Abstract
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 a single horizon through most of the area. Data from several modern high-bedload, low-sinuosity rivers show that, among logs with rootballs preserved, 83% are oriented with the rootballs on the upstream ends and 17% 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, suggesting that they were deposited by several 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 of varying degrees year to year, with variable effects on tree growth, and resulted in the formation of high-bedload rivers capable of undercutting, orienting, and transporting logs. The wider growth rings imply that the drying proposed for the later part of Chinle deposition may not have affected the trees; a drier climate could also result in trees growing closer to the streams, reducing their exposure to drought.