135-7 Growth Rings in Fossil Trees from the Chinle Formation, Petrified Forest National Park, Arizona, USA

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

Presenting Author:

Dr. Judith Totman Parrish

Authors:

Parrish, Judith Totman¹, Gillis, Robert J.², Fairley, Jerry P.³

(1) Univ Idaho Earth & Spatial Sciences, Viola, ID, USA, (2) Alaska Div. of Geological & Geophysical Surveys, Fairbanks, AK, USA, (3) Univ Idaho Earth and Spatial Sciences, Moscow, ID, USA,

Abstract:

Growth rings in 51 fossil logs from the Sonsela and Petrified Forest (PF) members of the Chinle Formation in Petrified Forest National Park were largely measured in field samples. We emphasized measuring growth rings in the field in order to maximize the lengths of the growth-ring sequences, so that our data provide information unattainable by measuring the short sequences observable in the restricted sizes of thin sections. Field measurements also minimized the removal of paleontological resources.  Mean ring widths ranged from 0.35 cm to 2.41 cm in 22 logs from the Sonsela Member and 0.18 cm to 2.33 cm in 29 logs from the PF Member. Overall, however, the growth rings are wider in woods from the PF Member, and the difference is statistically significant. The widest uninterrupted growth radius was 33 cm; 5 specimens had no growth interruptions. In both the Sonsela and PF members, a large variability in the expression of growth interruptions is observed. Previous workers concluded that the growth rings in the logs were determined by a combination of endogenous and environmental influences and did not represent growth rings in the conventional sense but, rather, growth interruptions. Our data support that conclusion, but further indicate that the environment was the dominant influence. The variability in the growth rings is consistent with trees growing at varying distances from the river channels and thus experiencing different degrees of drought in the highly seasonal and variable monsoonal climate.  The growth rings most likely reflect fluctuations in the water table and variable proximity of the trees to the water table as they grew. The most common taxon, Araucarioxylon, is known to have had taproots. Our data support the conclusion that the Chinle Formation was deposited in a strongly seasonal climate with respect to rainfall, which caused fluctuations in the water table year to year, with variable effects on tree growth. The wider growth rings from the younger PF Member trees imply that the drying proposed by others for the later part of Chinle deposition may not have affected their growth. Rather, a drier climate may have resulted in the trees growing closer to the streams, reducing their exposure to drought. This is supported by the lower mean sensitivities of the trees from the PF Member.

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

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