294-9 Unraveling a complex Miocene to Upper Pleistocene volcanic history in the Moon Reservoir and Iron Mountain Flat 7.5’ Quadrangles, Harney Basin, SE Oregon

Session: Geologic Mapping (Posters)

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Abstract:

Recent mapping in the adjacent Iron Mountain Flat (south) and Moon Reservoir (north) 7.5’ quadrangles within the High Lava Plains (HLP) province, SE Oregon, reveals a complex Miocene to upper Pleistocene volcanic succession not captured in previous maps. The oldest rocks crop out in the southern Iron Mountain Flat quadrangle and include Miocene(?) trachyte domes (Tmty), rhyolite (Tmro), and basalt lavas (Tmb1, Tmb2). These rocks are locally blanketed by the 7.05 Ma Rattlesnake Tuff (Tmtr), tuffaceous sedimentary rocks (Tmst), and a 7.08 Ma intermediate lava (Tmta). Miocene units are covered by the newly discovered 2.21 Ma basalt of Big Stick Road (Tpbs), vent deposits (Tpvb), and lower Pliocene to upper Miocene Harney Formation tuffaceous sedimentary rocks and (QTst) and basalt lava flows (QTb). The high-standing, 2.89 Ma Iron Mountain rhyolite lava dome (Tpri) penetrated many of these units. In the Moon Reservoir quadrangle, the Rattlesnake Tuff is completely covered by the Harney Formation (QTst, QTb), the newfound 2.36 Ma basalt of Moon Reservoir (QTbm), and vent deposits (QTvm). Bedrock units are locally mantled by upper Pleistocene(?) and Holocene sediments (Qc, Qaf, Qp, Qls, Qa, Qf). Bedrock units are displaced by ~N50°W-striking normal faults of the regional Brothers Fault Zone.

New detailed mapping is beginning to resolve some long-standing questions about this part of the HLP province. Previous studies assigned the Capehart Lake area (beneath the map area) as the probable source region for the regionally extensive Rattlesnake Tuff. Thickening of basalts and sedimentary rocks (QTst) in the center of the map area and prevalence of basaltic vents (over the site of the proposed Capehart Lake caldera), paired with significant thickening of the Harney Formation (~50-250 ft) and channelization of some lava flows eastward to Malheur Lake (east of the mapping area), could be consistent with filling of a buried caldera. The map area is situated where NW-trending normal and transverse faults of the Brothers Fault Zone intersect with normal faults of the Northwest Basin & Range Province, in a region proximal to multiple caldera eruptive centers. Ongoing mapping aims to resolve these complex relationships.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9001

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