Ancestral Rocky Research Articles

Paleozoic Structure of the Central Basin Uplift and Adjacent Delaware Basin, West Texas (1)

Structural cross sections and a regional tectonic map show that the Central Basin uplift consists of upthrust blocks of alternating vergence. The uplift is separated from the adjacent Delaware basin by a major fault that changes throw and structural style along strike, accompanying changes in block vergence found along the uplift. The west-facing basement blocks were thrust westward over the Delaware basin, whereas the adjacent blocks face eastward. The distribution and style of structures suggest that small amounts of left-lateral movement occurred along west-trending and northwest-trending faults that separated rising blocks of the uplift. Transfer of lateral movement from vertical, west-trending faults on the Central Basin uplift and the Ozona arch to the west-trending Mid-basin fault in the Delaware basin rotated blocks of the uplift in a clockwise sense. Faults and folds within the study area have trends and slip directions that are generally similar to those within the Reagan and Washita Valley fault zones in the Arbuckle uplift adjacent to the Anadarko basin. The difference in the style of deformation between the two uplifts relates to varied angular relationships between preexisting structure and the direction of late Paleozoic stress. The identity of structural trends within these two uplifts suggests that other late Paleozoic basins and uplifts of the Ancestral Rocky Mountains may have formed in a similar stress field and that crustal rotation was pervasive along the southwestern margin of the continent during the late Paleozoic Ouachita-Maratho deformation.

Preservation and paleoclimatic significance of eolian dust in the Ancestral Rocky Mountains province

Upper Paleozoic (Pennsylvanian-Permian) silty strata of the northern Pedregosa basin (southern Ancestral Rocky Mountains province) exhibit distinctive features that suggest origin of the silt as eolian dust prior to deposition in marine environments. Studies of mechanisms controlling modern eolian dust accumulation imply that combined marginal-marine-epeiric-sea environments constitute an ideal sink for eolian dust. Accordingly, Pedregosa siltstones may reflect the prevalent mode of eolian dust preservation downwind from coeval ergs, and significant volumes of eolian dust may be preserved within analogous epeiric strata. Because eolian dust reflects conditions of aridity and windiness, it bears on paleoclimatic and paleogeographic reconstructions. Recent studies within several basins of the Ancestral Rocky Mountains suggest an eolian derivation for many siltstones and thus corroborate inferences of increasing aridity and concomitant monsoonal conditions for western Pangea. Further regional analysis may provide valuable insights for refining concepts of the zonal to monsoonal transition that accompanied Pangean assembly.

Late Paleozoic extension in the Great Basin, western United States

Geologic mapping in the Toiyabe Range in central Nevada has revealed the existence of normal faults of probable mid-Mississippian to Early Permian age that strike roughly east-west and dip northward. Additional evidence of uplift and erosion followed by mafic volcanism and subsidence suggests that much of the central and southern Toiyabe Range was affected by late Paleozoic extension. Similar patterns of late Paleozoic uplift and subsidence, together with local basaltic volcanism, are widespread in the western United States, suggesting that the continental margin was dominated by extension or transtension in Mississippian to Permian time. This extension was coeval with convergence between North America and South America across the Ouachita and Marathon belts, and the dynamic interaction of these two margins may, by analogy with the Cenozoic tectonics of Asia, have given rise to complex late Paleozoic deformation in the Ancestral Rocky Mountains and adjacent areas of the interior western United States.

Determination of offset across the northern margin of the Wichita uplift, southwest Oklahoma

The Wichita uplift represents a tectonic element within the Ancestral Rocky Mountains, a series of basement-involved uplifts and associated basins within the foreland of the Ouachita orogen. Contrasting tectonic models for the evolution of the Wichita uplift are evaluated by (i) analyzing the suite of structures within the Frontal fault zone, which separates the uplift from the adjoining Anadarko basin, and (ii) undertaking a palinspastic restoration of Houton Group (Late Ordovician-Devonian) thickness trends between the basin and the Frontal fault zone to estimate the magnitude and sense of offset across the principal uplift-bounding faults. Cross sections from uplift to basin show basement-involved thrusts and folds similar to those recognized within the Laramide (Late Cretaceous-Tertiary) foreland of the Rocky Mountains. Structural patterns in map view possess elements that have been previously interpreted as resulting from reverse slip and/or left slip on the uplift-bounding faults. Palinspastic restoration of isopach patterns between the Frontal fault zone and Anadarko basin indicates that offset across the frontal faults may be explained by oblique slip (left reverse slip) on the uplift bounding faults. The component of left slip is nearly an order off magnitude less than proposed in some previous models of Carboniferous tectonism within the foreland of the Ouachita orogen.

Controls on synchronous ephemeral- and perennial-river sedimentation in the middle sandstone member of the Triassic Chinle Formation, northeastern New Mexico, U.S.A.

The middle sandstone member of the upper Triassic Chinle Formation in northeastern New Mexico consists of ephemeral-river deposits along the Front Range adjacent to the Ancestral Rocky Mountains and perennial braided-river deposits on the Plains to the east. The two fluvial systems were synchronous; the ephemeral rivers existed as transverse tributaries to a perennial trunk system that flowed from south to north. Proximal ephemeral-stream deposits are characterized by 60–80 m thick sequences of coarse-grained, siliciclastic sediments deposited principally by overbank sheetflooding. Facies associations are dominated by sandstone with horizontal stratification and subordinate sets of tabular-tangential and trough cross-stratification. Incised streamflood deposits are represented by channeled conglomerates and single-story, massive and trough cross-stratified sandstones. Distal ephemeral-stream deposits are distinguished by their lenticular geometries, thicknesses of 20–30 m, and sedimentary structures indicative mainly of lower-flow-regime stream flooding. Channeled conglomerates and trough cross-stratified and parallel-stratified sandstones are capped by trough cross-laminated and subordinate horizontally stratified sandstones. Facies associations in the deposits of the trunk system are comparable to those of the Platte and South Saskatchewan braided rivers. Sets and cosets of tabular-tangential or trough cross-stratified conglomerate and sandstone are separated by variable thicknesses of horizontally stratidied sandstone. These facies dominate 10–30 m thick, sheet-like, siliciclastic bodies that are traceable laterally for tens of kilometers and are separated by comparable or lesser thicknesses of red mudstone. Both extrabasinal (allocyclic) and intrabasinal (autocyclic) controls influenced sandstone body geometries in the middle Chinle Formation. Uplift and subsidence determined the position of base level that, in turn, determined the nature of autocyclic processes. Adjacent to the Ancestral Rocky Mountains, sediment yield was great resulting in accumulation of sheetflood deposits above base level, thereby promoting local incision. Under these conditions and with a continuously rising base level, lenticular, coarse-grained sediment body geometries were produced along the Front Range. On the Plains, sediment yield was moderate and subsidence rates were slow. Bedload sediment accumulated to, but not above base level, thereby promoting avulsion and the development of thin, sheet-like coarse-grained sediment bodies. The contrast between the two coexisting fluvial sedimentation styles is attributed to differences in runoff. Adjacent to the Ancestral Rocky Mountains, seasonal rainfall resulted in ephemeral flow, whereas on the Plains, flow was perennial due to groundwater discharge from the highlands to the west.

Controls on late Paleozoic and early Mesozoic eolian deposition of the western United States

The Oregon coastal dunes and Great Sand Dunes of Colorado illustrate the dynamic relationships between factors that enhance and inhibit eolian-sand deposition. Abundant sand supply, persistent sand-transporting winds, and a well-drained, low-relief surface are the most important enhancing factors. Sand supply is a function of source, concentration, and sand availability to the wind. Abundant precipitation does not preclude deposition of eolian sand. Inhibiting factors result in sand stabilization or reduction in sand supply and include water saturation to the surface of sand accumulation, freezing of sand moisture, formation of desert pavement, and, most importantly, vegetative cover. Inhibiting factors are directly or indirectly a function of climate. Through time, climate has been a function of atmospheric circulation vis a vis continental position. Vegetative cover is a function of climate and plant evolution. Evolution of land plants, angiosperms and grasses divide the geologic history of eolian sand deposition into four intervals of decreasing latitudinal and areal distribution and increasing climatic contrast. Two types of dunefields are identified on the basis of processes responsible for sand concentration and relation of paleowind to fluvial paleoslope. Type I eolian sands are concentrated by fluvial processes only. They are relatively thin, interbedded with, and commonly overlain by, fluvial sediments. Type II sands are concentrated by fluvial and littoral processes on coasts with onshore winds. They are relatively thicker than Type I sands, intertongue at their bases but are not interbedded with fluvial sediments, and commonly are overlain by marine sediments. Both types may be initiated by tectonic uplift or climatic change in the source area. Type II sands also may be initiated by change in sea level at the site of deposition. The Pangean interval is related to low stand in world sea level and corresponding widespread deposition and preservation of terrestrial sediments. Mesozoic eolian sandstones of the western interior are Type II with distal source in the Gulf of Mexico-Atlantic rift belts. Erg development was initiated by marine regression and terminated by marine transgression. The late Paleozoic eolian sandstones of the western United States are a combination of Type I and II with proximal source in the Ancestral Rockies.

Origins of upper Paleozoic quartzose sandstones, American southwest

In the latter half of the Paleozoic, all margins of the North American craton were orogenic highlands except for the Cordilleran miogeocline. Rivers that drained cratonic areas northwest of the Transcontinental arch probably flowed to the topographically low Cordilleran miogeocline, but little direct sedimentary record of these ancient fluvial systems remains. The voluminous upper Paleozoic quartzose sandstone deposits of the American west are indirect evidence for these systems. Northern and central portions of the miogeocline were flooded with quartzose sand in the Pennsylvanian, and by mid-Early Permian, littoral and wind processes were transporting large volumes of quartzose sand southward, along the miogeoclinal/cratonic hingeline. Winds drove some of these sands inland, onto the disrupted western end of the Transcontinental arch, where they mixed with locally derived detritus and collected in great ergs. Calcareous siltstones that were deposited downwind of major ergs probably represent desert loess accumulations. Some eolian sands spilled across the Transcontinental arch to interfinger with carbonate and evaporite deposits in basins of the southwestern United States. Most sandstones deposited by these ancient littoral and eolian systems are fine- to very fine-grained, potash-feldspar subarkoses. Bimodal sandstones are common. The distribution, composition and volume of these sandstones suggest that the northern craton was the major source area and cratonic uplifts of the Ancestral Rockies were a secondary source. The fine grain size, high degree of sorting, and grain-size bimodality suggest that these sands were the most mobile sediments in littoral and eolian settings. A perplexing aspect of these deposits is the lack of compositionally unstable silicates, such as plagioclase, and the lack of phyllosilicate clays. Possibly this reflects great amounts of physical abrasion and sediment winnowing during protracted transport in littoral and eolian systems, and it may partially reflect weathering processes in arid coastal settings. The petrology therefore reinforces the model of a northern cratonic source and protracted southerly transport, but the degree to which the petrology reflects provenance and various physico-chemical processes remains unresolved.

Sedimentation of the Late Triassic Higham Grit in a South SaskatchewanlPlatte River-Type Braided Stream Complex, Southeastern ldaho and Western Wyoming

The Late Tnassic Higham Gnt m southeastern ldaho and western H$aming IS comprised pmdommantly of coarse to medrum- grarned sandstone and pebble conglomerate wrth mrnor mudstone Lithofacies present include: Massive to crudely horizon- tally bedded pebbly conglomerate (Gm), trough crossbedded sandstone (St), planar crossbedded sandstone (Sp), horizon- tally stratified sandstone (Sh), ripple crosslaminated sandstone (Sr), and finely laminated sandstone and mudstone (FI). Deposition occurredm a South Saskatchewan/Platte River-type braided fluvialcomplex wrth the development of longitudinal bars and gravel lags (Gm), straightcrested transverse bars (Sp), and sinuouscrested transverse bars and dunes (St). Perrodic, high-velocity flow resulted m development of upper flow regime plane beds (Sh). Minor episodes of flood plain rnundation produced werbank deposits (FI). Application of the South Saskatchewan and Platte River brarded stream models to the Higham Gnt is m good agreement with provenance studres (Schmitt and Hazen, in preparation) which postulate a sediment source area in the Ancestral Rocky Mountains uplift, a distance of 500-700 km to the southeast Sediment transport distances in both the South Saskatchewan and Platte braided fluvial systems closely approximate this distance.

Polyphase deformation in Sierra del Cuervo, Chihuahua, Mexico: Evidence for Ancestral Rocky Mountain tectonics in the Ouachita foreland of northern Mexico

Sierra del Cuervo is a north-northwest–trending mountain range ∼30 km northeast of Chihuahua City, Chihuahua, Mexico. Rocks exposed in the sierra range from late Precambrian to Quaternary. It has long been recognized that Sierra del Cuervo contains one of the most lithologically different Paleozoic outcrops in Chihuahua, and the recent discovery of Precambrian rocks in the south-central part of the range distinguishes Sierra del Cuervo as one of only two known Precambrian outcrops in the state. Mapping and structural analysis in the south-central part of Sierra del Cuervo reveal that at least five major periods of deformation, ranging from Precambrian to late Tertiary, are represented. The most dramatic structures are a series of thrust faults and associated folds. Thrusting involved both Precambrian basement and Paleozoic sedimentary rocks, with tectonic transport toward the east-southeast. Stratigraphic relationships in Sierra del Cuervo restrict the age of this deformation only to post-Early Permian and pre-Cretaceous, but regional considerations suggest that it occurred during the middle Permian. The origin of these late Paleozoic structures has previously been related to the westward extension of the Ouachita frontal zone; however, their orientation is oblique to the postulated trace of the frontal zone and suggests a much more complex relationship with the Ouachita system. Paleocurrents and compositional variations in sandstones of the Rara Formation indicate the presence of a basement-involved uplift in west-central Chihuahua during the Late Pennsylvanian and Early Permian. This apparent unroofing of a western basement-cored block, followed by east-southeast–directed thrusting, is inconsistent with the tectonic style of the Ouachita frontal zone and suggests that the late Paleozoic structures in Sierra del Cuervo are related to Ancestral Rocky Mountain deformation. Additional evidence for this southern extension of the Ancestral Rocky Mountains is found in other parts of northern and central Chihuahua and indicates that most of the region was in the Ouachita foreland during the late Paleozoic.

Late Miocene reactivation of Ancestral Rocky Mountain structures in the Texas Panhandle: A response to Basin and Range extension

Research Article| February 01, 1987 Late Miocene reactivation of Ancestral Rocky Mountain structures in the Texas Panhandle: A response to Basin and Range extension Roy T. Budnik Roy T. Budnik 1Bureau of Economic Geology, University of Texas, University Station Box X, Austin, Texas 78713 Search for other works by this author on: GSW Google Scholar Author and Article Information Roy T. Budnik 1Bureau of Economic Geology, University of Texas, University Station Box X, Austin, Texas 78713 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1987) 15 (2): 163–166. https://doi.org/10.1130/0091-7613(1987)15<163:LMROAR>2.0.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Roy T. Budnik; Late Miocene reactivation of Ancestral Rocky Mountain structures in the Texas Panhandle: A response to Basin and Range extension. Geology 1987;; 15 (2): 163–166. doi: https://doi.org/10.1130/0091-7613(1987)15<163:LMROAR>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Structural and stratigraphic evidence from the Ogallala Formation (Neogene) documents late Miocene tectonic activity within the Great Plains. Field and subsurface studies in the Texas Panhandle indicate that parts of the Amarillo uplift, a major element of the Pennsylvanian Ancestral Rocky Mountains, were elevated as much as 150 m during initial deposition of the Ogallala Formation. Reactivation of these basement structures occurred in response to Basin and Range extension and opening of the Rio Grande rift in central New Mexico and Colorado. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Left-lateral intraplate deformation along the ancestral rocky mountains: Implications for late paleozoic plate motions

Abstract North America underwent synchronous orogenic events during the late Paleozoic along its eastern margin (Alleghanian orogeny), southern margin (Ouachita orogeny), and within the southwestern part of the continent (Ancestral Rocky Mountain orogeny). All three orogenies were initiated in the late Mississippian to early Pennsylvanian, reached the greatest intensity in the middle Pennsylvanian, and ended in the early Permian. The Alleghanian and Ouachita orogenies have been related to the closing of the proto-Atlantic and the collision between North America and South America-Africa: it is here proposed that the Ancestral Rocky Mountains were produced by a collision between eastern North America and Africa. The Ancestral Rockies were formed as the result of reactivation of the Wichita megashear, a preexisting zone of weakness that extends from southern Oklahoma to eastern Utah. Previous plate tectonic models have implied that the megashear was a zone of right-lateral strike-slip faulting and north-northwest-directed compression. However, structural and stratigraphic data from Oklahoma and Texas suggest that the Wichita megashear was a major left-lateral fault zone formed under east-northeast-oriented compression. Palinspastic reconstruction of pre-mid-Devonian strata across the megashear in Texas indicates that 120 to 150 km of left slip occurred during the Desmoinesian (middle Pennsylvanian). The proposed plate tectonic model for the Ancestral Rocky Mountain orogeny includes: 1. (1) movement of the North American plate eastward from a spreading center in the proto-Pacific; 2. (2) closing of the proto-Atlantic Ocean; 3. (3) collision of North America-Europe (Laurussia) and South America-Africa (Gondwana) resulting in the Hercynian, Alleghanian, and Ouachita orogenies; 4. (4) differential movement across the Wichita megashear and formation of a left-lateral strike-slip fault zone (Ancestral Rocky Mountain orogeny) as the result of east-west compression within the North American plate: 5. (5) relative northward movement of Gondwana against Laurussia producing the Marathon and Arbuckle orogenies; and 6. (6) development of a subduction zone to the west of the North American continent.

Paleozoic Stratigraphy and Tectonics of Northern Uncompahgre Front, Paradox Basin, Utah--An Alternative View: ABSTRACT

The Paradox basin is a complex pull-apart basin of major proportions that developed along intersecting basement fracture zones by strong east-west extensional pulses in the Middle Pennsylvanian. These stresses caused the Ancestral Rocky Mountains to emerge and the Paradox basin to subside. Oblique divergent strike-slip faulting along the Uncompahgre-San Luis uplifts allowed smaller subbasins to develop by orthogonal spreading along intersecting northeast-trending transform faults. The rate of basin-floor subsidence was related to combinations of normal reverse, and strike-slip faulting. The northernmost subbasin of the Paradox basin is bounded by the northwest-trending Uncompahgre uplift, the salt Valley diapiric feature, and the northeast-trending San Rafael and Cataract lineaments. Although generally straight on a regional scale, the Uncompahgre master fault system is complicated in detail. The zone consists of en echelon fault slices, thrust blocks, and detachment faults. Few Paleozoic tests have been drilled along the northern Uncompahgre front. Most structural interpretations have been based on seismic data that have disregarded empirical geologic data from the few deep tests in the area. Structural features such as the Thompson-Yellow Cat anticlines have been assumed to be salt bulges or pillow structures. Geologic and geophysical data strongly suggest these features may be low-angle detachment thrustmore » sheets. The true economic potential of the area also remains unknown; however, the structural style, burial history, and sedimentary rock types suggest that sizable accumulations of untapped hydrocarbons may exist in this portion of the Paradox basin.« less

THE TRANSCONTINENTAL ARCH AND ITS RELATION TO THE COLORADO OIL AND MINERAL BELT.

The Transcontinental Arch and its flanking basins dominated the landscape of North America before the rise of the Ancestral Rockies and the Wichita system. It is the SW extension of the Canadian Shield, and is partly of PreCambrian origin and partly a later development. It is found in a belt extending SW from Minnesota to New Mexico. This concealed structural feature is depicted here from comparisons of stratigraphic sections and well records. It was active from the Camnbrian through the Mississippian. During Cambrian time it was wide and uniform from the Canadian Shield to Arizona. Later, sags developed in Colorado and in Arizona. The changes in size and shape were partly positive and partly negative. PreCambrian rocks and structures have been recorded from geophysical data as well as from holes drilled for oil and gas in the intervening areas, particularly in the western half of the lowlands. The narrow NE‐trending Colorado mineral belt, containing the major mining districts of Colorado is closely related to this continental backbone. It is characterized by NE‐trending faults, and oil and mineral deposits of Laramide age.

Plate Tectonics of Ancestral Rocky Mountains: ABSTRACT

The Ancestral Rocky Mountains were intracratonic block uplifts that formed in Colorado and the surrounding region during Pennsylvanian time. Their development related to the collision-suturing of North America with South America-Africa, which also resulted in the Ouachita-Marathon orogeny. During Early Pennsylvanian time, suturing was taking place only in the Ouachita region, and foreland deformation took place largely in the Mid-Continent. By Middle Pennsylvanian time, the length of the suture zone had increased, and it was active from the Ouachita to the Marathon region. At this same time, deformation of the craton also increased in intensity and in areal extent, culminating in the Ancestral Rocky Mountains. By Late Pennsylvanian time, suturing was taking place only in he Marathon region, and cratonic deformation decreased areally, spreading southward into New Mexico and west Texas and west into the Cordillera miogeocline. The Ancestral Rocky Mountains, and related features over a broad area of the western United States were formed while an irregularly bounded peninsula of the craton (including the transcontinental arch) was pushed northward and north-westward by the progressive collision-suturing of North America and South America-Africa. This intraplate deformation is, in some respects, similar to the deformation of Asia in response to the Cenozoic collision with India. End_of_Article - Last_Page 853------------

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

The Paradox basin (Colorado Plateau province) is an intracratonic depression developed on continental crust. The elongate northwest-trending rhombic-shaped basin of Middle Pennsylvanian age is bounded on the northeast by the Uncompahgre-San Luis segments of the Ancestral Rocky Mountains and on the southwest by the less prominent Four Corners lineament. The basin sagged along intersecting basement fractures by strong east-west extension during Middle Pennsylvanian time. The master fracture system was the northwest-trending Olympic-Wichita structural lane. Oblique divergent strike-slip faulting along the Uncompahgre-San Luis segment created a tension-releasing bend where the Paradox pull-apart End_Page 867------------------------------ basin nucleated and subsequently developed throughout Middle Pennsylvanian time. Smaller subbasins developed by orthogonal spreading along intersecting northeast-trending transform faults, where the rate of basin-floor subsidence was related to combinations of normal and strike-slip faulting. The greater Paradox basin was episodically deepened during Middle Pennsylvanian time by rejuvenated extensional basement faulting. Vertical displacement was greatest along the Uncompahgre front, which caused tilting of the basin and deposition of an asymmetrically thick sedimentary sequence. By mid-Desmoinesian time, the rate of divergent strike-slip faulting slowed considerably. Folds caused by minor wrench movements provided shoaling conditions along the southwest shallow carbonate shelf where porous algal mounds developed. Meanwhile, continued tectonic movement and space-reduction of the basin floor may have triggered salt flowage and diapirism in the deep eastern pull-apart trough. As wrench tectonism diminished from late Desmoinesian through Early Permian time, the eastern portion of the basin continued to subside and was filled with marine and continental sediments. End_of_Article - Last_Page 868------------

Geohistorical Analysis of Paradox Basin: ABSTRACT

The Paradox basin is an elongate sedimentary basin, asymmetric in profile, extending across common corners of Utah, Colorado, Arizona, and New Mexico. Subsidence of the basin began in Desmoinesian time and was coincident with the development of the Ancestral Rocky Mountains. The Uncompahgre uplift formed the northeast boundary of the basin during Pennsylvanian and Permian times. Formation thicknesses and lithologies were obtained from lithologic and radioactivity logs from various parts of the basin. The stratigraphic column at each well, restored through the Upper Cretaceous, was backstripped and decompacted to reconstruct its depositional history. Decompacted geohistory diagrams and residual (tectonic) subsidence curves were then generated for each well. The Mobil 1 McCormick well, drilled in 1977, penetrates Pennsylvanian strata beneath reverse-faulted granitic basement; this indicates that the basin was flexed down in response to Pennsylvanian and Permian thrust faulting along the flank of the Uncompahgre uplift. However, close correspondence of the residual subsidence curves to theoretical thermal subsidence curves indicates that the basin formed by crustal extension. Consequently, development of the basin may have involved crustal stretching (transtensional?) beneath the basin floor, followed by thrusting (transpressional?) along the flank of the Uncompahgre uplift. End_of_Article - Last_Page 854------------

The San Luis Uplift, Colorado and New Mexico--An Enigma of the Ancestral Rockies

The San Luis uplift of the Ancestral Rockies has long been considered to be the source for the relatively clean quartzose sandstones of Morrowan through early Desmoinesian age of north-central New Mexico and southwestern Colorado. Yet, the exact location and structural nature of this uplift remains an enigma. Lithologic differences and stratigraphic sequence suggest the San Luis uplift is a separate distinctive structural complex formed from an earlier wrench fault system located southwest of the true Uncompahgre uplift. The San Luis uplift predated the Uncompahgre uplift according to this evidence. Both the San Luis and Uncompahgre uplifts are source areas which are terminated southeast­ward in New Mexico by the Picuris-Pecos fault. Sediments from the San Luis uplift may provide a viable exploration target in the Paradox basin.

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------------

Basin and Range-Age Reactivation of the Ancestral Rocky Mountains in Texas Panhandle: Evidence from Ogallala Formation: ABSTRACT

The Ogallala Formation (Neogene) is a widespread syntectonic alluvial apron that was shed eastward from the Rio Grande rift and related uplifts in Colorado and New Mexico during Basin and Range extension. In the Texas Panhandle, the Ogallala completely buried Ancestral Rocky Mountain (Pennsylvanian) structures. Renewed movement on these older structures during the Neogene influenced the thickness and facies distribution of the Ogallala. The Ogallala thickens into the Palo Duro, Dalhart, and Anadarko basins. Major distributary channels on Ogallala alluvial fans coincide with the axes of these basins, whereas major interchannel areas overlie intervening uplifts. Second-order structures subtly influenced the unit as well. For example, in the Carson basin, a Pennsylvanian rhomb graben along the Amarillo uplift, the Ogallala is over 250 m (820 ft) thick compared with 90 m (275 ft) in adjacent areas. Within the Palo Duro basin, local highs controlled the distribution of thin, interchannel fl od-basin and lacustrine deposits. Thicker, braided-stream channel deposits follow local lows. Later movement on the Amarillo uplift broadly folded the Ogallala. The southern high plains surface subtly reflects basement structure, with topographic highs overlying basement highs, suggesting post-Ogallala deformation within the Palo Duro basin. The Amarillo uplift is approximately perpendicular to the Rio Grande rift and parallel to the direction of Basin and Range extension. Thus, the stress field that produced the rift may have caused strike-slip movement and reactivation of the Carson basin along the Amarillo uplift. End_of_Article - Last_Page 459------------

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