ERUPTIVE CHRONOLOGY OF MOUNT BAKER REVEALED BY LACUSTRINE FACIES OF GLACIAL LAKE BAKER

Deposit facies of Glacial Lake Baker refine the chronology of Mount Baker's latest Pleistocene and Holocene activity. This newly recognized glacial lake occupied the Baker River valley, which drains the southeastern half of the volcano. The lake existed at least from the Everson Interstade (following the Vashon Stade of the Fraser Glaciation) to 3,205 BP, when a remnant stood ~370 ft below the earlier peak level of ~800 ft elevation.

Thick syneruptive fragmental flow assemblages were the final additions to Mount Baker--deposits of the Carmelo Crater eruptive period that began post-Vashon Stade (deposits overlie Vashon till). This eruptive period ended by 12,200 BP, the age of the earliest deposits of Glacial Lake Baker--volcaniclastic-free and macroflora-rich. A subsequent and the first of two volcaniclastic-rich facies of the glacial lake records minor eruptions ending before the Holocene. Sedimentation was again volcaniclastic-free by 9,975 BP.

Early Holocene quiescence was interrupted by a flux of basaltic scoria from a satellite cinder cone on the south flank of Baker, the initial event of the Schriebers Meadow eruptive period. The scoria, tephra set SC, is 8,850 BP in age. It contributed the second volcaniclastic facies of the glacial lake, beginning at that time and continuing after 8,655 BP. This volcaniclastic facies was subsequently deformed by the Sulphur Creek lava flow, which blocked the Baker River valley at modern Horseshoe Cove after 8,655 BP to form a new lake in the upper part of the Baker River valley that had originally been occupied by Glacial Lake Baker. The final event of the eruptive period was the collapse-runout Schriebers Meadow lahar (8,500 BP), originating from failure of the flank of Mount Baker upslope from the cinder cone. The lahar formed the flat at Schriebers Meadow surrounding the cinder cone.

The final two Mount Baker eruptive periods, Mazama Park (mid-Holocene) and Sherman Crater (1843-present), both included flank-collapse-runout lahars that raised or reestablished a blockage that impounded the natural lake that was present in the Baker River valley at the time of the 1959 reservoir inundation. The lahar deposits raised the height of a fan of post-Vashon-Stade coarse cobble gravel from Swift Creek where it entered the Baker River valley.