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

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 294.8^o, a 99% confidence cone error of ±4.9^o, and 95% confidence cone error of ±3.9^o. 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.