The Espinaso Formation (Oligocene) primarily consists of 430 m of water-laid, immature, volcaniclastic sandstones, conglomerates, and bounder conglomerates interbedded with matrix-supported pebble to boulder conglomerates that represent debris-flow deposits. Isolated occurrences of poorly welded, lithic- to crystal-rich deposits with pumic swarms represent pyroclastic-flow deposits. The geographic position, lithologic similarity with respect to intrusive rocks, coarseness of sediments, and lithofacies types suggest that the Espinaso was deposited by alluvial fans which radiated from magmatic centers in the Ortiz Mountains and Cerrillos Hills of north-central New Mexico (Fig. 1).
Exposures of the Espinaso occur as isolated outcrops near the Cerrillos Hills, Galisteo Creek and Santa Fe River, and in the Hagan basin. The largest and least deformed exposure of the formation crops out in the type area along Espinaso Ridge within the Hagan basin (Figs. 1, 2). Espinaso Ridge is the principle area of study for this investigation.