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Grounding line processes of the southern Cordilleran Ice Sheet in the Puget Lowland

By
Lauren M. Simkins
Lauren M. Simkins
Department of Earth, Environment and Planetary Sciences, Rice University, 6100 Main Street, Houston, Texas 77005, USA
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John B. Anderson
John B. Anderson
Department of Earth, Environment and Planetary Sciences, Rice University, 6100 Main Street, Houston, Texas 77005, USA
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Brian P. Demet
Brian P. Demet
Department of Earth, Environment and Planetary Sciences, Rice University, 6100 Main Street, Houston, Texas 77005, USA, and Schuepbach Energy, 2651 North Harwood, Suite 570, Dallas, Texas 75201, USA
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Published:
January 01, 2017
Publication history
01 August 2017

ABSTRACT

The northern Puget Lowland of Washington State, USA, provides an exceptional opportunity not only to examine grounding line processes associated with marine-based ice sheets, but also to relate subaerial outcrop to marine geological observations of grounding line landforms and sedimentary processes in Antarctica and the deglaciated Northern Hemisphere. During this trip, we visit outcrops that record the interaction of the Cordilleran Ice Sheet and its bed, starting with locations where the ice sheet slowly flowed across crystalline bedrock. We also visit locations where the ice flowed across unconsolidated deposits, allowing discussions of subglacial bed deformation and grounding zone wedge development. Evidence shows that grounding line retreat across Whidbey Island was punctuated by periods of grounding line position stability and local ice advance during the growth of multiple grounding zone wedges. We will discuss the criteria for identifying grounding zone wedges, including diamicton units with foreset bedding that downlap onto a regional glacial unconformity at the base, and are truncated at the top by localized unconformities indicative of ice advance across the foreset beds. Grounding zone wedge foreset beds are composed of debris flows sourced from a deformation till and from sediment transported to the grounding line by subglacial meltwater. The overlying surface unconformity is associated with a laterally discontinuous till and pervasive glacial lineations. Other field stops focus on iceberg scouring and evidence of subglacial meltwater drainage, as well as the transition from marine to subaerial conditions during retreat of the Cordilleran Ice Sheet from the northern Puget Lowland.

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Contents

GSA Field Guides

From the Puget Lowland to East of the Cascade Range: Geologic Excursions in the Pacific Northwest

Ralph A. Haugerud
Ralph A. Haugerud
U.S. Geological Survey c/o Department of Earth and Space Sciences University of Washington, Box 351310 Seattle, Washington 98195, USA
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Harvey M. Kelsey
Harvey M. Kelsey
Geology Department Humboldt State University 1 Harpst Street Arcata, California 95521, USA
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Geological Society of America
Volume
49
ISBN electronic:
9780813756493
Publication date:
January 01, 2017

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