New Mexico Geological Society Annual Spring Meeting — Abstracts

This study describes the eruption characteristics of the Pleistocene Cienega Cinder Cone. This volcano is located in the southeastern part of the Cerros del Rio volcanic field (CdR) west of Santa Fe, NM. The CdR is the largest (>700 km2) of several middle Pliocene to Pleistocene basaltic volcanic fields of the axial Rio Grande Rift in northern New Mexico. Eruptive centers are typically central vent volcanoes, ranging from low-relief shields to steep-sided, breached cinder and spatter cone remnants. The Cienega Cinder Cone is actually a volcanic complex that consists predominantly of tephra fall deposits as well as several vents, multiple intrusions, and numerous lava flow sequences. A slightly eroded northern vent is 230m in diameter and consists of inward bedded crater facies and periclinally bedded wall facies. A smaller southern vent is 95m in diameter and composed of steep N-dipping pyroclastic layers that suggest the development of a late-stage shallow cryptodome. Vent facies include vesiculated fragments, oxidized cinders, and spatter agglutinate interbedded with lava flows. Proximal wall facies are moderately sorted with a high proportion of coarse scoria and bombs while the distal wall facies are very well sorted with a high proportion of fine lapilli. Fluvial sand and gravel deposits as well as aeolian sand deposits within some of the pyroclastic layers illustrate the development of stream channels and exposed surfaces in between eruptions. One major N-striking anastomosing dike (>10 m long by 4 m wide) as well as several minor N-striking dikes (< 2 m long by 1 m wide) intrude the southern complex. Macrostructures (slickenlines, chatter marks, and Reidel shears) consistently show wall rock deformation having a North-sense of shear, indicating S-moving magma towards the inflating southern vent. Sample from all volcanic facies (vent, lava flows, proximal wall, and distal wall) contain major olivine (1-3%), pyroxene (1-3%), and plagioclase (5%) phenocrysts in an aphanitic matrix. Scoria cinders contain 20-50% vesicles in a holohyaline matrix. Our observations show that the Cienega Cinder Cone is a monogenetic volcanic complex that developed by endogenic and exogenic dome growth with short eruptive events that likely were derived from a rapidly evolving reservoir-conduit system.