Ancestral Rocky Research Articles

Thick- and thin-skinned contractional styles and the tectonic evolution of the northern Sangre de Cristo Mountains, Colorado, USA

AbstractThe Sangre de Cristo Mountains of southern Colorado and northern New Mexico, USA, contain an unusual combination of thick- and thin-skinned contractional structures involving both basement and cover rocks in the Laramide Rocky Mountain foreland. These structures are truncated by down-faulted extensional basins to the east and west. Together with synorogenic sediments, these structures preserve a record of the rise of the Ancestral Rocky Mountains, the Laramide orogeny, and Rio Grande rifting. Laramide structures within the mountains provide clues to processes that link the three events and to necessary conditions for thin-skinned and thick-skinned contractional structures to form together in continental interiors.To examine the full variety of structural styles, a portion of the northern Sangre de Cristo fold- and-thrust belt in Colorado was described and interpreted using geologic maps and structural cross-sections. Stratigraphic relations of the Ancestral Rocky Mountain highlands and basin fill were reconstructed from existing maps. These relations allow identification of faults inherited from the Ancestral Rocky Mountains, differentiation of thrust sheets, and in some cases, estimation of the magnitude of displacement. To examine relations between Laramide thrusts and Rio Grande rifting, kinematic data were collected from a thrust fault adjacent to rift faults.Three thrust fault styles were recognized: thin-skinned basement, thin-skinned cover rocks, and thick-skinned basement. Thin-skinned thrusts arising from a hinterland beneath the present San Luis Valley carried sheets of Proterozoic basement rocks northeast over a Laramide foreland. These basement thrusts are interpreted to be faults of the Ancestral Rocky Mountains that reactivated during the Laramide orogeny. The Laramide foreland consists of thin-skinned thrusts and folds in sedimentary cover rocks as young as 49 Ma. Both thin-skinned thrusts in basement and cover rocks are bounded by thick-skinned basement thrusts that moved intermittently throughout the Laramide orogeny.We infer that thin-skinned thrusts form in continental interiors where deformation is focused in weak strata of thick basin fill and in fluid-reaction weakened preexisting faults in basement rocks. Both conditions are met in the Sangre de Cristo Mountains. Basement thrusts adjacent to the San Luis Valley contain evidence of plastic contractional microstructures overprinted by extensional microstructures that may record the transition from Laramide contraction to Rio Grande extension of the crust.

The impact of continental shape in 3D geomechanical models of ancient orogenic belts: An example from the Ancestral Rocky Mountain orogeny

The orientations of ancient intra-cratonic uplifts are commonly used to interpret the boundary tectonism style along concealed, modified, or destroyed continental margins. An implicit assumption is that the tectonism-linked stress field strongly correlated with uplift orientations. However, among other uncertainties, the shape of a stressed domain likely has an influence upon the interior stress field. In many cases of pre-Cenozoic orogenesis, multiple feasible restorations of a continent margin exist. To explore the influence of continent margin position and shape on a hypothetical ancient continental stress field we developed a new 3D geomechanical finite element method (FEM) to evaluate scenarios for the late Paleozoic Ancestral Rocky Mountain orogen (ARMO). Tectonic scenarios were calculated across a range of domains constrained by geologic uncertainty in the pre-ARMO geometry of the continent. Uncertainty for continent geometry prior to the ARMO is estimated from several published palinspastic restorations. The results demonstrate that 10s–100s of km of uncertainty in margin position, as is the case of the ARMO, should not prevent the linkage of stress patterns to styles of boundary tectonism. This work shows it is plausible to simplify the analysis of the ARMO stress/strain field utilizing one continental geometry.

The edge of a Permian erg: Eolian facies and provenance of the Lyons Sandstone in northern Colorado

ABSTRACTFacies of the Permian Lyons Sandstone are described and interpreted based on analyses of 23 cores from Larimer and Weld counties, Colorado. Here, the Lyons Sandstone consists of very fine- to medium-grained sandstone with minor silt and mudstone interbeds. The unit has five recurrent siliciclastic facies that can be grouped into two facies associations (FA). FA1 consists of (1) high-angle, cross-laminated sandstone (Facies 1; interpreted as eolian dune remnants); (2) low-angle, cross-laminated and horizontally laminated sandstone (Facies 2; interdune); and (3) chaotically bedded to folded sandstone (Facies 3; lower dune flanks). FA2, in contrast, is mainly (4) wavy- to irregularly laminated silty sandstone (Facies 4; wet to damp interdune); and (5) massive to wavy-laminated silt-rich mudstone (Facies 5; ponded water areas between dunes) with minor amounts of high-angle, cross-laminated sandstone (Facies 1) and low-angle, cross-laminated and horizontally laminated sandstone (Facies 2). FA1 is hypothesized to have been produced in an eolian system akin to those that might exist in the dune-dominated portion of an erg, whereas FA2 was deposited in the intermittently wet portion of this eolian system, perhaps along erg margins or in flat dune-adjacent settings that were impacted by the water table. Isopach data suggests that the study area is on the fringe of a larger Lyons system that spans > 100,000 km2, and was deposited close to the Ancestral Rockies—a paleogeography consistent with deposition in erg to erg-margin paleoenvironments. Detrital zircon populations from nearby Colorado Front Range outcrops and from 12 correlative eolian units are dominated by small, well-rounded Paleoproterozoic and Mesoproterozoic grain populations that are remarkably similar between units, signaling a well-mixed system that also received an influx of distally sourced sediment from the Appalachian orogen. Detrital zircon-based maximum depositional ages of the Lyons Sandstone and its equivalents are internally consistent with deposition of the unit during the latest Artinskian to Kungurian.

Evaluating the Compatibility of Hematite (U‐Th)/He Data and Hematite‐Carried Secondary Magnetizations: An Example From the Colorado Front Range

AbstractAncient magnetization(s), often recorded by hematite (Fe2O3), provide key paleomagnetic constraints on plate interactions through time. Primary remanent magnetizations may be modified or overprinted by secondary processes that complicate interpretations of paleomagnetic data. Hematite (U‐Th)/He (hematite He) dating has the potential to resolve when secondary magnetizations were acquired. Here, we compare hematite He data and paleomagnetic results in Paleoproterozoic crystalline rocks, meters below a major nonconformity in the Colorado Front Range, USA. Prior work and new rock magnetic data indicate that pervasive hematite alteration records a secondary chemical remanent magnetization (CRM) during the Permo‐Carboniferous Reverse Superchron, coincident with the Ancestral Rocky Mountain orogeny. We target minor hematite‐coated faults cutting basement for (U‐Th)/He analyses because they are of sufficient hematite purity to yield geologically meaningful dates. Two samples yield overlapping and scattered individual hematite He dates ranging from ∼138 to 27 Ma (n = 33), significantly younger than the age of the late Paleozoic CRM. Scanning electron microscopy, electron probe microanalysis, and Raman spectroscopy indicate that aliquots have variable grain size distributions and fluorocarbonate impurities. Thermal history models support hematite on fault surfaces mineralized coeval with CRM acquisition during the late Paleozoic, and hematite He data scatter reflects variable He loss during Mesozoic burial owing to differences in grain size distribution from fault slip comminution and in chemistry among aliquots. Our results underscore the differences in temperature sensitivity and sampling requirements between paleomagnetic and hematite He investigations and illustrate that hematite He dates will usually be younger than preserved remanent magnetizations.

Magnetotelluric Imaging of the Lithospheric Structure of the Southern Oklahoma Aulacogen: Evidence for Long‐Term Weakening Caused by Rifting

AbstractMagnetotelluric data were used to study the lithosphere structure of the Southern Oklahoma Aulacogen (SOA). Inversion of the data revealed two low resistivity anomalies beneath the SOA. The first is located in the depth range 0–90 km in the crust and upper lithospheric mantle. The second extends from a depth 100 km to the base of the lithospheric mantle and extends away from the SOA to the ends of the profile. The cause of low resistivity anomalies is discussed in relation to the tectonic evolution of the region and recent laboratory experiments on rock conductivity. The first anomaly is attributed to the combination of (a) water present in mantle minerals and (b) the formation of hydrous mineral phases by interactions between a plume and the lithosphere during rifting. Grain size reduction and fabric alignment from deformation during the Ancestral Rocky Mountain (ARM) orogeny may have also contributed to the low resistivity. This enrichment phase may have mechanically weakened the lithosphere and allowed deformation to occur during the ARM orogeny. The low resistivity of the deeper anomaly is attributed to a fluorine‐enriched phlogopite layer that is also coincident with an observed seismic mid‐lithosphere discontinuity (MLD). A lithosphere keel of mantle minerals enriched in water underlies this layer and may have formed by accretion of the plume head to the lower lithosphere after rifting, which also rethickened the lithosphere to its present‐day depths. The MLD may then reflect a melt layer along a paleo lithosphere‐asthenosphere boundary entombed during the accretion.

Revised Paleozoic depositional history of the central Rocky Mountains (Utah and Colorado)

Precise sedimentological, stratigraphic, paleontological, and geochronological data collected from Paleozoic strata of the Dinosaur National Monument (DNM) area (NW Colorado and NE Utah) allow for comprehensive revision of the depositional history of that part of the central Rocky Mountains.Recovery of multiple species of the trilobite genus Elrathiella from sandstone beds in the middle of the Lodore Formation assigns those deposits to the mid-Miaolingian Ehmaniella Zone near the top of the Wuliuan Stage. This is significantly older than the previous “Dresbachian” age estimate for those coarse clastics, which attributed them to deposition during the late Miaolingian (latest Drumian or Guzhangian). This age revision re-positions the southernmost boundary of widespread Miaolingian inner detrital belt deposits of Wyoming to within 90 km of an extensive area of central Colorado where strata of this age are absent, and middle Furongian successions rest directly on basement. The boundary between these two regions, one with and one without Miaolingian strata, is nearly coincident with the Cheyenne belt, an east–west oriented Precambrian continental suture zone at the northern margin of the Yavapai Terrane. This association supports the idea that deep-seated crustal-scale features controlled the Cambrian paleotopography of the Rocky Mountain region and, in turn, the regional pattern of Cambrian transgression across western Laurentia.Early cursory paleontological work suggested a Devonian age for a thick sandstone interval near the top of the Lodore Formation, based on phosphatic fossils. We demonstrate that these fossils are in fact Cambrian obolid brachiopod shells. However, we provide multiple lines of evidence, including carbonate δ13C chemostratigraphic data, distinctive lithofacies, and stratigraphic relationships to argue that a thin interval of shale and dolostone, as well as a previously unrecognized paleokarst and regolith interval at the top of what was mapped as Lodore Formation, are in fact part of the Upper Devonian Parting Formation. The thick sub-Mississippian paleoregolith is consistent with elevated rates of weathering during the Late Devonian–Early Mississippian. δ13C chemostratigraphic data for the Madison Limestone include a large double-peaked Kinderhookian positive isotopic excursion (up to ~6.5 ‰), providing a precise estimate of the timing of Mississippian transgression in this part of the Rocky Mountain region.Detrital zircon geochronologic data for Cambrian to Pennsylvanian strata show remarkably consistent patterns dominated by ~1100 Ma, ~1400 Ma, and ~1700 Ma peaks, reflecting relative igneous quiescence over much of the Paleozoic for this region, a span of ~200 m.y., despite some orogenic activity (e.g., Antler Orogeny, Ancestral Rockies) during that time interval.

Large detrital zircon data set investigation and provenance mapping: Local versus regional and continental sediment sources before, during, and after Ancestral Rocky Mountain deformation

The increasing number and size of detrital geochronology data sets offer new opportunities for increased accuracy and resolution of sediment routing models. However, the new opportunities come coupled with challenges in large data integration and visualization. We address these challenges by outlining two novel approaches that aid in analyzing and interpreting large detrital geochronology data sets: (1) combination of bottom-up and top-down detrital zircon source modeling, and (2) sediment provenance mapping. Combining source-modeling methods provides guidance in identifying empirical detrital zircon sources and determining source proportions. Provenance mapping integrates source proportions from modeling results and complimentary geologic data (e.g., paleocurrents, paleogeography, and stratal thickness maps) to extrapolate provenance information through areas with sparse or ambiguous data, thus mitigating issues of data distribution heterogeneity. Sediment provenance maps also provide a synoptic view of data that, along with detrital zircon source modeling, aids in circumventing lengthy descriptions of individual age modes for data sets containing hundreds of samples, which can obscure underlying trends in the data. We apply this approach to late Paleozoic−early Mesozoic strata, using 329 published and new U-Pb detrital zircon samples, and document five sediment-routing episodes in the core zone of intraplate deformation in western Laurentia (i.e., the Ancestral Rocky Mountains (ARM)). The transitions between these episodes are defined by changes in sediment source distribution, which are illustrated by provenance maps that show (1) the degree and extent of ARM basin isolation from transcontinental sediment sources and (2) ARM-driven changes in transcontinental sediment routing systems. We map possible sediment pathways of distally derived sediment around the ARM core, illustrating that ARM uplifts diverted transcontinental systems around areas of intense intraplate deformation. Further, the evolution of sediment routing in western Laurentia before, during, and after ARM deformation provides an example of the interaction between transcontinental sediment routing and intraplate deformation.

Late Paleozoic cratonal sink: Distally sourced sediment filled the Anadarko Basin (USA) from multiple source regions

AbstractThe Anadarko Basin (south-central USA) is the deepest basin on the North American craton and occupies a region largely surrounded by major, late Paleo-zoic plate-margin (Marathon-Ouachita-Appalachian) and intraplate (Ancestral Rocky Mountains) orogenic systems, albeit a distal arm of the latter. The Anadarko Basin hosts an exceptionally voluminous record of Pennsylvanian strata, and much of this fill has been attributed to erosion of the adjacent Wichita uplift—composed of granitic and rhyolitic rocks of Cambrian age—separated from the basin by a fault zone exhibiting 12 km of vertical separation. This work incorporates thin-section petrography (102 samples) and U-Pb detrital zircon geochronology of sandstone samples (12 samples) from core and outcrop of the Middle Pennsylvanian Red Fork Sandstone (and equivalents) as well as slightly younger Upper Pennsylvanian units (Tonkawa, Chelsea, and Gypsy sandstones) in order to interpret drainage pathways and evolution of those pathways toward and into the Anadarko Basin (Oklahoma) and evaluate the relative importance of the major provenance regions.Our petrographic analysis indicates sandstones with arkosic compositions are limited to the region immediately adjacent to (north of) the Wichita uplift. All remaining samples, which reflect the vast bulk of sediment in the depocenter, including sediment on the northern and eastern Anadarko shelf, are litharenites. Analysis of kernel density plots of the U-Pb ages of detrital zircons together with multidimensional scaling analysis of the Middle Pennsylvanian samples indicate three groups of similar provenance: (1) samples dominated by Cambrian ages from locations directly adjacent to the Wichita uplift; (2) samples dominated by Neoproterozoic ages from locations along the northern shelf of the Anadarko Basin; and (3) samples dominated by Mesoproterozoic ages from locations along the eastern Anadarko shelf and the basin center. These samples are spatially discrete, indicating partitioning of drainage networks during the Middle Pennsylvanian, with two continental-scale fluvial systems entering the Anadarko Basin from the north (transversely) and east (axially). The lack of Cambrian ages in the depocenter and (northern) shelf samples indicate that the Wichita uplift supplied only limited sediment to the basin; sediment derived from the uplift was trapped in fringing fans directly adjacent to the uplift. In contrast to the patterns exhibited by the Middle Pennsylvanian samples, Upper Pennsylvanian samples exhibit more uniform U-Pb ages across the basin. This indicates the relatively rapid evolution of the Appalachian-derived northerly and easterly drainages into an integrated system that flowed axially across the (overfilled) mid-continent basins to the ultimate continental sink in the Anadarko Basin.

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.

Controls on Timing of Hydrothermal Fluid Flow in South-Central Kansas, North-Central Oklahoma, and the Tri-State Mineral District

Paleozoic sedimentary rocks in the southern midcontinent of the United States have been affected by multiple events of deformation and fluid flow, resulting in petroleum migration, thermal alteration, Mississippi Valley-type mineralization, and a complex diagenetic history. This record is a hidden history of how cratonal settings respond to tectonic and non-tectonic drivers. The aim of this contribution is to better understand the controls on fluid migration in Paleozoic strata to evaluate whether hydrothermal activity is forced by tectonic or non-tectonic processes. This paper summarizes and vets the distribution of published dates related to thermal events in the southern midcontinent. In addition, we present new U-Pb dates obtained by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) on calcite cements that were formed from hydrothermal fluids. These are from three samples from the Berexco Wellington KGS 1-32 core in Sumner County, Kansas; an ore sample from the Tri-State Mineral District, Neck City, Missouri; and a core sample from the Blackbird 4-33 well in Osage County, Oklahoma. Previous studies of these calcite samples provided evidence for hydrothermal fluid flow, with one of the Wellington samples possibly recording vertical hydrothermal fluid flow out of the basement. The sample from the Tri-State Mineral District (Missouri) yields a mid-Cretaceous age of 115.6±3.1 Ma. This age falls into the timing of the Sevier Orogeny along the west coast and the development of its foreland basin in the midcontinent. Calcites from the Mississippian interval in the Wellington KGS 1-32 core yield dates of 305±10.5 Ma and 305.1±9.1 Ma. Calcite in Mississippian strata from the Blackbird 4-33 core yields a date of 308.6±2.5 Ma. These dates from Mississippian calcite cements indicate hydrothermal fluid flow in the Late Pennsylvanian that coincides with the timing of the Marathon-Ouachita Orogeny or the Ancestral Rocky Mountains Orogeny. A calcite sample from the Ordovician Arbuckle Group from the Berexco Wellington KGS 1-32 core yielded an age of 5.6±1.6 Ma, coinciding with a time after high elevation uplift of the Rocky Mountains was already far advanced. We propose that this hydrothermal fluid flow may have been associated with increased meteoric recharge and increased regional fluid pressure in a basement aquifer that activated local seismic events far into the continental interior. The distribution of ages of hydrothermal fluid flow confirms a syntectonic driver during the Ouachita Orogeny and Ancestral Rocky Mountains Orogeny deformation. Continuation of hydrothermal fluid flow well into the Permian and tailing off early in the Triassic indicates a post-tectonic driver, where uplifted areas continued to provide the recharge from gravity-driven fluid flow, until the mountains were mostly beveled by the early part of the Triassic. A dearth of Triassic and Jurassic hydrothermal events suggests Gulf of Mexico rifting and extension were less important. Rejuvenation of hydrothermal fluid flow in the Cretaceous and continuing into the Paleogene indicates that elevation and regional flexure from both the Sevier and Laramide events continued to drive hydrothermal fluid flow far from the main sites of mountainous uplift and deformation. Finally, hydrothermal fluid flow associated with more recent uplift of the Rocky Mountains may have been activated by recharge events that pressurized a regional basement aquifer and triggered seismic activity.

Paleozoic evolution and heterogeneity of sediment provenance in the Permian Basin

Paleozoic sediment provenance of the Permian Basin remains controversial. This study presents new detrital zircon morphology, UPb ages, and Hf isotope data of eight Paleozoic sandstone samples, and integrates published zircon UPb data in the basin to illustrate the spatial and temporal changes of sediment provenance. The Cambrian sample has predominantly Mesoproterozoic (1500–1300 Ma) grains because of fluvial transport of basement detritus from the local Texas Arch. The Ordovician shoreface sandstone contains mostly rounded grains of Paleoproterozoic (1800–1600 Ma) age, mostly likely by longshore drift reworking detritus from the Transcontinental Arch. The Pennsylvanian sediment provenance shows heterogeneity. While samples from the Northwestern Shelf have both the Mesoproterozoic and Paleoproterozoic groups and low grain roundness, reflecting sediment delivery from basement rocks in the Ancestral Rocky Mountains, a sample in the Midland Basin contains a broad range of ages, including a major Neoproterozoic (800–500 Ma) group that were primarily from peri-Gondwana terranes. The difference suggests that there was limited sediment mixing in the Permian Basin. The similarity of zircon age distributions between the Midland Basin and Fort Worth Basin confirms synorogenic progradation of detritus from the Fort Worth Basin to the Eastern Shelf during the Pennsylvanian. During the early Permian Cisuralian epoch, the compiled UPb data show that the southern and eastern basin contains a significant fraction of Gondwanan detritus, and the amount reduced by the latest Cisuralian as the orogenesis waned. The middle Permian Guadalupian samples on the Northwestern Shelf contain well-rounded grains that are of mainly Grenvillian and Paleozoic ages, suggesting sediment recycling from Laurentia by wind and likely by river as well. The change of sediment provenance from the early to the middle Permian suggests that after the continental collision, Laurentian sediments dominated detritus in the Permian Basin.

Exploring the influence of Late Mississippian to Middle Pennsylvanian tectonics on sediment transport through detrital zircon geochronology, southwestern Kansas and northwestern Arkansas

This study explores the influence of the Alleghanian-Ouachita-Marathon Orogeny and Ancestral Rocky Mountains (ARM) uplifts on sediment dispersal in the Carboniferous through sandstone compositions and detrital zircon provenance analysis. We focus on Upper Mississippian to Middle Pennsylvanian siliciclastic intervals in two areas: (1) the Hugoton Embayment in southwestern Kansas, where six samples of Upper Mississippian and Lower Pennsylvanian sandstone were collected from two boreholes and (2) the Arkoma Shelf and foreland basin in northwestern Arkansas, where nine samples were collected from Lower through Middle Pennsylvanian outcrops. We report 1544 new concordant ages, 582 ages from Kansas core and 962 ages from Arkansas outcrop samples. UPb age spectra are characterized by major Grenville (900–1300 Ma) and Taconic-Acadian (350–500 Ma) age clusters and minor components of older age groups, suggesting a persistent source in the Appalachians tied to the Alleghanian orogeny. However, by Early Pennsylvanian, the Hugoton Embayment samples see the Appalachian source partially replaced by Yavapai-Mazatzal (1800–1600 Ma) and Granite-Rhyolite (1550–1300 Ma) grains, major components of the local basement, that suggest contributions from nearby ARM uplifts. In contrast, contributions from midcontinent basement sources are limited in the Arkoma Shelf and foreland basin samples. Sandstone composition data are also consistent across all Arkoma Shelf and Upper Mississippian Hugoton Embayment samples; whereas, Lower Pennsylvanian Hugoton samples show increases in feldspar and volcanic lithic grains, consistent with input from local midcontinent age provinces. Comparison with published data suggests a similar change in provenance, from extraregional to local sediment sources, is observed across western Laurentia where ARM deformation was active. Together, new and published data support development of a major transcontinental sediment dispersal system in the Late Mississippian, controlled by Alleghanian orogenesis on the eastern Laurentian margin; however, by the Early Pennsylvanian this system was disrupted by development of the ARM.

POTENTIAL ICE CRYSTAL MARKS FROM PENNSYLVANIAN–PERMIAN EQUATORIAL RED-BEDS OF NORTHWEST COLORADO, U.S.A.

ABSTRACTThe Pennsylvanian–Permian Maroon Formation of northwest Colorado is an up to 4,600 m thick succession of mainly siliciclastic continental red-beds deposited in equatorial intermontane basins of the Ancestral Rocky Mountains. Sedimentary surfaces of fluvio-lacustrine to eolian siltstones and fine-grained sandstones from various stratigraphic levels within the Maroon Formation preserve cm-sized straight to gently curved sediment-filled acicular structures referred to five morphological groups: single, branched, stellate, rosette, and bunched. Depositional environment, shape, and size of the structures are most similar to ice crystal marks that result from freezing of water-saturated fine-grained substrate at the sediment-air interface. They differ from other syngenetically produced crystals and crystal pseudomorphs in sedimentary rocks mainly by crystal shape and environmental conditions. The potential ice crystal marks of the Maroon Formation are notable for the fidelity and morphological diversity of the crystal casts and could be a key for the understanding of similar but hitherto often only called enigmatic structures of the sedimentary rock record. The ice crystal mark occurrences in the Maroon Formation suggest that night frost affected lower elevation equatorial areas during the climax of the Late Paleozoic Ice Age and may stimulate research on evolutionary adaptations of early terrestrial biota to overcome significant air temperature fluctuations.

Sediment routing and provenance of shallow to deep marine sandstones in the late Paleozoic Oquirrh Basin, Utah

The Oquirrh-Wood River Basin is a Pennsylvanian to Permian clastic and carbonate basin in northwestern Utah and southern Idaho, USA. The basin is commonly considered the northwesternmost expression of the Ancestral Rocky Mountains (ARM). Basin fill is locally up to 9 km thick. Facies range from shelf carbonates to deep marine debrites, and generally deepen from basin initiation to the beginning of the Permian. Unlike other ARM basins, no basin-bounding fault or highland has been identified as the source of these thick deposits and the tectonic drivers of basin subsidence are poorly understood.Tectonic subsidence analysis was performed on 10 stratigraphic sections across the basin, resulting in a two-phase tectonism model. Pennsylvanian subsidence is interpreted as being related to strike-slip deformation; whereas Permian subsidence in the western part of the basin may be related to the uplift and unconformity sequence documented in northeastern Nevada.Detrital zircon geochronology and thin section point counts are relatively uniform in Middle Pennsylvanian and early Permian samples, suggesting local tectonism did not alter dominant sediment routing patterns. Oquirrh Group sandstones are compositionally mature and contain sparse feldspar or lithic grains, leading to the interpretation that sample sediments were derived from recycled orogenic sources. UPb ages on detrital zircons from three Pennsylvanian and three Permian samples suggest sediment was overwhelmingly derived from multiple Laurentian basement provinces to the north and east, consistent with interpretations from the Wood River Basin. Paleocurrent data indicate a prevailing southward ocean current, suggesting a combination of sediment recycling and sediment transport across Laurentia from orogenic highlands in the northeast to the Oquirrh-Wood River Basin throughout the late Paleozoic.

Detrital U-Pb zircon and 40Ar/39Ar muscovite geochronology from Middle Pennsylvanian strata in the Anadarko Basin, Texas Panhandle, USA

The Late Mississippian-Permian Anadarko Basin formed in Texas and Oklahoma, USA as the result of inversion of Neoproterozic and Cambrian rift structures. Subsidence was driven by flexural loading of the Amarillo-Wichita Uplift, and this uplift may represent the easternmost element of the Ancestral Rocky Mountains system. The northwestern part of this basin has generally been interpreted to have been filled by sediment derived from the Ancestral Front Range Uplift, ~475 km to the northwest during the early stages of basin filling. We test this model using U-Pb detrital zircon and 40Ar/39Ar detrital muscovite results from three subsurface samples of the Morrow B sandstone in the northwestern part of the Anadarko Basin. We provide a new maximum depositional age of 310.9 ± 4.9 Ma that indicates the age of the Morrow B to be no older than late Atokan to early Desmoinesian Age (Moscovian), ~10 Myr younger than previously interpreted. In contrast to some previous interpretations, we propose that the most likely source for the sediment in the Morrow B is the Amarillo Uplift to the south. Detrital zircon and detrital muscovite data have age peaks at 900–1300 Ma, 1370 Ma and 1600–1800 Ma corresponding to derivation from Grenville, Granite-Rhyolite and Yavapai-Mazatzal basement provinces, respectively. A dominant detrital zircon peak at 1370 Ma suggests that Mesoproterozoic granites in the Amarillo Uplift were exposed by Middle Pennsylvanian time, and detritus eroded from the Amarillo Uplift dominated the lowstand sediment budget of the Texas Panhandle during this time; small volumes of sediment were likely sourced from the Ancestral Front Range. This study presents the first detrital geochronology data from the subsurface Anadarko Basin and the first detrital muscovite data from late Paleozoic southwestern Laurentia. The results presented here highlight the interpretive power of combined detrital zircon and muscovite datasets.

Tectonic controls on basement exhumation in the southern Rocky Mountains (United States): The power of combined zircon (U-Th)/He and K-feldspar 40Ar/39Ar thermochronology

AbstractThe Great Unconformity of the Rocky Mountain region (western North America), where Precambrian crystalline basement is nonconformably overlain by Phanerozoic strata, represents the removal of as much as 1.5 b.y. of rock record during 10-km-scale basement exhumation. We evaluate the timing of exhumation of basement rocks at five locations by combining geologic data with multiple thermochronometers. 40Ar/39Ar K-feldspar multi-diffusion domain (MDD) modeling indicates regional multi-stage basement cooling from 275 to 150 °C occurred at 1250–1100 Ma and/or 1000–700 Ma. Zircon (U-Th)/He (ZHe) dates from the Rocky Mountains range from 20 to 864 Ma, and independent forward modeling of ZHe data is also most consistent with multi-stage cooling. ZHe inverse models at five locations, combined with K-feldspar MDD and sample-specific geochronologic and/or thermochronologic constraints, document multiple pulses of basement cooling from 250 °C to surface temperatures with a major regional basement exhumation event 1300–900 Ma, limited cooling in some samples during the 770–570 Ma breakup of Rodinia and/or the 717–635 Ma snowball Earth, and ca. 300 Ma Ancestral Rocky Mountains cooling. These data argue for a tectonic control on basement exhumation leading up to formation of the Precambrian-Cambrian Great Unconformity and document the formation of composite erosional surfaces developed by faulting and differential uplift.

Tectonic subsidence analysis of the Ancestral Rocky Mountains from the interior to the southern margin

Southwestern Laurentia experienced intraplate deformation during the Pennsylvanian and early Permian, an orogenic event termed the Ancestral Rocky Mountains (ARM). Deformation consisted of uplifted Precambrian-cored basement blocks separated from adjacent basins by high-angle faults. The tectonic drivers of this orogen are debated, which is likely in part due to significant changes to the chronostratigraphy of the geologic time scale since many of the models have been proposed. To better understand the orogen, we assess spatial subsidence patterns with 1-D backstripped tectonic subsidence curves from eight ARM basins across the orogen using the 2020 Geologic Time Scale for temporal calibration. These patterns indicate the following: 1) basins largely initiated in the late Morrowan—early Atokan (Bashkirian); 2) most subsidence curves suggest flexure from a fixed-load foreland basin that steadily increased until the Missourian (Kasimovian) where most basins reached peak subsidence; 3) diminished subsidence occurred in most basins in the Late Pennsylvanian and shut off across most of the orogen by the end of the Wolfcampian (Asselian—early Artinskian); and 4) basin subsidence renewed in the southern basins during the Leonardian (late Artinskian—Kungurian). The synchronous start to basin subsidence is coeval to the initiation of highly oblique convergence between the Laurentia and Panthalassan plates along the southwestern margin. Influences from the Ouachita-Marathon margin are best reflected by Late Mississippian subsidence of the Anadarko Basin and transpressional deformation in the Orogrande Basin during the Late Pennsylvanian. The decrease in early Permian subsidence is coeval to the cessation of movement on most ARM faults and burial of Precambrian-cored highlands indicating that classical ARM tectonics is mostly a Pennsylvanian event. The increase in subsidence observed during the Leonardian (late Artinskian—Kungurian) in the southern basins may reflect renewed oblique convergence along the southwestern margin.

West Texas (Permian) Super Basin, United States: Tectonics, structural development, sedimentation, petroleum systems, and hydrocarbon reserves

The West Texas (Permian) Super Basin is the prototype super basin. The basin has produced 28.9 billion bbl of oil and 203 TCF of gas (63 billion BOE, 1920–2019). The US Geological Survey and Bureau of Economic Geology estimate this super basin has remaining reserves of 120–137 billion BOE, twice the volume produced during the first 100 yr of hydrocarbon production. During the past decade, the West Texas Super Basin has been the driver of production growth in the United States and has decades of remaining economic production and reserve growth. The West Texas Super Basin is a complex Paleozoic basin built on a varied Proterozoic crust. After Cambrian rifting, regional subsidence began in the Middle Ordovician and continued into the Devonian, forming the Tobosa Basin. The early Paleozoic Tobosa Basin subsidence terminated during Mississippian epeirogenic uplift. A later stage of subsidence began in the Late Mississippian accompanied by large-scale faulting and moderate folding. This tectonic and structural development was controlled by basement terrains, earlier tectonic, and structure reactivated by compression of the Ancestral Rocky Mountains and Marathon-Ouachita orogenic events. These formed the Permian Basin. The Marathon-Ouachita tectonic event ended in the Wolfcampian (early Permian). Subsidence continued to the end of the Permian (Ochoan). Periodic subsidence during the Mesozoic was likely caused by Rocky Mountain (Laramide) deformation. Cenozoic (late Paleogene–Neogene) western uplift tilted the basin to the east. Each of these events has a significant influence on the basin petroleum systems. The basin has multiple source rocks and petroleum systems formed during various stages of basin development. During the formation of the early Paleozoic Tobosa Basin, Simpson Group and Woodford Shale source rocks were deposited. During the transitional basin development phase, the Barnett Shale source rocks were deposited, and during Permian Basin subsidence, the Wolfcamp and middle Permian (Leonardian and Guadalupian) source rocks were deposited. Continued subsidence into the Mesozoic resulted in the deposition of additional strata. These Mesozoic intervals are now mostly eroded but provided sufficient burial depths for thermal development and increased the extent of thermal effect for maturation and migration of hydrocarbons within these Paleozoic petroleum systems. Leonardian and Guadalupian conventional reservoirs have produced 71% of the resources from all conventional West Texas Super Basin reservoirs. These reservoirs are typically most abundant on the shelf crest (shelf to edge), where reservoir development is maximized and becomes a focus of hydrocarbon migration from the deeper Delaware and Midland Basins source rocks and shallower, more-proximal shelf and platform source rock systems. Unconventional resource reservoir oil production in the West Texas Super Basin accounted for just under 90% of total basin daily production at the close of the last decade (2010–2019). Total West Texas Super Basin production peaked in March 2020 at 4.7 million BOPD. Since that time, production has declined because of lower rates of investment driven by lower product prices. The West Texas Super Basin economic oil and gas production has benefited from an extensive infrastructure, a large geologic and engineering community, regulatory and public support, open access, sufficient capital availability, and a scalable service industry. The paradigm toward new drilling, completion, and production technology has been driven by unconventional resource reservoir development in the basin. These West Texas Super Basin technological developments have lead industry technology for unconventional resource development worldwide. Maintaining talented human resources and capital are challenges that time will tell if individual firms and the industry will meet to develop the hydrocarbon resources within the basin.

Wrench faulting and trap breaching: A case study of the Kizler North Field, Lyon County, Kansas, USA

The Kizler North Field in northwest Lyon County, Kansas, is a producing field with structures associated with both uplift of the Ancestral Rockies (Pennsylvanian to early Permian) and reactivation of structures along the Proterozoic midcontinent rift system (MRS), which contributed to the current complex and poorly understood play mechanisms. The Lower Paleozoic dolomitic Simpson Group, Viola Limestone, and “Hunton Group” are the reservoir units within the field. These units have significant vuggy porosity, which is excellent for field potential; however, in places, the reservoir is inhibited by high water saturation. The seismic data show that two late-stage wrench fault events reactivated existing faults. The observed wrench faults exhibit secondary P, R’, and R Riedel shears, which likely resulted from Central Kansas uplift-MRS wrenching. The latest stage event breached reservoir caprock units during post-Mississippian to pre-Desmoinesian time and allowed for hydrocarbon migration out of the reservoirs. Future exploration models of the Kizler North and analog fields should be based on four play concepts: 1) four-way closure with wrench-fault-related traps, 2) structural highs in the Simpson Group and Viola Limestone, 3) thick “Hunton Group,” and 4) presence of a wrench fault adjacent to the well location that generates subtle closure but not directly beneath it, which causes migration out of reservoirs. In settings where complex structural styles are overprinted, particular attention should be paid to the timing of events that may cause breaches of seals in some structures but not others. Mapping the precise location and vertical throw of the reactivated wrench faults using high-resolution seismic data can help reduce the drilling risk in analog systems.

Late Paleozoic (Late Mississippian–Middle Permian) sediment provenance and dispersal in western equatorial Pangea

Late Mississippian to middle Permian sediment-dispersal networks of regional to continental scale in western equatorial Pangea, depicted here in a series of paleogeographic maps, developed in response to temporally and spatially changing influences of climate, eustasy, and a continent-wide late Paleozoic orogenic system. The orogenic system included linked Alleghanian, Ouachita-Marathon-Sonora collisional belts and associated foreland basin systems on Laurentia and magmatic arcs on Gondwana, intracratonic basement uplifts and basins of the Ancestral Rocky Mountains, flexural arches and intracratonic basins of the US midcontinent region, and basins and uplifts of the southwestern Laurentian transcurrent continental margin. Consideration of new and published U-Pb detrital-zircon datasets permits delineation of Laurentian sediment-dispersal networks of the developing supercontinent. The Transcontinental Arch deflected Late Mississippian transcontinental rivers with Alleghanian headwaters toward the southern midcontinent and nascent, deep-marine foreland basins along the Ouachita collision orogen. Pennsylvanian rivers likewise headed in the Alleghanian Orogen, transporting sediment southwest across the midcontinent and along the Alleghanian foreland basin to empty into the Arkoma Basin and Fort Worth Basin, which also received voluminous sediment from Gondwana and the Ouachita Orogen. Concomitantly, major growth of Ancestral Rocky Mountains uplifts yielded basement-derived sediment, much of which was retained in local flexural basins. Increased aridity drove ascendant eolian transport in early Permian (Artinskian) time, just as lowstand desiccation of a midcontinent seaway exposed unconsolidated silt and sand derived from eastern, western, and southern sources in an extensive interior desert sink. Eolian transport within the interior desert further mixed and deflated the already-cosmopolitan sediment, pushing it southwest toward the Permian Basin and westward beyond the Ancestral Rocky Mountains. Intercepted by newly developed monsoonal circulation, the deflated sediment came to reside in erg systems along the western marine margin of Pangea. Subtropical Pennsylvanian transcontinental fluvial networks were similar to those of modern big river systems of the eastern and midcontinent United States that drain toward the Gulf of Mexico. In contrast, Permian drainage networks yielding sediment to a continental desert more resembled Pleistocene sediment routes of the Arabian Plate; there, intermittent wadis draining western rift highlands and big rivers of the Mesopotamian foreland contribute sediment to Arabian eolian sands via zonal and monsoonal surface winds to create widespread sand seas of mixed Eurasian and Arabian provenance.