PLIOPLEISTOCENE PUMICE FLOODS IN THE ANCESTRAL RIO-GRANDE, SOUTHERN RIO-GRANDE RIFT, USA

At least four times during the late Pliocene and early Pleistocene pyr oclastic eruptions in the Jemez volcanic field, northern Rio Grande ri ft, flooded the ancestral Rio Grande with gravel-sized pumice. Followi ng as much as 400 km of fluvial transport, the pumice was deposited in beds 0.2 to 2.0 m thick in the Camp Rice Formation of the southern Ri o Grande rift. A combination of reversal magnetostratigraphy and singl e-crystal sanidine Ar-40/Ar-39 dating constrains the ages of pumice-cl ast conglomerates at 3.1, similar to 2.0, 1.6, and 1.3 Ma. The coarses t pumice beds (cobbles, boulders) were deposited as antidune-like bedf orms in a fluvial channel and as a crevasse-splay sheet. Granule and p ebble-sized pumice was deposited as dune bedforms in fluvial channels and as ripple bedforms on the floodplain. The abundance of pumice clas ts in the gravel fraction (60-100%) suggests very rapid transport down river, probably in a few days or weeks. The two older pumice-clast con glomerates correlate with the Puye Formation in the Jemez volcanic fie ld, whereas the younger two are coeval to the Lower Bandelier Tuff and Cerro Toledo Rhyolite.