Field characteristics of the El Cajete pumice deposit and associated southwestern moat rhyolites of the Valles Caldera
John A. Wolff, J. N. Gardner, and S. L. Reneau
Previous studies of the 50-60 ka southwestern moat rhyolites in the Valles caldera have underestimated the complexity of the eruption sequence they record. Based on new quarry exposures, and re-examination of roadcuts along NM-4, we recognize them as the products of a single eruption consisting of three cycles, each beginning with explosive activity and terminating with effusion of lava. The first cycle was dominated by plinian fallout, dispersed to the southeast of the vent which makes up part of the El Cajete pumice. A pyroclastic surge erupted during this cycle levelled a forest of tree trunks projecting from the initial fallout accumulation. This sequence ended with extrusion of a lava, progressively destroyed during explosive eruptions of the second cycle, during which activity gradually shifted from fallout-dominated to flow-dominated. The plinian falls are dispersed to the south, while the pyroclastic flows largely moved in a westerly direction along the caldera moat and into the head of San Diego Canyon to form the Battleship Rock ignimbrite. The last phase of this cycle produced a lava, found only in the VC-1 core. The third and last cycle began with eruption of an ignimbrite, but is mainly represented by the Banco Bonito lava, which dominates the modern topography of the southwestern Valles caldera moat.
- Wolff, John A.; Gardner, J. N.; Reneau, S. L., 1996, Field characteristics of the El Cajete pumice deposit and associated southwestern moat rhyolites of the Valles Caldera, in: The Jemez Mountains Region, Goff, Fraser; Kues, Barry S.; Rogers, Margaret Ann; McFadden, Les D.; Gardner, Jamie N., New Mexico Geological Society, Guidebook, 47th Field Conference, pp. 311-316.