9-10 SUBGLACIAL FLOOD DEPOSITS TRACK RETREATING CORDILLERAN ICE SHEET MARGIN ACROSS ADMIRALTY INLET, NORTHERN PUGET SOUND, WASHINGTON STATE

Session: Sedimentary Systems and Provenance in the Western Cordillera (Posters)

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Sea-cliff exposures along the north Quimper Peninsula coast (near Port Townsend) and north of Ledgewood Beach (on the west side of Whidbey Island) reveal apparently subglacial flood deposits similar to one described around Penn Cove on Whidbey Island by Polenz and others (2004). At all locations, the glaciofluvial unit stands out as a megabreccia that incorporates bedded gravel and large clasts, some >10 m across, of Vashon till (rounded) and older, interglacial sedimentary formations (angular). The unit’s base, extending discontinuously 5-7 km along the Quimper Peninsula and Penn Cove north shores, has an undulating profile, consistent with flow driven by hydraulic pressure under the confines of a glacier. Subglacial flow and hydraulic overpressure are also evident in the clastic dikes, sills, and lenses injected into deformed, disrupted, pre-deposit sedimentary beds to distances >10 m vertically and 100 m horizontally. The megabreccia deposits stratigraphically overlie Vashon till (~17 ka), which they locally eroded away completely, and are conformably overlain by Everson glaciomarine drift (~17-15 ka).

The megabreccia deposits are likely time-transgressive. Initially, a subglacial flood occurred across the north Quimper Peninsula coast, flowing mainly north, probably into marine water in the Strait of Juan de Fuca. A later underburst, north of Ledgewood Beach, flowed southwest into a seawater-filled Admiralty Inlet. Subsequently, from just north of Penn Cove, subglacial flow went mainly south toward marine water.

Along the Quimper Peninsula north coast, the flow deposit and eskers suggest a north-facing glacial margin paralleling the coast, contrasting with the series of south-facing ice sheet margins on Whidbey Island. Glacial ice on the Quimper Peninsula may have abutted the Juan de Fuca Lobe to the north until the marine incursion removed the adjacent Juan de Fuca ice. Marine water then displaced the ice in Admiralty Inlet, and the active ice sheet that remained on Whidbey Island retreated northward in steps. At all three places and times the megabreccia was deposited, the glacial margin was nearby, and the encroachment of marine water may have helped trigger the subglacial discharge.

The megabreccias help constrain how the Cordilleran Ice Sheet retreated from northern Puget Sound. They also provide rare, well-exposed examples of how subglacial, hydraulically driven flows of water can interact with weakly consolidated bodies of glacial and interglacial sediment.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3, 2026

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