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The Browns Park Formation of Miocene age consists dominantly of cross-bedded feldspathic sandstones and was deposited by a series of northward flowing rivers that headed in the vicinity of the San Juan Mountains. The sandstones were transported 150 to 250 miles to northwestern Colorado and south-central Wyoming.
Abundant plutonic and volcanic rock material show that the sandstones are largerly first cycle sediments. The quartz, feldspar, volcanic rock fragments, and heavy minerals all are considerably rounded; some are very well rounded. The freshness of much of the feldspar demonstrates that corrosion at the source is not responsible for the rounding, but the distance of fluviatile transport is too short to explain the degree of rounding. The quartz grains are commonly frosted and pitted, considered to be due to eolian action. The rounding of the sand grains apparently took place during intermittent periods of eolian activity. Fluviatile transport associated with eolian activity suggests a semi-arid climate.
Volcanic centers in northern Utah and southern Idaho have been suggested as a source of the volcanic material which occurs abundantly in the Browns Park Formation. Grain size determinations of quartz, feldspar and volcanic rock fragments showed an increase in size of all components toward the south and not toward the suggested western area. The mean grain size, sorting coefficient, and other grain-size parameters are similar in the quartz, feldspar and volcanic material. This relationship suggests that volcanic material was carried an appreciable distance with the other components in the sandstone. The grain-size characteristics of the volcanic-rich Browns Park sandstones indicate that most of this material was derived from volcanic centers in the San Juan region.
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