Big Bend Region Research Articles

Response of the West Florida Shelf circulation to climatological wind stress forcing

The response of the west Florida continental shelf circulation to monthly mean climatological wind forcing is investigated by using the three‐dimensional, primitive equation Princeton Ocean Model. Two basic seasonal patterns of circulation and sea surface elevation occur under a barotropic setting: a winter pattern from October to March and a summer pattern from April to September. An interesting finding in winter is an anticyclonic gyre over the northeastern (Florida Big Bend) region that merges with a northwestward flow from the south. The Big Bend Gyre is caused by a convergence of two separate flows: a nearshore along‐shelf southeastward flow and an offshore northwestward flow. Winter circulation characteristics also include offshore surface transport, coastal upwelling, and relatively low coastal sea level. The summer pattern features a continuous northwestward directed flow, onshore surface transport, coastal downwelling, and relatively high coastal sea level. Transitions between the two seasonal patterns show either a development or relaxation of the Big Bend Gyre. These three‐dimensional circulation patterns result from wind‐driven Ekman transports and their resulting surface‐slope‐induced geostrophic flows. Qualitative agreement exists in some respects with recent in situ observations, historical drift bottle retrievals, and tide gauge data. However, climatological wind stress forcing alone does not account for the seasonally varying southeastward currents observed at midshelf. Thus it may be concluded that for depths less than 50 m the seasonal winds may play a dominant role in the seasonal variation of the shelf circulation; and beyond this limit seasonal density related effect must also be a factor in the seasonally varying circulation on the West Florida Shelf.

Abstract: Depositional History and Sequence Stratigraphic Framework of Middle and Upper Albian Rocks In the Big Bend Region of West Texas and Mexico

Abstract: Depositional History and Sequence Stratigraphic Framework of Middle and Upper Albian Rocks In the Big Bend Region of West Texas and Mexico

Tectonic history of the metamorphic basement rocks of the Sierra del Carmen, Coahuila, Mexico

The Ouachita interior zone, arching through central Texas into the Big Bend region of the Rio Grande, has been defined chiefly on the basis of well samples and its associated gravity high. The only known exposure of rocks of the interior zone lies at the base of the Sierra del Carmen normal fault escarpment under approximately 1100 m of Lower Cretaceous carbonates in northern Coahuila, Mexico. These lower greenschist facies metamorphic rocks are thinly interlayered graphitic muscovite schist, graphitic marble, and minor quartzite with abundant quartz veins. The dominant structures shown by the metamorphic rocks are isoclinal folds (F2) that have a well-defined axial planar foliation (S2). Earlier (F1) folds refolded by F2 are locally preserved. At the microscopic scale, S2 is a crenulation cleavage of the S1 foliation that has been enhanced by pressure solution. Feldspar porphyroblasts overgrow a folded graphitic S1 foliation. Throughout the area a second crenulation cleavage (S3) is present on the S2 foliation surface as an intersection lineation (L3). Subsequent Laramide regionalto outcrop-scale folds and associated thrusts overprint these ductile structures. Lower Cretaceous carbonates form a large, east-verging overturned to recumbent anticline-syncline fold pair with a northwest-trending fold axis. The S2 metamorphic foliations define an antiform that is nearly coincident in trend to that of the fold in the overlying carbonates, indicating that the metamorphic rocks were also folded by this regional structure. A Basin and Range normal fault is interpreted to have reactivated the Laramide thrust fault responsible for the contractional deformation. One to two normal faults cut the carbonates and metamorphic rocks and truncate the anticline-syncline fold pair. The maximum offset along the normal fault is 1600 m. Rb/Sr data from the muscovite schist gives a six-point, mineralÐwhole-rock isochron age of 277 ± 10 Ma, indicating that polyphase deformation and metamorphism occurred during the late Paleozoic Ouachita orogeny. An initial Sr ratio of 0.721 suggests an ancient source for these metasediments. Common Pb ratios of the metamorphic whole rock and galena from the Puerto Rico Mine are isotopically distinct from those of the North American craton, suggesting that they were derived from a nonÐNorth American source and thus may be equivalent to unmetamorphosed Ouachita flysch sediments. This indicates that the metamorphic rocks of the Ouachita interior zone were derived from an ancient allochthonous source that was deformed and metamorphosed during the late Paleozoic Ouachita orogeny.

On the wintertime currents in the Florida Big Bend region

Moored current meter records and hydrographic sections obtained during the winter over 3 years on the inner shelf in the northeastern Gulf of Mexico indicate the following. The along-isobath flow component is primarily in the upper water column and directed towards the Florida Peninsula (eastward). Near the bottom the flow is primarily and consistently diabathic and shoreward. The eastward upper flow does not appear to be locally driven, but results from an offshore pressure gradient, the cause of which is not identified. The shoreward, near-bottom flow appears to result from the local offshore wind and from estuarine processes resulting from the freshwater input of rivers and springs. The former appears to dominate in the late fall and early winter, and the latter to take over in late winter or early spring, when the wind abates or is from the east. The hydrographic sections indicate that vertically mixed conditions resulting from passing cold fronts do occur, but are relatively short lived and that estuarine hydrographic conditions tend to arise in the intervals between the passages of cold fronts. The proximity of the inertial period to the diurnal tidal periods (the latitude of our moored measurements ≈ 30°N) did not result in the observation of pronounced inertial oscillations. It did result in reversal of the sense of rotation of the K 1 and O 1 tidal ellipses as the bottom was approached. The latter is explained in terms of concepts and a model used in Weatherly et al. (1980)( Journal of Physical Oceanography 10, 297–300). © 1997 Elsevier Science Ltd

Latest Cretaceous (Maastrichtian) aridity indicated by paleosols in the McRae Formation, south-central New Mexico

The McRae Formation in south-central New Mexico contains paleosols that indicate a significant increase in aridity occurred during the latest Cretaceous (late Maastrichtian). The McRae Formation is approximately 420 m thick and consists of interbedded fluvial channel sandstone and conglomerate, floodplain mudstone and fine sandstone, and siliceous ash-fall tuff. It has been divided into a lower Jose Creek Member and an upper Hall Lake Member. A Lancian dinosaur fauna is present in both members, indicating a late Maastrichtian age for the McRae Formation. Jose Creek paleosols are classified as argillisols and probably formed under humid to subhumid conditions. Paleosols in the Jose Creek member consist of A, ±E, and Bt horizons and are associated with numerous in situ petrified stumps, the largest of which is 1.7 m in diameter. The argillic B horizons (Bt) contain vertical clay-filled root traces, rhizoliths, blocky peds, embedded-grain argillans, and ped argillans. In contrast, paleosols in the Hall Lake Member are calcisols and vertic calcisols. These paleosols are characterized by Bw, Bt, Btk, Bk, and K horizons. The calcic horizons contain vertical calcite-filled tubules and carbonate nodules, in addition to calcans, spar coats, brecciated zones, alveolar-septal fabric and gypsum pseudomorphs. The vertic calcisols contain wedge-shaped peds and slickensides. The presence of calcic horizons in the Hall Lake Member suggest that the paleoclimate was significantly drier than during deposition of the Jose Creek Member. In addition, the vertic features indicate periods of wetting and desiccation (seasonality of precipitation?) during Hall Lake Time. Late Maastrichtian aridity, based on paleosols, has been interpreted for the Big Bend region of Texas, central Utah, and for Alberta, Canada, suggesting that the increase in aridity found in south-central New Mexico occurred throughout the Western Interior of North America.

Paleomagnetism and 40Ar/39Ar geochronology of gabbro sills at Mariscal Mountain anticline, southern Big Bend National Park, Texas: Implications for the timing of Laramide tectonism and vertical axis rotations in the southern Cordilleran orogenic belt

Mariscal Mountain anticline is a major Laramide asymmetric fold located along the frontal margin of the Cordilleran orogenic belt in the Trans‐Pecos region of southwest Texas. Exposed within the limbs and nose of the anticline are concordant mafic and felsic intrusions whose emplacement, based largely on their arcuate outcrop geometry, has been interpreted to predate folding. Paleomagnetic fold tests on samples from large (≥ 30 m thick) and thin (≤ l m thick) gabbro sills are negative at the 99% confidence level. Baked sandstones adjacent to the thick sill give sample directions identical to those of the gabbro, whereas sample directions from a site well removed from thermal effects of the gabbro are scattered. Results of the contact tests are consistent with a thermoremanent origin for the gabbro sill magnetization. The negative fold test indicates that sill intrusion at Mariscal Mountain anticline postdated Laramide folding. Two 40Ar/39Ar analyses of plagioclase from the gabbro give discordant age spectra but yield an inverse correlation age of 37.0 ± 1.3 Ma (2σ). This date is similar to those from rocks of similar composition elsewhere in Trans‐Pecos Texas, but significantly younger than whole rock and pyroxene K‐Ar dates of Late Cretaceous and early Eocene age commonly cited for the Mariscal Mountain gabbro. The paleomagnetic and geochronologic data indicate that Laramide deformation had clearly ceased by late Eocene/early Oligocene time, consistent with absence of contractional deformation in middle Eocene to lower Oligocene volcanic and sedimentary rocks elsewhere in the region. Although paleomagnetic data from the gabbro sills may not adequately average secular variation, the in situ group mean direction (D = 348.6°, I = 50.9°, k = 65.1, α95 = 4.3°, n = 18 sites) is indistinguishable from expected mid‐Tertiary reference directions. This indicates no evidence of significant vertical axis rotation in the Mariscal Mountain area. Analysis of paleomagnetic data from studies elsewhere in the Big Bend region indicates that significant problems exist with most data sets used to argue for the existence of local vertical axis rotations. Thus we conclude that existing paleomagnetic evidence regarding the timing and magnitude of rotations associated with Basin and Range extension is equivocal.

Javelinoxylon, an Upper Cretaceous dicotyledonous tree from Big Bend National Park, Texas, with presumed malvalean affinities

Well‐preserved petrified woods from the Maastrichtian of Big Bend National Park, Texas, with a combination of characters seen in the Malvales are described as Javelinoxylon multiporosum gen. et sp. nov. One log is over 70 cm in diameter and is riddled with termite galleries; this specimen provides additional documentation for the occurrence of dicotyledonous trees in the Late Cretaceous. These woods have structure different from any other known Cretaceous dicotyledonous wood and are advanced in the Baileyan sense as vessel elements are relatively short, perforations are exclusively simple, vessels are commonly in long radial multiples, and rays are storied. This is the earliest record for woods with storied structure. The occurrence of large dicotyledonous trees with advanced wood structure in Big Bend suggests there were differences between the Late Cretaceous vegetation of the northern Rocky Mountains and the Big Bend region.

A Laramide age push-up block: The structures and formation of the Terlingua-Solitario structural block, Big Bend region, Texas: Discussion and reply

Research Article| April 01, 1994 A Laramide age push-up block: The structures and formation of the Terlingua-Solitario structural block, Big Bend region, Texas: Discussion and reply CHARLES E. CORRY; CHARLES E. CORRY 1Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 Search for other works by this author on: GSW Google Scholar JAMES B. STEVENS; JAMES B. STEVENS 2Department of Geology, Lamar University, Beaumont, Texas 77710 Search for other works by this author on: GSW Google Scholar EUGENE HERRIN; EUGENE HERRIN 3Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75222 Search for other works by this author on: GSW Google Scholar RICHARD J. ERDLAC, JR.; RICHARD J. ERDLAC, JR. 4The University of Texas of the Permian Basin, 4901 E. University, Odessa, Texas 79762-0001 Search for other works by this author on: GSW Google Scholar CHRISTOPHER D. HENRY CHRISTOPHER D. HENRY 5Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas 78713-7508 Search for other works by this author on: GSW Google Scholar Author and Article Information CHARLES E. CORRY 1Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543 JAMES B. STEVENS 2Department of Geology, Lamar University, Beaumont, Texas 77710 EUGENE HERRIN 3Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75222 RICHARD J. ERDLAC, JR. 4The University of Texas of the Permian Basin, 4901 E. University, Odessa, Texas 79762-0001 CHRISTOPHER D. HENRY 5Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas 78713-7508 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1994) 106 (4): 553–559. https://doi.org/10.1130/0016-7606(1994)106<0553:ALAPUB>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 CHARLES E. CORRY, JAMES B. STEVENS, EUGENE HERRIN, RICHARD J. ERDLAC, CHRISTOPHER D. HENRY; A Laramide age push-up block: The structures and formation of the Terlingua-Solitario structural block, Big Bend region, Texas: Discussion and reply. GSA Bulletin 1994;; 106 (4): 553–559. doi: https://doi.org/10.1130/0016-7606(1994)106<0553:ALAPUB>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 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.

A mathematical model of push‐up block formation: An example from the Big Bend Region, Texas

Laramide age development of the Terlingua‐Solitario push‐up structure was investigated by field mapping, experimental study, and theoretical analysis. Field mapping suggested a four‐stage development involving Laramide age compression followed by Basin and Range age tension. The push‐up structure is composed of Cretaceous limestones, shales, and marls. Thick competent units of limestone (rigid plates) are imbedded within thick formations of thinner bedded incompetent shale and marl. Deformation occurred when the tectonic load rose to sufficient magnitude to overcome the strength of the competent units. The Santa Elena Limestone was investigated experimentally to determine its burial depth and mechanical properties. The critical buckling stress and the state of stress were then determined within the Santa Elena, using field mapping to establish “boundary conditions” for the theoretical work. Plate bending theory, linear elasticity, and fracture mechanics were used to describe incipient deformation of the push‐up structure. The critical stress required to initiate cylindrical bending was calculated at 7.9 MPa (79 bars) and reflects the regional Laramide tectonic stress acting on the outer edge of the Santa Elena plate. The state of stress within the plate was determined by linear elasticity theory combined with Laramide tectonic strain measurements. Compressive principal stresses of 209.2 MPa (2092 bars), 40.8 MPa (408 bars), and 18.6 MPa (186 bars) were calculated and are much higher in magnitude than the critical stress. However, near preexisting flaws (i.e., pore space, microcracks), tensile stresses of several hundred bars are required by fracture mechanics. This local tensile loading caused small cracks to propagate and eventually to coalesce to form joints and fractures, many of which later developed into strike‐slip and dip‐slip faults parallel tp Laramide σ1.

Fossil insect evidence for late Quaternary climatic change in the Big Bend region, Chihuahuan Desert, Texas

A series of 50 packrat midden assemblages from the Big Bend region of the Chihuahuan Desert, ranging in age from >36,000 yr B.P. to recent, yielded abundant, diverse arthropod faunas. The mesic nature of regional Wisconsin age climates is substantiated by the fauna from 30,000–12,000 yr B.P., especially during the middle Wisconsin (30,000–20,000 yr B.P.). Late Wisconsin faunas contained grassland species which are confined today to cooler, moister regions. Following 12,000 yr B.P., most of these temperate species were replaced either by desert species or by more cosmopolitan taxa, marking the climatic shift from late Wisconsin to postglacial time. Insects indicative of more severe aridity are first recorded at about 6000 yr B.P., but some temperate species persisted until about 2500 yr B.P. After this, only desert dwellers are recorded.

A Laramide-age push-up block: The structures and formation of the Terlingua-Solitario structural block, Big Bend region, Texas

The Terlingua-Solitario structural block represents a Laramide-age push-up block in the Big Bend region of Texas. Folding, thrust faults, monoclines, east-northeast-trending strike-slip faults and grabens, and tectonic stylolites are associated with push-up block development. Laramide-age east-northeast-directed compression caused sinistral movement on two east-trending, right-stepping, en echelon fault zones. This fault movement placed the area between fault overlap under transpression, which then responded by broad uplift on a northwest-trending fold axis. Local stratigraphic and structural controls suggest uplift initiation between 68-66 Ma with the completion of push-up block formation by about 50 Ma. This push-up block exists within a zone of transfer for Laramide sinistral faulting along the northern and southern boundary of the Texas Lineament zone. Laramide stress trajectories across the uplift as defined by tectonic stylolites indicate a single compressive event, but with stress, trajectory changes locally at developing structures. More recently, during Basin and Range age deformation (24 Ma), dextral movement along the two en echelon fault zones placed the structural block under transtension. Northwest-trending faults and grabens now cut through the eastern half of the push-up block and die out along the southern boundary of the uplift. Local dextral movement was also identified along some of these northwest-trending faults.

Lateral and Temporal Changes in Volcanogenic Sedimentation; Analysis of Two Eocene Sedimentary Aprons, Big Bend Region, Texas

ABSTRACT Middle to late Eocene volcanic activity in the Big Bend region of Texas induced dramatic changes in local depositional processes recorded by volcanogenic sequences in the Chisos, Canoe, and Devil's Graveyard formations. Analysis of lithofacies associations, clast lithologies, and paleocurrent trends of tuffaceous strata in these sequences allows recognition of two sedimentary aprons derived from two widely spaced volcanic centers; a southern apron shed from a center in Mexico, and an apron shed from the Christmas Mountains volcanic center. Each volcanogenic apron contains distinctive vertical sequences that reflect in different ways episodic explosive activity at the volcanic source area, and also distinctive records of lateral changes in sedimentation that correspond to distance from the volcanic source area. The southern apron contains intermediate-source deposits that are the product of alternating modes of deposition that correspond to aggradation during explosive volcanic activity as well as aggradation during volcanic quiescence. Mudflow and hyperconcentrated flood-flow deposition occurred during times of explosive activity, and fluvial deposition and pedogenesis occurred during volcanic quiescence. Stacked beds of lithic-clast deficient mudflow deposits are the result of rapid aggradation of intrabasinal-derived flows on a topography inundated with tephra. Distal-source deposits do not show evidence for alternating modes of deposition but instead were deposited almost entirely by traction and suspension processes in a fluvial system, and subsequently modified by pedogenesis. Eruptions associated with the Christmas Mountains caldera complex at 42 Ma produced abundant lava and pyroclastic deposits that are interstratified with sedimentary sequences. Proximal-source exposures within 8 km of the calderas consist mostly of ashflow and air-fall tuffs, and lava and debris flow deposits, occurring as five distinctive sequences that reflect the episodic eruptive history at this center. Intermediate-source exposures from 18 to 23 km are predominantly composed of interbedded mudflow and hyperconcentrated flood-flow deposits that are separated by disconformities. Alternating aggradation and degradation apparently resulted from drastically fluctuating sediment loads originating from the episodically active volcanic source. At distal-source exposures, 30 to 35 km from he caldera, there are no apparent disconformities in the apron. Initial aggradation was dominated by mudflows and pumice-fall, followed by traction deposition in sheet-floods and channels. Evidence independent of the volcanogenic sequences indicates that each apron aggraded under roughly the same tectonic and climatic influences. Hence, this study shows that volcanogenic sequences can record the explosive activity of a source center in 2 different ways in the same climatic and tectonic conditions. In addition, this study shows that this record of explosive activity may not be recorded at distal parts of the aprons.

Paleosols and the Cretaceous/Tertiary transition in the Big Bend region of Texas

A marked change in paleosols coincides with the Cretaceous/Tertiary transition in fluvial sediments of the Big Bend region in Texas. Early Paleocene paleosols exhibit thick, black epipedons and a greater depth to the argillic and petrocalcic horizons compared to Late Cretaceous paleosols. These features and comparison with modern soils suggest that early Paleocene soils developed under conditions of higher rainfall and cooler temperatures than did Late Cretaceous soils. The change in paleosols occurs abruptly at the highest occurrence of dinosaur bones in the section.

Geochemistry of tertiary alkaline rocks of the Eastern Trans-Pecos Magmatic Province, Texas

The Trans-Pecos Magmatic Province (TPMP) is an alkalic field that was active between 48-17 Ma. Rocks of two subprovinces in the eastern alkalic belt of the TPMP, the Big Bend region and the Davis Mountains, have been analyzed for major and trace element concentrations in order to determine what magmatic processes operated to influence the compositional evolution of the magmas, and to explore what relationship existed between the silica-oversaturated evolved rocks and the silica-undersaturated mafic rocks. Similar compositional trends exist in both subprovinces, implying that the evolved rocks are genetically related to the mafic rocks, and that the differentiation processes were broadly reproducible spatially and temporally.

Transgressive-Regressive Cycles and Environments of Coal Deposition in Upper Cretaceous Strata of Trans-Pecos Texas: ABSTRACT

ABSTRACT Paralic deposits of Late Cretaceous age are represented in Trans-Pecos Texas by the Aguja Formation in the Big Bend region and the correlative San Carlos Formation in the Sierra Vieja region. Although both units are broadly progradational, each records a single widespread transgressive pulse. This transgressive event began in Early Campanian time, and reached a peak during the middle Campanian. Regressive (progradational) deposits consist of prodeltaic shale and delta front siltstone and claystone, overlain by coalesced lenticular fluvial-dominated deltaic sand bodies. Trangressive deposits consist of local storm-dominated shoal sandstone, thin laterally extensive oyster-rich sublittoral sand sheets, and shelf mud. Transgression occurred relatively rapidly and was expressed differently depending on the distance traversed landward of the previous shoreline. Coal and lignite deposition occurred in restricted interdistributary areas during progradation, and in more widespread areas during peak transgression when the strandline was neither advancing nor retreating. There is no evidence for barrier island development during transgressive events; hence, the coal and lignite accumulated along a muddy non-barred coast in marshes directly adjacent to the sea. The coal-bearing strata consist of an alternating series of dark gray root-mottled carbonaceous claystone with sideritic ironstone concretions, coal or lignite, and well-laminated light gray pyritic shale with calcareous concretions bearing marine invertebrates. The cyclic alternation of these facies suggests the repeated inundation of coastal marshes by marine waters, followed by mud accumulation, subsequent regrowth of the marsh, and the deposition of lignite or coal.

Great earthquakes and great asperities, San Andreas fault, southern California

The big bend region of the southern San Andreas fault consists of two great asperities that rupture infrequently in great earthquakes. The eastern knot near San Gorgonio Pass, which has not ruptured historically in a large event, is the main locus of plate motion, appears to break in great events every few hundred years, and is more advanced in the cycle of strain accumulation than the western knot. The large size of these asperities results in the properties of these earthquakes being nearly invariant over many cycles of great shocks. An unusual sequence of moderate-size shocks within the eastern knot from 1940 to 1948 is an example of the type of precursory phenomenon that might precede a large earthquake.

Late Cenozoic Pull-Apart Graben Development, Big Bend Region, Texas: ABSTRACT

The Big Bend region is a giant rhomb-shaped structure with the Presidio (west) and Black Gap (east) pull-apart grabens marking the north-northwest-trending ends and the north and south branches of the Texas lineament (TL) marking its west-northwest-trending strike-slip ends. This large rhomb is broken internally by many faults (west, west-northwest, and north-northwest trends) that have generated numerous small to large pull-apart grabens. Black Gap graben is divided into segments by west-northwest faults that drop each segment deeper (250-1,050 m, 820-3,450 ft, structural relief) and southeastward in the United States part. The continuation into Mexico has not been studied. Presidio graben also has a complex bounding fault pattern of north-northwest-, west-northwest-, and west-trending segments. Depth of the graben in unknown, although outcrop and well data give a minimum of 800 m (2,600 ft). The internal shape is poorly known because of widespread pediment gravel cover. The Presidio and Black Gap grabens are the southeastern continuation of the Rio Grande graben system that terminates southward against the north-branch of the TL (that extends from El Paso to Valentine Black Gap across Texas). The south-branch of TL extends east-southeast from near Presidio across Mexico to the Gulf of Mexico near the mouth of the Rio Grande. Right slip along the west-northwest trends is demonstrated in the Sierra del Carmen by slickensides and by a first-motion study of the Valentine earthquake. Strike-slip displacement is presumably modest across the region (under 10 km, 6 mi, ?), but actual slips are indeterminate with the present data set. The Presidio graben lies along the eastern boundary of the Laramide Chihuahua overthrust belt. The Black Gap graben lies along the eastern boundary of the Laramide Rocky Mountain front. End_of_Article - Last_Page 510------------

Provenance and Depositional Environments of Middle Eocene Canoe Formation, Big Bend National Park, Brewster County, Texas: ABSTRACT

The middle Eocene Canoe Formation contains the first sedimentologic evidence of local volcanism in the Big Bend region. Sediments comprising the formation's lower member, the Big Yellow Sandstone, were deposited by sandy braided streams which were scoured by ancient carbonate highlands and volcanic terranes to the west. The unit represents a continuation of the depositional styles and compositional trends recorded in the Paleocene and early Eocene strata of the region. In contrast, sediments comprising the upper, unnamed member of the Canoe Formation were deposited as a volcanic sediment apron on the fringes of the newly forming Chisos Mountains volcanic center. The sandstones (feldspathic litharenites and lithic arkoses) are dominated by volcanic rock fragments and, as s ch, document an abrupt change in depositional style and sediment composition brought about by the onset of local volcanism. A comparison of Canoe Formation and earlier Tertiary sediment compositions results in the delineation of distinct petrologic trends which record the tectonic evolution of the early Tertiary sediment source area. The Paleocene sediments of the area were derived primarily from ancient magmatic arcs in northeastern Mexico. With the onset of the Laramide orogeny in late Paleocene-early Eocene, a new source of sediment--newly uplifted carbonate highlands--was added. Local volcanism in the middle Eocene produced yet another source of sediment, lava flows, ash flow tuffs, and sand-size pyroclastic materials from the Chisos Mountain volcanic center. Rapid erosion of these materials produced volcanic sediment aprons such as the one described here. As regional volcanic activity increased, typic l Paleocene and early Eocene depositional styles may have been completely abandoned, especially in areas proximal to the volcanic centers. End_of_Article - Last_Page 521------------

An Unusual New Chaetopappa (Asteraceae-Astereae) from New Mexico

Chaetopappa elegans, a narrow endemic from the White Mountains of south-central New Mexico, is unusual in Chaetopappa because of its double pappus of numerous bristles and narrow scales, comparatively long, lanceolate stylar appendages, crinkled stem pubescence, alveolate receptacle, faint cypsela nerves, long ligules, and occurrence on granitic rock. Its chromosome number is 2n = 9, In habit it resembles C. hersheyi from the nearby Guadalupe Mountains, but characteristics of the head are more like those of C. bellioides and C. pulchella from north-central Mexico and the Big Bend region of Texas, and Leucelene ericoides, widespread in arid southwestern North America. In its alveolate receptacle it resembles some species of Aster and the West Coast genus Pentachaeta, whereas its double pappus is reminiscent of some species of Erigeron and Aster (sect. Ionactis). The crinkly stem hairs are like those of some Erigeron species. A portion of the front ranges of southcentral New Mexico, the White and Sacramento mountains, form a continuous north-south oriented chain about 130 km long. The northern, primarily igneous portion, the White Mountains, rises to about 3660 m on Sierra Blanca, the southernmost glaciated peak in North America. The southern portion of the chain, the Sacramento Mountains, are primarily limestone and are considerably lower, barely exceeding 2740 m. This small mountain chain is isolated from other mountain ranges in the Southwest by fairly broad low gaps of arid grassland or desert, gaps that serve as barriers to migration of montane vegetation. To the north and south these gaps are relatively small and not particularly low (cf. map on p. 34 in Patterson 1980). During elevational fluctuation of vegetation in the late Pleistocene (Van Devender and Spaulding 1979) these gaps were narrowed even more and were, therefore, even less of a barrier to vegetation migration. On the one hand, the isolation of these mountains has produced an area of comparatively high endemism for New Mexico, while on the other the isolation has not been so complete that the vegetation has sharply differentiated. Plants from the higher elevations are mostly conspecific with those to the north; plants from lower elevations find their congeners mostly to the south. Most endemics in these mountains apparently have diverged but slightly from ancestral stock and show patterns of relationship similar to that of the general composition of the vegetation with which they are associated. The new species described herein adds one more endemic element to the White Mountains, one that is associated with a small genus of more southern affinity. Species of Chaetopappa are found in the mountains, hills, and plains in and surrounding the arid grassland regions of the northern portion of the Mexican Plateau, extending onto the southern Great Plains as far north as Kansas. This new Chaetopappa is unusual in that it occurs on granitic rocks at relatively high elevations, whereas its closest relatives all occur well to the south and then on limestone. It is also unusual in a number of morphological features. Shinners (1946a) redefined Chaetopappa to include a number of smaller genera previously distinguished by pappus differences. The publication of C. plomoensis B. Turner (Turner 1977) added a new kind of pappus to the genus. Our new species, with its pappus of numerous bristles and scales, further adds to pappus types known in the genus. In addition, it has stylar appendages, stem hairs, and a receptacle unique in Chaetopappa but similar in these characteristics to at least some species in Pentachaeta, Aster, and Erigeron. The faint nerves of the cypsela of C. elegans contrast to the prominent nerves on all other Chaetopappa. Chaetopappa elegans Soreng & Spellenberg, sp. nov. (fig. 1).-TYPE: U.S.A., New Mexico,

Electrophoretic variants of mojave rattlesnake ( Crotalus scutulatus scutulatus) venoms and migration differences of mojave toxin

E. D. Rael, R. A. Knight and H. Zepeda. Electrophoretic variants of Mojave rattlesnake ( Crotalus scutulatus scutulatus) venoms and migration differences of Mojave toxin. Toxicon 22, 980 – 985, 1984. — Mojave toxin was found in comparable quantities in venoms from Mojave rattlesnakes captured in the Big Bend region of Texas and southeastern Arizona. Toxicities in mice were also comparable. Electrophoretic profiles of venom differed significantly between the two groups, suggesting two genetic divergent groups. Immunotransfer revealed several electrophoretic variants of Mojave toxin among the Texas snake venoms, all of which migrated slower than Mojave toxin of venoms from the Arizona snakes.