TIMING AND NATURE OF POST-COLLAPSE SEDIMENTATION IN KULSHAN CALDERA, NORTH CASCADES, WA

Sedimentary rocks found in the 4 x 8.5 km Kulshan caldera of the Mount Baker volcanic field in the North Cascades, WA indicate that the basin hosted a lacustrine environment sometime shortly after the initial collapse 1.15Mya (Hildreth, 1996). The sedimentary rocks found in Wells Creek basin of Kulshan caldera, though well exposed, have been hydrothermally altered and contain abundant calcite and pyrite. However, the sedimentary rocks of Twomo basin in Kulshan caldera are well preserved in 130 meters of stratigraphic exposure. Field relations reveal that the depositional environment of the Twomo basin includes wall collapse facies; megabreccias to breccia debris flows dominate the lower strata. The layers above the breccia are predominantly tuffaceous sandstones interbedded with a fine grained lacustrine mudstone. The strata of tuffaceous sandstone and lacustrine mudstone vary in thickness from 1 cm to several meters. Additionally the strata of Twomo basin are divided by an andesite sill and cross cut by an andesite feeder dike. The sedimentary strata of Kulshan caldera preserve abundant sedimentary structures revealing some of its environmental history. Structures such as wave ripples and cross cutting beds indicate channeled flow. Similarly structures like mud cracks and raindrop impressions indicate shallow to no water. Sets of rhythmites may indicate deep water depositional processes. Drop stones are common, as are clastic dikes and soft sediment deformation. Timing of the post-collapse caldera lake and rates of deposition are being investigated by paleomagnetism. The Jaramillo sub-chron dated 1072 +/- 2 ka - 988 +/- 3 ka by Horng et al. (2002) is bracketed by post-collapse Kulshan lavas (Hildreth, 2004). Initial results from anisotropy of magnetic susceptibility (AMS) show that most of the sediments have retained their depositional fabric and at times indicate a northeasterly imbrication of magnetic grains.