BURRO MESA

Updated nomenclature:  The Burro Mesa Formation consists of the lower Wasp
Springs Tuff Member (fomerly the Wasp Spring Flow-Breccia) and the upper Burro
Mesa Rhyolite.  The formation was erupted between 29.4 and 29.0 Ma from several
lava dome volcanoes.  The Burro Mesa Formation is no longer considered to be
corrlative with the rhyolite found on top of Emory Peak, Casa Grande Peak, and
Lost Mine Mountain, which is now known as the Emory Peak Rhyolite Member of
the South Rim Formation. The Burro Mesa Formation consists of the Wasp Springs
Tuff Member underneath the Burro Mesa Rhyolite Member.  The Burro Mesa
Rhyolite Member consists of an upper and lower unit of potassium and sodium
(alkali) rich lava and ash-flow tuff,
separated by a mugearite lava flow (an orthoclase-rich basalt).
The Burro Mesa Formation apparently originated through eruptions of several
lava dome volcanoes.

Stratigraphy of Burro Mesa Pouroff

Burro Mesa Formation
	Burro Mesa Rhyolite Member (29 Ma, upper Burro Mesa - 29.4 Ma
	  according to circular 1327); 28 Ma, Cerro Castellan) - phyric
	  (>1% phenocrysts) and aphyric (<1% phenocrysts).  Comendite lava and
	  ash-flow tuff (comendite = sodic rhyolite).
	Wasp Spring Tuff Member (29 Ma - also 29.4 Ma according to circular
	  1327)
Unidentified conglomerate of Chisos Group
Chisos rocks with Bee Mtn Basalt

CHISOS MOUNTAINS

New stratigraphic names in Chisos Mtns:  South Rim Formation now includes the
Pine Canyon Rhyolite Member, the Boot Rock Member, and the Emory Peak Rhyolite
Member.  Dates from pinecanyoncaldera.pdf.

Burro Mesa Rhyolite --> Emory Peak Rhyolite, quartz-bearing rhyolitic ash-flow
			tuffs, 32 Ma. A series of E and NE trending dikes of
			chemically identical rock cut through Chisos Fm and
			younger rocks.
Wasp Spring Flow Breccia and Lost Mine Rhyolite --> Boot Rock Member, quartz
			trachyte to rhyolite lavas, ignimbrites, and maar
			surge deposits, ~32 Ma
Brown Rhyolite --> Pine Canyon Rhyolite, ignimbrite that formed Pine Canyon
			Caldera, 33-32 Ma

Summing up:

South Rim Formation
	Emory Peak Rhyolite Member - rhyolitic welded ash-flow tuffs (32 Ma)
	Boot Rock Member - quartz trachyte to rhyolite lavas, ignimbrites, mar
	surge deposits (32-33 Ma)
	Pine Canyon Rhyolite Member - ignimbrite (32-33 Ma)

All members are alkali-rich, with the Emory Peak Rhyolite being the least
alkaline.

CHISOS GROUP (*Dates from circular 1327, otherwise from pinecanyoncaldera.pdf)

Tule Mtn Trachyandesite			33 Ma*
Mule Ear Spring Tuff			33.6*
Bee Mtn Basalt				34-30  (interbedded with MEST and TMT)
					(33.7* according to circular 1327)
Ash Spring Basalt			42*
Alamo Creek Basalt			46.5-34.5 (46.5* according to 1327)