19-3 NEW PALEOENVIRONMENTAL PERSPECTIVES FROM THE MUD BETWEEN THE TOES OF THE DINOSAURS OF THE CHIGNIK FORMATION (LATE CRETACEOUS), ANIAKCHAK NATIONAL MONUMENT, SOUTHWEST ALASKA

Session: The Campanian Crucible: A synthesis of vertebrate paleobiogeography and ecosystem dynamics in Laramidia

Presenting Author:

Anthony Fiorillo

Authors:

Fiorillo, Anthony R¹, McCarthy, Paul J², Kobayashi, Yoshitsugu³, O'Sullivan, Paul⁴

(1) Department of Cultural Affairs, New Mexico Museum of Natural History & Science, Albuquerque, New Mexico, USA; Hokkaido University Museum, Sapporo, Japan, (2) University of Alaska, Fairbanks, Alaska, USA, (3) Hokkaido University Museum, Sapporo, Japan, (4) GeoSeps Services, Moscow, ID, USA,

Abstract:

The Chignik Formation (Late Cretaceous) in Aniakchak National Monument (ANIA; Alaska Peninsula) contains an extensive record of world-renowned dinosaur footprints. The Chignik Formation (CF) in ANIA is approximately 500–600 m thick, representing shallow marine to nearshore marine environments in the lower part and continental alluvial to estuarine environments in the upper part of the section. Paleomagnetic reconstructions suggest that the CF was deposited near its current latitude, almost 57^oN.  Biostratigraphic data suggest the CF straddles the Campanian-Maastrichtian boundary.  Preliminary detrital zircon data are consistent with this age determination.  New field observations of paleosols, insect herbivory frequency from leaf mat horizons, and crayfish burrows add paleoenvironmental insights for the CF.  Field observations of two thick red paleosols (1-2 m thick) have O-A-Bw-Bt-BC-C horizons.  There is evidence for a thin organic-rich A horizon, with fine carbonaceous and coaly root traces and fine subangular blocky structure.  Subangular blocky texture with clay cutans on ped surfaces and drab halos around root traces suggest the presence of a Bt horizon.  Modern soils with a Bt horizon develop under forest vegetation in temperate climatic zones. Previous paleoclimate data for the CF are consistent with this new paleosol interpretation.  Three leaf mat horizons were sampled through the section, with up to 100 leaves per horizon examined. Insect herbivory was identified as either punctures or trails. Toothed margin leaves dominated each sample, as did trail herbivory. Frequencies of insect damage ranged from 26% - 83% suggesting that environmental stress varied through time.  Crayfish burrows are preserved only in cross sections along bedding planes.  Diameters range from 8-10 cm, and the boundaries of the burrows are sharp.  More than half the burrows have a thin gray mudstone lining that is darker than the burrow fill.  The diameters as well as the linings are features consistent with fossil crayfish burrows reported elsewhere.  Modern crayfish are not found farther north than southern Ontario. Lastly, unionid burrows show that at least some river systems flowed with clear water and had stable substrates.  During deposition of the CF, varying frequencies of insect herbivory on fossil leaves through the stratigraphic section indicate environmental stress fluctuated through time. Crayfish burrows support palaeobotanical models of warmer temperatures than experienced in this part of Alaska today.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4, 2026

doi: 10.1130/abs/2026RM-13977

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