FLUVIAL CHANNEL ARCHITECTURE AND DEPOSITIONAL SETTING OF THE TUSCAN FORMATION, CHICO, CALIFORNIA

The late Pliocene age Tuscan Formation is the primary aquifer for the northeastern Sacramento Valley and is composed of volcaniclastic sediments including debrites, conglomerates, sandstones and siltstones. This study investigates the stratigraphic framework and depositional setting of the Tuscan Formation in the Chico, California area. This information is important for defining aquifer characteristics.

We measured and described five stratigraphic sections of the Tuscan Formation in Upper Bidwell Park east of Chico. The descriptions included bedding type, grain-size, angularity, color and stratigraphic structure. Sections were correlated on the basis of lithology. The results of our analysis indicate that the lower portion of the outcrop is mainly of fluvial sandstones and conglomerates deposited in shallow braided channels. These are overlain by a mixture of fluvial sandstones and conglomerates, with the addition of angular, matrix-supported debrites (lahars). The fluvial channel-fills in both the lower and middle portion of the Tuscan Formation have relatively low width to thickness ratios ranging from 15:1 to 20:1. The upper portion of the Tuscan Formation is almost exclusively of cobble to boulder sized, matrix supported debrites (lahars). This succession represents a classic volcanic fan apron with the medial fan facies consisting of fluvial sandstone and conglomerates, the medial/distal facies is a mixture of sandstone, conglomerate and debrites, while the proximal facies is represented by the presence of lahars. The vertical organization of these deposits suggests the volcaniclastic apron progressively prograded during Tuscan Formation deposition.