THE VOLCANIC, SEDIMENTOLOGIC, AND PALEOLIMNOLOGIC HISTORY OF THE CRATER-LAKE CALDERA FLOOR, OREGON - EVIDENCE FOR SMALL CALDERA EVOLUTION

Apparent phreatic explosion craters, caldera-floor volcanic cones, and geothermal features outline a ring fracture zone along which Mount Ma zama collapsed to form the Crater Lake caldera during its climactic er uption about 6,850 yr B.P. Within a few years, subaerial deposits infi lled the phreatic craters and then formed a thick wedge (10-20 m) of m ass flow deposits shed from caldera walls. Intense volcanic activity ( phreatic explosions, subaerial flows, and hydrothermal venting) occurr ed during this early postcaldera stage, and a central platform of suba erial andesite flows and scoria formed on the caldera floor. Radiocarb on ages suggest that deposition of lacustrine hemipelagic sediment beg an on the central platform about 150 yr after the caldera collapse. Th is is the minimum time to fill the lake halfway with water and cover t he platform assuming present hydrologic conditions of precipitation an d evaporation but with negligible leakage of lake water. Wizard Island formed during the final part of the 300-yr lake-filling period as sho wn by its (1) upper subaerial lava flows from 0 to -70 m below present water level and lower subaqueous lava flows from -70 to -500 m and by (2) lacustrine turbidite sand derived from Wizard Island that was dep osited on the central platform about 350 yr after the caldera collapse . Pollen stratigraphy indicates that the warm and dry climate of middl e Holocene time correlates with the early lake deposits. Diatom strati graphy also suggests a more thermally stratified and pbosphate-rich en vironment associated respectively with this climate and greater hydrot hermal activity during the early lake history. Apparent coarse-grained and thick-bedded turbidites of the early lake beds were deposited thr oughout northwest, southwest, and eastern basins during the time that volcanic and seismic activity formed the subaqueous Wizard Island, Mer riam Cone, and rhyodacite dome. The last known postcaldera volcanic ac tivity produced a subaqueous rhyodacite ash bed and dome about 4,240 y r B.P. The late lake beds with base-of-slope aprons and thin, fine-gra ined basin-plain turbidites were deposited during the volcanically qui escent period of the past 4,000 yr. Deposits in Crater Lake and on sim ilar caldera floors suggest that four stages characterize the postcald era evolution of smaller (less-than-or-equal-to 10 kin in diameter) te rrestrial caldera lake floors: (1) initial-stage caldera collapse form s the ring fracture zone that controls location of the main volcanic e ruptive centers and sedimentary basin depocenters on the caldera floor , (2) early-stage subaerial sedimentation rapidly fills ring-fracture depressions and constructs basin-floor debris fans from calderawall la ndslides; (3) first-stage subaqueous sedimentation deposits thick flat -lying lake turbidites throughout basins, while a thin blanket of hemi pelagic sediment covers volcanic edifices that continue to form concur rently with lake sedimentation; and (4) second-stage subaqueous sedime ntation after the waning of major volcanic activity and the earlier pe riods of most rapid sedimentation develops small siliciclastic basin b ase-of-slope turbidite aprons and central basin plains. Renewed volcan ic activity or lake destruction could cause part or all of the cycle t o repeat.