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Architecture of aquifers: Chile's Atacama Desert

Plus more Geosphere articles published online in August 2015

Geological Society of America

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Boulder, Colo., USA - The Loa River water system of northern Chile's Atacama Desert, in the Antofagasta region, exemplifies the high stakes involved in sustainable management of scarce water resources. The Loa surface and groundwater system supplies the great majority of water used in the region, and meets much of the municipal and agricultural demands. It is vital to regional copper mining, which constitutes ~50% of Chile's copper production, which in turn supplies one-third of the world's copper needs. However, a key property of the Loa system is the scarcity of surface water.

The aridity of the region sharply restricts the number of human inhabitants and the extent of native plants or animals. However, under different climate states during the past few millennia the water flux was greater than now; this leads to great uncertainty in estimations of how much of the current water flow is renewable versus fossil.

This study of the aquifers in the Calama Valley is motivated by the challenge of sustainable long-term management of the Loa coupled with the natural-human resource system. Authors Teresa Jordan and colleagues clarify the spatial distribution of the Cenozoic sedimentary rocks with properties favorable to function as aquifers and the distribution of water through those rocks. Their results identify where deeply buried aquifers likely exchange water with shallow aquifers or discharge to the surface water system.

FEATURED ARTICLE

Architecture of the aquifers of the Calama Basin, Loa catchment basin, northern Chile

Teresa Jordan et al., Dept. of Earth & Atmospheric Sciences and Atkinson Center for a Sustainable Future, Cornell University, Ithaca, New York, USA; Published online ahead of print on 5 Aug. 2015; http://dx.doi.org/10.1130/GES01176.1. This article is OPEN ACCESS.

Other recently published GEOSPHERE articles are highlighted below:

North America's Midcontinent Rift: When Rift met LIP

Carol A. Stein et al., University of Illinois at Chicago, Chicago, Illinois, USA. Published online ahead of print on 13 Aug. 2015; http://dx.doi.org/10.1130/GES01183.1.

An international team of geologists has a new explanation for how the Midwest's biggest geological feature -- an ancient and giant 2,000-mile-long underground crack that starts in Lake Superior and runs south to Oklahoma and to Alabama -- evolved. The 1.1 billion-year-old Midcontinent Rift formed in three stages: (1) it started as an enormous narrow crack in Earth's crust; (2) that space then filled with an unusually large amount of volcanic rock; and, finally (3) the igneous rocks were forced to the surface, forming the beautiful scenery seen today in the Lake Superior area of the Upper Midwest. Rifts are long, narrow cracks splitting Earth's crust, with some volcanic rocks in them that rise to fill the cracks. Large igneous provinces, or LIPs, are huge pools of volcanic rocks poured out at the Earth's surface. The Midcontinent Rift is both of these -- like a hybrid animal.

Drainage network reveals patterns and history of active deformation in the eastern Greater Caucasus

Adam M. Forte et al., School of Earth and Space Exploration, Arizona State University, 781 E. Terrace Mall, Tempe, Arizona 85257, USA. Published online ahead of print on 5 Aug. 2015; http://dx.doi.org/10.1130/GES01121.1.

The Greater Caucasus are a geologically young mountain range, with the main phase of mountain building beginning approximately five million years ago. Because of their youth, studying the location and evolution of active deformation within the range and how these are reflected in the topography of this mountain range can provide insight into the early stages of mountain building in a more general sense. Within the eastern part of the mountain range, two of the most distinctive features of the range are that its topography is very symmetric and that the drainage divide does not represent the highest elevations within the mountain range, two features which are at odds with predictions of mountain range topography from models and from what is seen in many older, more developed mountain ranges. Adam Forte and colleagues previously demonstrated that the separation of the divide from the high peaks of the range is likely the result of two zones of active deformation within the interior of the mountain range, but the order in which these two zones initiated remained unclear. Here they use a landscape evolution model that simulates the development of topography by simulating uplift and erosion to test various scenarios as they relate to the initiation of these two zones of uplift. They find that the northern zone of uplift, which is coincident with the peaks of the range, likely initiated second, with the zone of uplift which defines the drainage divide being older. This indicates that deformation has moved towards the interior of the mountain range over time, but that this is likely related to outward expansion of the deformation front of the mountain range as a whole. More generally, our results demonstrate that topographic features like the position of the drainage divide and topographic crest can be largely controlled by active deformation, at least for geologically short time periods, and that this can be used to help elucidate the location of active deformation within mountain ranges.

Modern and ancient hiatuses in the pelagic caps of Pacific guyots and seamounts and internal tides

Neil C. Mitchell et al., School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester, UK. Published online ahead of print on 13 Aug. 2015; http://doi.dx.org/10.1130/GES00999.1.

The movement of water over ridges sets off giant waves within the ocean interior (oscillations of internal density layers). These "internal waves" lose remarkably little energy, so they can travel vast distances. In this research, Neil C. Mitchell and colleagues found that sediments lying on the tops of seamounts (giant sea-mountains) have become eroded where the seamounts lie within beams of these ocean internal waves. Furthermore, scientists studying cores recovered by the Ocean Drilling Program from the sediments lying on the tops of these seamounts have found numerous hiatuses or levels where in the past the seabed was eroded. Mitchell and colleagues speculate that these reflect different internal waves of the past oceans and may originate from ridges surrounding the Pacific that had different depths, locations and orientations from today, as a result of plate tectonic movements.

Provenance of a Permian erg on the western margin of Pangea: Depositional system of the Kungurian (late Leonardian) Castle Valley and White Rim sandstones and subjacent Cutler Group, Paradox Basin, Utah, USA

Timothy F. Lawton et al., Centro de Geociencias, Universidad Nacional Autónoma de México, Querétaro, 76230, México. Published online ahead of print on 5 Aug. 2015; http:/dx.doi.org/10.1130/GES01174.1.

Linking large-volume sandstone deposits to the ultimate sources of their sediment is critical to understanding the paleogeography and mechanisms of sediment delivery in source-to-sink transport systems. Continental-scale rivers, the deposits of which may be no longer directly preserved in the stratigraphic record, can transport large volumes of sediment to submarine fan systems in sedimentary basins adjacent to the continent or to eolian sand seas on the continental margin itself. This study analyzes the sources for sand in the Permian White Rim erg, or sand sea, of the Paradox Basin of southeastern Utah. The White Rim Sandstone constitutes deposits of wind-blown sand equivalent in area to the dune fields of the Namib Desert in West Africa. Sandstone petrography, detrital zircon geochronology, and paleocurrent analysis of eolian sandstone and subjacent fluvial sandstone indicate that the principal source for fluvial sediment was the nearby Uncompahgre uplift, a block uplift of the Ancestral Rocky Mountain orogen. In contrast, most sand in eolian deposits of the White Rim Sandstone and correlative Castle Valley Sandstone was delivered from sources on the eastern edge of Pangea by large rivers with headwaters in the Appalachian region and possibly as far away as Baltica. The sand was reworked southward along the Permian marine margin by longshore currents and transported landward by northwesterly winds to form the White Rim erg.

Late Cenozoic displacement transfer in the eastern Sylvania Mountain fault system and Lida Valley pull-apart basin, southwestern Nevada, based on three-dimensional gravity depth inversion and forward models

Sarah B. Dunn et al., University of Texas at Dallas, Richardson, Texas, USA. Published online ahead of print on 5 Aug. 2015; http://dx.doi.org/10.1130/GES01151.1.

This paper by Sarah B. Dunn and colleagues serves to characterize the geometry of a complex system of subsurface structures in addition to providing constraint on the displacement budget for part of a larger network of faults underlying the Walker Lane in south-central Nevada. The article demonstrates a systematic approach for the determination of a subsurface fault geometry, and documents a methodology for the derivation of a displacement budget for an interconnected system of structures using techniques in gravity depth inversion and two-dimensional forward modeling of gravity. In addition to providing insight into gravity analysis, modeling, and structural characterization of extensional regimes, research findings suggest a much more complex structural history for the region than was previously hypothesized.

Dating synfolding remagnetization: Approach and field application (central Sierra Madre Oriental, Mexico)

Samantha R. Nemkin et al., University of Michigan, Ann Arbor, Michigan 48109, USA. Published online ahead of print on 13 Aug. 2015; http://dx.doi.org/10.1130/GES01187.1.

This article by Samantha R. Nemkin and colleagues focuses on determining the remagnetization history of folded Cretaceous carbonates of the Tamaulipas Formation in the central Sierra Madre Oriental (central Mexico). By combining Ar/Ar deformation dates and paleomagnetic fold test results, a quantitative age for synfolding remagnetization events can be determined. Results of the study indicate two distinct remagnetization events in the study area, one in the Late Cretaceous and one in the mid-Eocene. Remagnetization in carbonates is very widespread and this technique can be applied globally to many carbonate formations.

Surface uplift above the Jemez mantle anomaly in the past 4 Ma based on 40Ar/39Ar dated paleoprofiles of the Rio San Jose, New Mexico, USA

Michael A. Channer et al., Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico, USA. Published online ahead of print on 13 Aug. 2015; http://dx.doi.org/10.1130/GES01145.1.

This paper documents and interprets faulting and broad bending of the surface to be associated with a string of volcanoes known as the Jemez lineament in northwest New Mexico. This research suggests that these observations, located at the intersection of the Jemez lineament and a small river, could be manifestations of slow mantle-driven surface uplift over the past 4 million years on the order of ~135 meters.

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