Neogene Record of Rifting and Breakup in the Salton Trough, SE California: Limits on Sequence Analysis

Susan M. Kidwell

Spectacular exposures of a ~6 km-thick section of Mio-Pliocene strata in the western Imperial Valley illustrate ways in which structural activity and its cascading effects on basin topography and sediment sources can influence the anatomy, dimensions, and lateral traceability of sequences in tectonically active syn- and post-rift settings. Extensive block-faulting of crystalline basement, incised braidplain deposits, and conformable Miocene basalts produced a half-graben topography that was transgressed by Middle? to Late Miocene marine waters. Fanglomerates shed from upthrown footwalls grade distally and upward into marine siliciclastic sands and bioclastic limestones. In moderately deep half-grabens (approx. ~100 m fill), these facies are organized into retrogradational series of 10 m-thick fan delta cycles, but it is not possible to correlate flooding surfaces between half-grabens with confidence. Smaller half-grabens contain only a single marine parasequence, and deeper ones include non-cyclic small-basin turbidites of local provenance. At a horizon approximating the Mio-Pliocene boundary, local sections are punctuated either by unusual debris flows and mega- breccia or by an angular unconformity. Above this tectonically generated break-up unconformity (and its correlative conformity), sediment composition undergoes a rapid change to muds and quartz-rich sands, marking the coincidental initiation of the Colorado River as a major supplier to the nascent northern Gulf of California. Colorado deltaic deposits smother remnant block topography, and compris a single conformable progradational interval, which has been warped and faulted by San Andrean strike-slip faults only in the past 2 my. Delta-front strata are divisible into parasequences, but these are lenticular over a few km; the overlying (approx.) ~3 km of delta-plain deposits have proven to be indivisible from a sequence perspective.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California