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Description
At Picacho State Recreation Area a variable thick sequence of carbonate, sandstone, and gravel underlies the Oligocene Quechan volcanics and overlies the Jurassic Wash Formation. This dominantly elastic sequence is prominently exposed in gullies lying just to the east of Little Picacho Wash, and is herein informally referred to as the Little Picacho Wash formation. The lowest part of the Little Picacho Wash formation consists of about 3 meters of carbonate and interstratifled calcareous sandstone. Carbonate beds are mostly peloidal limestone and grade from pure limestone through sandy limestone to calcareous sandstone. Overlying this lower portion is a section of mostly paraconglomerate that reaches a maximum thickness of~ 150 meters. Subround to subangular gravel in the paraconglotnerate varies from pebble to boulder in size. Thin section study indicates that clasts vary from chlorite-grade greenschist facies metasandstones to metavolcanic derived from flows and pyroclastic material. Metavolcanic clasts on the conventional trace-element magma-series discrimination diagram range from basalt to andesite and rhyolite/dacite. On the trace element magma-series discrimination diagram, the compositional variants analyzed form the Little Picacho Wash formation cluster within fields defined by the chemistry of metavolcanic rocks analyzed from the Wmterhaven Formation. Hence, I conclude that the Winterhaven Formation must have been exposed at the Earth's surface during deposition of the Little Picacho Wash formation. Such a conclusion leaves open the possibility that the initial stages of growth of the Chocolate Mountains anticlinorium may have occurred during deposition of the Little Picacho Wash formation.
