2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 2
Presentation Time: 8:00 AM-12:00 PM


FILDANI, Andrea, Geological and Environmental Sciences, Stanford Univ, Stanford, CA 94305-2115 and HESSLER, Angela M., Geology, Grand Valley State Univ, 1 Campus Drive, Allendale, MI 49401, fildani@pangea.stanford.edu

Sedimentary fill in the Andean forearc basin of Talara, northwest Peru, records the influence of subduction tectonics on its stratigraphy. Also preserved within the Eocene section are petrographic and geochemical signals of changing source terranes, occurring at major unconformities. To supplement provenance information collected from sandstone petrography, we use major-, trace-, and rare-earth element (REE) geochemistry of mudstones to track components of the source area that may not have survived weathering and deposition as part of the sand fraction. Ternary comparisons of Al2O3, CaO+Na2O, and K2O (A-CN-K) measure the degree of weathering represented by the mudstones. Talara shales exhibit a consistent degree of weathering, enabling geochemical differences to be attributed to provenance rather than to climate and/or weathering changes. REE signatures in shale are considered the most reliable geochemical provenance tracers, owing to their relative insolubility during weathering and sedimentary transport. Talara shales do show significant shifts in REE patterns at major unconformities, pointing toward changing source terranes concurrent with tectonic events in the basin. Shales from the lower Eocene (San Cristobal-Lagoon) have a strong felsic REE signature, possibly reflecting a source in the nearby granite-intruded Amotape, confirmed by preliminary study of sandstone framework grains. At the Lagoon-Clavel unconformity, LaN/YbN fractionation decreases, indicating a more intermediate or mafic influence. There is a sharp shift at the Chacra (upper Echino)-Terebratula unconformity, with increasing influx of mafic material to the basin coincident with the beginning of the Andean Inca phase. With deposition of the Upper Middle Eocene Verdun Formation, where there is a parallel increase in observed granite and sandstone conglomerate clasts, REE signatures again become more felsic. Upper Eocene/Lower Oligocene shales (Carpitas-Mancora) mark a significant change from the underlying Verdun-Chira section, suggesting the strongest mafic influence of the entire Eocene section and perhaps reflecting renewed volcanism and/or uplift of Andean terranes. Provenance clues provided by REE data are further supported by immobile tracers Th, Sc, Cr, Zr, Ti, petrography, and conglomerate clast data.