Defiance Uplift Research Articles

Structure and thermochronology of basement/cover relations along the Defiance uplift (AZ and NM), and implications regarding Laramide tectonic evolution of the Colorado Plateau

Interpretation of the late Cretaceous to Eocene Laramide tectonic evolution of the Colorado Plateau is hampered by the difficulty of placing precise temporal constraints on fault-induced basement uplift, for there is a paucity of exposed basement from which thermochronologic ages might be obtained. The Defiance and Zuni uplifts, located on the southeasternmost Colorado Plateau, offer rare basement exposure that provide additional temporal insights. The Zuni uplift exposes abundant Precambrian basement rock, which has been the subject of previous apatite thermochronologic study to interpret low-temperature tectonic/exhumation history. In the Defiance uplift region, which is the main focus of this study, there are two outcrop areas of Precambrian-basement rock along the trace of the East Defiance monocline. Both sites are quarries. Uplift/erosion consequences of Pennsylvanian-Permian Ancestral Rocky Mountains (ARM) deformation are explicit in the geology of these quarries, for the contact between the Supai Group (Permian) and underlying basement (1,703 ± 1.3 Ma, zircon U-Pb) is a nonconformity. Here, we apply a combination of structural analysis of Laramide fold/fault relations and multi-method thermochronology to the exposed granitic basement of both the Defiance and Zuni uplifts. Zircon U-Pb, zircon (U-Th)/He, apatite fission-track, apatite (U-Th-Sm)/He, and hematite (U-Th)/He reveal a poly-phase thermo-tectonic history. Initially, the Paleo-Proterozoic 1.7 Ga basement cooled to ∼400 °C by at least 1.4 Ga, followed by 1.4–1.0 Ga unroofing to depths of ∼8 km. Following cooling, the Defiance-Zuni granitic basement experienced protracted residence at temperatures ≤200 °C (∼8 km) between <i>ca.</i> 900 and 600 Ma. Sedimentary evidence and hematite (U-Th)/He dates bracket the ARM event (400–200 Ma), which may suggest fluid-rock interaction or near-surface exposure associated with the ARM. Following the ARM, the Defiance uplift experienced heating (&gt;120 °C) associated with burial consistent with the stratigraphic overburden, until approximately 80 Ma. Finally, the Defiance-Zuni region experienced initial cooling at &lt;70 Ma, with the main phase of exhumation to the upper crust (&lt;2 km) at <i>ca.</i> 60–40 Ma. Detailed structural modeling along 15 normal-profile cross-sections across the east margin of the Defiance uplift reveals that Laramide trishear monoclinal folding was generated by an oblique-slip master fault that partitioned ∼8 km of strike-slip and ∼1.5 km of reverse-slip displacement. Inferred strike-slip compartmental faulting in the Zuni uplift appears to fit coherently within this overall kinematic model in relation to Laramide loading direction. In the context of geodynamic models for flat-slab subduction during the Laramide tectonic event, consideration of new data from the Defiance uplift (along with other recently reported thermochronology data) provides support for models that bring the flat slab beneath North America along an east-northeastward trajectory, rather than along a more northward trajectory.

Using detrital zircon ages and Hf isotopes to identify 1.48–1.45 Ga sedimentary basins and fingerprint sources of exotic 1.6–1.5 Ga grains in southwestern Laurentia

Globally rare 1.6–1.5 Ga zircons provide an underutilized correlation tool for Precambrian supercontinent reconstructions. Detrital 1.6–1.5 Ga zircons, long known from the Belt Supergroup, have recently been discovered in multiple metasedimentary successions in southwestern North America. Few igneous or metamorphic sources in this age range are known in Laurentia, implying non-Laurentian provenance. Combined U-Pb and Hf isotopic data offer a robust test of their provenance. New U-Pb detrital zircon ages from quartzite exposed in the Defiance uplift in northeastern Arizona indicate a maximum depositional age of ca. 1476 Ma and a predominance of locally derived ca. 1655 Ma and older detritus. The quartzite contains populations at 1570, 1554, and 1519 Ma that do not have known Laurentian sources and are similar to our new and recently published data from the ca. 1474–1436 Ma Blackjack Formation in south-central Arizona. Based on similarities between age spectra and depositional age constraints, we suggest a correlation between the two stratigraphic sections and postulate that a previously unrecognized, and now largely eroded, 1475–1450 Ma sedimentary basin or set of basins may have extended across much of southwestern Laurentia. We refer to this basin as the Yankee Joe-Defiance basin. Hf isotopic analysis of ca. 1.6–1.5 Ga detrital zircons from both localities yields positive epsilon-Hf (ɛHf) values of +0.2 to +12 indicating a near-juvenile Hf fingerprint with a mean of ∼ +7. This range overlaps with, but is more juvenile than, ɛHf values of +3 to +7.5 from the Belt basin. In terms of possible non-Laurentian source regions, Hf data from both the North and South Australia cratons overlap with, but are also generally less juvenile than, ɛHf values of Laurentian zircons. Only the North Australia craton contains the full range of ages and Hf isotope values recognized in the 1.6–1.5 Ga Yankee Joe-Defiance populations. This finding supports plate reconstructions involving Australia as a source for exotic detritus in western Laurentia ca. 1480–1450 Ma.

Seismic Structure and Seismic Stratigraphy of the Giant Aneth Field and Its Satellite Fields of Southeast Utah

Extensive seismic coverage in and around the greater Aneth field reveals much about the structure and stratigraphy of the Pennsylvanian Paradox Formation. The terminus of a northward-projecting nose of the Defiance uplift created a structural platform active throughout most of Late Pennsylvanian, and served as the foundation for the Aneth carbonate platform during upper Paradox deposition. Seismic and subsurface control demonstrate that the northwest-southeast-trending Four Corners lineament coincides with and probably acted as, a control on salinity concentrations in the marine environment. Higher salinities northeast of the Four Corners lineament, along with the positive structural position of the platform, contributed to conditions favoring growth of alga mounds in the Aneth area during deposition of Desert creek stage of Paradox Formation. The Aneth field is a complex of coalesced algal mounds and associated facies covering 75 mi[sup 2]. Surrounding Aneth field are numerous satellite mounds (from under 30 to over 200 ac in size) that are identifiable with high-resolution common-depth-point seismic. Seismic expression of the algal mounds are characterized by one or more of the following: (1) isochron thickening of the Desert Creek unit; (2) isochron thinning of the overlying Ismay unit; (3) amplitude dimming of the Desert Creek reflector; andmore » (4) a [open quotes]doublet[close quotes] development of the Desert Creek event. Calibrated amplitude variations within the mound seismic anomaly are in part a response to reservoir porosity and can be employed in mapping reservoir distribution.« less

Paradox Basin: A Model Pull-Apart Basin of Pennsylvanian Age: ABSTRACT

The Paradox basin of the east-central Colorado Plateau province is an elongate, roughly rhombic salt basin of Middle Pennsylvanian age. it is bounded on the northeast by the Uncompahgre-San Luis segments of the Ancestral Rockies. The writers have demonstrated previously that the End_Page 950------------------------------ basin sagged along basement rift zones by strong east-west extension during the Desmoinesian. The dominant zone of weakness was the northwest-trending Olympic-Wichita basement lineament that lies along the eastern margin of the Paradox salt basin and the southwestern edge of the Uncompahgre-San Luis uplifts. Less prominent northwest and northeast shear zones are ubiquitous, but are especially well developed in basement and Paleozoic rocks underlying the San Juan basin at the southeast termination of the Paradox basin. J. C. Crowell's classic model of a pull-apart basin along anastomosing transform fault zones is directly applicable to the Paradox basin, with the one exception that the Paradox is an intracratonic basin developed on continental crust. The primary zone of weakness, the Olympic-Wichita lineament, marks the abrupt eastern margin of the basin. The southwestern margin is less well defined along a broad zone of basement faults that trend northwesterly across the San Juan basin, through the southern margin of the salt basin, across the Monument upwarp at the anomalous Fish Creek structure and the Mille Crag Bend fracture zone, and on o the northwest through the Henry Mountains intrusives and the Fremont sag. The northwest termination is the expected irregular compressional (convergent) marg n at the Emery uplift (San Rafael swell), and the southeast limit of the basin is an irregular margin of normal faults and stretched attenuated floor (divergence) lying between the Hogback monocline and the House Creek fault. The complex intersections lying at the rhombic corners of the basin are in the San Juan Mountains on the southeast, the Defiance uplift on the southwest, the Fremont sag on the northwest, and the Oquirrh sag on the northeast. As the Paradox basin episodically deepened during the Middle Pennsylvanian by rejuvenation of basement faults, it was being filled contemporaneously with salt, which may have reached a thickness of 6,000-8,000 ft (1,800-2,400 m), and arkoses of 15,000-20,000 ft (4,600-6,100 m) thickness along the Uncompahgre front. A pull-apart of only about 5% of extension would account for a basin of this magnitude. By about mid-Desmoinesian time, the wrenching pull-apart was nearly completed. Folding caused by minor wrench movements formed shoaling conditions along the southwest shallow shelf of the basin where algal bioherms developed. Meanwhile, pull-apart stretching of the basin floor may have triggered salt flowage and diapirism in the eastern, deepest part of the basin. From the late Desmoines an through Permian, the basin filled with marine and nonmarine sediments as the wrench tectonism subsided. End_of_Article - Last_Page 951------------

Dineh-Bi-Keyah Field, Apache County, Arizona: ABSTRACT

The Dineh-Bi-Keyah oil field is located on the Navaho Indian Reservation in northeastern Apache County, Arizona, and is situated on the northwest end of the Toadlena anticline, a surface feature on the northeast flank of the Defiance uplift. The field is producing from a syenite sill which intruded Lower Pennsylvanian rocks. The discovery well was completed in January 1967 and as of October 1, 1979, the field has produced a total of 15,386,725 bbl of oil. The sill is of Tertiary age and contains both intercrystalline and fracture porosity. Primary minerals are sanidine, biotite, diopsidic augite, glass, and minor magnetite. Glass is the primary cementing material. The porosity, permeability, and oil-saturation values measured in the igneous rock are similar to the reservoir parameters of many oil-producing carbonate rocks. The sill is comparable in general appearance and mineral composition with plugs, dikes, and sills that crop out in the area. However, the igneous rocks exposed at the surface in the area are very fine grained and dense and have little, if any, porosity. Samples from the two igneous plugs which crop out at Roof Butte, 1 mi (1.6 km) southeast of the discovery well, are difficult to distinguish from core chips from the dense parts of the producing formation. End_of_Article - Last_Page 696------------

Deposition and Hydrocarbon Potential of Lower Cretaceous (Dakota) Sandstone Sequence, Chaco Slope, Southern San Juan Basin, New Mexico: ABSTRACT

The Chaco slope is the gently dipping, southern margin of the San Juan basin, bounded by the Zuni Mountains on the south, the Nacimiento Mountains on the east, the Defiance uplift on the west, and the main gas producing trend of the San Juan basin on the north. Approximately 6,800 sq mi fall within these arbitrary boundaries. The Lower Cretaceous sandstone sequence, or Dakota Formation, for the purpose of this paper, is defined as that sandstone and shale unit occurring beneath the base of the Greenhorn Limestone and above the top of the Jurassic Morrison Formation. It includes, among the more important locally named units, the Graneros sandstone and shale interval, the Tres Hermanos sandstone, the Burro Canyon sandstone, and the Twowells sandstone. The Dakota of the Chaco slope is generally considered to be of early Late Cretaceous age based on paleontologic evidence. Dakota deposition on the Chaco slope can be subdivided into four regional sandstone units and three regional shale units. The depositional environments of the sandstone units grade from fluvial in the west to deltaic and marine in the east. Accumulation of these sandstones occurred during stillstands of the northeast- to southwest-transgressing Dakota shorelines. Oil and gas production has been established in the deltaic (pointbar) and/or marine facies at Hospah, Lone Pine, Stoney Butte, Snake Eyes, Red Mountain, Five Lakes, and Crosswise areas. The significant producing areas at the present time are attributed to a combination of structural and stratigraphic conditions. Recent photogeologic-geomorphic mapping indicates surface manifestation of these features. To date, approximately 250 Dakota wildcats have been drilled across the subject area. The hydrocarbon potential of the Dakota Formation on the Chaco slope is analogous with the Muddy sandstone of the Powder River basin and the D and J sandstones of the Denver basin. End_of_Article - Last_Page 955------------

Oil and Gas on Toadlena Anticline, Arizona and New Mexico: ABSTRACT

The Toadlena anticline is a doubly plunging asymmetric fold in northeast Arizona and northwest New Mexico. The northwest-southeast-trending structure is approximately 35 mi long and ranges in width from 3 to 6 mi. On the east side of the fold the maximum structural relief is about 5,000 ft and at the south end of the fold the structural closure is about 1,250 ft. The sedimentary rocks in the area range in age from Cambrian through Tertiary and several Tertiary volcanic rock bodies are exposed at the surface. The anticline is on the northeast flank of the Defiance uplift and was formed during the Laramide orogeny. The Dineh-bi-Keyah field is at the northwest end of the Toadlena anticline. Oil production in this field is from a syenite sill which intruded Lower Pennsylvanian rocks. The sill is of Tertiary age and contains both intercrystalline and fracture porosity. The field was discovered in January 1967, and there are 15 completed oil wells on 160-acre spacing. Initial potential for an individual well was as much as 3,300 bbl/day of oil. Cumulative production to July 1, 1968 was 4,164,871 bbl. Nonflammable helium-bearing gas is trapped structurally in the Devonian McCracken Sandstone on the Toadlena anticline. There are two areas of accumulation with different gas-water contacts. These areas are separated by a saddle near the Arizona-New Mexico line. To September 1, 1968, 11 wells have tested gas from the McCracken Sandstone. One well, on the south end of the anticline, produced about 11,000 bbl of oil before it was shut in due to an excessive gas-oil ratio. End_of_Article - Last_Page 217------------

Dineh-bi-Keyah Field, Apache County, Arizona

The Dineh-bi-Keyah field on the Navajo Indian Reservation in northeastern Apache County, Arizona, is producing from a porous and fractured primary igneous intrusive rock. Production first was established in the field with the completion of the Kerr-McGee No. 1 Navajo in January 1967. As of December 1967, 14 oil wells had been completed on 160-acre spacing and the field still was being developed. The initial potential of individual wells is as high as 3,300 bbl of oil per day. Cumulative production to January 1, 1968 was 2,815,205 bbl of oil. The field is at the northwest end of the Toadlena anticline, a NW-SE-trending surface feature on the northeast flank of the Defiance uplift. The sedimentary section penetrated in the field conforms with that which has been described in the geologic literature for the Defiance uplift and surrounding area. The sedimentary rocks range in age from Cambrian through Tertiary. Several Tertiary volcanic rock bodies are exposed at the surface. The production at Dineh-bi-Keyah is from a syenite sill which intruded Lower Pennsylvanian rocks. The sill is of Tertiary age and contains both intercrystalline and fracture porosity. Primary minerals are sanidine, biotite, diopsidic augite, glass, and minor magnetite. Glass is the primary cementing material. The porosity, permeability, and oil-saturation values measured in the igneous rock are similar to the reservoir parameters of many oil-producing carbonate rocks. At the present stage of field development, the sill is known to cover approximately 3,000 acres.

Developments in Four Corners-Intermountain Area in 1967

The number of exploratory wells drilled in the Four Corners-Intermountain area during 1967 continued to decline, a trend that has persisted for several years. This decline was greatest in northern New Mexico. Oil discoveries in Utah's Uinta basin and in northeastern Arizona, and a gas discovery in the Salt anticline region of southwestern Colorado, should help to reduce this decline during 1968. Development drilling also decreased during 1967 in all parts of the area except Arizona. The largest decrease was in the San Juan basin of New Mexico followed by the Uinta basin in Utah. Increased leasing activity noted during 1967 should result in an increase of exploratory drilling during 1968, particularly in the Uinta basin, on the Defiance uplift, and throughout the Salt anticline region.

Kaiparowits and Black Mesa Basins--Stratigraphic Synthesis: ABSTRACT

The Black Mesa and Kaiparowits Basin area is located in the southwest portion of the Colorado Plateau Structural Province. The Black Mesa basin, encompassing approximately 6400 square miles, extends from the Kaibab upwarp on the west to the Defiance uplift on the east; from the Mogollon Rim on the south to the Utah-Arizona line on the north. The Kaiparowits basin covers approximately 8500 square miles and is bounded by the Sevier and Paunsaugunt faults which delineate the east edge of Basin and Range structures and by the Circle Cliffs upward on the east. To the north the Kaiparowits basin passes under the extensive Tertiary flows of the Aquarius Plateau. Throughout most of Paleozoic time, the Black Mesa and Kaiparowits basin area occupied an extensive shelf area between the Ancestral Sierra Grande on the east and the Cordilleran geosyncline on the west. Cambrian, Devonian, Mississippian, Pennsylvanian and Permian sedimentary units are present in the subsurface. No rocks of Ordovician or Silurian age are recognized. Rocks of Permian age and younger are commonly exposed. Paleozoic strata are products of marine sedimentation on the shelf which bordered the Cordilleran geosyncline. Stratigraphic or depositional strike was apparently controlled by adjacent low-lying land masses and embayments into the shelf area from the Cordilleran geosyncline. Orogenic activity, beginning in Late Pennsylvanian and continuing through the Tertiary affected sedimentation during this portion of geologic time as the seas periodically withdrew and encroached upon the shelf area. Large anticlinal features are present in both basins which appear to have had an early history and offer many favorable possibilities for oil and gas production in areas of favorable stratigraphy. End_of_Article - Last_Page 1874------------

Occurrence and Origin of Mississippian Gas in Four Corners Region

In the Four Corners region (junction of Utah, Colorado, New Mexico, and Arizona), uncommon amounts of carbon dioxide, nitrogen, and helium occur in carbonates of Mississippian age. About 80 per cent of the reservoir beds are gray, tan to brown, crystalline and sucrose, microcrystalline to very fine-grained, secondary dolomite. Vuggy, intercrystalline, fracture, earthy, and interoolitic porosity are present in the dolomite intervals. Vuggy and intercrystalline porosity are the most common; fracture porosity is probably more prevalent than has been noted. Tan to brown, partly dolomitic, microcrystalline to very fine-grained limestone beds constitute about 20 per cent of the reservoir intervals. Carbon dioxide ranges from 0.3 per cent to 97.5 per cent; nitrogen from a trace to 89.03 per cent; and helium from a trace to a maximum of 7.5 per cent. The amounts of nitrogen and helium decrease with distance from the Defiance uplift in the southern half of the area. There is a general increase in the percentage of nitrogen and helium with depth in individual boreholes in both Mississippian and Pennsylvanian rocks. A plot of helium and the sediment thickness from the top of the Mississippian to the top of the Precambrian shows high helium in the thinner stratigraphic intervals (450-650 feet). Nitrogen and argon were not separately plotted against the sediment thickness, but a similar relationship is believed to exist. A comparison of carbon dioxide and the same stratigraphic interval shows that seven of eight carbon dioxide occurrences in the 60-100 per cent range are 800-1,600 feet above the top of the Precambrian. In both Mississippian and Pennsylvanian rocks the N2/He ratio is nearly 10 : 1, the N2/A ratio nearly 100 : 1, and the ratio of He to A nearly 10 : 1. The relatively good correlation between the abundance of these gases suggests strongly that they were derived from a common source. It is concluded that this source was Precambrian basement rocks, particularly those of the Defiance uplift. The decay of nitrogenous material has undoubtedly contributed a little nitrogen to the Four Corners gases. The origin of the hydrocarbon gases in the Mississippian reservoirs has not been established with certainty. They are probably indigenous to the Mississippian reservoirs, but may have migrated into the Mississippian sediments from Devonian, or, more likely, from Pennsylvanian beds. Though there are a number of possibilities for the origin of the carbon dioxide, the writer favors: (1) an indirect intrusive source (by metamorphism of Mississippian carbonates during Laramide or younger intrusions); and (2) a contribution of environmental carbon dioxide. Methane, ethane, and carbon dioxide are not in chemical equilibrium. This disequilibrium state is attributed to either kinetic factors in the production of the gases and (or) to a mixing of gases at a temperature sufficiently low so that equilibrium has not been attained since the time of mixing.

Pennsylvanian Paleotectonics of Colorado Plateau

During the Pennsylvanian interval of time, the Colorado Plateau area underwent the most active period of tectonic activity of the Paleozoic era. Pennsylvanian paleotectonic relationships have been reconstructed from stratigraphic and lithologic data acquired from 264 control points. This was accomplished through the use of isopach maps, lithologic ratio maps, vertical sandstone distribution maps, field evidence, and paleontological data. Pennsylvanian tectonic elements of the Colorado Plateau area defined a northwest-trending parageosynclinal seaway. Greatest subsidence occurred in the north where the Paradox Salt basin formed as an asymmetrical half graben. On the south, depositional centers formed in the Pennsylvanian San Juan and Lucero basins. The seaway opened northwest onto the rapidly subsiding Oquirrh shelf, northeast into the northern Colorado trough, west into the Cordilleran geosyncline, southeast into the southern Colorado trough, and south and southwest into the Pedregosa basin and the Sonoran geosyncline. The seaway was bounded to the northwest by the Emery uplift, to the northeast and east by the Uncompahgre and San Luis uplifts, and to the west and southwest by the Kaibab-Zuni uplift. The Defiance upli t, a structural subsidiary of the Kaibab-Zuni lineament, projected north into the southern Paradox basin, where it strongly affected depositional patterns during Pennsylvanian time. Throughout most of Pennsylvanian time, the Emery uplift was a low, partly submerged, structural arch. A southeastern spur of tectonism, probably archipelagic in nature, affected depositional patterns from Desmoines to Wolfcamp time in the northwestern Paradox basin. Earliest and greatest Uncompahgre tectonism occurred in its southern part. Here it became a clastic source area in Atoka time, remained orogenically positive into the Permian, and during this time interval shed a tremendous coarse clastic delta to the southwest. The northwestern Uncompahgre responded to uplift later in the Pennsylvanian, but probably permitted marine access between the Eagle and Paradox evaporite basins during Desmoines time. Consequently, Paradox evaporites may be the composite product of two evaporite ba ins. The San Luis uplift, which was slightly positive in pre-Pennsylvanian time, did not respond to sharp uplift until late Pennsylvanian time. The Penasco uplift underwent early and late Pennsylvanian tectonism, but impressed only minor influence on the regional lithofacies. Although the Kaibab-Zuni uplift existed as a nearly continuous structural lineament during most of the Pennsylvanian, greatest and earliest tectonism occurred in the eastern, or Zuni segment. The Defiance uplift was probably positive in the Mississippian, and underwent successive tectonic pulses during Atoka-early Desmoines, late Desmoines, and late Pennsylvanian-early Permian times. Major hydrocarbon production in the Four Corners area displays close relationship to this tectonic element geographically and geologic lly. Because of sedimentary and tectonic barriers, a thick sequence of evaporites was deposited within the Paradox Salt basin. The Emery and Uncompahgre uplifts created restriction on the northwest, north, and east, and the Kaibab-Zuni uplift provided restriction on the southwest. The combination of a clastic barrier projecting into the Four Corners area from the northeast and the Defiance uplift projecting into the Four Corners area from the southwest formed nearly continous physical restriction across the southern Paradox basin and provided conditions necessary for cyclical evaporite deposition within the basin.

Pennsylvanian Paleotectonics of Paradox Basin: ABSTRACT

During Pennsylvanian and Permian time, the Colorado Plateau area underwent the most active period of tectonic activity of the Paleozoic and Mesozoic eras. A continuous parageosynclinal seaway extended from the Oquirrh sag of east-central Utah southeast into central New Mexico. Greatest subsidence occurred in the Paradox basin, an asymmetrical half-graben, where in excess of 6,500 feet of marine and restricted marine sediments accumulated during Pennsylvanian time. Peripheral to this seaway were the orogenic Uncompahgre-San Luis uplift, the epeirogenic Kaibab-Defiance-Zuni uplift, and the epeirogenic Emery uplift. Along the Uncompahgre and Kaibab-Zuni structural trends, tectonism originated earliest in their southeastern segments and progressed northwest. Two elements, the Defiance uplift and a southeastward projecting spur of the Emery uplift, bear major economic implications. The Defiance uplift, a structural subsidiary of the Kaibab-Zuni trend, projected north into the southern Paradox basin. It was active in pre-Pennsylvanian time and affected depositional patterns of marine sediments from Morrow? to Wolfcamp in age. Major hydrocarbon production in the Four Corners area bears a close relation to this element geographically and geologically. Because of Kaibab-Zuni tectonism, only the northern third of the Black Mesa basin appears favorable for hydrocarbon accumulations in Pennsylvanian rocks. The Emery spur projected into the northwestern Paradox basin and affected depositional patterns of sediments from Des Moines into Wolfcamp time. Geologic conditions east of the Emery uplift, and adjacent to its southeastern spur, appear more favorable for hydrocarbon accumulations in Pennsylvanian sediments than those present in the producing part of the Four Corners area. Recent discoveries on the western Paradox Basin shelf prove the presence of oil in marine sandstone stringers and reef-type build-ups within the Hermosa formation. Regional lithologic analyses indicate that this trend may be extended from the southern Emery flank to the northern Kaibab flank. End_of_Article - Last_Page 1096------------

Bleaching clay deposits, Sanders-Defiance Plateau District, Navajo country, Arizona

Bleaching clay deposits of the Sanders-Defiance Plateau district, northeastern Arizona, are an important commercial source of activated bentonite. High-grade deposits show a definite association with geologic structure, distribution of the host formation, type of parent tuff and degree of subsequent alteration, and both the pre- and post-clay erosion pattern.The district is located along the western and southern flanks of the Defiance Plateau. The DeChelly sandstone crops out in the central portion of the uplift and is successively overlain in places by the Moenkopi and Shinarump formations; the younger Chinle formation crops out in a belt around the flanks of the uplift. The Bidahochi formation of Pliocene age unconformably overlies the Chinle and older formations in the district.Accompanying the Laramide upwarping of the Defiance Plateau a broad shelf area was developed along the southwestern edge, flanked by a westward dipping monocline. The Defiance Uplift and the monocline restricted the areal distribution of the Bidahochi sediments to the shelf area. Channeling of the shelf area occurred during deposition of the lower member to the west. Volcanic ash falls occurred during deposition of the Bidahochi sediments. The ash was further accumulated in the depositional traps of the shelf area by erosion.The course of alteration of vitric tuff to bentonite is traced by field observation, microscopical examination, and x-ray analyses. The ratios of ions present in the tuff (latitic) and bentonite (calcium montmorillonite) are calculated from chemical analyses, and an equation is written for the chemical reaction between the rocks expressed in rock-cell formulas.Combination with water is the first step in the alteration. Mg has been added; and silica, Na, and K have been removed. Al is taken to have remained essentially static because it was presumably flocculated by the presence of excess divalent Ca and Mg ions.All known bleaching clay deposits of the district occur in close proximity to: (a) a belt confined to the shelf area, and (b) the approximate stratigraphic position of the medial volcanic member of the Bidahochi formation.The channel and basin structures, locale of the vitric ash accumulations, were developed as a result of regional structures. Within any depositional trap, smaller-scale structural features further controlled the accumulation of ash and subsequent-development of bleaching clays. These small-scale features may result from: (a) local squeezing, (b) inequalities of the depositional basin or channel, and (c) erosion having removed the clay since deposition and its alteration. The drainage pattern of the larger modern streams coincides somewhat with the Pliocene post-clay pattern. If by comparison the ancient channel was large, it probably removed the clay by an erosional cut-out feature.The best prospects for developing new deposits in the district lie within: (a) the areal extent of the shelf area, (b) those Bidahochi sediments immediately overlying this structural shelf in the older rocks, and (c) parent vitric ash accumulations that are latitic in composition. Favorable areas in the district are outlined.An estimated total output for the district (1924-1954) is 4,650,000 tons.

Pennsylvanian Stratigraphy of Paradox Salt Basin, Four Corners Region, Colorado and Utah

The Paradox Salt basin, located in southwestern Colorado and southeastern Utah, is bounded by the Uncompahgre uplift on the northeast, the San Juan basin contiguous to it on the southeast, the Defiance uplift on the south, the Black Mesa and Kaiparowits basins on the southwest, the Kaibab upwarp on the west, and the Uinta basin on the northwest. The Monument upwarp and the San Rafael swell are major associated structures which extend obliquely across the southwest and northwest parts of the Paradox Salt basin shelf, and exerted only slight sedimentational control within the confines of the basin prior to late Pennsylvanian time. Major marine facies gradations present in Pennsylvanian and Permian strata comprise limestone, dolomite, gypsum, anhydrite, salt, gray shale, and black shale grading to black siltstone and black limestone, quartzose sandstone, feldspathic sandstone, arkose, and arkosic conglomerate. These stratal variations were most strongly influenced not only by the major source area in the Uncompahgre-San Luis highland but also by the rate and periodicity of subsidence of the Paradox geosyncline. The structural history of the northwest-trending Paradox Salt basin, its shelves, and the ringing uplifts, is very complex, with epeirogenic movements prior to Pennsylvanian time responsible for the formation of widespread disconformities in pre-Pennsylvanian strata. Rapid subsidence to form the Paradox Salt basin (in a broad shelf area initially receiving allogenic sediments from the San Luis highland, the Defiance uplift, and the Kaibab upwarp, and carbonates generated within the basin area itself) was succeeded by the deposition of a unique sequence of evaporites which characterizes this most anomalous Pennsylvanian sequence known anywhere in North America. During evaporite deposition and continuing with major pulsations until Triassic time, the Uncompahgre uplift rose out of the n rtheast side of the Paradox basin to shed many cubic miles of low-grade graywacke sediments succeeded by arkose. In late Pennsylvanian and early Permian time, these coarse clastics interfingered with marine carbonates and fine-grained clastics in the deeper part of the basin while the correlative shelf carbonates were being altered during numerous diastems and times of non-deposition along the southwest shelf of the Paradox basin. Epeirogenic uplift of the entire region began during early Permian time and brought the Paradox Salt basin area above sea-level, with coarse clastics moving into the continuously downwarping continental basin southwest of the Uncompahgre uplift. Complexities of marine to non-marine sedimentation across the Permian-Pennsylvanian interface, with marine waters retreating northwestward from the region during early Permian time, have made the choice of a simple formational upper boundary for the Pennsylvanian system impossible. It is proposed that the Rico transition facies be considered a basal member of the Cutler formation despite the recognition that the Rico facies grades from Virgil to Hueco in age within the Paradox Salt basin region. The Hermosa formation is limited to the post-evaporite sequence below the Rico facies, whereas the Paradox evaporite sequence below the Hermosa formation is here considered a mappable formation. Pre-Paradox strata above the Molas shale are here given the new formation name Pinkerton Trail li estone inasmuch as it is a widespread mappable unit both in the subsurface of the Paradox Salt basin and on the surface in the San Juan Mountains.