Early neogene continental sedimentation in the vallecito and fish creek mountains, Western Salton Trough, California

Abstract

Lower Neogene sedimentary rocks in the Vallecito and the Fish Creek Mountains, along with locally intercalated basalts, were deposited in a half-graben Basin-and-Range setting. Three depositional environments are recognized: (1) braided stream; (2) alluvial fan; and (3) landslide. A Platte-type braided stream system, with local accumulation of seif dunes, drained a westward-tilted pediment surface. Coarse sand was deposited as transverse and linguoid bars, sand dunes and flat beds, and gravel was deposited as longitudinal bars, modified by tractive flow, in broad, shallow channels. This system joined a Donjek-type northward-flowing axial drainage characterized by channels with marked topographic relief. Deep channel reaches were infilled with longitudinal gravel bars and massive granular, very coarse sand from concentrated dispersions. Giant sand waves were rarely present in deeper channel reaches. In shallower channel tracts, coarse sand was deposited as transverse bars, sand dunes and flat beds, and gravel was deposited as small longitudinal bars. The alluvial fan deposits accumulated along the faulted western margin of the basin and prograded eastward over the axial drainage. They consist of debris flow, sheetflood and Donjek-type channel-fill deposits. Subenvironments are defined by the distribution of these deposits and their bedding geometries. The upper fan (above the intersection point) is characterized by debris flow and channel-fill deposits with numerous internal erosional contacts and laterally discontinuous beds. The middle fan (below the intersection point) is marked by crudely developed thickening-upward cycles of laterally continuous, repetitively interbedded, very thick debris flow and thinner bedded sheetflood deposits. The lower fan is represented by interbedded debris flow and sheetflood deposits that are thinner bedded and less organized than the middle fan subfacies. The landslide deposits locally top the sequence. They are massive, laterally continuous sedimentary megabreccias containing clasts that are up to 5 m in diameter and display a distinctive jigsaw-puzzle texture. These breccias formed as two cataclysmically emplaced, air-cushioned landslides. One was dislodged from the Vallecito Mountains and traveled eastward, while the other was released from the Fish Creek Mountains and traveled westward.