Llano Uplift Research Articles

Recently Discovered Overthrusting Northeast of Llano Uplift Along Extension of San Marcos Platform: New Exploration Frontier in Central Texas: ABSTRACT

TJB Resources recently completed gravity, magnetotelluric, and seismic geophysical surveys in San Saba and Mills Counties north of the Llano uplift, west of the Ouachita Disturbed belt. Significant results show overthrusting has occurred 60-70 mi west of the Ouachita Disturbed belt. Earliest thrusting could have occurred in the Silurian-Devonian from south or southeast. Faulting reoccurs during Pennsylvanian Ouachita thrusting and again in the Late Cretaceous. The primitive Llano uplift buttressed the allochthonous rocks moving north and west. Magnetotelluric and gravity data indicate the Llano uplift is a separate and distinct isolated remnant or terrain, with numerous igneous intrusions reaching to great depths. Associated with this thrusting is a major tectonic northwest-southeast lineament located parallel to the termination of the Cretaceous rocks, outcropping on the western edge of Mills and Lampasas Counties. This lineament is documented by east-west seismic lines displaying a 0.3-sec displacement and farther southwest a 1.1-sec displacement. Paleozoic fossils were found near the surface trace of the northwest-southwest lineament as observed on the east-west seismic line. This area was previously mapped as Cretaceous. Surface inspection found Paleozoic rocks containing crinoids, spirifers, and rugose corals, with beds dipping to 45/sup 0/, contrasted with the N2/sup 0/ regional dip surroundingmore » the lineament area. Well-log correlations from Marble Falls to Ellenberger range from 350 to 600 ft from east to west near the Llano uplift. The 60 to 70-mi frontal edge of the thrusting provides potential hydrocarbon traps along a north to northeast trend from central Texas to Oklahoma.« less

Rapid production of continental crust 1.7 to 1.9 b.y. ago: Nd isotopic evidence from the basement of the North American mid-continent

Nd isotopic analyses of Precambrian granitic rocks from the central United States were made to determine crust-formation ages and thereby to investigate the structure and evolution of central North America. Samples of anorogenic granites that have 1.4-b.y. crystallization ages yield model TDM ages of 1.77 to 1.98 b.y. that indicate a previously undocumented 0.4- to 0.6-b.y.-old precrystallization crustal history. These and previously published data approximately define the boundaries of a large segment of continental crust that was formed from the mantle during a relatively short interval ∼1.9 b.y. ago. Both 1.4- and 1.8-b.y.-old samples from the Penokean terrene of central Wisconsin yield distinctly older TDM model ages of 2.1 to 2.3 b.y. Approximately 1.1-b.y.-old samples from the Llano uplift in Texas yield crust-formation ages of 1.3 to 1.4 b.y. The data appear to delineate provinces with well-defined, nonoverlapping crust-formation ages that young to the south, indicating episodic formation of the North American continental crust during Proterozoic time. The initial 143Nd/144Nd ratios of the 1.4-b.y.-old granites are varied ( ϵ Nd = +4.8 to −2.0). Most of the samples analyzed probably represent crustal melts, but the existence of some high initial ϵ Nd values indicates an admixture of mantle-derived material. The isotopic patterns suggest that the 1.4-b.y.-old “anorogenic” plutonism may be an inland manifestation of subduction activity related to formation of Llano crust. The data also indicate that the Penokean terrane is not entirely 1.8-b.y.-old, mantle-derived material, nor simply reworked Archean crust. It may represent a mixture of Archean and juvenile mantle material added during the 1.84 Ga Penokean orogeny. Patterns of continental growth deduced from this study emphasize the episodicity of crust formation. Construction of an apparent crustal growth curve for North America indicates that the average age of the continent is ∼2.2 b.y., which is significantly older than the average age of ∼ 1.5 b.y. derived from Rb-Sr studies.

Magnetotelluric Soundings in Ouachita Thrust Belt of Central Texas: ABSTRACT

A 25-station, 84-line-mi, remote referenced magnetotelluric (MT) traverse across a portion of central Texas has been recorded and analyzed. Among the geologic and geophysical elements crossed are the Ouachita foreland, the frontal and interior zones, and the rimming gravity maximum. MT signatures of the allochthonous Ouachita facies have been established with the aid of well control and are correlated along the traverse. Both conductive foreland facies and the underlying resistive Precambrian can be traced beneath the resistive frontal thrust zone, but become indeterminate along the central part of the traverse, owing to abrupt thinning of the Precambrian resistor. Well control suggests that this resistive basement is correlative with the Grenville-age granitic basement of the nearby Llano uplift. A thick conductive interval of metasedimentary(?) basement material underlies Paleozoic rocks along the central portion of the traverse. This conductive basement appears to dip steeply beneath the Llano-type resistive basement and subcrops on the foreland flank of a bas ment antiform coincident with the rimming gravity maximum. Still another resistive basement interval appears deep within the basement near the central portion of the traverse, and shallows abruptly to the southeast to form the core of the basement antiform coincident with the rimming gravity maximum. Stratigraphic relationships within the Precambrian basement suggest that the resistive basement, which cores the basement antiform, may be older than the conductive metasedimentary(?) and resistive Llano-type basement, and that the basement beneath the Ouachita trend is of North American affinity at least as far south and east as the rimming gravity maximum. The MT signature of the subthrust foreland facies is truncated on the crest of the basement antiform coincident with the rimming gravity maximum. The geometry of the truncation suggests that the frontal thrust zone may have detached from the age-equivalent foreland facies near the present crest of the basement uplift, and that the distance between the foreland facies truncation and the foreland facies-frontal thrust zone boundary may serve as a crude minimum estimate of frontal thrust zone translation (about 60 mi). End_of_Article - Last_Page 1428------------

Magnetotelluric Sounding Method as Applied in Ouachita Thrust Belt, Central Texas: A Case History: ABSTRACT

A 25-station magnetotelluric (MT) traverse across central Texas has been recorded and analyzed. The 84 line-mi (135 line-km), northwest-southeast-trending traverse begins 13 mi (21 km) southeast of Lampasas and ends 6 mi (10 km) north-northeast of Lexington, Texas. Geologic elements crossed are the Ouachita foreland facies, frontal and interior zones, and the rimming gravity high described by Flawn et al, Nicholas and Rozendal, and others. Interpretation of the MT data, supported by 1- and 2-dimensional modeling, borehole data, and previously published gravity data, suggests northwest transport on the order of 22 mi (35 km) for the allochthonous Ouachita frontal zone. A shallow resistive zone in the Precambrian and the overlying autochthonous Ouachita foreland facies is traced southeast 46 mi (74 km) from the sites nearest Lampasas, but become indeterminate north of Taylor, Texas. Data from the Shell 1 Purcell (Williamson County) suggest the End_Page 510------------------------------ shallow Precambrian resistor to be correlative with granites outcropping in the Llano uplift. While indeterminate southeast of Taylor, autochthonous Ouachita foreland facies is interpreted to overly Precambrian basement of similar resistivity at least as far southeast as the rimming gravity high. Several other resistive and conductive zones are indicated in the Precambrian basement, including a resistive zone which rises to the southeast from a depth of approximately 50,000 ft (15,240 m) near Georgetown to 10,000 ft (3,050 m) into the core of an antiformal basement structure coincident with the rimming gravity high. End_of_Article - Last_Page 511------------

Sources, Distribution, and Mixing of Late Pleistocene and Holocene Sands on the South Texas Continental Shelf

ABSTRACT Gross grain shape and mineralogical analyses were used to determine the source, distribution, and mixing of late Pleistocene and Holocene sands on the South Texas continental shelf. Two major gross grain-shape types are found to be present in varying proportions in the sands of this shelf. The first sand type is associated with sands which are relatively enriched in plutonic-metamorphic quartz, volcanic quartz, feldspar and igneous and metamorphic rock fragments, and thus is considered to represent first-cycle sands derived from the Llano Uplift and the silicic volcanic rocks of the Southwest U.S. The second sand type is associated with more quartzose sands rich in sedimentary quartz and sedimentary rock fragments, and is considered to represent multicyclic sands eroded from the Texas Coastal Plain and High Plains. Variations in the relative proportions of these sand types in the sands of the nearshore/inner shelf region reflect both the changing sources of the sand and their transport, mixing, and convergence by modern nearshore processes. Three distinct sedimentary petrologic provinces, corresponding to three provinces previously defined by heavy mineral analysis (Van Andel and Poole 1960), are defined herein on the basis of their relative content of first- and multicyclic sand. Modern sands of the first of these provinces, the Rio Grande, are derived from the drainage basin of the Rio Grande and characteristically contain high proportions of first-cycle sand; such sands are presently found in the nearshore/inner shelf region between the mouth of the Rio Grande and the 27°N latitude to the north of the mouth of the river. Modern sands of the second province, the Texas Coast, are derived largely from the drainage basins of the South Texas coastal plain rivers and characteristically contain high proportions of multicyclic sand; such sands are presently transported southwestward by longshore currents to the 27°N latitude, where they converge with the northward-transported sands of the Rio Grande Province. Modern sands of the third province, the Western Gulf, are derived from the drainage basins of the Colorado and Brazos rivers and characteristically contain high proportions of first-cycle sands', such sands are also transported by longshore currents towards the southwest, where they are gradually diluted by Texas Coast Province sands. Sands of these three provinces are also found in the relict late Pleistocene fluvial and deltaic sediments on the outer part of the South Texas shelf. Previous studies had considered the fluvial and deltaic sediments on the northern part of the shelf to be entirely derived from the Western Gulf Province; however, this study shows that the relict fluvial deposits of certain South Texas coastal plain rivers which occur on the northern part of the shelf should be assigned to the Texas Coast Province. In all, multicyclic sands constitute approximately 48% of the late Pleistocene and Holocene sand of the South Texas continental shelf.

Uranium Occurrence in Major Rock Types by Fission-Track Mapping: ABSTRACT

Microscopic occurrence of uranium has been determined in about 50 igneous rocks from various locations, and in a genetically unrelated sandstone from south Texas. Precambrian granites from the Llano uplift of central Texas contain from a few ppm uranium (considered normal) to over 100 ppm on a whole-rock basis. In granite, uranium is concentrated in: (1) accessory minerals including zircon, biotite, allanite, Fe-Ti oxides, and altered sphene, (2) along grain boundaries and in microfractures by precipitation from deuteric fluids, and (3) as point sources (small inclusions) in quartz and feldspars. Tertiary volcanic rocks from the Davis Mountains of west Texas include diverse rock types from basalt to rhyolite. Average uranium contents increase from 1 ppm in basalts to 7 ppm in rhyolites. Concentration occurs: (1) in iron-titanium-oxides, zircon, and rutile, (2) in the fine-grained groundmass as uniform and point-source concentrations, and (3) as late uranium in cavities associated with banded, silica-rich material. Uranium in ore-grade sandstone is concentrated to more than 3%. Specific occurrences include (1) leucoxene and/or anatase, (2) opaline and calcite cements, (3) mud clasts and altered volcanic rock fragments, and (4) in a few samples, as silt-size uranium- and molybdenum-rich spheres. Uranium content is quite low in pyrite, marcasite, and zeolites. End_of_Article - Last_Page 499------------

Rb-Sr Ages of Glauconite Recrystallization: Dating Times of Regional Emergence Above Sea Level

ABSTRACT Rb-Sr ages of glauconite pellets commonly fall short of the presumed age of deposition by 10-20%, according to current estimates of the geologic time scale. In part, the young ages are the result of preferential loss of exchangeable radiogenic Sr. Consequently, in pellets where tightly bound, nonexchangeable Rb and Sr have remained a closed chemical system since deposition, removal of the exchangeable cations with ammonium acetate (NH4OAc) can raise the apparent age to coincide with the expected depositional age. In other cases the age of pelletal glauconite remains younger than the depositional age after chemical treatment. Analyses of NH4OAc-treated, disaggregated clay-size fractions invariably yield young but rather consistent apparent ages. These are interpre ed as recording the time of diagenetic recrystallization of the glauconite crystallites which compose the pellets. Glauconite recrystallization appears to have taken place in the Llano Uplift of central Texas at 428, 350, 328, and 265-280 my ago. These ages do not coincide with times of elevated temperatures, deep burial, or tectonism. Instead, they correlate with times of regional emergence above sea level. Apparently a change in pore-water chemistry due to an influx of meteoric water promoted recrystallization, and this inference is supported by the oxygen isotope composition of the glauconite. Diagenesis of glauconite by this mechanism offers an opportunity to date sea-level changes by isotopic methods.

Tectonic Map of Texas--A Progress Report: ABSTRACT

The Bureau of Economic Geology is compiling a new Tectonic Map of Texas--a detailed, up-to-date display at 1:500,000 of surface and subsurface structural history for the entire state and adjoining areas of Mexico and the continental shelf and slope. This is the first comprehensive structural mapping of since E. H. Sellards' Structural Map of Texas some 40 years ago. A companion illustrated text is also being compiled to systematically describe and summarize the tectonic evolution of the state from Proterozoic to Holocene. Deformed Precambrian crust is exposed in the Llano uplift and in western Trans-Pecos Texas. It is subdivided by the tectonic setting of the sedimentary rocks and the timing of intrusion and deformation. The late Paleozoic basins and the intervening fault-bounded basement uplifts of west are shown by 200-m and 100-m contours on the top of Precambrian or the top of Ellenburger, depending on the nature of well control. The principal features of the buried Ouachita Overthrust belt are displayed, along with a more detailed rendition of the exposed Ouachita rocks in the Marathon region. The East Texas, Maverick, and Sabinas basins and the inner Gulf coastal plain are shown by contours on the Edwards Limestone and the Austin Chalk; features due to salt tectonism, growth faulting, and C rdilleran deformation are prominent. Seaward of the Cretaceous shelf margin, the growth-fault trends of the Gulf Coast are shown with contours on the Tertiary formations most affected. Available offshore data have been integrated to provide a picture of the shelf, slope, and a corner of the Sigsbee abyssal plain. In the multiply deformed Trans-Pecos region, the surface expressions of structures related to Laramide folding and thrusting, middle Tertiary volcanism, and Miocene to Holocene basin and range faulting are shown, in addition to the Precambrian and Ouachita-Marathon structures, where exposed. The final edition will be multicolor, with colors emphasizing subsurface contour horizons and depths, tectonic units in deformed or volcanic areas, salt domes, igneous bodies, faults, and axial traces of folds. Faults will be identified, where possible, by their age. Inset maps will include basement age and lithology, gravity, magnetics, and topography. The map and text will provide a valuable summary of structural geology and suggest new approaches to the search for energy resources. End_of_Article - Last_Page 458------------

Metabasalts and metagabbros from the Llano Uplift, Texas: Petrologic and geochemical characterization with emphasis on tectonic setting

The 1.0-1.2 b.y. old rocks of the southeastern Llano Uplift, Texas include a 7 km thick sequence of amphibolite-grade, stratified, mafic metasedimentary rock (Packsaddle Schist) which is intruded by a varied suite of syntectonic and late-kinematic intrusions. The metasediments contain large blocks of serpentinized peridotite (Coal Creek serpentinite) and coarse hornblendite and metagabbro. Prior to the end of maximum deformation, the sequence was intruded by low to medium-K2O tholeiitic basalts (0.40-0.72% K2O). Late-kinematic low-K2O tholeiitic basalts (0.38-0.40% K2O) were intruded as dikes into the folded rocks. The Coal Creek serpentinite contains both syntectonic and late-kinematic low-K2O tholeiitic metabasalts (0.13–0.36% K2O). The Llano metabasalts and metagabbros are characterized by low Cr (67–378 ppm) and Ni (36–170 ppm), variable Rb (1.5–14.7 ppm), Sr (140–1229 ppm), TiO2 (0.40–2.20%), P (568–2707 ppm), and Zr (18–230 ppm), and Y (16–45 ppm), Co (40–57 ppm), and Sc (36–49 ppm) similar to modern MORB. The metabasalts have La abundances from 7 to 39 times chondritic and exhibit light REE enrichment with ¦La/Sm¦N from 1.13 to 1.45 and ¦La/Yb¦N from 1.12 to 2.99. The metabasalts show a strong correlation of increasing Zr, Ti, and Y and decreasing Eu/Eu* (1.56–1.00), CaO/TiO2, Al2O3/TiO2, and MgO/ MgO+FeO* with increasing REE enrichment (LaN). The Llano metabasalts and metagabbros have initial 87Sr/86Sr=0.7029±0.0001. A likely petrogenetic model for these metabasalts is an island arc in which events from early arc development to final late-kinematic intrusion were dominated by tholeiitic volcanism and intrusion. The chemical systematics of the Llano mafic metaigneous rocks suggest they are products of fractionation of olivine, clinopyroxene, and plagioclase from more primitive basaltic magmas generated beneath the island arc complex.

Comment and Reply on ‘Coal Creek serpentinite, Llano Uplift, Texas: A fragment of an incomplete Precambrian ophiolite’

Research Article| August 01, 1982 Comment and Reply on ‘Coal Creek serpentinite, Llano Uplift, Texas: A fragment of an incomplete Precambrian ophiolite’: COMMENT M. D. Thomas M. D. Thomas 1Gravity and Geodynamics Division, Earth Physics Branch, Department of Energy, Mines and Resources, Ottawa, Ontario K1A 0Y3, Canada Search for other works by this author on: GSW Google Scholar Geology (1982) 10 (8): 440–441. https://doi.org/10.1130/0091-7613(1982)10<440:CAROCC>2.0.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 M. D. Thomas; Comment and Reply on ‘Coal Creek serpentinite, Llano Uplift, Texas: A fragment of an incomplete Precambrian ophiolite’: COMMENT. Geology 1982;; 10 (8): 440–441. doi: https://doi.org/10.1130/0091-7613(1982)10<440:CAROCC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract No Abstract Available. 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.

Comment and Reply on ‘Coal Creek serpentinite, Llano Uplift, Texas: A fragment of an incomplete Precambrian ophiolite’

Comment and Reply on ‘Coal Creek serpentinite, Llano Uplift, Texas: A fragment of an incomplete Precambrian ophiolite’

Rb-Sr Dating of Illite Diagenesis: ABSTRACT

In the Woodford Shale (Upper Devonian), apparent Rb-Sr ages decrease as clay grain size decreases, which in turn correlates with increasing abundance of diagenetic illite. Analyses of the fine-clay size fraction (< 0.2µ) from widely spaced wells in the Delaware basin of west Texas, plot on a single isochron indicating an age of 300 ± 4 m.y. (Middle Pennsylvanian). At this time the Woodford was buried only 200 to 600 m; consequently diagenesis must have been triggered by a circumstance other than deep burial. Possibly diagenesis was accomplished by hydrothermal fluids moving toward the craton out of the Ouachita geosyncline, which at that time was experiencing horizontal compression. These fluids may have been responsible for petroleum migration and lead-zinc ineralization. In the Frio Formation (Oligocene) of the Texas Gulf Coast, samples of fine clay-size material (< 0.06µ) from the 3 to 5 km depth interval in a single well also provide a well-defined isochron corresponding to 21.6 ± 2.2 m.y. Burial here was possibly so rapid that transformation of smectite to illite approximated an episodic event over the entire depth interval. Alternatively, because the sediment is geopressured, the age might record the time of geopressure development which was accompanied by a rapid rise in temperature. Clay diagenesis at the surface is illustrated by a paleosoil developed on Pennsylvanian shale in the Llano uplift of central Texas. The paleosoil was buried by Cretaceous basal conglomerate and records the time of marine transgression 119 ± 3 m.y. ago. Constituents of the shale were degraded by soil-forming processes which erased previous isotopic memory, then reconstituted by coming in contact with marine water. This field relation offers a new way to date directly a time of sedimentary deposition. End_of_Article - Last_Page 610------------

Facies Analysis of Cretaceous Hensel Formation--Response of a Fluvial System to Marine Transgression: ABSTRACT

The Lower Cretaceous Hensel Formation in central and north-central Texas consists of predominantly terrestrial End_Page 1434------------------------------ deposits. Study of Hensel outcrops in Kimble, Gillespie, and Blanco Counties, south of the Llano uplift, reveals a facies evolution that is associated with a major marine transgression. This evolution is expressed in both a north-south facies tract and in the overall stratigraphic succession. Four major depositional systems are recognized. (1) Basal, valley-fill deposits are limited to localities proximal to the source area. (2) Low-sinuosity, bedload channel-facies overlie the valley fill and are widely distributed. (3) Low-sinuosity channel-facies evolved into more distal, somewhat finer grained, coastal-plain fluvial systems. (4) Both the fluvial and coastal-plain facies are characterized by extensive flood-basin muds and small, ephemeral arroyos. The pervasive development of calcr te, or caliche, within the overbank deposits, and the overall depositional style of the unit imply a semiarid, seasonal climate. As the marine transgression progressed, the sediment supply decreased and channel gradients lowered. Deposition of the shallow marine or lagoonal carbonates that overlie the Hensel resulted from the final inundation of the source area. End_of_Article - Last_Page 1435------------

Stress orientations from oil-well fractures in Alberta and Texas

Many oil wells in Alberta exhibit spalling of the walls (known as breakouts), which elongates the holes with the longer axis aligned northwest–southeast. This alignment is observed over an area in excess of 4 × 105 km2, in siltstones, sandstones, carbonate sediments, and one shale formation, through the stratigraphic column from Devonian to Cretaceous. It is unrelated to dip of the beds. We have elsewhere suggested that these breakouts are produced through stress concentration near the hole walls, in a stress field having large, unequal horizontal principal stresses and with the larger compression oriented northeast–southwest. It is probable that this northeast–southwest principal stress is σ1. This paper adds new data from oil wells in Alberta and northern British Columbia and shows that the breakouts, and by inference the stress orientations, are consistent through much of the western Canadian sedimentary basin. It also uses evidence from hydraulic fracturing in west-central Alberta and from steam-injection fracturing in eastern Alberta to support the view that σ1 is aligned northeast–southwest. The breakouts are consistent with either a thrust stress field or a strike–slip stress field, but the fractures formed by excess pressures in wells favour the latter. Recently Schafer has reported oil wells systematically elongated with a mean azimuth of N39°E in the Austin Chalk of southern Texas. We regard these elongations as formed by breakouts in a stress field with greater horizontal stress near N51°W, and this view is supported by overcoring measurements by Hooker and Johnson in granite of the Llano Uplift, 225 km from Schafer's wells, that reveal large horizontal compressions at the surface with the larger oriented N33°W.

Coal Creek serpentinite, Llano Uplift, Texas: A fragment of an incomplete Precambrian ophiolite

The Precambrian Coal Creek serpentinite, Llano Uplift, Texas, occurs within the upper part of the Packsaddle Schist, which appears to represent a thick sequence of shelf-edge volcaniclastic arc-flank metasediments. The serpentinite has a foliation defined by lizardite pseudomorphs after original foliated harzburgite tectonite, which deviates as much as 15° from the regional country-rock foliation. A few samples contain the relict assemblage olivine + orthopyroxene + anthophyllite reflecting re-equilibration to regional metamorphic conditions near 710 °C at ∼3.5 kb. Oxygen and deuterium isotopic data indicate the lizardite could have formed in equilibrium with magmatic water at about 300 to 400 °C during regional uplift. A likely petrogenetic model involves the olistostromal emplacement of a fragment of ophiolitic material, the Coal Creek ultramafic body, into the volcaniclastic arc-flank sediments. The island-arc model suggested for the Llano Uplift and the existence of ophiolitic material imply that brittle plate collisions were locally important 1,200 m.y. ago and that the orogenic event that affected the Llano terrane (that is, the Grenvillian orogeny) involved island-arc-continent interactions. The Llano Uplift contains the only exposed ophiolitic material along the entire “Grenvillian” orogenic belt.

Metamorphic Terrane Favored Over Rocky Mountains as Source of Claiborne Group, Eocene, Texas Coastal Plain

ABSTRACT The mineralogy of the lower Claiborne Group, Eocene, Texas Coastal Plain, is not similar to the mineralogy of updip time-equivalent sandstones at the foothills of the southern Rocky Mountains--the area from which they were most likely eroded, according to traditional views based on paleocurrent information. Numerous studies conclude that the Carrizo Formation and Newby Member of the Reklaw Formation in Texas were derived from three primary source types: 1) low-rank, pelitic metamorphic rocks; 2) high-rank, pelitic metamorphic rocks; and 3) plutonic rocks. Kyanite and staurolite are the most diagnostic heavy minerals found in these Texas sands. As seen in lower Eocene sandstones at the foothills of the southern Rocky Mountains, south-central Colorado, this part of the Laramide range su plied sediments rich in granitic and gneissic debris, with epidote and garnet being the two distinctive, associated heavy minerals. Source areas which could have supplied the metamorphic minerals to the lower Claiborne sandstones in Texas are the southern Rocky Mountains of New Mexico, the Llano Uplift, the Appalachian Mountains, or the Ouachita Uplift. There are difficulties with advancing each of these source areas. In terms of depositional considerations, the Ouachita Mountains are the best choice; however, selecting the Ouachitas as a provenance necessitates proposing an area of higher grade metamorphism than has previously been described.

Fluid-Inclusion Temperature Study of Paleozoic Carbonates, Llano Uplift, Texas: ABSTRACT

End_Page 1690------------------------------Homogenization temperatures of two-phase fluid inclusions in cements, recrystallized allochems, overgrowths, and vein and cavity fillings show that Paleozoic carbonates of the Llano uplift have experienced at least three phases of temperature-related, pervasive crystallization-recrystallization. These phases have modes at 70 to 80°C, 110 to 120°C, and 200 to 210°C, with ranges of 60°C, 90°C, and 60°C, respectively. These phases may represent distinct heating-cooling events, or they may be the results of a single, thermally complex event. The lack of any discernible regional trends in the data and the fact that Cretaceous rocks locally have been heated to at least 110°C suggest that heating was not related to regional metamorphism during the Ouachita orogeny. Yet, these temperatures seem too high to result merely from heating due to depth of burial associated with any reasonable geothermal gradient. We postulate that northeast-trending faults, which cut basement and overlying Paleozoic rocks in the region, provided avenues for pervasive heating by contemporaneous fluids from the lower crust. These faults, like those of the Balcones-Luling and Talco-Mexia zones to the east, may be genetically related to late Paleozoic-early Mesozoic opening of the Gulf of Mexico. Waning geothermal activity associated with this rifting would explain the localized heating of Cretaceous rocks. End_of_Article - Last_Page 1691------------

Cambrian paleomagnetism of the Llano Uplift, Texas

Late Cambrian sandstones and limestones sampled from various members of the Riley and Wilberns formations of the Llano uplift show a progression of paleomagnetic pole positions as a function of age. The members, ages, and poles are the following for the Riley Formation: the Hickory Sandstone, Lower Dresbachian, 34°N, 145°E; the Cap Mountain Limestone, Dresbachian, 33°N, 140°E; the Lion Mountain Limestone, Upper Dresbachian, 24°N, 146°E. For, the Wilberns Formation they are the following: the Welge Sandstone/Morgan Creek Limestone, Lower and Middle Franconian, 24°N, 151°E; and the Point peak Shale, Upper Franconian, 6°N, 159°E. These poles are based on thermal, chemical, and alternating field demagnetizations and on vector analysis. Most directions are interpreted to be of reversed polarity, but the Cap Mountain Limestone also yielded normal polarity directions; all directions resided in hematite with blocking temperatures up to 680°C. Almost all Cambrian poles appear to fall in a broad streak between the equator at about 155°E (e.g., the poles from the Tapeats Sandstone and from the Late Cambrian Point Peak Shale of the Wilberns Formation) and about 60°N, 90°E, where Cambrian poles have been obtained by Elston and Bressler (1977) and French et al. (1977). Although, at the present time, partial to complete remagnetization or nondipole behavior of the geomagnetic field are adequate ad hoc hypotheses to explain some of the data, it is suggested that the simplest and most unifying hypothesis to explain all data involves a Cambrian loop of apparent polar wander with respect to North America. This loop occurs before the middle Late Cambrian, with the poles from the Llano uplift falling on the return track.

Rb-Sr dating of Paleozoic glauconite from the Llano region, central Texas

Glauconite from eight stratigraphic horizons (Cambrian to Pennsylvanian) in the Llano Uplift, central Texas and two Cretaceous glauconites were analyzed by the Rb-Sr method. Only two untreated samples provide ages in agreement with those anticipated from current best estimates of the geologic time scale. With one exception all the other apparent ages fall short of the estimated age of deposition by as much as 22%. Low ages, the pattern customarily observed, are attributed to postdepositional loss of radiogenic 87Sr from expandable layers by weathering or during diagenesis. Detailed leaching experiments using a variety of reagents were performed to characterize the behavior of glauconite. The most promising treatment, which we recommend as standard procedure in all future studies, is with ammonium acetate which is able to purge the mineral of loosely-bound Rb and Sr while leaving tightly-bound components intact. After appropriate leach, three other Rb-Sr ages were brought into coincidence with their estimated ages of deposition. In contrast an Upper Cambrian glauconite was found to be extremely resistant to further alteration by chemical attack′, providing an age of 429 ± 17 M yr. Although 17% short of the age of deposition, this age is interpreted as the time of a real event: diagenetic recrystallization induced by burial. Comparison of data from four samples indicates that for Paleozoic glauconite, conditions exist in which the Rb-Sr system is less susceptible to mild disturbance than is the K-Ar system.

Rb-Sr and K-Ar geochronologic and isotopic studies, Llano Uplift, central Texas

Two major episodes are evident in the metamorphic and igneous Precambrian basement of the Llano Uplift, central Texas. Dynamothermal metamorphism was accompanied by minor basaltic and tonalitic syntectonic plutonism. This was followed by a second period of thermal overprinting accompanying emplacement of high-K2O, high-level major granite plutons. Extensive isotopic age work by Zartman, published in the mid-1960s, suggests that development of the basement complex, spanning an interval of 150 m.y. or more, began with deposition of Valley Spring Gneiss (the lowest unit) and terminated about 1,050 m.y. ago with final postmetamorphic cooling (indicated by retention ages of Ar and Sr in biotite). We have supplemented these data with more than 50 new K-Ar and Rb-Sr analyses. Two foliated plutons in the southeast are 1,167±12m.y. (2σ) old, with distinctly different initial 87Sr/86Sr ratios. Field relationships and isotopic data indicate that these plutons are the earliest yet known in the Uplift. Metamorphosed basalt dikes and gabbro bodies were emplaced immediately preceding and following the syntectonic plutons. Eleven of these rocks had extremely uniform initial 87Sr/ 86Sr=0.7029±0.0005. A Rb-Sr whole-rock isochron of the unfoliated Enchanted Rock pluton indicates an age of 1,048±34 m.y. with initial 87Sr/86Sr= 0.7048±0.0007. One of the northern unfoliated granites, the Lone Grove pluton, gives a whole-rock isochron age of 1,056±12 m.y., with initial 87Sr/86Sr = 0.7061±0.0003. All of the intrusive rocks have initial 87Sr/86Sr ratios consistent with a source in the mantle or lower crust, but not in ancient remobilized continental crust. Six K-Ar hornblende ages from metabasalts are 1,078±19 m.y. (1σ), in general agreement with K-Ar and Rb-Sr mineral ages elsewhere in the eastern Llano Uplift. A metasedimentary Valley Spring Gneiss sample from the western Uplift has a whole rock-muscovite Rb-Sr age of 1,129±9 m.y. Field and isotopic data are now sufficiently numerous to permit a moderately detailed reconstruction of the Precambrian history of the area.