The Cenozoic History of the Andean Foreland in Southeastern Peru

Abstract

<p>Results from two geographical study areas are presented in this dissertation, contributing important new information about the Cenozoic geological, environmental and biological regime. At the first locality along the Manu River (11.90°S, 71.34°W) I present a revised age and reinterpret the depositional environment. Age determination consisted of radiometrically dating detrital zircons, pushing the technical boundaries of this technique, paired with radiocarbon analyses of depositional organic material. Constraining the depositional environment relied on geochemical and sedimentological analyses. The revised age, 0.13±0.04 Ma instead of ~9 Ma, and depositional environment, as a fluvial overbank deposit instead of estuarine or marginal-marine deltaic deposits, have major implications for research previously published on the outcrop. </p><p>A second section is realized along the Alto Madre de Dios river and is situated in a piggyback basin belonging to the frontmost active deformation zone of the Sub Andean Zone, located approximately 12.8°S and 71.3°W. The section spans from the Paleocene to the Quaternary in time and allows a constraint on the influence of the Andean orogeny on the Madre de Dios foreland basin and the fauna and flora it contained through the Cenozoic. Field work has yielded a series of fossil localities that have been constrained in age through use of U/Pb dating of detrital zircons. Notable fossil finds include Early Miocene marsupials, xenarthrans, rodents, notoungulates, and bats. A new primate recovered from the section is the first record of a primate from the Early Miocene of the Amazon Basin. Reconstruction of the depositional environment for each formation was performed by a combined use of sedimentology, geochemistry, and paleontology. Stable isotopic analyses on depositional organic material indicates continental depositional settings for all formations, whereas stable isotopic analyses on abiogenic carbonates provide constraints on the degree of diagenetic alteration of sediments varying with age and structural setting. Sedimentary facies are consistent with deposits being formed in fluvial and overbank environments, consisting of fine-grained floodplain deposits, point bar deposits, conglomerate channels and fossil-bearing channel lag deposits. Average grain size for the section shows a coarsening trend towards younger deposits, consistent with the approach of, and incorporation into an active orogen, as supported by provenance data from detrital zircon analyses.</p>