High-Viscosity ‘Conglomerate’ Channel Deposits in Tertiary Lamprophyre Sill, Sacramento Mountains, New Mexico

Abstract

Three “conglomerate” channels are exposed in an approximately 12-m-thick (40 ft) Tertiary (44.2 ± 2.2 m.y.) lamprophyre sill in the northern Sacramento Mountains. The sill is intruded into the Pennsylvanian Gobbler Formation as two intrusions, the conglomerate occurring in troughs within the lenticular upper sill. The lamprophyre contains abundant fine phenocryst (0.25 to 8 mm) of hornblende, augite, and plagioclase (An40–45) in a groundmass of plagioclase (An35–40), chlorite, magnetite, and minor orthoclase. The bifurcating lenticular conglomerate is ca. 90 to 210 m (300 to 700 ft) wide and scours up to 5 m (15 ft) into the lower sill. The conglomerate has a chaotic and often nongrain-supported texture of poorly sorted xenoliths and phenocrysts in a groundmass of lamprophyre. The xenoliths consist of fragments of Precambrian schist, gneiss, and granite, Paleozoic sedimentary rock, and clasts eroded from the lower sill; they range in size from less than 1 cm to 0.5 m, with a mean size of 6 to 8 cm. The conglomerate channels are the result of local erosion along the top of the lower sill by a dense xenolith-laden magma current during intrusion of the upper sill. The chaotic texture of the conglomerates and the lack of any axial migration of the xenoliths and phenocrysts indicate transportation and deposition were by a highly viscous flow similar to a pebbly mudstone deposit.