Sedimentology, facies architecture and chemostratigraphy of a continental high-latitude Paleocene–Eocene succession—The Chickaloon Formation, Alaska

Abstract

The Paleocene–Eocene Chickaloon Formation of southern Alaska represents rapid deposition in a floodbasin setting, contains abundant and diverse fossil plant material, and spans the Paleocene–Eocene Thermal Maximum (PETM). Sedimentologic, lithofacies, and chemostratigraphic analyses of exposures of the Chickaloon Formation were conducted in order to provide a contextual framework for this, late Paleocene–early Eocene high-latitude sedimentary succession and to test if hyperthermal conditions are recorded in the unit. At the main study area in the Evan Jones Coal Mine, strata represent an alluvial sequence dominated by floodplain, channel sandstone, crevasse lobe, crevasse channel, and mire deposition. Floodplain environments are represented by medium-gray, commonly nodular and/or carbonate-cemented, shaly siltstone and centimeter-scale thick sheet-sandstone interbeds. Most channel sandstone deposits are interpreted to be crevasse channels and are generally small (2–5 m thick), rarely coarser than medium-grained sandstone, and sand grains are commonly coated with Fe-oxides. Carbonaceous shale and coal are localized and indicate a high-rate of organic carbon accumulation in the area. δ 13C analyses of bulk organic carbon from the succession at the Evan Jones Mine are internally consistent with this interpretation; they yield uniformly negative values that are typical of continental organic matter (− 21.7‰ to − 28.5‰; X̅ = − 25.4‰). Fossil plant material is also very common and includes leaves, wood, seeds, and resins from broad leaf angiosperms, conifers, ferns, and sphenopsids. Considered together, lithologic and paleontologic data suggest that these strata represent a floodbasin environment; the observed crevasse channels probably originate from a main channel located within several kilometers of the depocenter. Ash-fall tuffs punctuate the studied sequence and contain abundant reworked or recycled zircons, indicating that the depocenter was proximal to a volcanic source. Across all of these Chickaloon depositional environments, a significant negative isotopic shift (− 5.8‰) occurs, which we hypothesize to represent a short-lived perturbation associated with a global hyperthermal event.