Rocky Mountain Section - 64th Annual Meeting (9–11 May 2012)

Paper No. 17
Presentation Time: 8:00 AM-5:30 PM

LAVA FLOW EMPLACEMENT AND THE PREFERRED FLOW DIRECTION OF THE NINE-MILE HILL VOLCANO, GUNNISON COUNTY, COLORADO


TREE, Jonathan P., Geology Department, Western State College of Colorado, 600 North Adams Street, Gunnison, CO 81231 and STORK, Allen, Geology Department, Western State Colorado University, Gunnison, CO 81231, jonathan.tree@western.edu

Hinsdale Formation lava flows are exposed on the northern slope of the San Juan Mountains at Nine-Mile Hill, Gunnison County, Colorado. At this location the formation can be divided into a sequence of seven lava flows composed of biotite-hornblende, and clinopyroxene-hornblende latites that alternate with aphyric shoshonites. The lavas have integrated whole rock Ar-Ar ages of 19.17±.07 Ma from the basal flow to 19.22±.07 Ma from the highest flow. These flows filled early Miocene paleovalleys that currently cap mesas, representing a classic case of inverted topography. The first eruptions from a vent on the north edge of the field plugged a paleovalley. Subsequent eruptions from a vent to the south pooled in this drainage producing flattened spheroidal vesicles. Lava flows continued filling the drainage until gaining enough elevation (~60m) to overtop a drainage divide and flow 10 km down the slope of an adjacent paleovalley. These later flows show a preferred flow direction demonstrated by measurement of the plunge direction of elongated vesicles and small lava tubes. These data produced a mean vector with a flow direction of 294.8o, a 99% confidence cone error of ±4.9o, and 95% confidence cone error of ±3.9o. This result indicates that there was a strong preferred flow direction to the northwest parallel to the modern mesas, which then also defines the axis of the Miocene paleovalley.