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