Repeated sedimentation and exposure of glacial Lake Missoula sediments: A lake-level history at Garden Gulch, Montana, USA

• Lake-bottom glaciolacustrine sediments were subaerially exposed 12 times at the Garden Gulch section.
• Glacial Lake Missoula filled to, and fluctuated around, near its highstand position 7–12 times in the late Pleistocene.
• Cryogenic deformation was due to seasonal cold and desiccation. Features attributable to permafrost are not present.

Glaciolacustrine sediments record lake transgression, regression, and subaerial modification of the silty lake-bottom of glacial Lake Missoula in the Clark Fork River valley. The sequence preserved at Garden Gulch, MT documents lake-level fluctuations at >65% of its full-pool volume. Twelve sedimentary cycles fine upwards from (1) very fine-grained sandy silt to (2) silt with climbing ripples to (3) rhythmically laminated silt and some clay. The cycles are fine-grained turbidites capped locally by thin layers of angular gravel derived from local bedrock outcrops. The gravels appear to be the toes of mass wasting lobes carried onto the exposed lakebed surface during repeated lake-level lowerings.Periglacial wedges, small rotational faults, involutions, and clastic dikes deform the tops of eleven cycles. The wedges are 10–30 cm wide, penetrate 30–70 cm deep, are spaced <1 m apart, and contain vertically oriented gravel and massive to laminated sediment. Wedges split and taper in plan view. A few thin silt-filled dikes, which branch and taper downwards from wedges, are interpreted as filled frost cracks. One 10–20 cm-wide sand-filled dike protrudes upward from a sand bed; it is interpreted as a liquefaction feature consistent with a filling and draining lake. The deformed cycle tops preserve evidence of periglacial cold, subaerial exposure, seasonal frost heave, and the incipient formation of sorted polygons.The lowest five cycles are thicker and display more periglacial modification at their tops than the upper seven cycles. The Garden Gulch section may represent as few as seven and as many as twelve substantial fillings and partial to complete drainings of glacial Lake Missoula.

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