CRATER MORPHOLOGY IN MONOGENETIC VOLCANIC FIELDS OF WESTERN NEW MEXICO

The Red Hill, Bandera, and Mesa Chivato volcanic fields of western New Mexico occur along the northwest-trending Jemez Linament. The fields contain as many as 300 vents including maars, tuff rings, cinder cones and a possible explosive caldera (Mt. Taylor). Maars and tuff rings make up one-forth of the cratered structures. Maar and tuff ring rim-crest diameters range from 250 to 2200 m. Rim-heights range from inconsequential to greater than 30 m, and interior-depths range from a few meters to as deep as 50 m. Cinder cone heights range from 25 to 213 m, with basal diameters from 320 to 3600 m. The morphometry of 71 cratered structures exhibits a continuous gradation from maars, tuff rings, and cratered cinder cones.

Rim deposits of maars and tuff rings preserve the earliest pyroclastic base-surge- and fall-materials, representing the initial vent-opening episode. These deposits include a mixture of mantle xenoliths, lithic ash, and tuff breccia composed of fragments of subcrater lithologies traversed by the initial venting of gases from underlying magma. Maar-type craters formed during the initial eruptive phase may be followed by more voluminous eruptions of cinder or lava. Continued eruption builds internal cinder cones or fills the crater with lava. Still further eruption may completely engulf the crater as cinder cone basal-diameter grows to exceed that of the maar. Vent-opening breccia is probably present as the basal layer of most cinder cones. Maars and tuff rings are more common in small, monogenic volcanic fields than in larger fields where more abundant extrusions bury earlier structures. Monogenetic volcanic fields on Mars may provide fertile ground for the recognition of maar-type craters as possible sampling sites of underlying lithology.