Llano Uplift Research Articles

Detection of near-surface horizontal anisotropy in a weathered metamorphic schist at Llano Uplift (Texas) by transient electromagnetic induction

The use of transient controlled-source electromagnetic prospecting to detect buried, steeply dipping foliation in metamorphic rocks is illustrated with data acquired over the Precambrian Packsaddle schist in the Llano Uplift of central Texas. The azimuthal variation of the transient voltage at a given transmitter–receiver separation about a fixed central point is consistent with the forward model response of a homogeneous halfspace exhibiting horizontal electrical anisotropy. The loop–loop exploration configuration is ideally suited to probe horizontal anisotropy. A quantitative match of the forward response to the observed data produces reasonable electrical conductivity values and coefficient of anisotropy for resistive, crystalline geological materials. The most conductive direction consistently lies within a few degrees of the geologically mapped foliation strike direction. The electrical anisotropy is strongest below the near-surface weathered layer, within the more competent bedrock. The agent responsible for generating the anistropy cannot be definitively determined, because it is likely to be a combination of geological factors, such as weathering, compositional banding and microcracking, all of which enhance electrical conductivity parallel to the plane of foliation. The transient electromagnetics is supplemented by DC resistivity and seismic surveys. The elastic anisotropy is evident in the near-surface weathered layer, but it may not persist very deep into the underlying competent schist. The exposure of foliated schist at the surface is not sufficient to rule out a possible role for systematically aligned macrofracture sets as a secondary cause of the observed anisotropy.

Drowning of the Upper Marble Falls carbonate platform (Pennsylvanian), central Texas: A case of conflicting “signals?”

Marble Falls and Smithwick Formation carbonates and shales were deposited on the western margin of the slowly subsiding Fort Worth Basin of central Texas during Early Pennsylvanian time. The Upper Marble Falls Member carbonate platform back-stepped towards the west–southwest as Smithwick outer shelf–slope black shales drowned platform margin areas to the east. The shelf-to-basin profile observed from subsurface data from the Fort Worth Basin and outcrops from the Llano Uplift area records platform drowning; however, the platform began to shoal upwards as it back-stepped towards the west–southwest, due to the presence of the Llano Uplift paleobathymetric high. The geometry of these facies tracts was controlled primarily by a combination of subsidence and second-order sea-level change, while the development of sequences and unconformities was controlled by third-order sea-level changes and environmental factors, and not subsidence. The termination of platform deposition in the Fort Worth Basin was caused by the interplay of third-order sea-level changes, environmental deterioration in advance of siliciclastic burial, and a lack of accommodation space. The depositional history of the Upper Marble Falls carbonate platform does not fit “classic” examples of drowned platforms described from other areas, but may be typical of platforms in slowly subsiding basins.

Terrane transfer during the Grenville orogeny: tracing the Amazonian ancestry of southern Appalachian basement through Pb and Nd isotopes

Whole rock Pb isotope data can be used to determine the provenance of different blocks within the Rodinia supercontinent, providing a test for paleogeographic reconstructions. Calculated isotopic values for the source region of the Grenville-deformed SW Amazon craton (Rondônia, Brazil), anchored by published U–Pb zircon ages, are compared to those from the Grenville belt of North America and “Grenvillian” basement inliers in the southern Appalachians. Both the SW Amazon craton and the allochthonous Blue Ridge/Mars Hill terrane are defined by a similar Pb isotopic signature, indicating derivation from an ancient source region with an elevated U/Pb ratio. In contrast, the Grenville Province of Laurentia (extending from Labrador to the Llano Uplift of Texas) is characterized by a source region with a distinctly lower, time-integrated U/Pb ratio. Published U–Pb zircon ages (ca. 1.8 Ga) and Nd model ages (1.4–2.2 Ga) for the Blue Ridge/Mars Hill terrane also suggest an ancient provenance very different from the rest of the adjacent Grenville belt, which is dominated by juvenile 1.3–1.5 Ga rocks. The presence of mature continental material in rocks older than 1.15 Ga in the Blue Ridge/Mars Hill terrane is consistent with characteristics of basement rocks from the SW Amazon craton. High-grade metamorphism of the Blue Ridge/Mars Hill basement resulted in purging of U, consistent with observations of the rest of the North American Grenville province. In contrast, the “Grenvillian” metamorphic history of the Amazon appears to have been much more heterogeneous, with both U enrichment and U depletion recorded locally. We propose that the Blue Ridge/Mars Hill portion of the Appalachian basement is of Amazonian provenance and was transferred to Laurentia during Grenvillian orogenesis after ∼1.15 Ga. The presence of these Amazonian rocks in southeastern Laurentia records the northward passage of the Amazon craton along the Laurentian margin, following the original collision with southernmost Laurentia at ca. 1.2 Ga.

Thermochronology of the Grenville Orogeny in west Texas

Abstract The Carrizo Mountain Group (CMG) comprises the westernmost exposures of the metamorphic core of the Grenville Orogen in Texas. Unlike the correlative Llano uplift, 500 km to the east, the CMG underwent one phase of medium P/T metamorphism; peak T was attained during D2 in all CMG exposures. The peak metamorphic paleotemperatures decrease to the northwest, in the overall direction of tectonic transport, from staurolite to biotite grade. Garnet in the southeast Carrizo Mountains grew isothermally and probably during decompression (i.e. part of a clockwise P–T loop) at ca. 520 °C, in between biotite-grade D1 and D3 events. Hornblende in this region cooled below closure T, during D2, at 1057±6 Ma, within error of muscovite closure in the northwest Carrizo Mountains at 1059±2 Ma. Hornblende in higher grade exposures to the southeast cooled through closure T as late as 1002±7 Ma. Muscovite in the garnet-grade and higher exposures closed to Ar diffusion at 992±4 Ma to 981±3 Ma. Biotite and (partially recrystallized) hornblende from the northwest Carrizo Mountains cooled through closure in this same time period. We interpret these data as signifying: (1) Ar loss due to peak metamorphism in hornblende (southeast Carrizo Mountains) and muscovite (northwest Carrizo Mountains) at ca. 1059 Ma; 2) post-Tmax cooling through 500 °C in the higher grade outcrops until ca. 1000 Ma; and (3) cooling of all CMG exposures through 300 °C, due to uplift along the Streeruwitz thrust and erosion, from 1000 to 980 Ma. These ages significantly post-date known tectonism in the Llano uplift and extend the duration of Grenville orogenesis in Texas by ca. 115 million years. The 1000–980 Ma time period is a time of thrusting in frontal regions of the Grenville Province of Canada and the northeastern US, and shearing in the Natal Province, South Africa, suggesting ultimate closure of the Grenville Orogen and supporting recent Rodinia reconstructions.

Kinematic Constraints on Rodinia Reconstructions from the Core of the Texas Grenville Orogen

Mesoproterozoic metamorphic rocks exposed in the Llano Uplift, central Texas, were involved in a ∼1.15–1.12 Ga Grenville‐age collisional orogenic event along the southern margin of Laurentia. This event is characterized by polyphase ductile deformation synchronous with transitional amphibolite‐, granulite‐ to eclogite‐facies metamorphism. Results of detailed structural mapping in the southeastern part of the Llano Uplift show that these rocks record a deformational history involving northeast‐directed ductile thrusting and regional‐scale polyphase folding, all related to northward collision of a southern continent and an arc with the southern margin of Laurentia. We use the structural, metamorphic, and kinematic history of the southeastern Llano uplift to critically evaluate collisional models for the Grenville orogen in Laurentia and discuss the implications for reconstructions of Rodinia.

Structure of the Carrizo Mountain Group, southeastern Carrizo Mountains, west Texas: A transpressional zone of the Grenville orogen

The west Texas segment of the Grenville orogen includes an oblique, dextral, transpressive zone. The orogen exposed near Van Horn, Texas, displays a transition from the mid‐amphibolite‐facies metamorphic core to a foreland fold and thrust belt. In this paper, detailed structural mapping and analyses, presented for key exposures of the metamorphic rocks, the Mesoproterozoic Carrizo Mountain Group, show a southeastward increase in number of deformational phases and intensity of deformation. The central portion is dominated by oblique northwest convergence, which produced two phases of northwest verging folds and synchronous, oblique reverse and dextral shearing. The thick parts of pre‐tectonic mafic plutons, which had been emplaced as locally discordant sills, control the spatial distribution of structures. Two dominant phases of folds form Type 0 interference patterns (essentially coaxial and coplanar; i.e., fold tightening was dominant). This central belt also contains en echelon reverse to oblique shear zones and comprises a transfer zone within the orogen between structurally higher and lower major shear zones. Overall, the mapped part of the central Carrizo Mountains represents localized dextral transpression within a larger region dominated by reverse motion, possibly indicating a change through time from orthogonal to oblique contraction. Further south in the Bass Canyon region, the earlier fold generations are refolded into Type 2 (“mushroom”) interference patterns. These structures show a westward vergence and are kinematically linked to those further north across a zone of disharmonic folding and heterogeneous shear. The structure and kinematics of the polyphase deformation and ductile shearing in the metamorphic portion of the orogen is compatible with that seen in the foreland fold‐thrust belt, suggesting that in both foreland and hinterland, transpression was a characteristic feature of the west Texas Grenville orogeny. The timing and kinematics of deformation in the Van Horn exposures contrasts with that seen in the Llano uplift, but compares favorably with those of the Natal belt, South Africa, a relationship with implications for recently proposed reconstructions of Rodinia.

Ion microprobe age and geochemistry of southern appalachian basement, with implications for Proterozoic and Paleozoic reconstructions

Ion microprobe U–Pb analyses of zircons from basement units in the southern Appalachians, combined with supporting isotopic compositions and major and trace element geochemistry, have delineated a granitic magmatic pulse ∼1165–1150 Ma. The pulse is manifested by the Watauga River Gneiss (western Blue Ridge), Toxaway, Wiley, and Sutton Creek gneisses (eastern Blue Ridge), Pilot Mountain and Grassy Creek gneisses (Sauratown Mountains window), and possibly the Forbush gneiss (∼1140 Ma, Inner Piedmont or Sauratown Mountains window) and Cranberry-Mine Layered Gneiss (∼1190 Ma, western Blue Ridge). Additional samples analyzed include the Blowing Rock Gneiss (∼1080 Ma, Grandfather Mountain window), and the Carvers Gap Granulite Gneiss (∼1.8 Ga) and Cloudland Granulite Gneiss (detrital cores ∼1.2–1.8 Ga) from the Mars Hill terrane. Age data were evaluated by calculating concordia ages and concordia probability plots using 206 Pb*/ 238 U and 207 Pb*/ 206 Pb* data simultaneously. Rocks in the main magmatic pulse are granitic (63–72 wt.% SiO 2 ), but elevated in K and incompatible trace elements compared to typical subduction-related magmas, and initial Nd ratios cluster tightly near CHUR. Mars Hill terrane samples are distinct in age, geochemistry (poorer in K and incompatible elements), and isotopic compositions (ε Nd −7.6 and −5 at 1.0 Ga). Zircons from almost all samples have metamorphic rims that yield ages ∼1030 Ma, with the exception of the Blowing Rock Gneiss. Ages of Grenvillian magmatism and metamorphism are similar to reported ages from the northern Blue Ridge of Virginia, the Adirondack Highlands, the Central Metasedimentary Belt (Canadian Grenville Province), and the Llano uplift of Texas. This suggests the entire southeastern margin of Laurentia has a similar history ∼1.2–1.0 Ga. Although consistent with known ages in Laurentia, the presence of ∼1.8 Ga rocks and T DM ages commonly >1.6 Ga is inconsistent with the inferred 1.6 Ga margin of Laurentia. This suggests either that the 1.4–1.5 Ga mid-continent terrane separates older portions of Laurentia, that this region was exotic, or that it was a rifted fragment of Laurentia reattached during Grenville orogeny. Surprisingly few Paleozoic metamorphic zircon rims have been identified, but the few analyses obtained from samples in the eastern Blue Ridge yield late Acadian ages (∼350 Ma). The similarity of basement units across the southern Appalachians suggests a relationship between these provinces and that the Piedmont terrane is not exotic to Laurentia during the Appalachian orogenic cycle.

Scales of disequilibrium and rates of equilibration during metamorphism

Mounting evidence suggests that partial disequilibrium-meaning disequilibrium for some elements, but not for others-may be a common but rarely detected phenomenon during metamorphic mineral growth, even in ordinary prograde reactions that progress to completion. Detailed examination of compositional variations in garnet crystals over a range of metamorphic grade suggests distinctly different scales of equilibration for common elements, with strong temperature dependence. Under lower greenschist-facies conditions, Fe and Mg may equilibrate at hand-sample scale, whereas Mn and Ca may not equilibrate even at millimeter-scale. Although Mn may achieve hand-samplescale equilibration under upper greenschist-facies conditions, Ca and many trivalent cations (e.g., REEs) may not do so until temperatures exceed those of the middle amphibolite facies. Even in the lower granulite facies, some elements (e.g., Y, Yb) show indications of disequilibrium at sub-centimeter- scales during garnet growth. Analysis and numerical modeling of undisputed disequilibrium textures demonstrate that the most common impediment to equilibration during metamorphism is the sluggishness of intergranular diffusion, the same mechanism known to govern porphyroblast crystallization in many metamorphic environments. Despite this importance to petrology, few quantitative determinations exist of intergranular diffusion rates under metamorphic conditions. Using numerical simulations of coupled intergranular and intracrystalline diffusion processes in coronal textures around partially resorbed garnet crystals from the Llano Uplift, U.S.A., a very precise and relatively accurate estimate is obtained for the rate of intergranular diffusion of Al in fluid-undersaturated systems. This result and an earlier estimate for fluid-saturated systems provide bracketing values for Al diffusivity during metamorphism.

Topographic Irregularities on the Base Zuni Supersequence Boundary and their Initial Cretaceous Sediment Fill, Central Texas

ABSTRACT The Cretaceous-preCretaceous unconformity, i. e., base Zuni supersequence boundary, crops out across much of central and west Texas. Topographic features on the unconformity surface are exhumed along the erosional out-crop edge of the nearly flat-lying Cretaceous strata that overlie the surface within Lampasas and Mills counties, Texas. Digital elevation data displayed as a topographic profile atop Pennsylvanian outcrops adjacent to the Cretaceous outcrop edge indicate that paleotopography was influenced by northwest-dipping Pennsylvanian strata that underlie the unconformity surface. Differential erosion of Pennsylvanian limestone, sandstone, and shale prior to Cretaceous deposition created a trellis network of northeast-trending valleys and divides. Local tectonic features, climate, and rising sea level influenced the nature of the Cretaceous fluvial fill that directly overlies the unconformity surface. The Sycamore Conglomerate is the earliest Cretaceous deposit, and accumulated in a sub-humid or drier climate as a downfilling braided alluvial wedge shed off the Llano Uplift. The Travis Peak Formation, a conglomeratic sandstone, onlaps and disconformably overlies the Sycamore Conglomerate, and was deposited as an aggradational meandering river complex during sea level rise.

Introduction to special section: New Views of the Moon II, a series of papers related to the lunar science initiative “New views of the moon enabled by combined remotely sensed and lunar sample data sets”

The Gulf Coast province provides an array of scientific dilemmas ranging from the origin of the gulf itself to the causes and effects of long‐lasting circulation of hot, deep waters throughout the thick sedimentary section. The nature of the underlying crust and superjacent sediments and their contained waters; the precise timing of rifting; depositional history and diagenesis of the sedimentary sequence; fluid dynamics; geochemistry; hydrocarbon generation and migration; thermal history, including unusually high thermal gradient; and the fluid pressure regime in the deep sedimentary section are too poorly understood to permit quantitative analysis of processes that are of enormous scientific and practical importance. The area centered on DeWitt and Victoria counties, Texas, on the southeastern extension of the San Marcos arch, is probably the best location for a deep borehole to investigate these important phenomena and problems. The arch extends southeastward from exposed Grenville‐age basement rocks of the Llano uplift and separates the deep South Texas and Houston embayment salt basins. Seaward of the Llano uplift, highly deformed and slightly metamorphosed rocks of the Ouachita‐Marathon orogen have been intersected beneath Cretaceous sediments. The inferred edge of continental crust underlies an extensive Lower Cretaceous reef trend southeast of known Ouachita orogen rocks. Rapid thickening of Tertiary and possibly of Cretaceous sediments southeast of the shelf edge, together with geophysical indications of a relatively shallow Moho, suggests that “transitional continental crust” underlies sediments basinward of the inferred edge of continental crust. This transitional crust, the ultimate objective for a proposed deep well, could be rifted Grenville basement, buried rocks of the Ouachita trend, an island arc related to the Ouachita trend, or exotic continental basement related to a proto‐South American continent. To achieve optimum results to guarantee adequate basement penetration, a borehole should be designed to penetrate a relatively thin succession of lower Mesozoic synrift, graben‐fill sediments seaward of the shelf edge. Because of expected high temperatures and pressures, new technologies will need to be developed to successfully drill and test the well.

Geology and geochronology of Grenville-age rocks in the Van Horn and Franklin Mountains area, west Texas: Implications for the tectonic evolution of Laurentia during the Grenville

New U-Pb zircon and 40 Ar/ 39 Ar geochronological data for the Mesoproterozoic rocks at Van Horn, far west Texas, indicate that metarhyolite, quartzite, phyllite, and metabasalt of the Carrizo Mountain Group were formed ca. 1380–1330 Ma and are probably a southern part of the Southern Granite–Rhyolite province of W.R. Van Schmus and coworkers. These rocks were thrust over the ca. 1250 Ma carbonate-basalt-rhyolite sequence of the Allamoore and Tumbledown Formations and the younger Hazel Conglomerate (ca. 1035 Ma). The Castner Marble and Mundy Breccia of the Franklin Mountains near El Paso are coeval, and probably correlative, with the Allamoore-Tumbledown sequence. The ca. 1120 Ma granite and rhyolite boulders in the Hazel Formation are coeval with the Red Bluff Granite Suite and rhyolite of the Thunderbird Formation, both occurring in the Franklin Mountains, indicating that formation of granite and rhyolite was widespread in far west Texas at this time. Although these rocks are temporally related to the classic Grenvillian events of eastern North America and the Llano uplift of central Texas, they contrast starkly in their much lower (lowest greenschist) metamorphic grade and degree of deformation. It is likely that the Allamoore-Tumbledown-Castner-Mundy sequences, with their shallow-water sedimentary rocks and bimodal volcanic assemblages, accumulated ca. 1250 Ma in rift basins that formed on the southern margin of Laurentia during Grenville orogenesis. Renewed or continued extensional tectonism is implied by the presence of ca. 1120 Ma alkaline granitic and/or basaltic rocks in the Franklin Mountains, at Pajarito Mountain in New Mexico, in the Apache Group of southern Arizona, and in the Pahrump Group of the Death Valley region of southeastern California; all of these occurrences have been attributed to accumulation in rift basins. The striking alignment of these occurrences suggests that they were formed along a zone of transcurrent faulting that extended along the southern margin of Laurentia during Grenvillian (Mesoproterozoic) time.

Mesoproterozoic chronostratigraphy of the southeastern Llano uplift, central Texas

The Llano uplift of central Texas exposes Mesoproterozoic crystalline rocks affected by a Grenvillian orogeny between 1.2 and 1.1 Ga. We report four new U-Pb zircon protolith ages of 1247 ± 4, 1257 ± 3, 1272 +8 / −5 , and 1366 ± 3 Ma for four quartzofeldspathic rocks from the regionally defined Packsaddle Schist and Valley Spring Gneiss in the southeastern part of the uplift. The 1366 ± 3 Ma gneiss also yields a metamorphic age of 1325 ± 5 Ma. These new U-Pb ages, in conjunction with previous U-Pb age data, demonstrate that several regionally defined map units contain constituents with widely disparate ages. We have identified four age suites for protoliths of metamorphic rocks in the southeastern part of the Llano uplift that appear to represent rock packages of distinct tectonic origins: (1) the northern, 1366–1272 Ma felsic gneisses of volcanic, plutonic, and continentally derived sedimentary origin (Valley Spring Gneiss and an older gneiss component), (2) the geographically intermediate, ca. 1257–1247 Ma (and possibly older) basinal sequence that formed along a continental shelf and slope near an arc (Packsaddle Schist), (3) the southern, 1326–1275 Ma remnant of an exotic, ensimatic arc complex (Big Branch Gneiss and Coal Creek Serpentinite and plutonic complex), and (4) the 1239–1232 Ma tectonized felsic rocks that intruded the first two suites. The first three suites of rocks were structurally imbricated during Grenville orogenesis, and the fourth suite may either represent early synorogenic crustal melts or be related to the volcanic rocks in the Packsaddle Schist. In addition, these new data show that, contrary to previous reports, younger Packsaddle Schist lies in structural contact above older components of the Valley Spring Gneiss. Moreover, one of the dated meta-igneous units is the oldest unit yet found in the uplift and records an early period of metamorphism prior to formation of most Llano uplift rocks. Its 1366 ± 3 Ma protolith age is coeval with rocks of the Western Granite-Rhyolite terrane. Age and geochemical similarities link this unit of unknown regional extent with the Western Granite-Rhyolite terrane of known Laurentian affinity. Thus, this component of the Llano uplift may be the basement on which the younger components of the Valley Spring Gneiss and Packsaddle Schist formed in a tectonic setting proximal to Laurentia and south of the Western Granite-Rhyolite terrane.

Mesoproterozoic chronostratigraphy of the southeastern Llano uplift, central Texas

Research Article| February 01, 2000 Mesoproterozoic chronostratigraphy of the southeastern Llano uplift, central Texas Joseph F. Reese; Joseph F. Reese 1Department of Geological Sciences, University of Texas, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar Sharon Mosher; Sharon Mosher 1Department of Geological Sciences, University of Texas, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar James Connelly; James Connelly 1Department of Geological Sciences, University of Texas, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar Robert Roback Robert Roback 1Department of Geological Sciences, University of Texas, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar GSA Bulletin (2000) 112 (2): 278–291. https://doi.org/10.1130/0016-7606(2000)112<278:MCOTSL>2.0.CO;2 Article history received: 30 Jun 1997 rev-recd: 19 Feb 1999 accepted: 11 Mar 1999 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Joseph F. Reese, Sharon Mosher, James Connelly, Robert Roback; Mesoproterozoic chronostratigraphy of the southeastern Llano uplift, central Texas. GSA Bulletin 2000;; 112 (2): 278–291. doi: https://doi.org/10.1130/0016-7606(2000)112<278:MCOTSL>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 SocietyGSA Bulletin Search Advanced Search Abstract The Llano uplift of central Texas exposes Mesoproterozoic crystalline rocks affected by a Grenvillian orogeny between 1.2 and 1.1 Ga. We report four new U-Pb zircon protolith ages of 1247 ± 4, 1257 ± 3, 1272 +8/−5, and 1366 ± 3 Ma for four quartzofeldspathic rocks from the regionally defined Packsaddle Schist and Valley Spring Gneiss in the southeastern part of the uplift. The 1366 ± 3 Ma gneiss also yields a metamorphic age of 1325 ± 5 Ma. These new U-Pb ages, in conjunction with previous U-Pb age data, demonstrate that several regionally defined map units contain constituents with widely disparate ages.We have identified four age suites for protoliths of metamorphic rocks in the southeastern part of the Llano uplift that appear to represent rock packages of distinct tectonic origins: (1) the northern, 1366–1272 Ma felsic gneisses of volcanic, plutonic, and continentally derived sedimentary origin (Valley Spring Gneiss and an older gneiss component), (2) the geographically intermediate, ca. 1257–1247 Ma (and possibly older) basinal sequence that formed along a continental shelf and slope near an arc (Packsaddle Schist), (3) the southern, 1326–1275 Ma remnant of an exotic, ensimatic arc complex (Big Branch Gneiss and Coal Creek Serpentinite and plutonic complex), and (4) the 1239–1232 Ma tectonized felsic rocks that intruded the first two suites. The first three suites of rocks were structurally imbricated during Grenville orogenesis, and the fourth suite may either represent early synorogenic crustal melts or be related to the volcanic rocks in the Packsaddle Schist.In addition, these new data show that, contrary to previous reports, younger Packsaddle Schist lies in structural contact above older components of the Valley Spring Gneiss. Moreover, one of the dated meta-igneous units is the oldest unit yet found in the uplift and records an early period of metamorphism prior to formation of most Llano uplift rocks. Its 1366 ± 3 Ma protolith age is coeval with rocks of the Western Granite-Rhyolite terrane. Age and geochemical similarities link this unit of unknown regional extent with the Western Granite-Rhyolite terrane of known Laurentian affinity. Thus, this component of the Llano uplift may be the basement on which the younger components of the Valley Spring Gneiss and Packsaddle Schist formed in a tectonic setting proximal to Laurentia and south of the Western Granite-Rhyolite terrane. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The concentrations and distributions of U and Th in Paleozoic aquifers surrounding the Llano Uplift area, central Texas, U.S.A.: application to the sources of Ra and Rn in groundwater

Interactions between groundwater and aquifer rock can result in anomalously high levels of radium (Ra) and radon (Rn) in groundwaters. An understanding of aquifer mineralogy and determining the sources of natural radionuclides are, therefore, essential to design possible means for improving groundwater quality. The Cambrian Hickory Sandstones and Cap Mountain Limestones constitute important aquifers in the area surrounding the Llano Uplift in central Texas. Groundwaters produced from these aquifers, however, have anomalous concentrations of Ra and Rn, much greater than the maximum contaminant levels (MCL) suggested by the USEPA. To determine the sources of Ra and Rn, the abundance, distribution, and nature of occurrence of U and Th, parent radionuclides of Ra and Rn, were examined in cored aquifer rock samples.

Mesoproterozoic tectonic evolution of the western Llano uplift, central Texas: The story in an outcrop

A small, but perfectly exposed outcrop of Mesoproterozoic rocks in the western Llano uplift displays lithologic and structural relations from which a sequence of geologic events has been determined. Detailed geologic mapping and petrologic studies combined with new U-Pb ages of key units are used to constrain the tectonic evolution of this little-studied region and provide a basis for comparison with the previously studied southeastern uplift. The outcrop consists of layered mafic and felsic gneiss cross-cut by three generations of granitic dikes and sills. The sequence of events as deduced from cross-cutting relations and U-Pb geochronology is as follows: (1) generation of compositional layering (S 0 ) and parallel biotite foliation (S 1 ) in the gneisses at 1256 Ma; (2) emplacement of the first set of granitic sills and dikes at 1253 Ma; (3) transposition, isoclinal folding, amphibolite facies metamorphism, and fabric formation (S 2 ) between 1253 Ma and 1126 Ma; (4) emplacement of pegmatitic granitic dikes at 1126 Ma; (5) open folding and boudinage between 1126 Ma and 1076 Ma; and (6) emplacement of aplitic granitic dikes at 1076 Ma. Four U-Pb ages of titanite, two each from the gneiss and pegmatitic granite, are concordant at ∼ 1114 Ma indicating that the region had been sufficiently hot to reset titanite ages in the gneiss and that the region had cooled below the titanite blocking temperature by this time. The tectonic evolution of this outcrop is remarkably similar to that of other areas of the Llano uplift, suggesting that the entire uplift experienced a similar evolution.

Late thermal evolution of Proterozoic rocks in the northeastern Llano Uplift, central Texas

The Llano Uplift of central Texas, USA contains large exposures of polydeformed Grenvillian metaigneous and metasedimentary rocks that were intruded by late- to post-orogenic granites. New U–Pb and 40 Ar/ 39 Ar single mineral ages and Rb–Sr mineral isochrons document the retrograde cooling history that followed Grenville-age metamorphism in the north-central and northeastern Llano Uplift, and explain discrepancies between previous U–Pb crystallization ages and younger determinations based on Rb–Sr and K–Ar isotopic systems. Dating has been carried out in the Valley Spring Gneiss (VSG) and the post-tectonic Lone Grove Pluton (LGP) in three areas of the Llano Uplift, from west to east: VSG rocks in the Babyhead area north of Llano, Texas, yielding hornblende, muscovite and biotite 40 Ar/ 39 Ar ages and Rb/Sr mineral isochron ages from ∼1098 to ∼1014 Ma; a sample from the LGP at Petrick Quarry, yielding U–Pb titanite and hornblende and biotite 40 Ar/ 39 Ar ages from ∼1096 to ∼1076 Ma; and VSG samples from the Inks Lake area in the eastern Llano Uplift, yielding U–Pb titanite, hornblende and biotite 40 Ar/ 39 Ar , and Rb-Sr mineral isochron ages from ∼1087 to ∼1029 Ma. Titanite from the Oatman granite to the southwest of these areas yields a U–Pb age of 1119±6 Ma. These ages, combined with closure-temperature estimates, document cooling in the Llano Uplift from ∼1100–1000 Ma at rates between 5 and 14°C Ma −1. Although rapid uplift following early high-pressure metamorphism has important regional thermal effects, the cooling rates documented in this study are interpreted to represent the relaxation of local geotherms following local magmatic activity. The low-temperature history documented in this study is probably unrelated to uplift rates. This supports the suggestion that magmatic heating was an important factor in the late high-temperature/low-pressure metamorphism of the Llano region. It further suggests that the late pressure and temperature conditions recorded in the rocks were likely not synchronous across the region.

Alteration of magnetic properties of Palaeozoic platform carbonate rocks during burial diagenesis (Lower Ordovician sequence, Texas, USA)

Abstract Palaeomagnetic and sedimentological investigations of samples from two sections of correlative Iapetan platform carbonate rocks from Texas, USA, were made to test whether their magnetic properties reflect diagenetic alteration associated with regional and local tectonism. The Honeycut Formation (Llano Uplift area, central Texas), in close proximity to the late Palaeozoic Ouachita orogenic belt, exhibits a distinct correlation between magnetization intensity, magnetization age (direction) and lithofacies. Mudstones preserve their weak primary Early Ordovician magnetization, whereas dolo-grainstones carry a strong Pennsylvanian magnetization residing in authigenic magnetite. Fluid migration associated with the Ouachita Orogeny has been focused in lithofacies with high permeability and caused dolomite recrystallization and pervasive remagnetization. Magnetization intensity trends covary with fluid/rock ratios. However, aquitards were either not affected or less affected by these fluids. Unlike the Honeycut Formation, permeable rocks of the El Paso Group (Franklin Mountains, west Texas) carry only a non-pervasive Pennsylvanian magnetization. Therefore, a larger percentage of El Paso Group samples retain a primary Early Ordovician signature. This area is further removed from the Ouachita front, and, thus, the influence by Pennsylvanian orogenic fluids was less pronounced.

Tectonic evolution of the southern Laurentian Grenville orogenic belt

Research Article| November 01, 1998 Tectonic evolution of the southern Laurentian Grenville orogenic belt Sharon Mosher Sharon Mosher 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1998) 110 (11): 1357–1375. https://doi.org/10.1130/0016-7606(1998)110<1357:TEOTSL>2.3.CO;2 Article history first online: 01 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 Sharon Mosher; Tectonic evolution of the southern Laurentian Grenville orogenic belt. GSA Bulletin 1998;; 110 (11): 1357–1375. doi: https://doi.org/10.1130/0016-7606(1998)110<1357:TEOTSL>2.3.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 SocietyGSA Bulletin Search Advanced Search Abstract The Grenville orogenic belt along the southern margin of Laurentia records more than 300 m.y. of orogenic activity culminating in arc-continent and continent-continent collision ca. 1150–1120 Ma. Exposures in Texas provide a unique profile across the Grenville orogen from the orogen core to the cratonal margin. In the Llano uplift of central Texas, ca. 1360–1232 Ma upper amphibolite–lower granulite facies, polydeformed supracrustal and plutonic rocks represent the core of the collisional orogen. This exposure contains a suture between a 1326–1275 Ma exotic island-arc terrane and probable Laurentian crust and records A-type subduction. In west Texas, 1380–1327 Ma amphibolite to greenschist facies, polydeformed supracrustal rocks are thrust over ca. 1250 Ma carbonate and volcanic rocks along the cratonal margin. The carbonate and volcanic rocks form a narrow thrust belt with post–1123 Ma synorogenic sedimentary rocks, which grade into undeformed sedimentary rocks northward on the Laurentian craton.The Texas basement reveals a consistent but evolving tectonic setting for the southern margin of Laurentia during Mesoproterozoic time. This paper summarizes recent advances in our knowledge of the Texas basement and proposes plate models to explain the tectonic evolution of this margin during Mesoproterozoic time. The orogenic history is strikingly similar to that of the Canadian Grenville orogen and requires a colliding continent off the southern Laurentian margin during the assembly of Rodinia. This content is PDF only. Please click on the PDF icon to access. 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.

Oolitized fragments of filamentous calcimicrobes and the pseudofossil affinity of Nuia Maslov from the Upper Cambrian rocks of central Texas

The origin and affinity of the problematic Nuia Maslov is reappraised from exceptionally preserved specimens in Upper Cambrian (Sunwaptan) rocks of the Llano Uplift, central Texas. Nuia is suggested to be a radial calcitic ooid that has nucleated on fragments of reworked filamentous calcimicrobes rather than a separate microorganism as has been assumed previously. Nuia is found predominantly in intrareef grainstone accumulations, as well as in interreef grainstones, invariably accompanied by other bioclastic debris showing similar radial calcite ooid overgrowths.

The Argentine Precordillera: its odyssey from the Laurentian Ouachita margin towards the Sierras Pampeanas of Gondwana

Abstract The Argentine Precordillera (Cuyania terrane) and the Marathon/Solitario basin (west Texas), both underpinned by Grenvillian Laurentian basement, evolved together during Early Cambrian to Mid-Ordovician times. Ages, Pb-isotope and geochemical data for Precordilleran and for west-central Texas basement (including the Llano uplift) are strikingly similar. Carbonate platform sequences developed on both sides (e.g., El Paso and Chica de Zonda) of the Marathon/Solitario outer-shelf to slope basin and hosted homologous reef organisms which were unique to the Laurentian Ouachita margin. The Marathon/Solitario basin received sediments from both north and south; much detritus came from the northern shelf, including olistoliths bearing shelf fauna. Erosional vacuities on the platforms correlate with coarse detrital basin deposits. Cuyania constituted the long-sought southern source. Stratigraphy and structures of the Precordillera/Marathon/Solitario basin are consonant with plate reconstructions that place southern Laurentia near western Gondwana from Late Cambrian into mid-Ordovician time. During Caradoc time, Cuyania moved beyond range of faunal exchange with Laurentia, and tholeiitic basalts were intruded into off-shelf turbidites in the western Precordillera. The attenuated, thermally weakened, Laurentian slab broke apart with continued extension and right-oblique separation of Laurentia and Gondwana. Severance of Cuyania from Laurentia was complete before the onset of Taconic or Ocloyic orogenesis.