New Mexico Geological Society Annual Spring Meeting — Abstracts

Mapping during the past three field seasons has clearly delineated the previously unknown source cauldron for the tuff of Turkey Springs (24.3 Ma) the youngest ignimbrite of the northeastern Mogollon-Datil volcanic field. The cauldron is located in the northwestern San Mateo Mountains just southwest of Mt.Withington encompassing the southern 3/4 of the Bay Bucks Peaks quadrangle and small parts of the adjoining Grassy Lookout, Welty Hill and Oak Peak quadrangles to the east, south and west. Approximately 80% of the cauldron has been mapped in detail at 1/24000. A small part is exposed but unmapped on Welty Hill quadrangle and the remainder is concealed beneath piedmont alluvium on the west flank of the range.

The cauldron is about 10 mi (17 km) across, nearly circular and exhibits classic resurgent-cauldron map patterns. A central resurgent dome, exposing at least 1000 ft of intracauldron tuff of Turkey Springs, is bisected by a north-trending apical graben and surrounded by a sea of unwelded tuffs, volcaniclastic sediments and rhyolitic domes (moat deposits). The cauldron margin is an eroded topographic scarp buried by the moat deposits except at the eastern edge where tilting and erosion have exposed faults (possibly the ring fracture) separating tuff of Turkey Springs from precauldron rocks (South Canyon Tuff). A sequence of rhyolite domes follows the main ring fracture in the north and west and several intrusions cut the resurgent dome. Several have been dated by ⁴⁰Ar/³⁹Ar method and fall between 24.3 and 24.1 Ma-- the ages respectively of tuff of Turkey Springs and the highest dated tufaceous unit in the moat sequence. Post-eruptive activity was rapid, intense and overwhelmed the cauldron quickly. A final dome or group of domes was emplaced at about 21.7 Ma and the cauldron largely undisturbed but for slight westward tilting and erosion since.

The Bear Trap Canyon cauldron sits in the extreme western end of the much larger Mt. Withington cauldron, source for the South Canyon Tuff (27.4 Ma) and contrasts strongly with it in structural style. Whereas, the Bear Trap Canyon cauldron exhibits classic undeformed morphology, the older Mt. Withington cauldron is dominated geologically by north-trending, tilted fault blocks and the cauldron geology must be inferred through the screen of extensional deformation. It is possible to interpret the strong difference to indicate deformation waned before 24.3 Ma, however, mapping is not complete enough to exclude the alternate possibility that the cauldron simply coincides with the boundary between strongly and less strongly extended areas.

Bear Trap Canyon offers easy accessibility and excellent exposures of the buried topographic margin, moat deposits (tuffs, sediments and domes), and intracauldron tuff of Turkey Springs. Tentatively, therefore, the name Bear Trap Canyon Formation, previously used in another context for these domes and unwelded tuffs (Deal, E.G. and Rhodes, R. C., 1975, NMGS, Special Pub. No. 5, pp. 51-56), is retained and the name Bear Trap Canyon cauldron applied to the structure.