Abstract The Brushy Basin Member and the upper part of the Westwater Canyon Member of the Morrison Formation in northwest New Mexico are nonmarine sedimentary rocks of Late Jurassic age. This stratigraphic interval consists of as many as four lithofacies deposited in fluvial and playa-lake environments. Lithofacies A is composed of crossbedded feldspathic sandstone and was deposited by braided streams on an alluvial plain. Lithofacies B is composed of crossbedded feldspathic sandstone and tuffaceous mudstone, and was deposited by braided and anastomosing streams at the distal end of the alluvial plain. Lithofacies C is composed of calcareous, tuffaceous mudstone and was deposited on a mudflat between the alluvial plain and a playa lake. Lithofacies D is composed of zeolitic, tuffaceous mudstone and was deposited in a playa lake. The distribution of diagenetic facies in mudstones and tuffs in the Brushy Basin Member and upper part of the Westwater Canyon Member reflects the pH and salinity gradients common to fluvial/playa-lake systems. The abundant vitric ash in the sediments reacted to form montmorillonite in the fluvial facies. Calcite and montmorillonite were the reaction products where the fluvial and outermost playa facies met. Vitric ash reacted to form clinoptilolite and heulandite along the playa margins. In the center of the playa facies, analcime replaced clinoptilolite, an early zeolite. These early diagenetic minerals were replaced by albite, quartz, and mixed-layer illite-montmorillonite where the Brushy Basin Member and upper part of the Westwater Canyon Member have been deeply buried in the San Juan basin.

Abstract The Westwater Canyon Member of the Morrison Formation (Upper Jurassic) has previously been interpreted as consisting of fluvial “channel systems” tens of kilometers wide and tens of meters thick. Reinvestigation of the member indicates that these “channel systems” actually represent post-depositional aquifer conduits, defined instead by differing sandstone colors, rather than primary depositional features. The member is composed of amalgamated, individual fining-up sandstone sheets each about 5-10 m thick. The absolute widths of these sheet sandstone bodies are at least 1 km and possibly exceed several kilometers. The width.thickness range of the sandstone sheets are well within the typical values of sandstone body dimensions reported from other fluvial sandstones, and are interpreted to represent aggradational channel-belts. Sandstone bodies thicker than about 12 m are the result of amalgamation of these individual unit sandstone bodies, and do not represent individual channel belts as interpreted previously. Internally, the sheets contain abundant concave-up troughs typically 30 m wide and 5 m thick, filled both laterally and vertically with inclined parallel- to low-angle cross-stratified sandstone, in places exhibiting parting lineation. The laterally-limited extent of these large troughs and nature of their internal fills suggest that they represent short-lived scour fills rather than confined elongated channels. The concave-up erosional base, a negative feature, was most likely formed due to large-scale flow separation within a wider and shallower channel. Physical conditions similar to stream-flow convergence at channel confluences may be responsible for their formation. The abundant preservation of these troughs in the Westwater Canyon Member is consistent with the expected poor preservation of positive barforms in a sweeping, sandy-braided channel belt. Review of the literature indicates that inferred channelbelt sandstone bodies mostly fall within the thickness range of 1 to 12 m, irrespective of their interpreted fluvial style. Post-depositional large-scale reservoir conduits are also expected to fall within this range for sandy fluvial systems-. Deviations from this range are due to amalgamation of the sandstone bodies or increased grain-size heterogeneity, resulting in an increase and decrease, respectively, of the conduit size.

Abstract The Westwater Canyon Member of the Morrison Formation, the main ore-bearing sandstone in the San Juan basin, consists of a sequence of vertically stacked braided stream deposits. Three fluvial units within the sequence can be delineated in the basin. Volcanic pebbles are abundant in the middle fluvial unit, in a zone that forms a crude time line. A pronounced thickening of sandstone in the Westwater Canyon Member north of Gallup, once believed to be the apex of a large alluvial fan, is now thought to merely reflect a greater accumulation of sediment in response to downwarping of the basin in that area. Provenance studies suggest that highlands that contributed detritus to Westwater Canyon streams were located several hundred kilometers to the west and southwest of the San Juan basin, and thus fan apices would also have been several hundred kilometers upstream. The fluvial units recognized in the basin may well be coalesced distal fan deposits, but are probably best interpreted as vertically stacked braided stream sequences. Facies changes in fine-grained interbeds of the Westwater Canyon probably have greater significance in terms of localizing ore than any special attribute of the fluvial sandstones themselves. Uranium ore generally occurs in sandstones that are inter- bedded with greenish-gray lacustrine mudstones. Pore waters that were expelled from these mudstones are thought to have been the source of the pore-filling organic matter (humate) associated with primary uranium ore in nearby sandstones.

Abstract Approximately 1,800 geophysical logs and 100 measured sections provided data for several types of isopleth maps of the Westwater Canyon and Brushy Basin Members of the Upper Jurassic Morrison Formation in the southern San Juan basin, New Mexico. These types of maps include: isopach, sandstone:mudstone ratio, percent sandstone, net sandstone, and average number of mudstone interbeds per 100 ft (30 m) of section. We also constructed a paleotopographic map on the base of the Westwater Canyon and a structure contour map on the base of the Upper Cretaceous Dakota Sandstone. These maps illustrate depositional unit geometry, sandstone depocenter distribution, and large-scale lithofacies variations within the units. The Westwater Canyon is thinner and less sandy over paleotopographic highs and is thicker and sandier along paleotopographic lows, which suggests active structural control of facies distribution during deposition. Sedimentation of the Brushy Basin Member was also affected by some of the same active structural elements. Detailed reflection seismic studies have defined basement faults that were periodically reactivated since the Precambrian. These faults exerted a significant influence on depositional patterns in the Morrison Formation. Primary uranium ore in the Westwater Canyon +Member is restricted to sandstone depocenters associated with east-southeast-trending isopach thicks. Remnant ore deposits are relict primary deposits that lie in oxidized ground updip from a regional oxidation-reduction (redox) interface. Sedimentologic controls seem to be similar to those for primary ore, and in general these deposits have been preserved from oxidation by stratigraphic variations and by structures. Redistributed ore deposits are also concentrated in the vicinity of isopach thicks, but in rocks with relatively low sand-stoneimudstone ratios. However, the location of redistributed ore is much more closely related to the position of the regional redox interface. The geographic form of this interface was influenced regionally and locally by Laramide structures.

Abstract A newly developed data-directed numerical method is used to estimate the undiscovered uranium endowment of the Westwater Canyon Member of the Morrison Formation in the San Juan basin, New Mexico. Data from 1,800 geophysical logs, chip samples from 40 test wells, and 40 measured surface sections provided the basic geologic information. Using these data, we evaluated the favorability of uranium concentration in each of 2,068 cells defined within the basin. Favorability was based on the correlated similarity of the geologic characteristics of each cell to the geologic characteristics of five deposit models. Estimates of the undiscovered endowment for each cell were categorized according to deposit type, depth, cutoff grade, and resource area. The undiscovered uranium endowment of the Westwater Canyon Member is estimated at 2.6 x 10 6 tonnes U 3 O 8 with an estimated error equal to 0.25 x 10 6 tonnes of U 3 O 8 . This estimate is roughly twice that obtained by the U.S. Department of Energy in the NURE program. The total uranium endowment, which is the sum of the discovered and undiscovered endowments, is estimated to be 3.5 x 10 6 tonnes of U 3 O 8 .

Abstract Petrographic study of the Mariano Lake-Lake Valley cores reveals three distinct zones of postdepositional alteration of detrital Fe-Ti (iron-titanium) oxide minerals in the Westwater Canyon Member of the Upper Jurassic Morrison Formation. In the uranium-bearing and adjacent portions of the Westwater Canyon, these detrital Fe-Ti oxide minerals have been thoroughly altered by leaching of iron. Stratigraphically lower parts of the Westwater Canyon and the underlying Recapture Member are characterized by preservation of Fe-Ti oxide grains, primarily magnetite and ilmenite, and of hematite, and by an absence of uranium concentrations. Partly destroyed Fe-Ti oxide minerals occupy an interval between the zones of destruction and preservation. Alteration patterns of the Fe-Ti oxide minerals are reflected in bore-hole magnetic susceptibility logs. Magnetic susceptibility response in the upper parts of the Westwater Canyon Member is flat and uniformly < 500 /xSI units, but at greater depths it fluctuates sharply, from <1,000 to nearly 8,000 μSI units. The boundary between uniformly low and high magnetic susceptibility response corresponds closely to the interval that divides the zone of completely altered from the zone of preserved detrital Fe-Ti oxide minerals. The alteration pattern suggests that solutions responsible for destruction of the Fe-Ti oxide minerals originated in the overlying Brushy Basin Member of the Morrison Formation. Previous studies indicate that these solutions were rich in soluble organic matter and perhaps in uranium. Uranium precipitation may have been controlled by a vertically fluctuating interface between organic-rich solutions and geochemically different fluids in which the detrital Fe-Ti oxide minerals were preserved.

Abstract Early diagenesis in the Morrison Formation resulted in the formation of the world’s largest sandstone-hosted uranium deposits. Distribution of diagenetic alterations in ore-bearing sandstones of the Westwater Canyon Member suggests that these alterations were strongly influenced by pore waters expelled from fine-grained units in the overlying Brushy Basin Member. A moderately high pH created by hydrolysis and dissolution of volcanic ash enabled these fluids to dissolve and mobilize humate in lower Brushy Basin and upper Westwater Canyon sediments. When these fluids mixed with connate water in sandstones of the middle to lower parts of the Westwater Canyon Member, tabular uranium orebodies were formed. A strong diagenetic overprint related to Laramide tectonism and late Tertiary oxidation obscured early alteration patterns and resulted in the local redistribution of primary uranium ore and dissolution of previously formed authigenic cements. Similarities between ore mineralogy and postdepositional alterations in the Morrison Formation of the Grants uranium region and in the Morrison of the northern part of the Colorado Plateau suggest that these ore deposits have a common genesis. The apparent replacement of the organic matrix in the Grants region by chlorite and locally by a chlorite-coffinite mixture in ore zones suggests that, where a chlorite- dominated assemblage is now present, carbonaceous uranium ore once existed. This observation leads to the hypothesis that the organic-carbon-rich, locally chlorite- bearing Grants uranium ore and the organic-carbon-poor, chlorite-rich ore of the northern Colorado Plateau are end members of the same mineralization process.

Abstract A seismic reflection survey, using conventional field techniques and a computer-intensive data processing program, has identified basement structures that had an effect on the deposition of the uranium-bearing Upper Jurassic Morrison Formation. The structural style is one of block faulting defined by northwest- and northeast-trending high-angle normal faults with major periods of movement occurring in the Late Permian and Late Jurassic. The seismic survey, combined with detailed subsurface stratigraphic studies, shows that thickness and sand content of the Westwater Canyon Member increase within a downdropped block. Most of the deposits of the Church Rock uranium district are located within rocks of the Westwater Canyon Member in this block.

Abstract This drilling project included 12 holes along a north-south-trending line from Mariano Lake to Lake Valley, New Mexico, near the southern margin of the San Juan basin. Of a total 33,075 ft (10,088 m) drilled, 4,550 ft (1,388 m) were cored in the strati- graphic interval that included the basal part of the Dakota Sandstone, the Brushy Basin and Westwater Canyon Members of the Morrison Formation, and the upper part of the Recapture Member of the Morrison Formation. The project objectives were (1) to provide cores and geophysical logs for study of the sedimentology, petrography, geochemistry, and mineralization in the uranium- bearing Westwater Canyon Member; (2) to provide control for a detailed seismic study of Morrison stratigraphy and basement structures; (3) to define and correlate the stratigraphy of Cretaceous coal-bearing units; (4) to supply background data for studies of ground-water flow pattern and ground-water quality; and (5) to provide data to aid resource assessment of uranium and coal. The project design included selection of (1) drill-hole locations to cross known ore and depositional trends in the Morrison Formation; (2) a coring interval to include the uranium-bearing unit and adjacent units; geophysical logs for lithologic correlations, quantitative evaluation of uranium mineralization, qualitative detection of coal beds, preparation of synthetic seismograms, and magnetic susceptibility studies of alteration in the Morrison; and (3) a high-salinity mud program to enhance core recovery. A regional stratigraphic correlation chart, brief lithologic descriptions, and descriptions of geophysical log responses of units in and adjacent to the cored interval aided in the interpretation of a geophysical log correlation diagram. Uranium mineralization was detected in four drill holes; coal beds and carbonaceous shales were encountered in all holes; and artesian flows of oil mixed with water were encountered in two holes.

Preliminary examination of the distribution, texture, and chemical composition of clay minerals in the Morrison Formation suggests that the sandstone of the Westwater Canyon served as a conduit for potassium- and aluminum-rich, possibly warm, fluids that moved updip from the center of the basin toward the basin margin, giving rise to mineral zonation within the sandstone over a lateral distance of approximately 35 km. The observed patterns include: (1) Pure smectite occurs as grain coatings in the Westwater Canyon Member in the shallowest core; this smectite is texturally similar to mixed-layer illite-smectite found in the deeper cores. (2) Expandability of the illite-smectite decreases toward the center of the basin and is more expandable near the upper and lower sandstone-mudstone contacts than at the center of the sandstone. This illite-smectite is generally highly ordered and frequently exhibits well-defined superlattice peaks. (3) Iron-rich chlorite occurs texturally on top of the smectite and illite-smectite and therefore must be later. (4) Smectite in the overlying Brushy Basin Member and in the underlying Recapture Member remains 100% expandable through the area analyzed. (5) Kaolinite, the latest clay mineral to form, is most abundant in the middle cores, decreasing in the shallow cores and in the deeper cores. The fluid movement event probably occurred subsequent to the basin formation during Laramide time. Diagenetic reactions are more extensive in the deeper sections, near the basin center, and have obliterated some chemical and mineralogical relationships which are still observable in the shallower cores.