EVALUATING PYROCLASTIC SEDIMENTARY ROCKS: EXAMPLES FROM THE OLIGOCENE OF CENTRAL COLORADO

Pyroclastic sedimentary rocks are seldom studied and poorly known; they are largely absent from southeastern United States. Nonetheless, in parts of the country, they are locally common and provide great opportunities for understanding basic sedimentary relationships. In central Colorado, the Oligocene West Elk Breccia is a pyroclastic agglomerate or diamictite composed of large volcanic clasts and/or stream gravels in a volcanic-ash matrix. Up to 14 compositionally distinct subunits are present in the entire formation, some of which are erosionally superimposed on others in paleovalleys cut into the Mesozoic country rock. Stratigraphy within valleys allows the construction of a chronology of deposition and correlation from valley to valley. Many of the flows show liquefaction structures and rafts of stream gravels, indicating that they were water-saturated and probably deposited as lahars in stream valleys. Others with meter-size clasts and no liquefaction structures probably represent volcanic avalanche deposits that flowed in a more unrestricted fashion across several valleys and intervening divides. Similar units occur at high elevations in the mountains due to reversal of topography and at low elevations in the Gunnison Basin, permitting the calculation of ancient stream gradients and the mapping of some Oligocene stream networks. Plant fossils in water-laid tuffs between some of the flows also allow for climatic interpretations. A change in flow composition from dacitic to andesitic through time may indicate differentiation in the magma chamber and suggest emptying of the chamber and consequent caldera formation. In fact, later andesitic flows may represent collapse and final denudation of a tall stratovolcano. The most distal flows are 39 km from the purported source, and using this distance as the maximum run-out and knowing the median ratio for height-to-run-out for debris avalanche deposits suggests that the source volcano was at least 4300 m tall. As this study suggests, pyroclastic sedimentary rocks may contain important information about paleotopography, paleoclimate and the nature of volcanism at a given time and place.