Boulder, Colo., USA - Turquoise has had cultural significance for Native American peoples in the southwestern United States and Mexico for more than a millennium, and turquoise artifacts have been recovered from archaeological sites hundreds of kilometers distant from known sources of the mineral. Evidence for pre-Hispanic turquoise mining has been recognized across much of the southwestern United States and northern Mexico, including as far north as Leadville, Colorado, and as far south as Zacatecas.
Detailed archaeological studies of ancient turquoise mines are rare, and little is known about the timing of their exploitation or the cultural identities of the miners In this study, Alyson M. Thibodeau and colleagues show that many geological sources of turquoise in the southwestern U.S. and northern Mexico can be distinguished from each other through the measurement of lead and strontium isotopic ratios. These isotopic analyses thus provide a new way to investigate the mining and movement of this mineral in prehistory.
Isotopic evidence for the provenance of turquoise in the southwestern United States
A.M. Thibodeau et al., Dickinson College, Carlisle, Pennsylvania, USA. Published online ahead of print on 3 June 2015; http://dx.
Other GSA BULLETIN articles posted 3 and 10 June 2015 are listed below.
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Sea level controls on carbonate beaches and coastal dunes (eolianite): Lessons from Pleistocene Bermuda
M.P. Rowe and C.S. Bristow, Birkbeck, University of London, Bloomsbury, London, UK. Published online ahead of print on 10 June 2015; http://dx.
Emergent Pleistocene littoral calcarenites are beautifully exposed on Bermuda's coastline. Shoreface and foreshore successions such as these are rarely preserved in the global geological record. This study exploits a unique opportunity to establish the connection between ancient beach deposition and relative sea level (RSL) change, which is recorded by shifting elevation of newly identified sea-level-indicator facies. M.P. Rowe and C.S. Bristow propose an allocyclic model for the evolution of beach-dune systems at Bermuda, which differs from prevailing autogenic and transgressive models. The new model represents a useful analogue for studying the fate of modern beach-dune systems in the face of changing RSLs. It comprises beach progradation and barrier aggradation at a peaking RSL, followed by protosol formation and the release of expansive mobile dunes as RSL falls at the termination of a highstand. Furthermore it is inferred that sediment generation and mobilization are enhanced by low order, sub-interstadial, RSL fluctuations.
A lithologic control on active meandering in bedrock channels
K.N. Johnson and N.J. Finnegan, University of California, Santa Cruz, California, USA. Published online ahead of print on 3 June 2015; http://dx.
The cause of sinuosity in bedrock river canyons has been a catalyzing question in the field of geology for over a century, and helped motivate the study of landforms as records of past climate, tectonics, and surface processes. Despite continued interest, no hypothesis for how a river could move laterally and meander within bedrock canyon walls had yet been proposed, limiting the ability to interpret the record these rivers hold. In fact, it is commonly assumed that active meandering in bedrock is not possible and sinuosity must be inherited from a preexisting floodplain river. Here, K.N. Johnson and N.J. Finnegan present clear evidence that some rivers do indeed meander within bedrock canyon walls. They then show how this meandering process works and propose a framework for further climatic interpretation, and warnings for common misinterpretation of these classically awe-inspiring river canyons that hold the promise to teach us about past climate and tectonic conditions.
Shock pressure estimates in target basalts of a pristine crater: A case study in the Lonar crater, India
A. Agarwal et al., Institute of Applied Geosciences, Karlsruhe Institute of Technology, Karlsruhe, Germany. Published online ahead of print on 10 June 2015; http://dx.
The present study employs rocks magnetics (natural remanent magnetization and anisotropy of magnetic susceptibility) and microfractures to predict shock pressures in a pristine impact crater, the Lonar crater, India. Rocks magnetic properties of the basalts in the crater rim were compared with basalts collected away from the rim. Magnetic fabric was compared with the magmatic fabric. The study provides information on acquisition of natural remanent magnetization, development of magnetic fabrics and geometry and chronology of shock induced fractures in the basalts at the Lonar crater.
Early Eocene climatic optimum: Environmental impact on the North Iberian continental margin
A. Payros et al., University of the Basque Country (UVP/EHU), Leioa, Spain. Published online ahead of print on 10 June 2015; http://dx.
The Early Eocene Climatic Optimum (EECO) constituted the peak of the long-term early Cenozoic global warming and is therefore relevant to past and future climate studies. It had a significant impact on the environmental evolution of terrestrial and oceanic areas but, surprisingly, its influence on continental margins is poorly known. A. Payros and colleagues provide new insights from a multiproxy study carried out in deep-marine Eocene sediments accumulated on the North Iberian continental margin, which are now exposed along coastal cliffs. Based on this analysis, the environmental influence of the EECO started 52.6 million years ago and lasted approx. 2.3 million years. The onset of the warming was related to the addition of light carbon into the ocean-atmosphere system. A hotter climate and a perennial rainfall regime increased the supply of terrestrial clays, organic matter and iron oxides into the sea. Eventually, these changes affected the deep-sea bottom, creating conditions in which opportunistic organisms thrived and leading to increased methanogenesis in the subsurface, which caused the formation of siderite. A subsequent gradual recovery culminated abruptly at 50.3 Ma with a global cooling episode
Insights into the mechanics of fault-propagation folding styles
A.N. Hughes and J.H. Shaw, Harvard University, Cambridge, Massachusetts, USA. Published online ahead of print on 3 June 2015; http://dx.
This study by A.N. Hughes and J.H. Shaw demonstrates the important roles that rock strength and rock layer strength contrasts play in determining the specific geometry of folds and faults that develop in compressional tectonic settings such as mountain belts and accretionary prisms. This study focuses on a particular type of fold that is common to these settings that forms as faults propagate upward toward the earth's surface, termed "fault-propagation folds"; as this type of structure is common to both areas that are seismically active and areas of significance for petroleum exploration, understanding the details of how these structures grow over time improves understanding of seismic hazard and petroleum systems. Numerical model results are compared with observations from natural examples of these structures in locations including offshore Nigeria, southeast Asia, and Argentina, allowing the authors to demonstrate that rock layer strength contrast is the primary factor that determines the specific fold and fault geometry that develops in the deforming rocks.
Collapse of the Late Triassic megamonsoon in western equatorial Pangea, present-day American Southwest
Lee Nordt et al., Baylor University, Waco, Texas, USA. Published online ahead of print on 3 June 2015; http://dx.
Lee Nordt and colleagues show from field properties, geochemical transfer functions, and isotopic analysis of paleosols in the Chinle Formation of Petrified Forest National Park (PEFO), Arizona that the megamonsoon in Western Equatorial Pangea had collapsed by 214.7 million years ago (Ma). The most notable environmental shift is the appearance of carbonate-enriched paleo-Inceptisols and paleo-Vertisols after 214.7 Ma and paleo-Aridisols after 210 Ma. The evolving Cordilleran magmatic arc forming a rain shadow as the cause of aridification is supported by recent magnetostratigraphic work substantiating that the region remained in the tropics through the Triassic. However, temperatures were subtropical to temperate perhaps because of elevation. A regionally defined faunal turnover may have been a response to rapidly changing climates in the region.
Constraints on plateau architecture and assembly from deep crustal xenoliths, northern Altiplano (SE Peru)
Alan D. Chapman et al., University of Arizona, Tucson, Arizona, USA. Published online ahead of print on 10 June 2015; http://dx.
How do large orogenic plateaus form? Addressing this question requires knowledge of what rocks reside beneath the surface of plateaus and when these rocks formed. Given that exposures of rocks from the middle and deep crust are lacking from the central Andes, our understanding of the processes that shaped the Peruvian Altiplano, the world's second largest orogenic plateau, is incomplete. In this paper, Alan D. Chapman and colleagues present new data from fragments of middle and deep crustal rocks erupted from Quaternary volcanoes on the Peruvian Altiplano. These data suggest that the Altiplano began to grow approximately 45 million years ago (some 20 million years earlier than previously thought) by a combination of tectonic compression and input of magma derived from the mantle below.