Abstract
Fossil logs from the Sonsela Member of the Chinle Formation in the Long Logs area of Petrified Forest National Park are exposed in plan view and are likely derived from closely spaced horizons through most of the area. Data from several modern high-bedload, low-sinuosity rivers show that, among logs with rootballs preserved, 60% are oriented with the rootballs on the upstream ends and 12% are oriented with the rootballs on the downstream ends. Applied to the assemblage at Long Logs, flow in the river that deposited them was generally eastward. Spatial analysis shows the logs at Long Logs do not have a consistent pattern, supporting the hypothesis that they were likely deposited by sequential flood events. Previous workers have concluded that the growth rings in the logs from Petrified Forest National Park were determined by a combination of endogenous and environmental influences and did not represent growth rings in the conventional sense but, rather, growth interruptions. This paper supports that conclusion but further concludes that the environment was the dominant influence; growth rings most likely reflect fluctuations in the water table and variable proximity of the trees to the water table as they grew. Mean ring widths ranged from 0.35 cm to 2.41 cm in logs from the Sonsela Member, and 0.18 cm to 2.33 cm in logs from the Petrified Forest Member. However, statistics on all rings indicate that, overall, the growth rings are wider in woods from the Petrified Forest Member, and the difference is statistically significant. The data from both the growth rings and the log assemblage 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, and resulted in the formation of high-bedload rivers capable of undercutting, transporting, and orienting logs. The wider growth rings from younger Petrified Forest Member trees imply that the drying proposed for the later part of Chinle deposition may not have affected their growth; a drier climate could also result in trees growing closer to the streams, reducing their exposure to drought. This is supported by the lower mean sensitivities of the trees from the Petrified Forest Member.