New Mexico Geological Society Annual Spring Meeting — Abstracts

Fluvial, debris flow, and colluvial wedge deposits that lie below and within the 1.8 to 1.2 Ma San Diego Canyon and Bandelier tuffs are well exposed in Cañon de San Diego, Virgin Canyon, and Paliza Canyon in the southwestern Jemez Mts. We recognize at least four packages of Pliocene(?) to Pleistocene fluvial sediment. The oldest gravel (Cochiti) is below the San Diego Canyon (SDC) Tuff and rests on rocks ranging from Permian Abo Formation to Miocene Paliza Canyon Andesite. The axial fluvial gravels contain a variety of clasts, including Paliza Canyon volcanic rocks, Proterozoic granitioids and metamorphics, Permian sandstone, and Pedernal Chert. The Proterozoic component decreases toward the east. The presence of Pedernal Chert implies a source region either in the northern Jemez Mts. or in the San Pedro Parks. Paleocurrents indicate a meandering stream with a strong component of southerly paleoflow.

The 1.8 Ma San Diego Canyon Tuff locally filled in a drainage to the west of the modern Jemez River. This drainage persisted during multiple episodes of small-volume SDC eruption, as indicated by erosion of ignimbrites “A” and “B.” Abundant pumice clasts in a red sandstone at least 5 m below ignimbrite “A” may signal an early phase of the eruption. Gravels within the SDC Tuff generally have the same composition as the older gravels. In many places, the SDC Tuff gravels are as much as ~300 m above modern grade, but immediately west of Jemez Springs, the gravels are only about ~122 m above modern grade. The paleocurrent directions are highly variable, ranging from northeast to southwest.

Gravels between the SDC Tuff and the 1.6 Ma Otowi member of the Bandelier Tuff are rare and are dominated by clasts of Paliza Canyon volcanics and mafic lithics locally reworked out of the SDC Tuff. The eruption of the Otowi member of the Bandelier Tuff changed the landscape of the southwestern Jemez Mountains dramatically, totally disrupting the pre-existing drainage pattern. Only one pre-Otowi stream, located near the edge of the outflow sheet in Virgin Canyon, quickly became re-established following the eruption. At this remarkable outcrop, the Guaje pumice, the basal unit of the Otowi member, is crossbedded and eroded, indicating that the pumice fell into an active stream. This stream then cut a paleo-Virgin Canyon that is located west of the modern drainage. The Tshirege member of the Bandelier Tuff subsequently filled this paleocanyon, which was nearly as deep as the modern Virgin Canyon.

A considerable paleotopography developed during the 400,000 years between the Bandelier eruptions. Stream gravels dominated by Paliza Canyon volcanic clasts, reworked lithic clasts out of the Otowi, and a few Permian sandstones are preserved between the Bandelier tuffs southeast and northwest of Jemez Springs, on Mesa de Guadalupe, and on Cat Mesa. On Cat Mesa and on Virgin Mesa, Tshirege member filled in SW-trending paleocanyons cut into the Otowi member.