Timing of Slope System Evolution and Intra-Basinal Sediment Recycling in the Magallanes Retroarc Foreland Basin (Chile) From Detrital Strontium Isotope Stratigraphy

Abstract

The Patagonian fold-thrust belt and adjacent Magallanes basin in southern Chile offer an excellent natural laboratory for investigating retroarc foreland basin evolution. Studies spanning the last several decades have provided a general interpretation of an elongate orogen-parallel foredeep that was filled sequentially in a dominantly north-to-south basin axial fashion by channel-levee deposits (Cerro Toro Fm), slope deposits (Tres Pasos Fm), and deltaic and shallow-marine deposits (Dorotea Fm). Within this general basin context, however, fundamental aspects about the relative timing and spatial distribution of distinct depositional systems remain unresolved. For example, the temporal relation of the Tres Pasos Fm to the underlying Cerro Toro Fm remains poorly constrained. The Tres Pasos Fm is characterized by mass wasting, slope channel, and abundant hemipelagic siltstone deposits, and has been interpreted as a high-relief slope system that was fed by deltas, the deposits of which are preserved in the genetically related Dorotea Fm. The inferred high energy flows and general slope instability are hypothesized to contribute to a paucity of preserved volcanic ash beds and in situ fossils that are typically used for age constraint in other parts of the basin fill. We employ a new approach to strontium isotope stratigraphy, which interprets resedimented (detrital) carbonate shell material as a maximum depositional age (MDA) rather than an exact age in the case of a single in situ specimen. Using populations of detrital shell material from lag deposits and shell-hash conglomerates, we provide new MDAs for the Tres Pasos Fm from samples collected at eight positions along a 100 km basin axial transect. Basal Tres Pasos strontium isotope ages are consistent with two newly dated volcanic ashes from the basal Tres Pasos and uppermost Cerro Toro Fm. Ages are also consistent with and bracketed by published detrital zircon ages in Cerro Toro and Dorotea deposits. The resulting ages constrain: 1) age of onset of Tres Pasos slope sedimentation at ∼80 Ma; 2) duration of Tres Pasos deposition (∼10 Myr); and 3) age relationships along the longitudinal profile of the basin both within the Tres Pasos Fm and relative to underlying and overlying formations. Additionally, this novel detrital strontium isotope stratigraphy approach provides insight about intra-basinal reworking of material that resulted from excavation into and/or basin-margin uplift of older basin fill.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016