The Jaquey-Brena Complex: A New Type of Maar Volcano from the Guadiana Valley Volcanic Field Durango, Mexico

Abstract The Quaternary, Guadiana Valley volcanic field covers nearly 2000 km2 northeast of Durango, Mexico and contains more than 100 basaltic vents. Its most interesting feature is the field’s only maar, which together with an older scoria cone and an adjacent collapse zone form the Jaquey-Brena complex. The complex was formed when the eruptive activity at a pre-maar scoria cone was altered by runoff from nearby mountain arroyos to produce a series of gigantic steam explosions. Although the Jaquey-Brena complex is unique, it is analogous in many ways to a maar and nearby collapse pit formed on Iwo Jima in 1957. El Jaquey is a funnel-shaped maar 0.75 km in diameter. Pyroclastic surge beds line the inside of the vent and extend onto the surrounding plain in all directions but west. La Brena is an immediately adjacent, nearly circular, collapse structure 1.4 km in diameter. An older scoria cone and petrographically similar basaltic lava flow are exposed in the walls of both El Jaquey and La Brena, and the caldera’s floor is covered by a post-collapse spatter cone and an associated lava flow. The volcanic history of the complex began with construction of a large scoria cone over the present site of La Brena caldera. Lava eventually flowed from the failed east flank of the cone. Shortly afterward groundwater encroachment triggered a series of phreatic explosions that formed El Jaquey maar while removing supporting material from beneath the scoria cone. A detailed study of ejected lithic fragments indicates that the explosion chamber did not grow vertically but enlarged laterally as it ejected a constant ratio local basalt and an underlying ash-flow tuff. After preventing the westward dispersal of pyroclastic surges, the breached scoria cone foundered to form La Brena caldera. The volcanic history ended with the eruption of basaltic lava from a spatter cone complex on the caldera’s floor.