Stirring the Detrital Pot: Intrabasinal Sediment Recycling and Mixing Revealed Through Strontium Isotopes, Cretaceous Magallanes Basin, Chile

Abstract

Provenance information, such as age or composition, is an important component of source-to-sink analysis and is widely used to interpret changes in sediment source area characteristics and/or configuration. However, sediment mixing during transfer from the hinterland to the basin can introduce complex detrital age distributions where distinct contributions from various source terranes can be difficult to resolve. In addition to mixing related to sediment routing system configuration (e.g., connectivity via longshore transport), there can also be mixing that results from recycling of previous accumulated material within the basin. Such intrabasinal recycling could further complicate detrital age signatures by combining mixtures of different depositional ages. Here, we use strontium isotopes ages of calcium carbonate shells that were reworked from older strata to investigate spatiotemporal patterns of intrabasinal sediment recycling.

The Patagonian fold-thrust belt and related Upper Cretaceous strata of the Magallanes Basin in southern Chile offer an excellent natural laboratory for investigating source-to-sink sediment routing. We leverage the exceptional chronostratigraphic context in this basin based on volcanic-ash geochronology, detrital zircon (DZ) geochronology, and strontium isotope stratigraphy (SIS), with provenance analyses and extensive stratigraphic mapping to test and apply the novel approach of detrital strontium isotope stratigraphy (dSIS). Rooted in the fundamental principles of SIS, dSIS offers the opportunity to use resedimented, intrabasinally sourced carbonate shell material to elucidate sediment storage and recycling signals in deep-marine deposits. Results from a 100 km-long basin-axial transect through stratigraphically younging clinoform deposits reveal a signature of progressively older and more proximal (shelf-derived) sediment sources. Intrabasinal recycling via excavation and/or uplift within the depositional basin introduced sediment into the sediment routing system that was deposited up to 8 million years earlier. Clinoform deposits therefore record compound mixtures of sediment sourced from the active drainage basin mixed at 10² – 10⁴ yr time-scales with intrabasinally sourced sediment >5 myr older than the active depositional system, which was potentially just as well mixed when deposited. Such mixing is a critical, and rarely addressed consideration when interpreting detrital grain signatures from terminal basin deposits.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018