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Interfingering Of An Imperial Group Sediment With Volcanic Flows Of The Alverson Formation In The Coyote Mountains, Western Salton Trough, Southern California


Woodring (1931) was looking at the Tertiary sediments in the Salton Trough when he noted on the “southeastern” edge of the Coyote Mountains that Imperial Formation sediments interfingered with basaltic flows of the Alverson Formation. Considering the unconformity that is usually mapped between the two formations and the age difference that have been published [lower Imperial: ∼6.2 Ma (Dorsey, 2011) and lower Alverson: 17.1 ± 2 Ma (Morgan et al, 2012)], most workers, including us, have ignored Woodring's observations. While mapping the Coyote Mountains at 1:2000 scale, Woodring's outcrops were rediscovered on the southeastern edge of the Coyote Mountains west of Bataca Canyon. This is the only place, so far, in the Coyote Mountains where this relationship has been observed. A siltstone containing typical Imperial Group marine shells is found between two flows of what appears to be upper(?) Alverson. A single zircon sample from a siltstone associated with the Imperial Group gave a U/Pb date of 16.9±1.7 Ma (n=5)In Alverson Canyon/Fossil Canyon an Imperial Group unit with marine fossils interfingers with a green fanglomerate. This fanglomerate can be traced eastward to near Bataca Canyon where it is capped by flows of the Alverson. These two outcrops indicate that a lower part of the Imperial Group in the Coyote Mountains may be older than Dorsey's (2011) ∼6.2 Ma date. The continuing work being done to date zircons from tuffs in the area may provide further constraint on the age of these Imperial Group rocks.