CONSTRAINING THE TIMING OF SEDIMENT DELIVERY TO A CRETACEOUS DEEP-WATER SLOPE SYSTEM VIA ZIRCON GEOCHRONOLOGY AND DETRITAL STRONTIUM ISOTOPE STRATIGRAPHY, MAGALLANES BASIN, CHILE

Deep-water slopes serve as key sinks towards the ends of continent-scale sediment routing systems on many basin margins. However, the temporal properties of associated sedimentary systems remain poorly constrained. To elucidate the temporal evolution of slope systems, we present new chronostratigraphic data from outcropping strata of the Magallanes foreland basin, Chile. The succession is up to 2.5 km thick and can be traced for 100 km along depositional dip. The deposits record linked shelf-slope evolution along a deeply subsided foredeep during the Campanian-Maastrichtian. The objectives of this study are to: (1) use mapping and the chronologic data to construct a new stratigraphic framework for the ancient slope system; and (2) determine timing of slope evolution phases to decipher the controls on sediment delivery to the slope.

Chronologic data was derived from >6300 detrital zircon ages (N = 13), 2 volcanic ash zircon ages, and 80 detrital strontium ages obtained from numerous stratigraphic intervals within the succession. Zircon ages were determined from ²³⁸U/²⁰⁶Pb ratios via LA-ICP-MS and TIMS; detrital strontium ages were determined from ⁸⁷Sr/⁸⁶Sr ratios in detrital carbonate material (inoceramid and oyster shells).

Results indicate that the slope system underwent four evolutionary phases (0.5-4.6 Myr in duration), which record the shift from an out-of-grade, mass-wasting-dominated slope to dominantly graded clinoform growth over >10 Myr. Zircon age populations show that all phases have statistically indistinguishable proportions of ages >90 Ma; this suggests that no major tectonic changes impacted sediment supply. Detrital strontium ages are concentrated at 87 Ma and 79-80 Ma, which indicates that older intrabasinally sourced deposits were reworked during slope deposition.

We interpret that slope system initiation and subsequent progradation was linked to a decline in basin subsidence. Early evolutionary phases were affected by inherited slope topography, which was fully healed by the later phases. Within this framework, punctuated coarse-grained sediment delivery is plausibly tied to sea-level change. The geochronologic methods represent a novel approach to the study of basin evolution; the results provide key insight into the evolution of basin-scale depositional trends worldwide.