Reports: DNI852480-DNI8: Evaluating the Mechanisms for Episodic Filling and Erosion in Wedge-Top and Proximal Foreland Environments in the Precordillera, San Juan Province, Argentina

Gregory D. Hoke, PhD, Syracuse University

This research explores the extent to which climate or tectonics controls sediment generation in the highlands and subsequent storage and excavation in the sedimentary basins (e.g. Hilley and Strecker, 2005) using paleo-erosion rates derived using terrestrial cosmogenic nuclides (TCN). The paired Iglesia “wedge top”and Bermejo foreland Basins of San Juan Province, Argentina present an unparalleled opportunity to explore these interactions as they have been the subject of intense prior study, have well-established deformation histories and excellent stratigraphic age control (Jordan et al. 1993; Allmendinger, 1990; Ré et al. 2003; Milana et al. 2003; Johnson et al. 1986; Jordan et al. 2001; Cardozo and Jordan, 2001). The Iglesia Basin contains extensive perched remnant alluvial fills including several with areal extents exceeding 10s of square kms in the northern parts of the basin. Combined, these fills strongly suggest complete filling of the wedge top basin by up to 1200 m of sediment that was later excavated.  The Coulomb critically tapered wedge theory (Dahlen, 1984) suggests that changes in the mass distribution in an orogen controls where faulting occurs in a fold-and-thrust belt. Knowledge of how transient sediment storage and excavation in wedge-top basins may exert a control on the spatial position of deformation in fold-and-thrust belts, by changing the stress balance, enhances our understanding of how petroleum producing foreland basins evolve. This study will construct an 8 to 2 Ma record of paleoerosion rates in the dated foreland, wedge-top basin sediments as well as constrain the age and paleoerosion rates of the perched Plio-Pleistocene fills. The goal is to compare the coupled wedge top and foreland records and determine whether tectonics or climate is driving variations in paleoerosion rates.

During the first year of ACS-PRF funding extensive geomorphic analysis of the landscape was performed using river profiles and the first field season was completed with over 30 samples collected for potential erosion rate determinations in three different paleomagnetically dated stratigraphic sections (Sierra de Mogna, Huaco and Iglesia sections).  Sampling focused on the local minima, maxima and inflection points in the accumulation rate curves in the Huaco and Mogna foreland sections, while the Iglesia wedge top section was sampled for optimal temporal overlap with the foreland sections. A serendipitous volcanic ash deposit at the base of the fill and a TCN depth profile from a stable surface were collected from the areally extensive perched sedimentary fills at the northern end of the Iglesia wedge top basin.  All samples are currently being processed for 10Be concentration measurements or conventional geochronology.

This project supports PhD Pedro Val’s field and analytical expenses and a postdoctoral research associate, Devin McPhillips, in the department of Earth Sciences at Syracuse University. This expansion of my research group and creates comfortable critical mass where group synergies can feed off of one another. During the project’s first year collaborations have been established with Juile Fosdick (Indiana University), Manfred Strecker (Potsdam University, Germany).