REGIONAL CORRELATION OF THE SONSELA MEMBER OF THE CHINLE FORMATION, PETRIFIED FOREST NP, ARIZONA TO GRAND STAIRCASE-ESCALANTE NM, UTAH

A series of measured sections from Petrified Forest National Park (through The Navajo Nation, Lees Ferry, and Glen Canyon National Recreation Area) to Grand Staircase-Escalante National Monument done in conjunction with Northern Arizona University Geology Field Camp firmly establishes previous preliminary correlation of the Sonsela Member of the Upper Triassic Chinle Formation from east-central Arizona to south-central Utah; the resulting correlation greatly expands the known size of the fluvial basin to all of northeastern and north-central Arizona and adjacent parts of Utah and New Mexico. Based on fluvial paleocurrent patterns and abrupt lateral changes in fluvial style, the NW-trending line of sections cuts obliquely to perpendicularly across the Sonsela fluvial complex, thus crossing several trunk river systems. Despite the suspected presence of separate river systems exposed across the outcrop belt, most sections show three vertically successive fining-upward sequences within the Sonsela Member. The base of each sequence consists of beds composed of cross-stratified sandstone and local conglomerate. Sharp, scoured contacts with several meters of local relief mark their bases. Pebbles comprise quartz, quartzite, chert, and volcanic rock fragments. Significant variation in bed thickness, cross-bed type, and architectural pattern suggest that several different styles of major river channels deposited these beds. The channel deposits fine both upwards and laterally into heterolithic sandstone-mudsone overbank complexes.

At all locations, the Sonsela Member disconformably overlies distinctive blue-gray mudstone of the Blue Mesa Member and is conformably(?) overlain by dominant mudstone and subordinate thin sandstone of the Petrified Forest Member. Sandwiched between two muddy sequences, coarse deposits of the Sonsela represent a sharp contrast in fluvial history during deposition of the medial Chinle Formation. The presence of three fining-upward fluvial sequences that appear to correlate basin-wide across laterally diverse river systems suggests widespread controls acted across the basin, source area, or both, to influence Sonsela Member architecture.