Description
Sedimentation styles and patterns for lower Neogene continental sedimentary rocks exposed in the Vallecito, Split, Fish Creek, and Superstition Mountains are examined. These rocks are part of a previously unrecognized strati-graphic sequence that includes locally intercalated, lower to middle Miocene, predominantly basaltic, volcanic rocks. The sequence rests on an older (Paleocene?) pediment surface cut across crystalline basement rocks of the Southern California Batholith: locally, however, this low-relief surface has been deeply incised by early Neogene drainage. In turn, the sequence is unconformably overlain by upper Neogene marine and continental strata associated with opening of the Gulf of California. Formal stratigraphic terminology proposed to date is controversial and inadequate. Therefore, it is recommended the name Anza Formation be used to encompass the lower Neogene stratigraphic sequence; other nomenclatural recommendations are also offered. Lower Neogene continental sedimentary rocks consist of three environmental facies: (1) braided stream, (2) alluvial fan, and (3) landslide. The braided stream facies is typified by lenses of conglomeratic sandstone. Vertical sedimentary sequences, paleocurrent indicators, and clast populations indicate that a Platte-type braided stream drained westward off a pediment surface cut into a "back-tipped" basement block and joined a Donjek-type axial drainage system that flowed northward. The alluvial fan facies consists of debris flow, sheetflood, and channel-fill deposits with subfacies defined by the distribution of these deposits along with bedding geometry. Proximal subfacies is characterized by debris flow and channel-fill deposits with numerous internal erosional contacts and laterally discontinuous beds. Midfan deposition below the intersection point is marked by laterally continuous, regularly interbedded, very thick debris flow and thinner-bedded sheetflood deposits. Distal subfacies is represented by interbedded debris flow and sheetflood deposits that are thinner bedded and less organized than the midfan subfacies. Stratigraphic relationships and paleocurrent data suggest that alluvial fans were deposited along a down-faulted western basin margin and eventually prograded eastward over the axial braided stream system. The upper facies is a massive, laterally continuous, sedimentary megabreccia containing clasts up to 15 feet in diameter. This facies is thought to represent a catastrophically emplaced, air-cushioned landslide. Palinspastic removal of 230 miles of northwestward translation along faults of the San Andreas system, and 90 degrees of clockwise rotation, restore the early Neogene half-graben basins of the western Salton Trough to north-western Sonora, Mexico. Furthermore, these basins align with the Basin-and-Range tectonic trends of southwestern North America, suggesting early Neogene continental sedimentation may have occurred in a Basin-and-Range basin.
