267-9 Spatial-Statistical Analysis of Fossil Logs in the Chinle Formation, Petrified Forest National Park

Session: Advances in Fluvial Processes and Sediment Transport (Posters)

Presenting Author:

Authors:

Abstract:

The orientations of logs from the Chinle Formation in the Long Logs area of Petrified Forest National Park were analyzed to obtain information on the direction and character of the paleocurrent at the time of deposition. The logs are exposed in plan view, and most were likely deposited in a single horizon or a few closely spaced horizons. Of the 554 logs included in this study, 140 were preserved with intact rootballs. Data from several modern high-bedload, low-sinuosity rivers show that 60% of logs with preserved rootballs are oriented with the rootball at the upstream end, while 12% are oriented with the rootballs at the downstream end. When applied to the Long Logs assemblage, the orientations of the rootballs indicate flow was generally eastward.

In an attempt to extract more detailed information on flow patterns from the log orientation data, we represented the 360° orientation azimuths as range-limited (0° to 180°) complex numbers (u + iv, where i = √(-1)) and developed model variograms for the functions u and v with the gstat package from the statistical computing environment R. On the basis of the inferred spatial correlation structure, and employing the measured log orientations as conditioning data, we used the GSLIB program kt3d to determine the most probable directions for flow at gridded locations for resolutions from 2 X 2 m to 10 X 10 m. The flow direction vector fields support the generally eastward flow direction inferred from the rootball orientations but indicate clear flow patterns in some areas and confused or contradictory flow in others. This suggests the logs were deposited by multiple flood events, during which time the channel location may have migrated, resulting in overlapping or overprinted depositional patterns. In agreement with previous studies, data from the present study support the conclusion that the Chinle Formation was deposited in a strongly seasonal climate with respect to rainfall, resulting in the formation of high-bedload rivers capable of undercutting, transporting, and orienting logs.

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

doi: 10.1130/abs/2025AM-5262

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