RECONNAISSANCE GEOLOGY OF MEXICO'S COPPER CANYON REGION: A PRELIMINARY REPORT

A reconnaissance study of major ignimbrite sheets and calderas is currently underway in a 5,000 km2 region of Mexico’s giant, Sierra Madre Occidental volcanic field. The area under study is located in western Chihuahua and extends southward from the previously mapped Tomochic caldera complex to the well-known tourist destination site of Copper Canyon.

Field investigations to date have shown that the area contains approximately six major ignimbrite sheets abundantly interlayered with lava rock ranging in composition from rhyolite to basaltic andesite. Three resurgent calderas have thus far been identified based on the occurrence of thick, intracaldera ignimbrites facies surrounded by typical moat sedimentary and igneous lithologies. The largest caldera, possibly 40 kms in diameter, is centered on the Sierra Gasachic where several hundred meters of red, eutaxitic ignimbrite is exposed. A second caldera is located on the southeastern periphery of the Gasachic caldera and centered on the Sierra Manzanita. The Sierra Manzanita’s intracaldera ignimbrite is distinctive in its abundance of lithic fragments, particularly flow-banded rhyolite lithic fragments ranging upward in size to many meters in diameter. The highway to Copper Canyon passes between the resurgent domes of these calderas and across their more easily eroded moat lithologies. The highway continues on to San Juanito, passing just south of the San Juanito caldera, with it’s own resurgently exposed lithic-rich intracaldera tuff and surrounding moat.

At Divisadero, on the edge of Copper Canyon, the upper third of the volcanic section consists mostly of a pyroclastic sequence capped by a distinctive, crystal-rich tuff. A similar (perhaps the same) crystal-rich ignimbrite is found near the top of the volcanic section across much of the region and a caldera source for this unit is currently being sought. Work also continues on relating the other major ignimbrites to the three calderas already identified.