And other new GSA Bulletin articles published online ahead of print on March 10 and 26, 2015
Monica Pondrelli and colleagues investigated the Equatorial Layered Deposits (ELDs) of Arabia Terra in Firsoff crater area, Mars, to understand their formation and potential habitability. On the plateau, ELDs consist of rare mounds, flat-lying deposits, and cross-bedded dune fields. Pondrelli and colleagues interpret the mounds as smaller spring deposits, the flat-lying deposits as playa, and the cross-bedded dune fields as aeolian. They write that groundwater fluctuations appear to be the major factor controlling ELD deposition.
Figure 1 is from Pondrelli et al. (A) Location map of the study area on MOLA-based shaded relief map. Topographic contours (in black, 1000 m spacing) are indicated. (B) High Resolution Stereo Camera (HRSC) mosaic of the mapped area. Topographic contours (in white, 500 m spacing) are indicated. (C) Excerpt of the geological map by Scott and Tanaka (1986) on an HRSC mosaic. Geologic units (see text for more details): Npl1--Noachian cratered unit of the plateau sequence; Npl2--Noachian subdued crater unit of the plateau sequence; Hr--Hesperian ridged plains material. (D) Footprints of High Resolution Imaging Science Experiment (HiRISE) coverage on an HRSC mosaic. The white fi lling indicates stereo pairs. The area is fully covered by Context Camera (CTX) imagery. Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) scenes used in this study are recognizable by the hourglass shape. Click on the figure for a larger image.
Credit: Pondrelli et al. and GSA Bulletin
Pondrelli and colleagues also note that the ELDs inside the craters would likely have originated by fluid upwelling through the fissure ridges and the mounds, and that lead to evaporite precipitation. The presence of spring and playa deposits points to the possible presence of a hydrological cycle, driving groundwater upwelling on Mars at surface temperatures above freezing. Pondrelli and colleagues write that such conditions in a similar Earth environment would have been conducive for microbial colonization.
As a basis for their research, Pondrelli and colleagues produced a detailed geological map of the Firsoff crater area. The new map includes crater count dating, a survey of the stratigraphic relations, and analysis of the depositional geometries and compositional constraints. They note that this ELD unit consists of sulfates and shows other characteristics typical of evaporites such as polygonal pattern and indications of dissolution.
Equatorial layered deposits in Arabia Terra, Mars: Facies and process variability
M. Pondrelli et al., International Research School of Planetary Sciences, Università d'Annunzio, Pescara, Italy. Published online ahead of print on 10 Mar. 2015; http://dx.
Other GSA BULLETIN articles published online in March 2015 are highlighted below.
GSA BULLETIN articles published ahead of print are online at http://gsabulletin.
Representatives of the media may obtain complimentary copies of articles by contacting Kea Giles. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA Bulletin in your articles or blog posts. Contact Kea Giles for additional information or assistance. Non-media requests for articles may be directed to GSA Sales and Service, firstname.lastname@example.org.
Sedimentology and geochemistry of Archean silica granules
E.J.T. Stefurak et al., Stanford University, Stanford, California, USA. Published online ahead of print on 10 Mar. 2015; http://dx.
In the modern ocean, amorphous silica occurs only in the forms of opaline skeletons of select marine organisms or as volcanic glass. Geologists have long predicted that this material could have precipitated directly from seawater on the ancient Earth, prior to the evolution of silica-biomineralizing organisms. Elizabeth J.T. Stefurak and colleagues present the first compelling evidence of the occurrence of this process -- in the form of sand-sized grains of primary amorphous silica. They look to modern analogues to build a hypothesis for how these new chemical sand grains may have formed. This presents a fundamental advance in the understanding of the nature of sedimentation in ancient oceans, providing a link between theoretical predictions and observations in the rock record.
Evaluating rare earth element (REE) mineralization mechanisms in Proterozoic gneiss, Music Valley, California
S. Tyson McKinney et al., University of California, Santa Barbara, California, USA. Published online ahead of print on 26 Mar. 2015; http://dx.
This paper by S. Tyson McKinney and colleagues presents the first systematic age data from, and geochemical model for, the formation of economically important rare earth element (REE) deposits in the Music Valley region of southern California. This area represents one of two known major REE deposits in the continental United States and is therefore of potential significance given the recent increase in demand for these technologically important metals. This work indicates that the deposit formed approximately 1710 million years ago along with crystallizing granitic rocks in the area. The ore-bearing rocks have subsequently been subjected to at least two phases of alteration that have partly re-mobilized and redistributed the REE. These data provide a basis to understand the mechanisms of by which REE are concentrated in such rocks.
Characterization and quantification of aquifer heterogeneity using outcrop analogs at the Canadian Forces Base Borden, Ontario, Canada
G.S. Weissmann et al., University of New Mexico, Albuquerque, New Mexico, USA. Published online ahead of print on 10 Mar. 2015; http://dx.
Ground-based LiDAR and high-resolution photography of excavated exposures in a sand quarry allow for detailed 3-D mapping of sediments that comprise the well-studied Borden aquifer. The Borden aquifer has been the site of numerous high-resolution groundwater tracer tests, yet a comprehensive evaluation of the sedimentology of the site has not been completed until now. In this paper, G.S. Weissmann and colleagues describe the use of complete outcrop surveys using ground-based LiDAR and sedimentologic field studies to map lithofacies and hydraulic properties in the aquifer material. Using these data, they quantified the 3-D variability of aquifer sediments and provide these results for use by other researchers studying the spatial distribution of heterogeneous hydraulic properties in aquifers. These results will help improve the understanding of the controls of aquifer heterogeneity on tracer tests at this site and other similar geologic provinces.
Along-strike variation in crustal shortening and kinematic evolution of the base of a retroarc fold-and-thrust belt: Magallanes, Chile 53°S-54°S
P. Betka et al., Lamont-Doherty Earth Observatory, Palisades, New York, USA. Published online ahead of print on 26 Mar. 2015; http://dx.
This paper presents new field mapping, structural data, and two new balanced cross sections from a ~100-square-kilometer region of the southern Patagonian fold-thrust belt near Tierra del Fuego. The results of this study are discussed in the context of understanding how antecedent geology influences the development of thrust-belts at "Andean-style" plate margins. The paper describes the structure of two décollement horizons that decoupled plastically deformed "metamorphic basement" from the overlying fold-thrust belt and discusses how mechanically layered stratigraphy can control the development of a fold-thrust belt. The field observations and data in this paper are important because the study area is one of the few places in the Andes where the lower levels of the orogen have been exhumed and can be directly observed, thus they may be used as analogs for elsewhere in the Andes where the base of the orogen cannot be directly observed.
Depositional history, tectonics, and provenance of the Cambrian-Ordovician boundary interval in the western margin of the North China block
P.M. Myrow et al., Colorado College, Colorado Springs, Colorado, USA. Published online ahead of print on 10 Mar. 2015; http://dx.
Cambrian and Ordovician (~450 to 500 million year old) sedimentary rocks exposed in the Inner Mongolia region of northern China are part of the western margin of the country's main tectonic provinces, the North China Block (NCB). The strata record a major unconformity that covers the Cambrian-Ordovician boundary, an interval that includes a globally important mass extinction. The unconformity is of similar timing and duration to one recorded along the ancient northern Indian continental margin. Thus, the western margin of the NCB may have been affected by a regionally significant tectonic event that affected part of the supercontinent Gondwana. Therefore, the Inner Mongolian region was likely a continuation of the northern Indian margin, and part of Gondwana. This stands in contrast to most recent paleogeographic reconstructions that place the NCB in areas outside of Gondwana, either as an isolated terrane or one closely associated with Australia or Antarctica.
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