Geology of the Las Vegas area
We document a rich Upper Triassic bonebed in Borden County, Texas in either the uppermost Tecovas Formation or, more likely, the Trujillo Formation of the Chinle Group, that represents one of the highest stratigraphic occurrences of the aberrant archosauromorph reptile Trilophosaurus. The associated fauna includes indeterminate coprolites, unionid bivalves, osteichthyans, indeterminate reptiles, a possible ornithischian dinosaur, and the putative theropod dinosaur Spinosuchus caseanus Huene.
Hydrogeology and Hydrogeochemistry of the Shallow Alluvial Aquifer Zone, Las Vegas Valley, Nevada
In Las Vegas Valley, Nevada, the shallow alluvial aquifer zone is a possible source of contamination to the principal alluvial aquifers that provide 30% of the public drinking water supply for the valley. Development of the principal aquifers has lowered pressure head in the principal aquifers and created the potential for downward seepage from the shallow aquifer zone. This study was undertaken to characterize the hydrogeology and hydrogeochemistry of the shallow alluvial aquifer zone and to compare the hydrogeochemistry of the shallow and pr'lnc'ipal alluvial aquifer zones A 37 well shallow ground water monitoring network was established and water-level, water-quality, and isotopic data were collected between June, 1988 and December, 1989. Water levels fluctuate seasonally and are influenced by land use practices. Irrigation influenced water levels are higher in fall and lower in winter while the natural water level pattern has lows in the fall and highs in the winter. Water temperature, pH, and EC appear to be unaffected by local land use practices. Temperatures are high in fall and low in spring. pH remains fairly constant near neutrality throughout the year. EC appears to be controlled by alternating varible length cycles of concentration and dilution of saline water near the water table. Water quality evolves along flow path from a fresh Ca+-Mg+-Hco3type water with TDS around 300 mg/l in the north to a moderately saline Ca2+ • Mg+-so4 2type water with TDS around 8000 mg/1 in the southeast near Las Vegas Wash. TDS varies temporally but does not follow a seasonal pattern. Ion ratios remain constant throughout the year. Water samples are generally oversaturated with respect to calcite, dolomite, and quartz, but are undersaturated with respect to gysum and amorphous silica. Delta D and o18 0 indicate that the water in the shallow aquifer zone originated as principal aquifer zone water. Comparison of shallow to principal aquifer zone data reveals that Ca2+, Mg2+, Cl-, sof-, TDS, Si02, TOC, Po]-, B, Mn, Se, and tritium are all suitable for use as natural tracers for tracing the downward leakage of water from the shallow to the principal aquifer zone. Comparison of historical shallow aquifer zone data to the data generated during this investigation reveals that water levels in the shallow aquifer zone rose by an average of about 0.5 meter from 1972 to 1989 and that TDS increased by around 570 mg/1 between 1981 and 1989.
Taphonomy of Isisfordia duncani specimens from the Lower Cretaceous (upper Albian) portion of the Winton Formation, Isisford, central-west Queensland
Taphonomic analysis of fossil material can benefit from including the results of actualistic decay experiments. This is crucial in determining the autochthony or allochthony of fossils of juvenile and adult Isisfordia duncani, a basal eusuchian from the Lower Cretaceous (upper Albian) distal-fluvial-deltaic lower Winton Formation near Isisford. The taphonomic characteristics of the I. duncani fossils were documented using a combination of traditional taphonomic analysis alongside already published actualistic decay data from juvenile Crocodylus porosus carcasses. We found that the I. duncani holotype, paratypes and referred specimens show little signs of weathering and no signs of abrasion. Disarticulated skeletal elements are often found in close proximity to the rest of the otherwise articulated skeleton. The isolated and disarticulated skeletal elements identified, commonly cranial, maxillary and mandibular elements, are typical of lag deposits. The holotype QM F36211 and paratype QM F34642 were classified as autochthonous, and the remaining I. duncani paratypes and referred specimens are parautochthonous. We propose that I. duncani inhabited upper and lower delta plains near the Eromanga Sea in life. Their carcasses were buried in sediment-laden floodwaters in delta plain overbank and distributary channel deposits. Future studies should refer to I. duncani as a brackish water tolerant species.