Sym - Abstracts - University of Texas at El Paso

ABSTRACTS: GEOLOGY/INVEREBRATE ZOOLOGY
PARKER, DON F.
A RECENTLY EXPOSED, COMPLETE SECTION OF GOMEZ TUFF IN THE NORTHERN DAVIS MOUNTAINS, TRANS-PECOS TEXAS.
UN DESCUBRIMIENTO RECIENTE, LA SECCION COMPLETA DEL HORIZONTE DE GOMEZ EN EL NORTE DE LA MONTAÑAS DAVIS, EN
TRANS – PECOS, TEXAS. Don F. Parker, Dept. of Geology, Baylor University, Waco, TX 76798 .--New roadcuts through a Toreva
block expose a complete, 80 m section of Gomez Tuff, including a basal vitrophyre, flow-banded granophyre, an upper nonwelded zone, and an upper block and ash
flow. The Gomez Tuff is overlain by an 8 m tuffaceous mudstone, which, in turn is overlain by Adobe Canyon Rhyolite lava. Four whole-rock analyses of the lower
Gomez units are peralkalic and pantelleritic, including one sample with 10.47 percent ac, and 1.12 percent ns; glass compositions are comparable. Samples from
the upper part of the lower unit are enriched in incompatible trace elements by about 10 percent. Phenocrysts include Or38 anorthoclase, Na-cpx, and ilmenite.
The Gomez Tuff (>200 km3) may be the largest ash-flow tuff of this type in the world.
Los nuevos cortes de carretera realizados en el block de Toreva descubrieron una completa sección de 80 metros del horizonte de Gómez que incluye vitropiritas
basalticas, bandas de flujo de granopirita, una zona superior no fucionada y una parte alta de block y lava. El horizonte de Gómez esta delineado por lodo de piedra
de la roca madre el cual resulta ser lava de riolita en el Cañón del Adobe. Cuatro análisis completos de rocas de la parte baja de las unidades de Gómez resultaron
ser peralquicas y panteleriticas incluyendo una muestra de 10.47% de ac, y 1.2 ns; las composiciones de vidrios son comparables. Las muestras de la parte superior
de la unidad baja estan enriquecidas por un número incompatible de elementos trazas en aproximadamente el 10%. Los fenocristos incluyen anortoclase Or38, Na –
cpx, e ilmenitas. El horizonte de Gómez (>200 km 3 ) puede ser el horizonte de lava más grande de este tipo en el mundo.
URBANCZYK, KEVIN.
COLORADO CANYON AND THE MARGIN OF THE SAN CARLOS CALDERA.
EL CAÑON COLORADO EL MARGEN DE LA CALDERA DE SAN CARLOS. Kevin Urbanczyk and John C. White
, Department of Earth and Physical Sciences, Sul Ross State University, Alpine, TX 79832.--Recent mapping in the Colorado Canyon
area, approximately 15 miles west of Lajitas, Texas, has revealed new details regarding the northern margin of the San Carlos caldera. The eruptive product of the
caldera is the San Carlos tuff, a thick, crystal-rich ash-flow tuff whose maximum outcrop distribution has been used by previous workers to define the San Carlos
caldera margin. Using this criterion, we have located the apparent margin of the caldera in Colorado Canyon, where a steep flow front of San Carlos tuff exhibits
control over the present channel of the Rio Grande. In this location, the overlying Santana tuff appears to “drape” over this steep flow front at the exact location
where the river channel is diverted by the flow front. Immediately east of this outcrop, a fluvial channel, marked by rounded cobbles composed predominantly of
San Carlos tuff is incised into the tuff.
El mapeo reciente en el área del Cañón Colorado, aproximadamente a 15 millas al Oeste de Lajitas, Texas ha revelado nuevos detalles acerca del margen Norte de la
Caldera de San Carlos. El producto eruptivo de esta caldera es el horizonte de San Carlos, delgado, rico en cristal con flujo de cenizas quien de acuerdo a su
distribución más amplia ha sido utilizado en trabajos previos para definir los márgenes de la caldera de San Carlos. Usando este criterio, nosotros hemos localizado
el aparente márgen de la caldera en el Cañón Colorado donde el frente del flujo del horizonte de San Carlos presenta un control sobre el presente canal del río
Grande. En este lugar, se sobrepone el horizonte de Santa Ana el cual parece que se pliega en este punto en la localización exacta donde el canal del río es derivado
por el flujo frontal. Inmediatamente al Este de este punto un canal fluvial, marcado por bordes redondeados compuestos principalmente por el horizonte de San
Carlos se inserta dentro de la roca madre.
WHITE, JOHN C. AND KEVIN URBANCZYK.
GEOCHEMICAL EVOLUTION OF THE LEYVA CANYON MEMBER OF THE RAWLS FORMATION, A TRACHYTIC SHIELD VOLCANO IN
SOUTH PRESIDIO COUNTY, BIG BEND RANCH STATE PARK, TEXAS. John C. White (trachyte@brooksdata.net) and Kevin M. Urbanczyk
(kevinu@sulross.edu). Dept. of Earth and Physical Sciences, Sul Ross State University, Alpine, TX 79832. --The Leyva Canyon Member of the Rawls Formation is
a sequence of Oligocene (27.3 to 27.1 Ma) silicic lavas, tuffs, and volcaniclastic rocks that comprise a trachytic shield volcano in the central Bofecillos Mountains
in Big Bend Ranch State Park. These silica oversaturated, felsic lavas erupted within a volcanic field that was otherwise dominated by silica undersaturated, mafic
to intermediate lavas. The petrogenetic relationship between silica under- and oversaturated rocks is a long-standing problem both in the field of igneous petrology
in general and to the Trans-Pecos Magmatic Province (TPMP) specifically. Several models of the geochemical evolution of the Leyva Canyon volcano will be
evaluated and their implication for similar processes in the rest of the TPMP and other similar systems will be discussed.
VI. INVERTEBRATE ZOOLOGY
ANDUAGA, SOFIA, ET AL
DUNG BEETLES ACTIVITY (COLEOPTERA : SCARABACIDAE : SCARABAEINAE) IN A CATTLE RANCH UNDER HOLISTIC RESOURCE
MANAGEMENT IN DURANGO, MEXICO.
ACTIVIDAD DE LOS ESCARABAJOS ESTERCOLEROS (COLEOPTERA: SCARABAEIDAE: SCARABAEINAE) EN UN RANCHO GANADERO
BAJO EL MANEJO HOLISTICO DE LOS RECURSOS NATURALES EN DURANGO, MEXICO. Sofia Anduaga, Armando Giner, and Luis Carlos Fierro.
Institute de Ecologia, A.C., Centro Regional Durango, Apartado postal 632. C.P. 34 000, Durango, Dgo. Mexico.--The study was conducted in the state of Durango,
Mexico on a cattle ranch (2,150 ha), under Holistic Resource Management, divided in 40 pastures with a stocking rate of 250 animal units per year. Dung beetles
were collected to determine species and population densities, using pitfall traps during the rainy season (July, September, and October). Total capture numbers were
3,432, identifying eight species : Euoniticellus intermedius, Digitonthopagus gazella, Onthophagus mexicanus, 0. lecontei, Phanaeus quadridens, Copris
sierrensis, Dichotomius carolinus and Canthon humectus. Based on density numbers, the dominant species were exotics of Indo-African origin: E. intermedius and
D. gazella. All species followed the rotational schedule for cattle (3 d/pasture) harvesting fresh feces. Apparently, dung beetles densities (31 individuals/kg of fresh
feces) are not sufficient to process the amount of fecal material deposited on each pasture, due to the high concentration of cattle numbers on such restricted area.
The implications of this, are related to cattle fly densities, internal parasites, forage availability, soil nutrients, and permeability.
El estudio se realiz6 en la regi6n estatal de Durango en una extension de 2,150 ha. dividida en 40 potreros con una carga de 250 unidades animales. Se efectuaron tres
muestreos (julio, septiembre y octubre), por medio de trampas pitfall cebadas con estiercol, cuadrantes de 50 m 2 y colocaci6n de bonigas de peso controlado (1kg).
Se capturo un total de 3,432 ejemplares que corresponden a ocho especies: Euoniticellus intermedius, Digitonthophagus gazella, Onthophagus mexicanus, 0.
lecontei, Phanaeus quadridens, Copris k. sierrensis, Dichotomius carolinus y Canthon aff. humectus. Por su abundancia las especies dominantes fueron exoticas
(origen Indo-Africano) E.intermedius y D. gazella. Los escarabajos siguieron el pastoreo rotacional del ganado (3 dias) en busca del estiercol. Aparentemente las
densidades encontradas ( =31) individuos/kg estiercol) no son suficientes para procesar la cantidad de estiercol depositada en cada potrero, esto es tal vez debido a
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