VIEJO FORMATION: A NEW FORMATION FOR THE IMPERIAL GROUP SEDIMENTS IN THE SOUTHERN COYOTE MOUNTAINS, SALTON TROUGH , SOUTHERN CALIFORNIA

Kidwell and Winker (1996) describe the Andrade Member of the Latrania Formation, Imperial Group as a marine progradation (transgression) over the non-marine fanglomerate of their Garnet Formation, Split Mountain Group in the Coyote Mountains (CM). Our detailed mapping in the CM indicates that their Garnet Formation is composed of several (4+) distinct fresh-water fanglomerates. We also have divided their Andrade Member into several (8+) transgression-regressional (T-R) marine (Imperial Group) sequences that bracket some of our fresh-water fanglomerates. The transgressions have a recognizable, predictable and repeated marine sedimentary sequence consisting of a basal conglomerate followed by sandstone and then a mudstone. These marine sedimentary sequences are capped by unconformities representing regressions. At least one of our fresh-water fanglomerates interfingers with marine sediments. Fresh water limestones are associated with at least two of our fanglomerates. Our fresh-water fanglomerates and T-R marine sequences make up the Viejo Formation (VF).

The lower part of the VF has a siltstone (Woodring’s Outcrops) that is interbedded with basalts of the Alverson Formation (17 Ma, Morgan et al., 2012). The top member of the VF has a distinctive basal conglomerate, described as having resistant, well-rounded, quartz rich pebbles in a course arkosic matrix with granitic lithic fragments. The lithic fragments are responsible for the desert varnish that is often found on this member. We believe this top member may be reworked from a marine extension of Dibblee’s (1954) lower rock-slide breccia found in the Split Mountain area and is the first Imperial unit to cover the CM. This would make the VF older than the Latrania.

Winker and Kidwell describe the Latrania Formation as marine sediments in a shallow subsiding basin in the Fish Creek-Vallecito-Split Mountain area. The VF is made up of marine and intermingling fresh-water sediments deposited at sea level on the subsiding CM. The VF was deposited in the Basin and Range extensional environment when the CM were in the State of Sonora, Mexico. We believe there are enough differences between the Latrania and the VF to separate them. This would make Christensen’s (1954) Garnet Formation, which is bracketed by T-R sequences, a member of the VF.