GEOLOGY OF MID-OLIGOCENE NATHROP VOLCANICS, CENTRAL COLORADO VOLCANIC FIELD: MODELS FOR EXTRUSION, ALTERATION, AND EROSION

The Nathrop Volcanics consist of rhyolite lava and pyroclastic deposits located on the eastern shoulder of the Arkansas graben in south-central Colorado and are part of the late Eocene-Oligocene Central Colorado Volcanic Field (McIntosh and Chapin, 2004). Deposits at Ruby Mountain consist of a lower lithic-rich lapilli tuff (ca. 3 m thick) with multiple layers that are reversely graded overlain by ~30 m thick lithic-poor tuff breccia containing pumice blocks up to 1 m in diameter. The upper portion of the tuff breccia transitions into a 5 m thick, moderately to densely welded tuff (vitrophyre), which in turn is overlain by a 20 m thick flow-banded rhyolite. A similar stratigraphic sequence is found at Sugarloaf Mountain (<1 km to the NNE), and also portions crop out as faulted and eroded blocks in the valley between the two mountains. Their origin has been linked to exogenic lava dome growth; pyroclastic facies (fall overlain by flow) followed by lava extrusion. This study considers three scenarios to explain their origin. Pivotal to each is the cause of welding and the stratigraphic position of the vitrophyre. The three scenarios are: 1) the tuff breccia and flow-banded rhyolite are not from the same eruption and welding occurred in a thick pyroclastic flow that was eroded to the level of the more resistant vitrophyre followed by eruption of the rhyolite as a lava; 2) the flow-banded rhyolite was erupted as a lava immediately after the pyroclastic flow which caused the welding; 3) the sequence represents a single short-lived eruptive event in which the pyroclasts accumulated rapidly enough to weld and flow rheomorphically (cf. Bachmann et al., 2000). This study evaluates all three models based on field relationships.

References

Bachmann, O., Dungan, M.A., Lipman, P.W., 2000, Voluminous lava-like precursor to a major ash-flow tuff: low-column pyroclastic eruption of the Pagosa Peak Dacite, San Juan volcanic field, Colorado, Journal of Volcanology and Geothermal Research, 98, 153 – 171.

McIntosh, W.C., Chapin, C.E., 2004, Geochronology of the central Colorado volcanic field., New Mexico Bureau of Geology and Mineral Resources, Bulletin 160, 205-238.