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Alamo impact crater: New study could double its size

New Geosphere articles posted online Jan. 14, 2015

Geological Society of America

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IMAGE: This is a map view of the impact region outlining the Alamo impact crater and the possible location (dotted lines and question marks) of its complex crater features. view more

Credit: Retzler et al. and Geosphere

Boulder, Colo., USA - Carbonate rock deposits found within the mountain ranges of south-central Nevada, USA, record evidence of a catastrophic impact event known as the Alamo impact. This event occurred roughly 382 million years ago when the ancient seafloor was struck and a submarine crater was formed. The crater was filled-in with fragmented rock, and later with more typical ocean deposits, as the energy from the impact lessened and the environment returned to normal. By studying the distribution and features of the post-impact ocean deposits and fragmented rock that filled the crater, Andrew J. Retzler of Idaho State University and colleagues present a new map characterizing the size and shape of the Alamo crater. Their results indicate that only about half of the Alamo impact crater and its related deposits are now exposed within the region, and they estimate its total diameter to be between 111 and 150 km. This is more than double previous estimates and, if correct, places the Alamo crater as one of the largest marine impacts in the last 550 million years, conservatively larger than the well-studied Chesapeake Bay impact crater (about 35 million years old) on the eastern shore of North America.

FEATURED ARTICLE

Post-impact depositional environments as a proxy for crater morphology, Late Devonian Alamo impact, Nevada

Andrew J. Retzler et al., Idaho State University, Pocatello, Idaho, USA. Published online on 14 Jan. 2015; http://dx.doi.org/10.1130/GES00964.1.

Other GEOSPHERE articles posted online on 14 Jan. 2015 (see below) cover such topics as

1. The origin and evolution of the Colorado River system;

2. A new three-dimensional look at the geology, geophysics, and hydrology of the Santa Clara ("Silicon") Valley; and

3. The makeup of the western Brooks Range, Alaska, USA.

All GEOSPHERE articles available at http://geosphere.gsapubs.org/. Representatives of the media may obtain complimentary copies of GEOSPHERE articles by contacting Kea Giles at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOSPHERE in articles published. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

Constraints on the evolution of vertical deformation and Colorado River incision near eastern Lake Mead, Arizona, provided by quantitative structural mapping of the Hualapai Limestone

Gustav B. Seixas et al., Stanford University, Stanford, California, USA. Published online on 14 Jan. 2014; http://dx.doi.org/10.1130/GES01096.1. Themed issue: CRevolution 2: Origin and Evolution of the Colorado River System II.

The 12- to 6-million-year-old Hualapai Limestone was deposited in a series of basins that lie in the path of the Colorado River directly west of the Colorado Plateau and has been deformed by a pair of faults that have slipped due to tectonic extension of the Earth's crust over this same time period. Therefore, this rock unit represents an opportunity to study the sedimentary geology and deformational history over which the Colorado River first flowed after becoming a through-flowing river in the western Grand Canyon and Lake Mead. In this study, we have quantified the deformation of the Hualapai Limestone using high-resolution aerial photography, detailed field studies of the stratigraphy, and numerical modeling of the deflections of the stratigraphy during faulting. Specifically, we hypothesize that the Colorado River in the eastern Lake Mead region initially flowed over a low-relief surface developed over sediment-filled basins; the surface was warped as the faults continued to accrue displacement over the past six million years, causing spatial variations in apparent incision rate of the Colorado River as it incised through the deforming structure. We further explore physically meaningful estimates of fault geometry that may have been responsible for the observed deflections using mechanical modeling. Our study thus examines linkages between fault geometry, fold geometry, and landscape evolution in a tectonically-active region.

A summary of the late Cenozoic stratigraphic and tectonic history of the Santa Clara Valley, California

V.E. Langenheim et al., U.S. Geological Survey, Menlo Park, California, USA. Published online on 14 Jan. 2015; http://dx.doi.org/10.1130/GES01093.1. Themed issue: A New Three-Dimensional Look at the Geology, Geophysics, and Hydrology of the Santa Clara ("Silicon") Valley.

We summarize the stratigraphic and tectonic history of the Santa Clara (Silicon) Valley during the past ~18 million years, which is a tale of three basins. The present-day basin has been uniformly subsiding for the past 1-1.5 million years, receiving sediment from the neighboring mountains. These deposits form the main groundwater aquifer and conceal two deep basins that formed within the broader San Andreas fault system. These basins, largely older than four million years, illustrate how this older geologic history hidden beneath urban areas has seismic-hazard implications because of simulations that show amplification of ground shaking within these basins from not only nearby earthquakes, but also distant sources.

Structural superposition in fault systems bounding Santa Clara Valley, California

R.W. Graymer et al., U.S. Geological Survey, Menlo Park, California, USA. Published online on 14 Jan. 2015; http://dx.doi.org/10.1130/GES01100.1. Themed issue: A New Three-Dimensional Look at the Geology, Geophysics, and Hydrology of the Santa Clara ("Silicon") Valley.

Santa Clara Valley is bounded on the west and east by active faults of the San Andreas fault system, that broad zone of faults that forms the boundary between the North America and Pacific tectonic plates. On both sides of the valley, these faults are on top of older faults, earlier parts of the San Andreas system. Movement on the older faults formed deep, sediment-filled basins that are concealed beneath the young sedimentary deposits that make up the valley floor. Though hidden, the basins are detectable by tiny changes in Earth's gravity (too small for humans to feel, but measurable with sensitive detectors). Geologic evidence shows that the older faults on the west probably died out about eight million years ago, whereas those on the east probably stayed active until about 2.5 million years ago. After, or perhaps while, the old faults died out, the new faults formed, and the old faults were concealed. Some geologic observations that did not make sense to geologists at first are explained now that we know about the hidden older faults and sediment-filled basins.

Provenance and detrital zircon geochronologic evolution of lower Brookian foreland basin deposits of the western Brooks Range, Alaska, and implications for early Brookian tectonism

Thomas E. Moore, et al., U.S. Geological Survey, Menlo Park, California, USA. Published online on 14 Jan. 2015; http://dx.doi.org/10.1130/GES01043.1.

In their new paper in the journal Geosphere, Moore and others report on the composition of sediment eroded from the rising east-west-trending ancestral Brooks Range in northern Alaska in the Late Jurassic and Early Cretaceous (160 to 115 million years ago) to determine the tectonic forces that produced this mountain range, one of the most important in the Circum-Arctic region. They collected sandstone samples from progressively younger thrust sheets in the Brooks Range and from the somewhat younger deposits of the Colville basin that underlies much of northern Alaska to the north and counted the types of sand grains in the samples. They also dated grains of the mineral zircon, which can be used as a tracer for the source area of the sand and their subsequent burial history. The results show that much of the sand from the oldest deposits was derived from thrust-emplaced oceanic and volcanic arc rocks exposed at the highest level of the Brooks Range today, but surprisingly, that some of the sand in these samples and most or all in the other samples was not derived from the Brooks Range or northern Alaska but instead originated from erosion of Triassic (201 to 252 million years ago) strata exposed in the Russian Far East (the data also indicate that the Triassic deposits themselves were originally derived from erosion of the Uralian and Taimyr mountain belts in Eurasia). Moore and others conclude that the results indicate that the ancestral Brooks Range was formed by a collision of a volcanic arc and the continental margin of northern Alaska at a subduction zone in the ancestral Pacific Ocean in the Late Jurassic and Early Cretaceous and that the resulting mountain belt continued westward into Russia where it attained its largest size, probably due to the accretion of large thicknesses of the Triassic sediment. Detritus eroded from this area was transported eastward along the northern front of the mountain belt for as much as 750 kilometers into low areas in northern Alaska where it became involved in the thrusting as the Brooks Range gradually advanced northward. It wasn't until later, during the latest Early Cretaceous (about 105-100 million years ago), that sand began arriving from the metamorphic rocks in the southern Brooks Range, suggesting that Alaskan source areas had been uplifted and eroded and had again regained importance as the major source of sediment in the Colville basin. This significant change in sand composition corresponds to the end of collisional tectonics and a transition into a period of extensional exhumation and uplift that previously has been documented in the southern Brooks Range at about this time.

An integrated geophysical imaging of the upper-crustal features in the Harney Basin, southeast Oregon

M. Khatiwada and G. Randy Keller, ConocoPhillips School of Geology and Geophysics, University of Oklahoma, Norman, Oklahoma, USA. Published online on 14 Jan. 2015; http://dx.doi.org/10.1130/GES01046.1.

In this paper, we presented an integrated geophysical image of the upper crustal features of Harney Basin. While doing so, we used 2-D seismic lines to generate the 3-D seismic tomography of the basin. Extensive gravity coverage and gravity data analysis was employed and integrated with the seismic results. In addition, geological data, magnetic data, and geospatial data were also used. We have explained the basin-forming process as possible caldera collapse. This paper should provide the scientific community with a good overview of the upper crustal features in the Harney basin and its structural and tectonic evolution.

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