Llano Uplift Research Articles

Source‐to‐sink tandem geochronology reveals tectonic influences on the Cambrian Transcontinental Arch of Laurentia

AbstractCompeting hypotheses attribute the regional loss of 1.2–1.0 Ga detrital zircon from the Cambrian Sauk Sequence in southwestern North America to differing tectonic controls on surface topography. We test three hypotheses with source‐to‐sink detrital zircon provenance analysis via tandem in situ and isotope dilution U–Pb geochronology paired with geochemical and Hf‐isotope tracers. Our data indicate that the lower‐to‐middle Sixtymile Formation in Grand Canyon was derived from ca. 1.1 Ga rocks of the Llano Uplift and the ca. 539–523 Ma Wichita igneous province, approximately 1400 km away. In contrast, new U–Pb geochronology links the upper Sixtymile and Tapeats formations to the 513–510 Ma Florida Mountains intrusive complex, southern New Mexico, and proximal 1.4 and 1.7 Ga basement approximately 650 km away. We attribute a regional provenance shift to plume–lithosphere interactions on the Iapetan margin, tectonism along ‘leaky’ intracratonic transverse fault zones and the rift‐to‐drift transition on the Cordilleran margin.

GEOLOGY AND HXRF GEOCHEMISTRY OF METAMORPHIC XENOLITHS IN TOWN MOUNTAIN GRANITE AT “THE SLAB” SWIMMING HOLE, LLANO UPLIFT, KINGSLAND, TEXAS

Abstract Metamorphic xenoliths in the Town Mountain Granite are exposed at “The Slab,” a low-water crossing in Kingsland, Texas, that is popular both for recreation and as a stop on regional geology field trips to the Llano uplift. Difficulty in sampling the pavement-style outcrops limits observation and analysis to that which can be completed at the outcrop. We use structural orientation measurements and a handheld X-ray fluorescence spectrometer (hXRF) to study xenolith orientation in context of regional fabrics, assess evidence of xenolith assimilation into the Town Mountain Granite, and investigate the origin of the xenoliths based on bulk composition. A nearly 1:1 correlation between the orientations of the xenoliths’ long axes and their internal foliation suggests that granite intrusion along foliation strongly controlled the xenoliths’ shapes. The orientation of the xenoliths’ foliation is consistent with dominant regional foliations. The hXRF analyses are able to discriminate between the compositions of the granite and the xenoliths. Xenolith compositions are consistent with a mafic protolith and suggest they have the same or similar sources; different analytical techniques for previously published bulk compositional analyses prevent direct comparison with our data or confident identification of a source unit. Limited assimilation of the xenoliths into the granite is suggested by magma injections into xenoliths, mafic residue from partial melting, chemical gradations at contacts between the granite and xenoliths, and possible enrichment of xenoliths in FeO + MgO relative to bulk compositions of the metamorphic country rock. These results encourage future use of hXRF for outcrop-based geological studies.

Fluvial response to Late Pleistocene–Holocene climate change in the Colorado River drainage, central Texas, USA

AbstractWe documented the impact of Late Pleistocene–Holocene climate change on terrace deposits and preserved channels in the unglaciated drainage of the Colorado River in central Texas (south-central United States) using integrated channel morphology and provenance analysis. Detrital zircon (DZ) U-Pb ages (n = 1850) from fluvial terrace deposits and new quantitative analysis of fluvial channel morphology based on LiDAR data were used to reconstruct sediment provenance and shifts in paleohydraulic conditions during Late Pleistocene to Holocene aridification. These data reveal a reduction in fluvial channel size and discharge temporally coupled with a rapid shift in erosion locus and dominant sediment sourcing, from the Southern Rocky Mountains to the Llano area, during the glacial-interglacial transition. Geomorphic mapping and morphometric analysis show narrowing of river channels linked to diminishing Colorado River discharge. DZ data show an abrupt shift to erosion in the lower drainage basin and the remobilization of older terraces due to river incision and lateral channel migration. We attribute these systematic changes to upper-basin contraction caused by drainage reorganization and aridification during the Late Pleistocene, as well as the onset of enhanced convective precipitation sourced from the Gulf of Mexico, driving focused erosion along the topographic edge of the Llano uplift in central Texas since the early to mid-Holocene.

Tectonomagmatic evolution of southwestern Laurentia: Insights from zircon U-Pb geochronology and hafnium isotopic composition of the Red Bluff Granite Suite, west Texas, USA

AbstractWe provide laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS) and high-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb ages and Hf isotopic compositions of zircons from the Red Bluff Granite Suite and mafic dikes in the Franklin Mountains of El Paso, Texas, USA. Granitoids exposed in the Franklin Mountains were previously divided into five magmatic stages based on cross-cutting relationships. Major and trace element compositions showed that these granitoids are ferroan, alkaline, and A2 type. Homogeneity in the whole-rock geochemistry suggests that the granite stages are genetically related and share similar petrogenetic histories. Weighted mean zircon 206Pb/238U dates from the older magmatic stage 1 alkali-feldspar quartz syenite and stage 2 alkali-feldspar granite are 1112.36 ± 0.35 and 1112.46 ± 0.37 Ma, respectively. The weighted mean εHf(t) values varying from +5.3 to +7.2 are similar to those of other regional ca. 1.1 Ga magmatic rocks throughout southwestern Laurentia. Geochemical characteristics, petrological modeling, and enriched Hf isotopic composition suggest fractional crystallization of a basaltic magma that was produced by melting of an enriched mantle reservoir. However, zircon inheritance ages of ca. 1.3 Ga and 1.26–1.15 Ga are consistent with a minor contribution from felsic crustal basement. Our data and regional geology are consistent with a post-collisional slab break-off that facilitated asthenospheric upwelling and partial melting of the enriched mantle, possibly subcontinental lithospheric mantle, extending from Llano Uplift, Texas, in the southeast to California to the northwest. Magma thus generated upon differentiation produced ferroan and A-type granitoids.

Covering the Great Unconformity in southern Laurentia during Rodinia breakup: Detrital zircon studies of provenance evolution during Cambrian marine transgression (Llano Uplift, Texas)

The Llano Uplift in central Texas, USA, exposes the southernmost expanse of Laurentian crystalline basement in North America and the overlying lower Paleozoic strata deposited on the Great Unconformity. Systematic detrital zircon (DZ) U-Pb provenance analysis of the Hickory Sandstone—the basal unit of the Cambrian Riley Formation that onlaps the Mesoproterozoic core of the Llano Uplift—yielded locally variable DZ U-Pb signatures with ages ranging from 1800 Ma to 485 Ma (n >1700). The Hickory Sandstone zircons are dominated by 1550−1300 Ma (50%) and 1300−1000 Ma (46%) ages. These two dominant Mesoproterozoic DZ age components likely were sourced from local Grenville metamorphic and igneous basement of the Llano Province (1300−1000 Ma) and from the extra-regional Granite-Rhyolite Province basement to the north and northwest of the Grenville Front, which suggests both local sourcing and a regional drainage system supplying sediment to the southern Laurentian margin during the Cambrian. This interpretation is supported by a minor component of 1800−1600 Ma zircons, sourced from the Yavapai-Mazatzal Province, and zircons with Early Cambrian ages that were likely sourced from the Wichita Igneous Province, which is located to the northwest and to the north of the Llano Uplift, respectively. Documented NNW-trending topographic ridges in the Mesoproterozoic basement surface of the Llano Uplift could have funneled aeolian and fluvial sand supply to the southern Laurentian coast that was reworked in the marginal marine environment. Samples from the western Llano Uplift are dominated by regionally sourced, early Mesoproterozoic Granite-Rhyolite Province DZ ages, whereas samples from the eastern Llano Uplift exhibit a dominant locally sourced Grenville signature. Hickory Sandstone samples also contain a small number of Neoproterozoic (850−600 Ma) and Cambrian (541−487 Ma) zircons. Sources for Neoproterozoic zircons likely were located along the Laurentian continental margins and derived from extension-related magmatism associated with the breakup of Rodinia. Cambrian zircons are most common in Hickory Sandstone samples on the southern and western flanks of the Llano Uplift. Some are reasonably sourced from the Wichita Igneous Province to the north, but the younger Cambrian zircons suggest sources to the west. Upper Hickory strata are dominated (>60%) by 1550−1300 Ma grains with up to 10% >1600 Ma Paleoproterozoic grains. Upper Cambrian sandstones of the Wilberns Formation also contain a significant contribution of >1600 Ma grains, which suggests a possible enlargement of the fluvial headwaters over time extending into older Laurentian provinces to the north and northwest. Differences among DZ populations over relatively limited distances may have been related to NW-oriented ridge and corridor landforms that developed on the Great Unconformity surface, which were products of the Proterozoic tectonic and lithologic architecture affected by aeolian and fluvial processes at this unusual stage in Earth history. Paleocurrent data and atypical detrital grain characteristics for a Hickory Sandstone locality in the southeastern Llano Uplift and its DZ age similarities with Cambrian sandstones of the Argentine Precordillera suggest a common provenance prior to or during Rodinia breakup.

Geoelectric survey of the Granite Gravel aquifer, Llano Uplift, Central Texas, to determine locations for water wells

An electrical geophysical survey was completed within a small area of the Llano Uplift of central Texas to determine locations to install two water wells in the Granite Gravel aquifer (GGA). Electrical resistivity tomography (ERT) was performed along two 188-m long profiles that intersected at the approximate center of a 100-m by 100-m self-potential (SP) map. The ERT survey was completed to map two-dimensional (2D) electric resistivity distributions in the GGA and the underlying Precambrian Town Mountain Granite (TMG) bedrock, whereas SP mapping was performed to delineate apparent streaming potential anomalies at the land surface that appeared correlated to the subsurface resistivity distributions, which exhibited strong lateral heterogeneity in the upper 35 m of the weathered layer that comprises the GGA. The depth to TMG bedrock, as shown by the resistivity distributions, varied substantially over relatively small profile distances and surface areas; however, the general electrical structure showed resistivity increasing with depth, beginning with a thin electrically conductive layer at the surface characterized by resistivity in the range of about 30–100 Ω-m, followed by a resistivity increase to 300–500 Ω-m at a depth that coincided with the water-table depth observed in the installed water wells. Resistivity of the TMG bedrock was generally greater than 500 Ω-m and exceeded 1000 Ω-m in some locations of the tomograms. Electrical structure beneath the survey area, as shown by the 2D resistivity distributions beneath the ERT profiles, delineated a relatively thick weathered section of GGA that spatially aligned with a conspicuous negative anomaly observed in the SP map after electrode-drift and terrain corrections were made. The combination of ERT and SP mapping guided selection of locations for two productive water wells within the small survey area despite substantial heterogeneity in the weathering profile of the GGA beneath the survey area.

Using Ambient Noise Seismic Interferometry and Local and Teleseismic Earthquakes to Determine Crustal Thickness and Moho Structure of the Northwestern Gulf of Mexico Margin

AbstractThe northwestern part of the Gulf of Mexico has undergone two episodes of continental rifting and collision and produced structural artifacts that are now buried under many kilometers of sediments, complicating investigations of the region. The deep sedimentary package precludes outcrops and points to a need for the application of seismic techniques, but low rates of seismicity in the region and sparse seismic monitoring limit the utility of traditional seismic methods. We therefore use diverse data to perform two‐dimensional seismic tomography across the dry land portion of the margin. Data are gleaned from teleseismic and regional earthquakes, postcritical SsPmp arrivals, and direct P wave energy identified with seismic interferometry that were recorded by a broadband, three‐component array and partially overlapping short‐period, vertical‐component array. The Pn and postcritical SsPmp phase help constrain the Moho discontinuity, which a previous receiver function study suggested was absent beneath the seaward portion of this transect. A high‐velocity body is observed in the crust at the same location as the Houston Magnetic Anomaly, possibly marking rocks from the Alleghenian continental assembly. The crust thins from NW to SE, indicating that extension occurred mostly to the south of the Ouachita orogeny. Our model indicates that the margin's sediment package reaches a maximum thickness of ~15 km at the coast and becomes unresolvably thin near the Llano Uplift.

Controls on microbial and oolitic carbonate sedimentation and stratigraphic cyclicity within a mixed carbonate‐siliciclastic system: Upper Cambrian Wilberns Formation, Llano Uplift, Mason County, Texas, USA

AbstractThe upper Cambrian Wilberns Formation in central Texas records deposition on a low‐gradient shelf within a mixed carbonate–siliciclastic tidal‐flat system that changes offshore to subtidal shelf and open‐marine oolitic skeletal shoals with large microbial mounds. Siliciclastic sediment is interpreted to have been delivered to the tidal flat by aeolian processes because of the narrow range in grain size and paucity of clay. Tidal influence is dominant as evidenced by reversing currents and desiccation on the tidal flat, and megaripples with reversing current indicators in offshore shoals. Intraclastic conglomerates were deposited in broad channels on the tidal flats during storm surges. Microbialite deposition is interpreted to be controlled by accommodation favouring amalgamated thin biostromes developed in the tidal flat vs. larger mounds with greater synoptic relief in the offshore, and current energy resulting in preferential elongation of offshore mounds in a NE–SW orientation. Intertidal mounds and biostromes grew in the presence of significant siliciclastic flux and trapped it within their structure, whereas offshore large buildups incorporated little siliciclastic component. Oolite and skeletal grainstone formed in tide agitated shoals associated with large subtidal microbial mounds. Storms extensively recycled and redistributed skeletal and oolitic sands from the offshore shoals across the shelf as thin sand sheets. Spatial mixing of siliciclastic and carbonate sediment occurred across the tidal flat and shelf. Low‐frequency and intermediate‐frequency stratigraphic cycles were driven by shifts in the shoreline and changes in rate of siliciclastic flux in response to relative sea‐level fluctuation. Random facies stacking and the lack of metre‐scale cyclicity are interpreted to reflect stratigraphic incompleteness and an episodic signal introduced by storms.

Pollen-spore assemblages of the Navarro Group (Maastrichtian) of Texas, USA: biostratigraphical and palaeoecological significance

Diverse assemblages of spore–pollen floras are present in all formations in the Navarro Group (Maastrichtian) in outcrops of the Austin Section and the subsurface Frio Section. The stratigraphic ranges and relative abundance of spore, pollen, dinoflagellate cysts and acritarchs were used to establish four biozones (A, B, C and D). Biozone A corresponds to the undifferentiated Neylandville Marl–Nacatoch Sand Formation; Biozone B corresponds to the Corsicana Marl Formation; Biozone C corresponds to the lower half of the Kemp Clay Formation; and Biozone D corresponds to the upper half of the Kemp Clay Formation. Biozonal boundaries do not always correspond to formation boundaries. Biozones A, B, C and D of the Austin section are correlated with the subsurface Frio section. The zonal boundaries based on spore–pollen distribution in the Austin and the Frio sections correlate well with the timelines established in these two sections based on dinoflagellate cysts and acritarchs. All Navarro Group formations except the Olmos were deposited in marine environments ranging from intertidal to outer shelf. Olmos Formation sediments were deposited in a range of coastal terrestrial environments with occasional minor influences of the sea. Three transgressive phases (T1, T2 and T3) separated by four regressive phases (R1, R2, R3 and R4) are recognized. During the Maastrichtian the floral composition in Texas changed from predominantly pteridophytic and bryophytic in the early Maastrichtian to predominantly angiospermous flora in the middle and late Maastrichtian. Tropical to subtropical climates prevailed in Texas throughout the Maastrichtian but highland regions such as Llano Uplift and nearby Marathon and Arbuckle Mountains were cooler regions with a temperate to sub–temperate climate. The upper part of the Navarro Group records a gradual cooling trend suggested by the dominance of temperate angiosperm pollen taxa.

Stratigraphic architecture and petrophysical characterization of formations for deep disposal in the Fort Worth Basin, Texas

Disposal of hydraulic fracturing flowback and produced water into Ordovician and Cambrian formations of the Fort Worth Basin (FWB), coupled with an increase in observed seismicity in the Dallas-Fort Worth area, necessitates an understanding of the geologic character of these disposal targets. More than 2 billion barrels (Bbbls) of wastewater have been disposed into the Ordovician Ellenburger Group of the FWB over the past 35 years. Since the implementation of the TexNet Earthquake Catalog (1 January 2017), more than 20 earthquakes of local magnitude ML2.0 or greater have been detected in the area, with depths ranging from 2 to 10 km (approximately 6500–33,000 ft). The cited mechanism for inducement of these earthquakes is reactivation of basement faults due to pore pressure changes, either directly related to proximal disposal or due to disposal volume buildup over time. Here, we present a stratigraphic and petrophysical analysis of FWB disposal targets and their relation to basement rocks. The Ellenburger consists of alternating layers of limestone and dolomite, with minor siliciclastics above the basement toward the Llano Uplift. Matrix porosity averages <5 porosity units (p.u.), with higher porosity in dolomitic layers than in limestone. Dolomite dominates at the top of the Ellenburger, which was exposed at the end of both the Lower and Upper Ordovician. Where crystalline basement rocks are penetrated, the composition ranges from granitic to chlorite-bearing metamorphosed lithology. The basement-sediment interface is frequently marked by increased porosity. An updated map of structure on top of basement indicates elevations ranging from outcrop at the Llano Uplift to more than [Formula: see text] ([Formula: see text]) subsea toward the northeast. The disposal zone pore volume is estimated from thickness and porosity maps and ranges from [Formula: see text] to [Formula: see text] billion barrels per square mile ([Formula: see text]).

Home range of the Columbian mammoths (Mammuthus columbi) and grazing herbivores from the Waco Mammoth National Monument, (Texas, USA) based on strontium isotope ratios from tooth enamel bioapatite

Waco Mammoth National Monument (WMNM) is a central Texas, Late Pleistocene fossil site dating to ~67 ka. At least 23 Columbian mammoths (Mammuthus columbi), along with the remains of 12 other vertebrate genera have been discovered at this locality. Mammoth teeth were micro-sampled at a high resolution with a computerized micromill, whereas bison and horse teeth were serially sampled with a handheld rotary tool. Serial samples from these individuals offer insights into the movement patterns (87Sr/86Sr) of WMNM herbivores. Regional vegetation and local sediments were also collected to construct an 87Sr/86Sr isoscape to track past movement of mammals, derived from Sr analyses of teeth enamel. 87Sr/86Sr in WMNM herbivores often reflects multiple grazing provenances, and sources. A series of mixing models are proposed to understand the contribution of different surface Sr sources to herbivore diets. These data indicate one mammoth spent some time at the Llano uplift area ~180 km to the SSW during the formation of molar enamel. The remaining individuals (three mammoths, a bison and a horse) lived in an area more proximal to the WMNM, ≤70 km SE of the site. 87Sr/86Sr from WMNM mammoths indicate individual fidelity to at least two geographic ranges, with at least one mammoth, potentially unrelated to the possible matriarchal herd, indicating complex population dynamics prior to mass mortality. This analysis shows that at least 25 mammals, including the horse and bison, congregated at the confluence of Brazos and Bosque rivers at ~67 ka ago, prior to their demise. A resource shortage related to drought is consistent with this pattern.

Tomography, Seismotectonics, and Mantle Dynamics of Central and Eastern United States

AbstractWe study the 3‐D P wave velocity structure of the crust and mantle down to 1,000‐km depth beneath the central and eastern United States. A 3‐D velocity model is obtained by conducting a joint inversion of 236,670 arrival times of local earthquakes and 870,455 relative traveltime residuals of teleseismic events recorded by the EarthScope/USArray Transportable Array. Significant low‐velocity (low‐V) anomalies are revealed in the crust beneath the eastern arm of the Midcontinent Rift and the Triassic Basins along the East Coast, whereas a prominent high‐velocity (high‐V) anomaly is visible beneath the Llano Uplift in central Texas. The stable North American Craton exhibits high‐V anomalies at depths of 65–250 km. Low‐V anomalies exist along the eastern and southern margins of the craton, which may reflect relatively thin lithosphere there. A prominent low‐V anomaly is revealed at depths of 50–200 km beneath the New Madrid Seismic Zone, which is bounded by high‐V anomalies to its southeast and northwest. This feature reflects a weak lithosphere surrounded by relatively strong cratonic regions and stress concentration caused intraplate seismicity in the New Madrid region. Two high‐V bodies appear in the mantle transition zone (410‐ to 660‐km depths) beneath the Interior Low Plateaus, the central Great Plains, and the Central Lowland, which may reflect the subducted Farallon plate or delaminated lithosphere. At depths of 800–1,000 km, a high‐V anomaly is visible beneath the southeast United States, which may be the subducted Hess Rise conjugate.

Crystalline-rock aquifer system of the Llano Uplift, Central Texas, USA

The crystalline-rock aquifer system of the Llano Uplift (Central Texas, USA) hosts an important local water resource that has been relatively little studied. The Uplift is a structural dome of Precambrian granitic and metamorphic rocks. Late Paleozoic normal faulting and fractures, decompressive fractures, weathering, lithology, and rock fabric control the aquifer properties. Data from driller reports (over 2,000 wells) show that wells in granites have higher median yields than wells in metamorphic rocks. There is a weak correlation of well yield with regolith thickness, and median regolith thickness is greater over granites than over metamorphic rocks. Fracture permeability, which is very heterogeneous, is the major control. Wells are shallow (generally <100 m depth), but more recent wells have been drilled more deeply. Permeability data imply decreased open-fracture density and aperture with depth, although sample bias is a consideration. Diamond drill cores show that many near-surface fractures with significant aperture are filled by rock fragments from weathered surrounding rocks and that fracture skins are thinner and contain iron oxides in a more reduced state with depth. Fracture skins can be porous, with porosity ranging to over 10%. There is a need to compare crystalline-rock aquifer systems to assess weathering, tectonics, fractures, and mineralogy/petrology to assess the characteristics of these systems, which are critical water resources in large areas of the world.

Endemic plants of the central grassland of North America: distribution, ecology, and conservation status

This paper enumerates the endemic plants of the Central Grassland of North America. The Central Grassland encompasses the full extent of the tallgrass, mixed-grass, and shortgrass prairie ecological systems of North America plus floristically related plant communities that adjoin and/or interdigitate with the midcontinental grasslands including savanna-open woodland systems, shrub-steppe, and rock outcrop communities. There are 382 plant taxa endemic to the Central Grassland, 300 endemic species (eight of which have multiple subspecific taxa endemic to the region) and 72 endemic subspecies/varieties of more widely distributed species. Nine regional concentrations of endemic taxa were identified and are described as centers of endemism for the Central Grassland: Arkansas Valley Barrens, Edwards Plateau, Llano Estacado Escarpments, Llano Uplift, Mescalero-Monahans Dunes, Niobrara-Platte Tablelands, Raton Tablelands, Red Bed Plains, and Reverchon Rocklands. In addition to hosting localized endemics, these areas are typically enriched with more widely-distributed Central Grassland endemics as well as peripheral or disjunct occurrences of locally-rare taxa, making them regions of high floristic diversity for the Central Grassland. Most of the endemics (299 or 78%) are habitat specialists, associated with rock outcrop, sand, hydric, or riparian habitats. There is a strong correlation between geology and endemism in the Central Grassland, with 59% of the endemics (225 taxa) associated with rock outcrop habitat. Of the 382 Central Grassland endemics, 124 or 33% are of conservation concern (NatureServe ranking of G1/T1 to G3/T3). Of these at-risk taxa, 78 or 63% are primarily associated with one of the centers of endemism identified in the study. It is hoped these findings will be useful in focusing conservation action on the habitats, ecological associations, and regions of the Central Grassland that host the highest concentrations of unique and at-risk plant species and associated biological diversity.

U‐Pb Geochronological and Thermochronological Time–Temperature Constraints of 40Ar/39Ar Hornblende Reference Material HB3gr

Hornblende from the Lone Grove Pluton, Llano Uplift, Texas, has served as an irradiation reference material in 40Ar/39Ar studies for decades. In order to evaluate the apparent age bias that currently exists between the U‐Pb and 40Ar/39Ar systems, zircon and titanite were dated by isotope dilution‐thermal ionisation mass spectrometry (ID‐TIMS) from the same rock from which the hornblende 40Ar/39Ar reference material HB3gr is derived. Zircon U‐Pb data indicate initial crystallisation at 1090.10 ± 0.16 Ma (2s), a date that is 1.7% older than the accepted K‐Ar date (1072 ± 14 Ma, 2s) for HB3gr; an offset that exceeds the typical 0.5–1% bias between the two systems, though remaining within uncertainty due to the large uncertainties in the 40K decay constant. Zircon data are presented using both EARTHTIME tracers ET535 and ET2535 and are statistically indistinguishable. Single grain titanite analyses range between 1082 ± 0.75 and 1086 ± 0.81 Ma (2s) and are interpreted to record the subsequent cooling following crystallisation at rates between 30 and 50 °C Ma−1. This is supported by the observation that hornblende 40Ar/39Ar dates corrected for decay constant bias are resolvably younger than the zircon U‐Pb date and in good agreement with titanite U‐Pb dates, permitting the conclusion that both titanite U‐Pb and hornblende 40Ar/39Ar systems provide a record of cooling.

Early Miocene continental-scale sediment supply to the Gulf of Mexico Basin based on detrital zircon analysis

The early Miocene was a period of major continental margin progradation in the Gulf of Mexico Basin that accompanied prominent tectonic and climatic changes in North America. However, sediment pathways from continental upland sources to deep basinal sinks remain poorly constrained. This study presents 2192 new detrital zircon U-Pb analyses from 19 Lower Miocene samples spanning the entire northern Gulf of Mexico margin to elucidate early Miocene sediment provenance and paleodrainage systems. The U-Pb age patterns indicate that the Great Plains, southern Rocky Mountains, and mid-Cenozoic volcanic field were the major source terranes for the western-central Gulf of Mexico coast, whereas the Appalachian foreland basin and Appalachian Mountains mainly contributed sediment to the eastern Gulf of Mexico coast. Local source terranes included the Llano uplift and Edwards Plateau in central Texas and the Ouachita Mountains and foreland basin in Oklahoma and Arkansas. A comparison to previous detrital zircon studies around the Gulf of Mexico indicates that sediment recycling was important during the early Miocene. Sediment associated with major paleorivers, including the Rio Bravo, Rio Grande, Houston-Brazos, Red, Mississippi, Tombigbee, and Apalachicola Rivers, can be differentiated using the detrital zircon U-Pb analyses. These data help to better define the early Miocene source-to-sink system in the northern Gulf of Mexico, by relating the basin fill to hinterland tectonic and geological evolution. In comparison to the Paleocene−Eocene Wilcox drainage system, the early Miocene drainage system of the northern Gulf of Mexico was smaller and received less input from western Mexico arc terranes and Archean basement in Wyoming. This drainage area reduction, related to regional thermal uplift and Basin and Range−Rio Grande rifting, likely explains the reduced sediment volume of the Lower Miocene strata in the Gulf of Mexico relative to the Paleocene−Eocene Wilcox Group.

The role of subgrain boundaries in partial melting

Evidence for partial melting along subgrain boundaries in quartz and plagioclase is documented for rocks from the Lost Creek Gneiss of the Llano Uplift, central Texas, the Wet Mountains of central Colorado, and the Albany-Fraser Orogen, southwestern Australia. Domains of quartz or plagioclase crystals along subgrain boundaries are preferentially involved in partial melting over unstrained domains of these minerals. Material along subgrain boundaries in quartz and plagioclase has the same morphology as melt pseudomorphs present along grain boundaries and is commonly laterally continuous with this former grain boundary melt, indicating the material along subgrain boundaries can also be categorized as a melt pseudomorph. Subgrain boundaries consist of arrays of dislocations within a crystal lattice, and unlike fractures would not act as conduits for melt migration. Instead, the presence of former melt along subgrain boundaries requires that partial melting occurred in these locations because it is kinetically more favorable for melting reactions to occur there. Preferential melting in high strain locations may be attributed to strain energy, which provides a minor energetic contribution to the reaction and leads to preferential melting in locations with weakened bonds, and/or the presence of small quantities of water associated with dislocations, which may enhance diffusion rates or locally lower the temperature needed for partial melting.

A Naturalist’s Guide to the Texas Hill Country by Mark Gustafson

Reviewed by: A Naturalist’s Guide to the Texas Hill Country by Mark Gustafson Thomas R. Simpson A Naturalist’s Guide to the Texas Hill Country. By Mark Gustafson. College Station: Texas A&M University Press, 2015. vii + 339 pp. Illustrations, references, index. $24.95 cloth. Author Mark Gustafson remarks in the introduction of his book that the Hill Country is perhaps the best-loved region of Texas. It is a crossroads region of cultures, plants, and animals from the bordering ecoregions of South Texas brush country, western arid lands of the Trans-Pecos, and northern prairies of the Rolling Plains. The region’s distinctiveness derives from its unique geology and topography, producing picturesque hills, canyons, and a multitude of perennial spring-fed rivers. Because of these influences, the flora and fauna of the Texas Hill Country are rich and diverse. With 430 species of birds alone, choosing representatives from any of the wildlife categories (woody plants, flowers, grasses, birds, mammals, reptiles, amphibians, fish, and invertebrates) presents a daunting task to any author preparing a guide for this region. In this effort, I find the author has included species that characterize and reflect the diversity of the region, are taxonomically varied, and range from the abundant and obvious to the obscure. Importantly, species of conservation concern, such as the black-capped vireo, Cagle’s map turtle, and Texas horned lizard, are among the listed species. Also, I find the inclusion of nonindigenous (exotic and, in some cases, invasive) species to be refreshing. Many guides to wildlife and plants ignore these ecologically important species. In addition to the wild flora and fauna, this guide contains a wealth of information on geology, topography, and cultural aspects of the Hill Country. With simple and straightforward discussions on limestone karst, granite of the Llano Uplift, streams, rivers, and aquifers, Gustafson conveys the region’s complex geological history. Added to this is the human cultural element, which has impacted plants and animals beginning with Paleoindians and historical native tribes such as the Apaches and Comanches, and stretching to European settlers and their modern descendants. The inclusion of brief, basic descriptions of selected state parks and recreational areas is a nice touch that helps to familiarize readers with Hill Country nature. Along with the many positives of this book, there are missed opportunities. Some of the species’ narratives are mystifyingly short. The addition of a sentence or two could have provided interesting natural history or conservation information. For example, facts about ecological problems created by nonindigenous and invasive species are generally lacking. However, additional information can be found through listed references to detailed species accounts. This nicely produced volume with a wealth of vivid photographs is not a traditional “field guide” for identification of wild plant and animal species through distinguishing characteristics and distributions. This guide will find its best use by accompanying the target audience of amateur naturalists exploring the Texas Hill Country and offering glimpses and descriptions of the region’s exceptional beauty. Thomas R. Simpson Department of Biology Texas State University, San Marcos Copyright © 2016 Center for Great Plains Studies, University of Nebraska–Lincoln

An Unusual Late Aboriginal Assemblage from the Wilson Site (41SS186), San Saba County, Central Texas

The late aboriginal component in the Wilson Site in San Saba County is unusual in that most of the assemblage is consistent with that of Classic Toyah, but the diagnostic projectile point is an unnotched triangular arrow point instead of the typical Perdiz point. The absence of Perdiz points suggests that this component is associated with non-Toyah people and possibly dates to after 1700. Archaeological testing by the Llano Uplift Archeological Society (LUAS) to find supporting evidence for a historic date identified an Austin phase shell midden and a “Late Component” composed of triangular arrow points, end scrapers, a beveled biface and bone-tempered sherds, but no items of European manufacture. Complicating matters, the luminescence dating on a ceramic sample opens the possibility that the Late Component predates the currently accepted beginning of the Toyah phase.

Zircon U–Pb ages and Hf isotopic compositions indicate multiple sources for Grenvillian detrital zircon deposited in western Laurentia

Combined U–Pb ages and Hf isotopic data from 1.0 Ga to 1.3 Ga (Grenvillian) detrital zircon in Neoproterozoic and Cambrian siliciclastic sedimentary rocks in southwest North America, and from igneous zircon in potential Mesoproterozoic source rocks, are used to better assess the provenance of detrital zircon potentially transported across Laurentia in major river systems originating in the Grenville orogenic highlands. High-precision hafnium isotopic analyses of individual ∼1.1 Ga detrital zircon from Neoproterozoic siliciclastic sedimentary rocks in Sonora, northern Mexico, reveal that these zircons have low εHf(0) (−22 to −26) and were most likely derived from ∼1.1 Ga granitic rocks embedded in local Mojave Province Paleoproterozoic crust. In contrast, Grenvillian detrital zircons in Cambrian sedimentary rocks in Sonora, the Great Basin, and the Mojave Desert, have generally higher εHf(0) (−15 to −21) as demonstrated both by high precision solution-based, and by lower precision laser ablation, ICPMS data and were likely derived from more distal sources further to the east/southeast in Laurentia. Comparison to new and existing zircon U–Pb geochronology and Hf isotopic data from Grenvillian crystalline rocks from the Appalachian Mountains, central and west Texas, and from Paleoproterozoic terranes throughout southwest North America reveals that zircon in Cambrian sandstones need not entirely represent detritus transported across the continent from Grenville province rocks in the vicinity of the present-day southern Appalachian Mountains. Instead, these zircons could have been derived from more proximal, high εHf(0), ∼1.1 Ga, crystalline rocks such as those exposed today in the Llano Uplift in central Texas and in the Franklin Mountains of west Texas. Regardless of the exact source(s) of the Grenvillian detrital zircon, new and existing whole–rock Nd isotopic data from Neoproterozoic to Cambrian siliciclastic sedimentary rocks in the Mojave Desert demonstrate that the occurrences of higher εHf(0), Grenvillian detrital zircons are decoupled from the sources of the bulk of the sedimentary detritus in which the zircons are entrained. The Cambrian Wood Canyon Formation and the underlying “off craton” Neoproterozoic Johnnie Formation and Stirling Quartzite all contain higher εHf(0), Grenvillian detrital zircon, in some cases as the dominant detrital zircon population. However, only portions of the Wood Canyon Formation have whole rock Nd isotopic compositions consistent with a bulk sediment source in ∼1.1 Ga sources rocks. Whole rock Nd isotopic compositions of the remaining portions of this unit, and all of the Johnnie Formation and Stirling Quartzite, require bulk sediment sources principally in Paleoproterozoic continental crust. We consider the observed decoupling in the sources of Grenvillian detrital zircon and bulk sediment in the Wood Canyon Formation and underlying siliciclastic sediments as a demonstration that detrital zircon U–Pb and Hf isotopic data alone can provide an incomplete picture of the source of sediments that comprise a given siliciclastic stratigraphic unit.