COLORADO RIVER DELTA: 5 MYR-OLD, TIDE-DOMINATED, BIG-RIVER DELTA IN A TECTONICALLY EVOLVING, OBLIQUE RIFT BASIN

In contrast to many of the world's big-river deltas characterized by late Cenozoic progradation and outbuilding of continental margins, the Colorado River delta has remained more or less stationary during most of its ~5 Myr history as the Salton Trough rift widened transtensively. The delta was initiated during Pliocene basin-margin low-angle detachment faulting, but persisted through the change to modern basin-centered "leaky transform" extension. The delta prograded across the Salton Trough during its first 1 Myr, and has remained a barrier to further marine incursion throughout the last 4 Myr. While Quaternary deltaic deposits are growth-faulted and mineralogically altered by hydrothermal plumes at right-stepping offsets of the plate-bounding transform fault system, no corresponding evidence of extensive growth faulting or hydrothermal plumes has been observed in exposed Pliocene deltaic deposits.

Pliocene facies exposed in the west-central Salton Trough correspond broadly to modern deltaic environments (tide-dominated shallow marine, delta plain, lacustrine); the Pliocene vertical succession reflects northwestward tectonic transport through these environments rather than simple progradation. In contrast to many passive-margin and foreland-basin counterparts, the Pliocene transition from marine to nonmarine is gradational and appears to lack through-going master erosion surfaces, probably due to high sedimentation rates. Offlapping, upward-coarsening delta-front parasequences are limited to a relatively narrow interval between underlying prodelta and overlying tidal flat deposits. Most of these parasequences are capped by high-energy shoal deposits of cross-bedded sandstone and oyster-shell coquina with mostly unimodal, offshore-directed paleocurrents. The lack of through-going master surfaces and limited range of parasequences thus make standard sequence stratigraphic approaches problematic. Counterparts to these parasequences have not yet been recognized in the modern Colorado delta, but have been described from the modern Mahakam delta, where they represent aggradation of tidal channel-mouth and channel-margin shoals, providing a valuable analog for this aspect of Pliocene Colorado delta stratigraphy.