Geology of the Sierra Foothills Melange and adjacent areas, Amador County, California

Abstract

Detailed outcrop mapping in the western Sierra foothills of Amador County, Calif., has resulted in some major changes in the interpretation of stratigraphy and structure. The Amador Group was originally defined at its type locality on the south bank of the Cosumnes River in Amador County to include the Cosumnes Formation and the conformably overlying Logtown Ridge Formation, but the new data indicate that the lower boundary of the type Logtown Ridge should be located 600 m farther west (downsection) than originally designated and that this boundary is a fault. The strata that were originally called the Cosumnes Formation are part of a lithologically diverse assemblage of tectonically intermixed rocks that constitute a newly recognized melange and thus are not a formational rock-stratigraphic unit as the earlier workers believed. Thus, the names Cosumnes Formation and Amador Group are both inappropriate in their type area and are abandoned. The Logtown Ridge Formation is here divided into four members, some of which cross what earlier was considered to be a formational boundary of the Logtown Ridge with overlapping pyroclastic strata. The outcrop mapping requires additional changes, although of lesser importance, in the identification and correlation of other Mesozoic rocks in Amador County. The newly recognized melange forms a 4-km-wide belt underlying the Logtown Ridge Formation. In addition to the type section of the abandoned Cosumnes Formation and scattered fault-bounded blocks of strata of Cosumnes lithology, the melange comprises rocks heretofore mapped as "western belt of Calaveras Formation," considered to be of Paleozoic age. Single clasts of this huge tectonic breccia range from a few centimeters to a few kilometers in maximum dimension. Distinctive strata are generally disrupted, and pervasive shearing is common. In the absence of fossils, no age of original deposition can be assigned to any clast or matrix of the melange, but on the basis of indirect structural evidence, the intermixing that formed the melange probably took place during the Late Jurassic or before, and therefore the now sheared and faulted strata must originally have been at least this old. Available data are ambiguous but suggest that rocks were intermixed to form the melange when the strata were horizontal or nearly so. Similarly, the overlying Logtown Ridge and Mariposa Formations were faulted when these rocks were essentially horizontal. The entire section was subsequently tilted to its present, nearly vertical position. Traditional syntheses of the tectonic history of the Sierra foothills argue that the faults there have always been steeply dipping. Although this may be true for some faults, the new interpretation suggests that most faulting occurred before the section was steeply tilted. Neither suggestion can yet be proved, but we maintain that the highly deformed rocks mapped in Amador County represent primarily the effects of subduction at a continental margin, possibly augmented by gravity tectonics in a trough of sediment accumulation there. On the basis of the ages of affected strata, this period of subduction was Late Jurassic but possibly began at an earlier time. If this interpretation of the melange in Amador County is correct, a belt of similarly deformed rocks should extend far beyond the limits of the study area.