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.
Teleseismic Travel-Time Delays in the Las Vegas Basin
A temporary broadband seismic array was deployed in the Las Vegas basin (LVB), home to one of the fastest growing communities in the United States, to investigate structure in this deep (∼5 km) sedimentary basin. To constrain basin velocity structure, we measured the differential travel time of teleseismic P waves by waveform cross correlation relative to a station near the basin's edge. The range of the travel-time delays is significant (up to 0.5 sec), and the pattern of travel-time delays is independent of the back azimuth of the incoming energy, suggesting that the near-surface structure controls the delay times. Assuming the reported basin geometry of Langenheim et al. (2001), we modeled the average delay times at the basin stations to estimate the average P-wave velocity structure within the basin. The average times can be modeled with relatively fast P-wave velocities (4:5 km=sec) in the deepest part of the basin (below 2 km), which is in agreement with the P-wave velocities of the deep part of the basin from recent seismic refraction profiling (Snelson et al., 2004) and low velocities (1:5 km=sec) in the shallow basin (200 m). We also performed computations based on the fast marching method approach to solve the forward problem and inversion for basin geometry. This method is used to map the travel- time residual information extracted from the array to variations in subsurface seismic structure. While the coverage of teleseismic data is insufficient to independently re- solve the steeply dipping footwall of the basin in its eastern part, we found that the footwall block is likely to be shifted farther west than indicated by the gravimetry- based model. The basin edge is probably related to the Frenchman Mountain fault and its inferred location closer to Las Vegas will result in stronger ground motion during an earthquake. We report site response from teleseismic earthquakes and compare it with pre- viously published site response from regional earthquakes using the standard spectral ratio method. The useful bandwidth of large teleseismic and regional events for stan- dard spectral ratio measurements is 0.1-1.0 and 0.2-5.0 Hz, respectively. Remarkably, we find excellent agreement between the two measurement types within the overlap- ping frequency band (0.2-1.0 Hz). This indicates that the amplification arises from the structure in the immediate vicinity of the recording station, regardless of the nature of the incoming energy—vertically propagating teleseismic S body waves or horizon- tally propagating regional surface waves. The results of these investigations indicate that low velocities are present near the surface in LVB, likely related to relatively re- cent (Quaternary) alluvial and lakebed sediments at the surface. Fast velocities in the deeper basin probably result from older formations.
Palaeocarpinus (Extinct Betulaceae) from Northwestern China: New Evidence for Paleocene Floristic Continuity between Asia, North America, and Europe
Spiny-bracted fruits of Palaeocarpinus Crane, a genus of Betulaceae previously known only from Europe and North America, have been discovered in the Paleocene of Altai, northwestern China. The fruits are found in association with abundant Corylus-like foliage and staminate catkins containing Corylus-like pollen. Because of problems distinguishing leaves of Corylus from those of extinct betulaceous genera, we place these leaves in the genus Corylites Gardner ex Seward & Holttum. The fruits, staminate catkins, pollen, and leaves from Altai were probably all produced by a single species. Palaeocarpinus orientalis is nearly identical in its morphology to Palaeocarpinus joffrensis from the Paleocene of North America except in the more rounded primary bracts of the associated staminate catkins. The species is less similar to Palaeocarpinus laciniata from the Paleocene of southern England.