T231 On the Edge—New Insights into Ice-Marginal Conditions I

Thursday, 29 October 2020
5:30 PM - 8:00 PM
Meeting Rooms

EROSIONALLY-STREAMLINED SUBGLACIAL BEDFORMS, AND ICE MARGINAL PRESSED MORAINES ON THE BED OF A PALEO ICE STREAM: GREEN BAY LOBE, WISCONSIN USA: LIDAR MAPPING OF A SOFT-BED ICE STREAM LANDSYSTEM

Paper number: T231-2
Program: Technical Sessions
Time: 5:35 PM – 5:45 PM

Authors

Abstract

The Green Bay Lobe (35,000 km2) of the Great Lake sector of the Late Wisconsin Laurentide Ice Sheet was a late glacial paleo ice stream flowing southwest out of the Michigan Basin along a topographically-confined depression west of the Niagara Escarpment and Door Peninsula in present day Wisconsin. It is bounded to the south by the Driftless Area which remained unglaciated at that time. We employ high resolution (0.5 m) LiDAR data to map the entire bed of the paleo ice stream using machine learning to identify subglacial morphotypes that includes classical drumlins and megascale glacial lineations and other more complex intermediate forms mapped previously as `drumloids`. Their form and spatial arrangement are consistent with erosion of pre-existing sediment bodies; LiDAR data now allows recognition of original landforms such as overridden hummocky kettle moraine, and former moraine ridges deposited previously along the margin of the adjacent west-flowing Michigan Lobe. These residual sediment bodies form higher standing parts of the bed along the ice stream lateral margins and were reduced in size and elevation by subglacial erosional streamlining culminating in a wide variety of drumlin forms, and ultimately megascale glacial lineations under the axial faster flowing parts of the Green Bay Lobe. Subglacial debris eroded from streamlined bedforms was swept to the ice margin to be extruded and pressed into a variety of till-cored moraines including classical hummocky moraine, indicating a buoyant ice margin resting on soft till. This paper sets out an approach to the mapping, description and analysis of ancient Great Lake ice streams and provides significant glaciological information in regard to the evolution of basal drag.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-359117

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