In the Hagan basin of north-central New Mexico, Espinaso Ridge contains the largest and least deformed exposures of the Espinaso Formation (Oligocene). The Espinaso primarily consists of 430 m of volcanic detritus eroded from eruptive centers in the Ortiz Mountains and Cerrillos Hills. The Espinaso appears conformable and gradational with the underlying Galisteo Formation and is overlain unconformably by the Santa Fe Group.

Sedimentary structures, facies relations, and upward coarsening sequences indicate that the Espinaso was deposited on prograding alluvial fans by braided streams and laharic flows.

Clasts in the conglomerates range from andesitic near the base of the formation to latitic near the top. A chemical analysis from an interbedded lava flow in the upper half of the section shows a normative composition of nepheline latite. The sandstones are composed mostly of feldspar and lithic fragments. The most distinctive petrologic characteristics of the Espinaso are high P/F ratios, low quartz, lack of any lithic fragments other than volcanic fragments, and high percentages of microlitic volcanic fragments.

The sedimentary record suggests that the commencement of volcanic activity in late Eocene time coincided with deposition of upper Galisteo sandstones. By Espinaso time, significant volcanic activity caused the progradation of coalescing fans over a region of low physiographic relief. The fine grain size of uppermost Espinaso sediments suggests the waning of volcanic activity in the latest Oligocene.

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