maximum extent of the paleo-San Juan Basin was
uplifted and sediments were deposited on a vast floodplain that was tilted to
the west and northwest. According to Colbert (1950), this floodplain was a
vast lowland traversed by streams and dotted with scattered forests and
lakes. Facies coarsen generally southward toward the Mogollon region. At the
same time, volcanism occurred south of the Aztec Quadrangle area, and the Uncompaghre Uplift was rejuvenated to form the northern boundary of the
basin. The slightly positive Zuni and Defiance Uplifts of Arizona and New
Mexico, and the Uncompaghre Uplift, Colorado, served as important sediment
sources for the Triassic Chinle Formation (Ridgley and others, 1978). In the
Aztec Quadrangle, the Chinle Formation consists of sandstone, siltstone, and mudstone of fluvial and lacustrine origin.
Relatively stable conditions marked the change from Triassic to Jurassic
time. An erosional period is evidenced in the area by a widespread
unconformity between Triassic and Upper Jurassic rocks. The erosion surface
was covered by the vast dune fields of the Entrada Sandstone. After eolian
and sabkha deposition of the Entrada Sandstone, the area was covered by a
large lake. In this lake was deposited the limestone and gypsum facies of the
Todilto Limestone and the equivalent units such as the Pony Express Limestone. Uplift to the southwest, in the Mogollon region (Craig and others, 1955), preceded deposition of the Jurassic Morrison Formation and is believed to be
the source for much of the sediment in the Morrison Formation. The various
members of the Morrison Formation were deposi~ed on a vast plain in a variety
of fluvial and lacustrine environments. Deposition was also accompanied by
volcanic activity and much of the Morrison contains volcanic debris (Ridgley and others, 1978).
During deposition of Jurassic sediments the basin sank and land masses to
the west and south rose. These tectonic movements caused folding and flexing
in the basin, especially around the southern margin. The presence of folds
and flexures probably influenced the course of sedimentation in the basin. Although the folds and flexures developed at this time may not be directly related to all uranium deposits, their influence on sedimentation patterns may have permitted accumulation of sediments favorable as sites for subsequent uranium deposition (Ridgley and others, 1978).
Sediments of the transitional period from Jurassic to Cretaceous time are
absent over most of the area because of pre-Cretaceous erosion. Sometime in
Late Jurassic to Early Cretaceous time the southern part of the basin was
uplifted and beveled. It was a time dominated by the Sevier Orogeny to the
west, and intense overthrust faulting and mountain building in the Cordilleran geosyncline. Only in the northern part of the San Juan Basin and in the Chama
Basin are rocks of Early Cretaceous age preserved. Here rocks of the
Cretaceous Burro Canyon(?) Formation reflect deposition under fluvial and
local lacustrine conditions. The basin uplift and beveling of sediments
resulted in the subsequently deposited Cretaceous Dakota Sandstone resting on progressively older rocks from north to south within the basin and basin margin.
The Aztec area, in Late Cretaceous time, was a broad, subsiding plain
(Hilpert, 1969). It was traversed by streams and dotted with swamps in which
carbonaceous siltstones and coals accumulated. As subsidence continued the
area was covered by marine waters entering from the north. Numerous
transgressions and regressions followed (Peterson and Kirk, 1977), depositing intertonguing sequences of shale and sandstone and lesser amounts of limestone
and coal. These sediments were deposited in marine, nearshore marine, beach,
paludal, and fluvial environments and compose the stratigraphic sequence from the upper part of the Dakota Sandstone through the Pictured Cliffs
Sandstone. The Pictured Gliffs Sandstone was deposited as shallow marine and
beach sands during the final regression of the sea from the area (Ridgley and others, 1978).
As the sea withdrew, the area was once again dominated by terrestrial
sedimentation. The area was a vast alluvial plain traversed by streams and
dotted with swamps. The Cretaceous Fruitland Formation and Kirtland Shale
were deposited in this environment.
At the end of the Cretaceous Period or early in the Tertiary Period there was renewed tectonic activity accompanied by a mountain-building pattern which consisted of essentially vertical basement uplifts of the Rocky Mountain
geosyncline. These tectonic events are referred to as the Laramide orogeny
and they shaped much of the present-day configuration of geology and
physiography in the Aztec Quadrangle. During this period, the final emergence
of the San Juan, Nacimiento, San Pedro, and Brazos Uplifts occurred around the
margins of the area. The San Juan and Chama Basins formed and were separated
by the Gallena-Archuleta Arch.
Rocks of Cenozoic Age
Fluvial conditions persisted into the Tertiary Period in both the San
Juan and Chama Basins. However, as a result of their structural separation
during the Laramide Orogeny, both basins were the sites of deposition of
different, unrelated sediments. In the San Juan Basin, fluvially deposited
conglomerate, sandstone, siltstone, and mudstone characterize the Paleocene Animas Formation (also Late Cretaceous in part), Ojo Alamo Sandstone, and
Nacimiento Formation. In the Chama Basin, rocks of similar age are absent due
to erosion or nondeposition.
During the early Tertiary Period, numerous structural features formed,
especially along the margins of the San Juan Basin. Deepening of the San Juan
Basin occurred, and its margins were beveled. The Eocene San Jose Formation
was deposited over the beveled surface of earlier Tertiary rocks in the San
Juan Basin. During the same period in the Chama Basin, arkose, sandstone, and
conglomerate of the Blanco Basin Formation were deposited on the beveled
erosion surface in the northern part of the basin. At about the same time
along the east side of the basin, the El Rito Formation was deposited as a fanglomerate derived from highlands to the north and east (Smith, Budding, and others, 1961).
At the end of Eocene time the San Juan Basin was tilted to the north. Deposition continued into Oligocene and Miocene time and was accompanied by renewed volcanic activity in the San Juan Mountains, Mogollon Highlands, and
Jemez Mountains. Volcanic debris was contributed to fluvial systems, and lava
flows of basic composition covered large areas. The Abiquiu Tuff of the Santa
Fe Group was deposited during this period in the Chama Basin.
During late Miocene or early Pliocene time there was a broad regional
uplift, and large areas of the San Juan and Chama Basins were eroded. Large
amounts of Tertiary and pre-Tertiary rocks were removed. This period was also
accompanied by faulting or rejuvenation of old faults. Mobilization and
redeposition of uranium in the Grants mineral belt on the southern edge of the San Juan Basin probably occurred at this time (Ridgley and others, 1978). Structural features that formed during Oligocene-Miocene time include the Nacimiento Uplift and Rio Grande Trough (Pl. lOA).
Volcanic activity in the Aztec Quadrangle during Miocene time was centered in the Jemez Volcanic Field and on the Taos Plateau (Pl. lOC). Igneous rocks of pyroclastic and tuffaceous composition and flows of
intermediate to silicic composition covered parts of the eastern half of the
quadrangle. Widespread basalt flows along with local silicic, alkali rhyolite
flows spread out as sheets over erosional surfaces. In the Aztec Quadrangle,
these igneous rocks include the Lobato Basalt, the dacite and latite members of the Tschicoma Formation, the basalts and andesites of the Hinsdale
Formation, and the tholeiite basalts of the Servilleta Formation. Lamphrophyre dikes and sills were also intruded during this time.
Rocks of Quaternary Age
Quaternary rocks in the Aztec Quadrangle are primarily surficial deposits
of alluvial, landslide, and glacial origin. In the southern part of the
quadrangle, Quaternary rocks also include the nonwelded to densely welded
rhyolitic tuff of the Pleistocene Bandelier Tuff. Chenoweth (1974b) has
speculated that the weakly radioactive Bandelier Tuff may have been the source of uranium for many of the small uranium deposits in the southern part of the
Chama Basin and in the northern Nacimiento area. (J. L. R., M. W. G., and P.
G. L. S.)
DEFINITION OF FAVORABILITY AND CRITERIA USED FOR EVALUATION
All rocks to a depth of 1500 m (5000 ft) within the Aztec Quadrangle have been evaluated as either favorable or unfavorable for the occurrence of
uranium deposits which could contain at least 100 metric tons of
u
3o
8 with anaverage grade not less than 100 ppm (0.01 percent)
u
3o
8 • The evaluation hasbeen conducted according to DOE as stated in Mickle and Mathews (1979). Accordingly, rock formations (or environments within formations or rock bodies) can be classified as favorable if (1) there are known uranium
occurrences; (2) there are uranium anomalies detectable by hydrogeochemical, ground, or aerial surveys; (3) the potential host rock possesses geologic and geochemical characteristics similar to known uranium-producing, environments; and/or (4) there is evidence, based on the above criteria, for the potential
to contain uranium deposits in the formation (or environment) of minimum grade and tonnage as specified above.
In this folio, several recognition criteria have been used to delineate and evaluate each formation (or environment) insofar as time and data have
allowed. These criteria include the recognition criteria established by BFEC
(1979) as well as criteria which, in the experience of the investigators, are
pertinent to uranium occurrence in rock units of the quadrangle. These
include the following:
(a) Lithology of the formation/environment. This includes consideration
of mineralogic composition, texture, and diagenetic alteration of potential host rocks.
(b) Environment of deposition of the rock. This includes consideration
of depositional conditions, climatic factors, tectonic factors affecting depositional patterns and energy levels, and resultant sandstone to-siltstone (shale) ratio and configuration; it also includes any other depositional factors related to and controlling facies distribution and host-rock geometry.
(c) Presence of uranium and/or gamma radioactivity. This includes
consideration of the relative background radioactivity of the formation (or environment) away from mineralized ground; the size, geometry, quantity, grade, and distribution of mineralized ground; and the possible sources of uranium.
(d) Presence of reductants. This includes consideration of the type,
quantity, origin, and distribution of organic matter with respect to uranium, as well as, other potential reductants and uranium precipitation.
(e) Transmissivity. This includes consideration of porosity,
permeability, cementation, and fracturing of potential host rocks and the lateral and vertical continuity of aquifer conduits for the flow of mineralizing and/or oxidizing solutions.
(f) Tectonic framework. This includes consideration of local and
regional strike and dip, faulting, and folding affecting the rock formation (or environment) and uranium.
(g) Preservation conditions. This includes consideration of oxidation
history, and the availability of confinement and barrier
mechanisms (i.e., caprock, shale, siltstone). It also includes
consideration of the relative age of the host rock, its uranium deposits, and the possibility that deposits may have been
destroyed with time and adverse geologic and geochemical conditions.
Areas designated as favorable are shown on Plate 1. The areas have not
been subdivided into areas of relative favorability since subdivisions would, in most areas, be based either on proprietary information or on geologic and geochemical criteria which are speculative or transitional in nature.
Boundaries of favorable areas shown by dashed line indicate only the approximate limits of a favorable area or environment as determined geologically or by the 1500 m (5000 ft) overburden contour.
Rocks or environments within favorable areas exhibit a majority of the recognition criteria. They also show evidence of the existence of uranium
deposits of minimum grade and tonnage as specified. Such evidence includes
the presence of known orebodies or high-grade uranium in outcrop occurrences
and/or drill holes. Boundaries of the favorable areas are placed to include
these mineralized areas and adjacent areas in which geologic and geochemical characteristics are similar. All areas outside of favorable areas are
considered unfavorable even though they may· locally contain rocks or environments which exhibit a few favorable recognition criteria.
Areas designated as unfavorable commonly contain one or more favorable
criteria for uranium mineralization. However, from present data the
unfavorable environments show little evidence of containing deposits of the
minimum grade and tonnage. Additional information, not presently available
for this evaluation, could conceivably change some unfavorable evaluations of
rocks or their environments to the favorable category. A few units are
considered geologically more favorable but lack sufficient evidence of
extensive uranium mineralization. These units include the Permian Cutler
Formation, the Cretaceous Dakota Sandstone, and the Tertiary Nacimiento and
San Jose Formations. They include scattered uranium occurrences, favorable
facies and environments of deposition, but no evidence of large deposits to
warrant designating them as favorable. These formations, therefore, are
discussed in greater detail in this report than other formations considered unfavorable.
ENVIRONMENTS FAVORABLE FOR URJu~IUM DEPOSITS
AREA FAVORABLE FOR URANIUM DEPOSITS IN THE BURRO CANYON(?)FORMATION
The Burro Canyon(?) Formation is considered favorable for sandstone-type uranium deposits (Class 240, subclass 241) based on the recognition criteria established by Austin and D'Andrea (1978), and Mathews and others (1979).
Stratigraphy and environments of deposition
The Burro Canyon(?) Formation crops out in the southern part of the Chama Basin (Pl. lOA) and is present in the subsurface in the rest of that basin and
in the northeastern part of the San Juan Basin. It is absent south of Cuba,
New Mexico, where it wedges out under the pre-Dakota erosion surface, and east
of the Chama Basin due to erosion or nondeposition. In the subsurface in the
western part of the Aztec Quadrangle, the Burro Canyon(?) is not easily
recognized and, thus, is generally lumped with the Dakota Sandstone in surface and subsurface correlations (Pls. 8, 8A).
As defined by several workers (Swift, 1956, Ridgley, 1977; Saucier, 1974), the Burro Canyon(?) Formation in the Chama Basin is the conglomeratic sandstone, sandstone, and mudstone interval occurring stratigraphically between the Brushy Basin Member of the Morrison Formation and the Dakota
Sandstone (Fig. 2). The exact correlation of this lithologic sequence and the
extension of the Burro Canyon nomenclature into the Chama Basin has been the
subject of controversy, due primarily to the absence of datable fossils. In
the past, some workers (Lookingbill, 1953; Sears, 1953; Woodward and
Schumacher, 1973; and Woodward and others, 1976) have included this sequence with the Morrison Formation, whereas others (Grant and Owen, 1974;
Muehlberger, 1967; Smith and others, 1961) have included it with the Dakota. The latter workers recognized the possible equivalence of this stratigraphic interval the Burro Canyon Formation in Colorado.
Although the determination of age and correlation of this stratigraphic interval may seem purely academic, there are also some economic
considerations. Because of the stratigraphic relation of this interval to the
Brushy Basin and Dakota Sandstone, there has been a tendency for many
geologists to correlate this interval with the Jackpile sandstone (of economic
usage). However, based on paleocurrent measurements and facies relationships,
Grant and Owen (1974), Owen and Siemers (1977), Ridgley (1977), and Saucier (1974) suggest that this interval is not equivalent to the Jackpile and, at present, should be considered equivalent to the Burro Canyon(?) Formation in Colorado.
In the Chama Basin the Burro Canyon(?) may be divided into t\vo distinct
lithologic sequences--basal and upper. The basal sequence consists of 18 to
35 m (60-115 ft) of conglomerate, conglomeratic sandstone, and sandstone. The
conglomerate and sandstone are generally buff, tan, orange, or white at the
outcrop; they are usually gray or yellow gray in the subsurface. The
sandstones may be subarkosic, feldspathic, or quartzose and are fine to coarse
grained. They consist mainly of subrounded quartz with variable amounts of
feldspar, chert, and accessory minerals. Kaolinite is present throughout the
sequence. It occurs especially near the top in the southern part of the
basin.
Sedimentary structures in the basal sequence consist mainly of trough
cross-stratification, ripple lamination, and parallel lamination. The ratio
is also characterized by numerous fining-upward sequences that begin with a conglomerate and end with a medium- to fine-grained parallel- or
ripple-laminated sandstone. The contact between the sandstone and overlying
conglomerate is commonly sharp and may be gradational or scoured. Clay-gall
zones and thin gray-green mudstone seams occur at the scour interfaces in places. Laterally, the basal sequence is very persistent, may form steep cliffs, and has an almost sheet-like geometry.
The upper sequence of the Burro Canyon(?) consists of a basal red mudstone interval, a white to pink friable sandstone interval, and an upper
pale-red, pale-purple, or pale-green mudstone interval. Proportion of
mudstone to sandstone is variable; thickness of this sequence ranges from a few meters to 23m (75ft). In the southern part of the basin much of this
interval is absent owing to pre-Dakota erosion. In the subsurface in the
eastern part of the basin, electric logs show that sandstone is the dominant lithology of this interval and that the sandstone horizon is somewhat
persistent laterally. The sandstone of the upper sequence is fine to medium grained and is composed of subangular to subrounded quartz and minor amounts
of feldspar. Kaolinite alteration is abundant at the outcrop, and has also
been found in the subsurface where the sandstones are various shades of gray. Mudstones at the outcrop are siliceous and contain kaolinite and illite. The red and green colors found in mudstones at the surface are also present in the subsurface.
The Burro Canyon(?) is unconformably overlain by the Dakota Sandstone. The contact is generally sandstone on sandstone but may also be sandstone on
mudstone. The Burro Canyon(?) may disconformably overlie the Brushy Basin
Member of the Morrison Formation. This contact, where exposed, is generally
sandstone on gray-green mudstone but may also be sandstone on sandstone. From outcrop studies by Ridgley, the basal part of the Burro Canyon(?) Formation is interpreted to have been deposited by a series of high-energy
braided streams. This interpretation is based on the dominance of trough
cross-stratification, on the geometric relation of conglomerate to sandstone intervals that indicate the presence of channel bars and channel-fill
deposits, and on the lateral continuity of the sandstone interval, which includes very little of fine-grained overbank deposits. Measurements of
crossbeds indicate a NNW to NNE direction of transport, with a probable source
area to the south and southwest (Ridgley, 1978). These observations are in
agreement with those reported by Grant and Owen (1974), and Saucier (1974). The upper sequence of the Burro Canyon(?) reflects deposition by lower energy, braided to meandering streams. Lower energy conditions are reflected by an increase in proportion of mudstone, the presence of finer grained
sandstone, and a decrease in the lateral persistence of the main sandstone interval.
Uranium mineralization and host rock characteristics
Little has been reported on uranium mineralization in the Burro Canyon(?)
Formation. There are no known surface uranium occurrences of any consequence
in the Burro Canyon(?), and aerial radiometric and hydrogeochemical studies
did not indicate the presence of any uranium anomalies. In the course of
mapping in the southern part of the basin, Ridgley located two very small
uranium anomalies that were associated with clay galls. Samples contained 6
and 27 ppm
u
3o
8 • In this area the Burro Canyon(?) is extensively oxidized atthe outcrop. Uranium occurrences purported to be in the Burro Canyon(?) were
examined and were found to occur in the underlying Brushy Basin mudstones or in overlying distributary-channel sandstone of the Dakota Sandstone.
Uranium occurrences, however, have been found in the subsurface on the east side of the basin. The following discussion of these occurrences is based in part on the author's interpretations derived from examination of a
limited number of electric and lithologic logs provided by the TVA and in part
on a report by Saucier (1974). Uranium minerals in the Burro Canyon(?) at
this area occur as numerous ore pods that are associated with oxidation-reduction interfaces, thus indicating the deposits are roll-type uranium deposits (Saucier, 1974).
Uranium occurences have been found at all levels in the Burro Canyon(?), although the largest concentration of uranium occurs in the lower part of the basal sandstone sequence and to a lesser extent in the top of the upper
sandstone sequence. Gamma-logs also indica~e that uranium occurs locally in
an underlying Brushy Basin sandstone and in overlying Dakota Sandstones. Depths of anomalous uranium range from 60 to 180m (200-600 ft).
The variation in depth of uranium mineralization reflects the variation in position of oxidation-reduction interfaces within the thick lower and upper
sandstone sequences of the Burro Canyon(?). Variations in permeability within
the sandstone have probably affected the rate at which the oxidation front has
travelled through any portion of the thick sandstone bodies. Gamma-logs
indicate that many of the thinner radioactive intervals are associated with
slight breaks in spontaneous potential (SP) and resistivity curves. These
breaks do not appear to be associated with discrete mudstone intervals.
Corresponding lithologic logs do not show any mudstone breaks within the thick sandstone sequences, but they do indicate periodic changes in grain size. These changes in grain size are probably similar to those observed at the outcrop in the fining-upward sequences, that were previously described.
Nothing is known about the uranium mineralogy. Uranium appears to be
associated with small amounts of carbonaceous and "humicn material and
pyrite. Alteration of the host rock in the subsurface includes the presence
of kaolinite and trace amounts of hematite and limonite. At the outcrop the host sandstones are extensively oxidized; the sparse amounts of iron-bearing
minerals have been oxidized to hematite and limonite. No sulfides or
carbonaceous material have been observed at the outcrop; only local pockets of silicified wood have been found near the base of the formation.
According to Saucier (1974) some of the ore pods lie above the water table and are oxidized, while others lie at or below the water table and have
not been oxidized. He reported that ore pods in the lower sandstone interval
have a south to southwest orientation, suggesting ground-water movement to the northwest, and that those in the upper sandstone interval are related to
ground-water movement to the north and northeast.
There appears to be a good relationship of the position of the uranium
occurrences to regional and local structural features. The regional dip of
pre-Tertiary strata in the area is to the north and northwest and is generally
less than 5°. Locally dips may be to the northeast and dips in the vicinity
of a major fault in the mineralized area are to the southeast (Smith and
others, 1961). The relationship of this major fault to uranium occurrences is
unknown. It is possible that the fault is post-mineralization as ore pods
near it on the upthrown side (northwest side of northeast-trending fault) are oxidized.
Structure contours drawn on the base of the Dakota Sandstone in the area indicate the presence of several broad, gentle folds that plunge to the
northwest (Smith and others, 1961). These folds are bounded on the east by a
homocline that has dips of 10o to 12°. The pattern of mineral occurrences
appears to be coincident with ground water moving in the direction of the regional dip, with perhaps local modification of ground-water movement in the vicinity of the shallow folds.
The age of mineralization and source of uranium are not known.
Mineralization, however, appears to be related in time to basin-margin uplift
to the south and possibly east. Uplift of the basin margin as well as other
regional and local structural features, previously mentioned, are related to
tectonic events of late Cretaceous and early Tertiary times. This period of
deformation was followed by extensive erosion, which is recorded in the
southeast-dipping unconformity below the El Rito Formation of probable Eocene
age. In turn, the El Rito Formation ~v-as eroded prior to deposition of the
overlying Miocene Abiquiu Tuff.
Chenoweth (1974b) and Saucier (1974) have speculated that the >veakly
radioactive Pleistocene Bandelier Tuff was the source for uranium. This would
mean that the uranium deposits in the Burro Canyon(?) are no more than 2 m.y.
old. As an alternative, Ridgley proposes that the Abiquiu Tuff of Miocene age
should also be considered as a possible source of the uranium.
The Abiquiu Tuff is composed of light-gray to pale-orange silty tuff, micaceous, tuffaceous sandstone, and conglomerate, and may be as much as 390 m
( 1300 ft) thick. The average uranium content of the tuffaceous portions is
unknown. A few scattered uranium anomalies were found in the Abiquiu Tuff by
aerial radiometric and hydrogeochemical surveys (Pls. 3, 4). Source areas of
the Abiquiu Tuff were in the San Juan Mountain region (Smith and others, 1961).
