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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Two massive, very thick (165 ft or 50 m), essentially tabular sedimentary megabreccia beds, exposed in the Vallecito Mountains, Split Mountain, and Fish Creek Mountains, are comprised of very poorly sorted, large boulders up to 33 ft (10 m) in diameter, suspended in a comminuted silty sand matrix. Many boulders can be visually reconstructed, like pieces of a giant jigsaw puzzle. Upper bed boundaries are hummocky, and lower bed contacts are undulatory. Where present, subjacent sedimentary strata are typically disrupted, locally deformed into mega-flaps (-flames) and also locally occur as large rip-up blocks. Each megabreccia bed is thought to represent a catastrophically emplaced, air-cushioned landslide, perhaps triggered by a strong seismic event.
The stratigraphic position, paleotransport data, and provenance suggest that these catastrophic landslides were deposited during mid-Neogene tectonic readjustments in the Salton Trough. The lower megabreccia bed culminates early Miocene nonmarine sedimentation in a Basin-Range(?) rift basin and was derived from the Vallecito Mountains and transported eastward. The upper megabreccia bed occurs within the lower Pliocene basal marine and nonmarine deposits of the Gulf of California, and is though to have been transported southward from a "phantom-porpoise" structure of the San Jacinto fault zone, indicating a minimum early Pliocene age for the San Jacinto fault.
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