MAJOR BUTTRESS DISCONFORMITY WITHIN THE MIOCENE SPLIT MOUNTAIN GROUP, SALTON TROUGH, SOUTHERN CALIFORNIA

The Salton Trough underwent an episode of Late Neogene-Quaternary detachment faulting and subsidence that abruptly ended with the inception of the currently active Elsinore, San Jacinto and related right-lateral strike slip faults. Dorsey & others (2011) interpret the timing of detachment faulting as 8.0±0.4 to 0.95 Ma, based on the ages of the basin fill sediments. However, the oldest unit used to bracket this timing—the Elephant Trees Fm, an alluvial fan deposit in the Miocene Split Mountain Group—has been interpreted by Winker & Kidwell (1996) as interfingering with volcanic rocks of the 17 Ma (Morgan & others, 2012) Alverson Fm, implying an age much older than 8 Ma.

In an attempt to resolve this age discrepancy, we conducted field research in the southwestern Fish Creek Mountains, where Winker & Kidwell had reported the interfingering relationship. We observed two nearly identical alluvial fan deposits of distinctly different ages, separated by a buttress disconformity: (1) older primarily volcaniclastic conglomerates which fill isolated paleocanyons and are interbedded with the 17 Ma Alverson Fm, and (2) younger mixed clast conglomerates which we interpret as the Garnet Fm, an alluvial fan deposit within the Split Mountain Group that Winker & Kidwell (1996) correlate with the Elephant Trees Formation but do not report as occurring in the Fish Creek Mountains. Both of these conglomerates are composed of a mixture of volcanic clasts and “basement” (metamorphic and plutonic) clasts; thus neither resembles the Elephant Trees Fm, which is composed exclusively of metamorphic and plutonic clasts.

Thus there are, indeed, alluvial fan deposits interbedded with the 17 Ma Alverson Fm, but these alluvial fan deposits are not correlative with the Elephant Trees Fm. These older alluvial fan deposits are restricted to paleocanyons and thus predate detachment faulting and associated regional subsidence. The younger Elephant Trees Fm, as well as the Garnet Fm, both of which mark the beginning of regional extension in the area, could be as young as 8 Ma, as interpreted by Dorsey & others (2011).

Dorsey & others, 2011, GSA Bulletin, v. 123, p. 771-793.

Morgan & others, 2012, GSA Abstracts with Programs, Vol. 44, No. 3, p. 21.

Winker & Kidwell, 1996, Field Conference Guidebook, AAPG Annual Convention, p. 295–336.