(A)SYNCHRONOUS STRATIGRAPHY? RESOLVING SEQUENCE-STRATIGRAPHIC RELATIONS WITH CA-TIMS ZIRCON GEOCHRONOLOGY OF CENOMANIAN TOPSET STRATA, COLVILLE FORELAND BASIN, ALASKA

Chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-TIMS) zircon geochronology has created new opportunities for precise and accurate chronostratigraphic calibration. Here, we use tandem dating, with in situ screening by laser ablation and final analyses by CA-TIMS, of tephra (TZ) and detrital (DZ) zircon to correlate between Nanushuk–Seabee Formations sections that crop out at Rooftop Ridge and Ninuluk Bluff (central North Slope). The Nanushuk–Seabee transition is lithostratigraphically well expressed at both localities and is regionally regarded to record rapid and relatively synchronous transgression, terminating sandy shoreface sedimentation (Nanushuk) as muddy offshore deposition (Seabee) commenced above an intra-transgressive systems tract (TST) transgressive surface of erosion (TSE; i.e., Nanushuk–Seabee contact). Although traditional views of sequence-stratigraphic surfaces as timelines have largely shifted to more nuanced discussions of diachroneity, high-precision tests are rare. In this case, a TZ age from 4.2 m above the TSE at Ninuluk Bluff is 94.909 ± 0.032 Ma (2σ internal), yet a TZ age from ~170 m above the TSE at Rooftop Ridge is 95.009 ± 0.033 Ma. Furthermore, a single-grain DZ constraint of ≤96.874 ± 0.066 Ma from 0.2 m above the TSE at Rooftop Ridge indicates that the surface there is ≤~2 Ma older than it is at Ninuluk Bluff. This diachronous Nanushuk–Seabee transition along the ~115 km depositional-strike distance between the two exposures is bolstered by an age–depth synthesis that incorporates eight CA-TIMS results (~96.2–94.5 Ma) from ~205 m of section at Ninuluk Bluff and five CA-TIMS results (~98.9–95.0 Ma) from ~680 m of section at Rooftop Ridge. Pending analyses of two additional TZ samples from Rooftop Ridge will be integrated into a Bayesian model to further refine the correlation with Ninuluk Bluff and permit a posterior ages comparison for the base-of-TST transgressive surface at each locality. Establishing accurate stratal-age frameworks is also critical for correlating with well-calibrated regional and global datasets that themselves bear on stratigraphic cyclicity, and such comparisons may delineate causal factors reflected in the rock record. The extraordinary temporal resolution necessary for this work would not be possible without Jim Mattinson’s chemical abrasion technique.