MAZAMA IGNIMBRITE SURFACE FEATURES REVEALED BY LIDAR MAPPING OF CRATER LAKE NATIONAL PARK, OREGON

The surfaces of voluminous ignimbrites are rarely observable because youthful examples typically are buried, heavily vegetated, or undersea. During the ~7.7 ka Mazama climatic eruption, pyroclastic flows coursed down the flanks of Mount Mazama and partially filled valleys with ~100 m of compositionally zoned ignimbrite. Surface features of the Mazama deposits revealed beneath conifer forest by lidar (J.E. Robinson, 2012, USGS Data Series 716) illustrate primary ignimbrite deposition, secondary flowage, gas venting, compaction, and reworking by water. North of the caldera, turbulent high-energy flows carved two groups of deep parallel grooves in climactic pumice fall on the lee slope of Timber Crater shield volcano. Energetic flows also sculpted wedge-shaped yardangs between the grooved areas and on the north upper slopes of Mazama. Hummocks of lithic breccia and dense juvenile clasts are near-source bedforms stratigraphically equivalent to valley filling ignimbrite. Valley margins preserve ignimbrite benches, such as north of Timber Crater where a trio of benches was left when centers of successive flows drained away. Transverse 1‑m-high megaripples (~20 m wavelength) are present on low longitudinal ridges on ignimbrite in Crater Creek valley. Several 1–2-m-high, broad ridges (~60 m spacing) south of Crater Lake parallel to a valley axis wrap around a moraine remnant; there, and elsewhere near terminations of late flow units, low ridges appear similar to lineated landslide deposits. Arcuate breakaway scarps 1–10 m high and up to 900 m long mark sources of secondary flows. Closely spaced degassing pits ~1–4 m deep and up to 30 m across coincide with emplacement over water-saturated ground. Cracks and pull-aparts are associated with transitions from thin to thick, compacted ignimbrite. Crack networks suggestive of polygonal patterns are scattered in low-relief terrain, apparently over less-compacted ignimbrite or pre-existing highs. Surface water locally reworked and eroded the ignimbrite, as in lower Annie Creek where eight benches and an abandoned stream course are preserved. The lidar survey greatly expanded the range in scale of often-subtle morphologic features that can be detected, measured, and related to ignimbrite deposition and modification processes at Crater Lake.