135-10 New paleoenvironmental data for the dinosaur-bearing Chignik Formation (Late Cretaceous), Aniakchak National Monument, southwest Alaska

Session: Science and Stewardship of U.S. National Park Service Paleontological Resources

Presenting Author:

Authors:

Abstract:

The Chignik Formation (Late Cretaceous) in Aniakchak National Monument (ANIA), located on the Alaska Peninsula, is now recognized for an extensive record of dinosaur footprints, primarily attributable to hadrosaurs, or duck-billed dinosaurs. The Chignik Formation (CF) in ANIA is approximately 500–600 m thick, representing an up-section shift from shallow marine to coastal/tidal flat to fluvial depositional environments.  The entire succession is interpreted as a transgressive-regressive sequence, consistent with a tide-dominated estuary fill. Paleomagnetic reconstructions suggest that the CF was deposited near its current latitude, almost 57^oN.  New field observations of two thick reddish paleosols, changes in insect herbivory frequency from leaf mat horizons, and putative crayfish burrows add paleoenvironmental insights for the CF.  Three leaf mat horizons were sampled through the section, with up to 100 leaves per horizon examined. Insect herbivory was identified as either puncture or trail herbivory. 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, over 12^o to the south.  Thus, 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 Program. Vol. 57, No. 6, 2025

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