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Profile B is representative of facie observed in the northern end of the tombolo.

B

Profile A is representative of the northern end of the tombolo. A

Four radar facies are interpreted from the GPR transect across the Grand Island Tombolo.

Radar Facies­ Radar StratigraphicDescription Interpretation
1
The upper radar facies consists of northward dipping inclined reflections with a 3-5m thickness across the length of the profile, and downlaps lower facies. The modern strandplain. This radar facies matches radar stratigraphic descriptions of other Lake Superior strandplains, (Johnston, et. al, 2007).
2
Concordant with lower reflections, a third radar facies with a thickness of 5-7m at the southern end of the transect and 20-22m at the northern end of the transect consists of continuous subhorizontal to inclined reflections. Shoreface progradation during lake level regression.   Subhorizontal reflections were deposited on top of the underlying deposits and grade to inclined reflections which are interpreted as clinoforms building out from the underlying deposit.  
3
A thin, 2-4m, radar facies downlaps onto radar facies 1 and consists of northward dipping inclined reflections. This radar facies is only found on the southern portion of the transect. A historic strandplain. Modern Lake Superior strandplains have been extensively studied and show lakeward dipping reflections with an average thickness of 5m (Johnston, et. Al, 2007) which match reflections observed in radar facies 2.
4
The deepest facies is below 16m and likely descends below the depth of penetration. This radar facies is composed of sigmoid and hummocky reflections which top lap above facies. Hummocky reflections are truncated by sigmoid reflections. Sigmoid reflections are 5-10m apart. The facies are found only on the southern part of the transact. The lowest radar facies is interpreted as shoreface progradation during lake level transgression. Sigmoid lower reflections are interpreted as erosional surfaces, while horizontal hummocky reflections suggest aggradation (Larson, et al, 2007).   Together these reflections represent periods of erosion and aggradation resulting in net progradation.   Lake level transgression provided the erosive energy needed to form sigmoid reflections, while other unidentified processes episodically deposited sediment.
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