137-3 Absolute chronology and carbon isotope stratigraphy of Paleocene strata from Sagwon Bluffs, Alaska, and implications for high latitude Earth systems

Session: Joint SGD-SEPM-IAS Focus on the Sedimentary Record of Climate Change

Presenting Author:

William Craddock

Authors:

Craddock, William H.¹, Lease, Richard Oliver², Wycech, Jody³, Connors, Christopher D.⁴, Roberts, Dana⁵, Dreier, Mark⁶, Gooley, Jared T.⁷, Smith, Rebecca⁸

Abstract:

Recently reported absolute chronologies from sediment cores and outcrops around the Arctic have facilitated pioneering reconstruction of Cretaceous-Cenozoic Earth systems at high latitudes. However, gaps of 10s of Myr remain, notably including much of the Paleocene. Here, we present a new absolute chronology of a ~260-m thick succession of mostly delta plain strata at Sagwon Bluffs in the central Colville foreland basin, Alaska. Recent analysis of fossil pollen and other plant matter indicates the presence of both Danian (~66–62 Ma) and Selandian-Thanetian (~62–56 Ma) intervals within this section, and flora characteristic of modern mid-latitudes that flourished at these times. New geochronology by laser-ablation inductively-coupled-plasma mass spectrometry of zircons separated from three ash layers indicates that the lower part of the section is ~63 Ma. Ongoing work focuses on refinement of this geochronology by high precision chemical-abrasion isotope-dilution thermal-ionization mass spectrometry of zircon. The entire section was described, and bulk sediment samples were collected at 0.5-m spacing for the purpose of establishing a total organic carbon isotope (δ¹³C_org) stratigraphy (determined via continuous flow-isotope ratio mass spectrometry).  From bottom to top, the work resolves an 80-m basal section with relatively stable values of ~-26‰; a ~70-m covered interval; a ~60-m interval that records a positive shift to values of ~-25‰; a 20-m interval that records a negative shift to values approaching -28‰; and an upper, unconformably overlying 30-m interval of boulder conglomerate that is mostly barren of organic carbon.  The entire carbon isotope stratigraphy contains punctuated negative carbon isotope excursions (CIEs) of ~1‰ that are recorded over a few meters of section and probably represent ~10⁵ yrs of time. The δ¹³C_org stratigraphy is qualitatively similar to global reference records of the Paleocene, and our working interpretation is that this section spans a land-based record of both the late Danian CIE and the Paleocene Carbon Isotope Maximum (PCIM), as well as many shorter-lived, minor CIEs. The upper unconformity may represent a sequence boundary associated with the Paleocene-Eocene Thermal Maximum that marked termination of the PCIM. Importantly, this outcrop lies within a grid of seismic reflection data and wells that facilitates projection of this age framework into stratigraphic records of the lesser-known contemporaneous marine realm at outcrops, and within cored/cuttings intervals in petroleum exploration wells.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9453

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