LITHOSTRATIGRAPHIC AND STRUCTURAL CONTROLS OF CANYON MORPHOLOGY, BIG CHICO CREEK, CA

The morphologic characteristics of Big Chico Creek Canyon located NE of Chico, CA, are influenced by lithologic factors related to processes active during deposition of the late Pliocene to Pleistocene Tuscan Formation and subsequent flexure along a series of cross-cutting faults referred to collectively as the Chico Monocline.

Slope gradient domains determined from closely spaced computer generated transverse topographic profiles are projected onto a longitudinal slope cross section along the canyon. This profile is compared to measured columnar sections at several locations to determine the relationship between slope angle and lithofacies. Schmidt hammer impact readings used to estimate degree of induration of lithotypes comprising the wallrocks of the canyon suggest a major difference between the matrix of cliff forming diamict volcanic breccia, which record relative values above 30, and the matrix of fluvial conglomerate and sandstones with values below 25. The matrix of the breccia consists primarily of sand sized volcanic rock fragments and crystals in grain contact, and is generally devoid of fine clastic constituent. Petrographic and X-Ray analyses of the breccia and associated tuffaceous sandstones indicate that incipient crystallization of secondary minerals, such as zeolites, enhance adhesion at grain and crystal contacts resulting in greater induration, increased resistance to erosion, and higher slope gradients. In general, greater surface slope gradients in Big Chico Creek canyon correspond to higher percentages of volcanic breccia within the Tuscan. Distinct breaks in slope separate steeper, breccia rich upper Tuscan from fluvial sediment dominated lower Tuscan at approximately the Ishii tuff horizon (ca 2.7 Ma).

Anastomosing vertical fractures related to the N-S trending Chico Monocline allow infiltration of surficial water into the volcanic breccia, decreasing Schmidt values to less than 20 in weathered zones. Lateral discontinuities in slope profiles and stream length gradients characterize the zone of highest fracture density.

Our initial results suggest that detailed slope gradient analysis may be useful as a means of inferring lithofacies and structural variation in unmapped, poorly accessible portions of the aerially extensive Tuscan Formation of northeastern California.