LATE PLEISTOCENE BEDROCK CHANNEL LATE PLEISTOCENE BEDROCK CHANNEL INCISION OF THE SUSQUEHANNA RIVER, INCISION OF THE SUSQUEHANNA RIVER, HOLTWOOD GORGE, PENNSYLVANIA HOLTWOOD GORGE, PENNSYLVANIA A Masters Thesis Presented by Lucas Jonathan Reusser to The Faculty of the Geology Department of The University of Vermont
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LATE PLEISTOCENE BEDROCK CHANNEL INCISION OF THE SUSQUEHANNA RIVER, HOLTWOOD GORGE, PENNSYLVANIA A Masters Thesis Presented by Lucas Jonathan Reusser to.
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LATE PLEISTOCENE BEDROCK CHANNEL LATE PLEISTOCENE BEDROCK CHANNEL INCISION OF THE SUSQUEHANNA RIVER, INCISION OF THE SUSQUEHANNA RIVER,
Cosmic ray Cosmic ray bombardment bombardment in situin situ
Cosmic ray Cosmic ray bombardment bombardment in situin situ
OO
SiSi
26Al, 21Ne, 3He
10Be, 14C, 3HeQuartzQuartz
5.2 atoms/g/yr5.2 atoms/g/yr
Model Age ~ Terrace Abandonment
Dating Bedrock Terraces
High 10Be Concentration
Older Model Age
Lower 10Be Concentration
Younger Model Age
Significance of ResearchSignificance of Research• River incision through bedrock has important
implications for landscape development.
• Recent efforts to directly measure or model rates of incision with numerical simulations may not capture the timing and/or nature of dominant erosional processes.
• Very little is known about how, when, or why passive margin rivers incise through bedrock.
• Age control is mandatory to decipher how passive margin rivers respond to glaciation, climate, land-level, and/or sea-level.
Goals of The Project:
• Estimate the age of bare-rock strath terraces within Holtwood Gorge.
• Investigate the spatial patterning of erosion.
• Determine the timing and rate of incision within the gorge.
• Consider potential drivers of incision.
• Refine this new application of cosmogenic dating.
Field Work in Holtwood GorgeField Work in Holtwood Gorge
Long-Profile of the Susquehanna
~0.5 m/km
~1.0 m/km
Holtwood Gorge
Oversteepening:
Flexural response toOffshore sediment loading
AndIsostatic response to
denudation
Holtwood Gorge Field AreaHoltwood Gorge Field Area• Located approximately 50 km upstream from
Chesapeake Bay and immediately below Holtwood Dam.
• Carved into the Wissahickon Schist of the Appalachian Piedmont.
• Harbors three distinct levels of bedrock terraces.
• Accessible by canoe.
• Abundant extractable quartz.
~ 80 Bedrock Samples~ 80 Bedrock Samples
3 Well Preserved Terraces3 Well Preserved Terraces
Timing and rate of bedrock Timing and rate of bedrock incision within Holtwood incision within Holtwood
GorgeGorge
Average Terrace Ages
How Old Are The Holtwood Terraces??
Lower LimitingAges
>90 ka
Mean Age = 20 +/- 3 ka
Mean Age = 36 +/- 7 ka
Mean Age = 14 +/- 1 ka
When and How Quickly Did The Susquehanna River Incise??
Acceleration at ~35 kaLevel 3 Abandonment
Maximum Rate Since ~100 ka
Increase at ~20 kaLevel 2 Abandonment
Incision Ceased ~14Level 1 Mean Age
Are Rates of Downcutting At Specific Are Rates of Downcutting At Specific Locations Similar to Gorge Wide Averages?Locations Similar to Gorge Wide Averages?
Are Rates of Downcutting At Specific Are Rates of Downcutting At Specific Locations Similar to Gorge Wide Averages?Locations Similar to Gorge Wide Averages?
Bedrock strath terraces record a pulse of rapid incisionBedrock strath terraces record a pulse of rapid incision
Downcutting increased between ~50 and ~35 ka;Downcutting increased between ~50 and ~35 ka;Incision ceased near the Pleistocene/Holocene transitionIncision ceased near the Pleistocene/Holocene transition
Correlation to GISP2 climate proxy recordsCorrelation to GISP2 climate proxy records
Influence of glacial retreat on Susquehanna River;Influence of glacial retreat on Susquehanna River;But regional first order drivers for the But regional first order drivers for the
initiation of incision on both rivers.initiation of incision on both rivers.
Implications and Future ResearchImplications and Future Research
1010Be dating is a useful tool for investigating the tempoBe dating is a useful tool for investigating the tempoand style of bedrock channel incision around the globe and style of bedrock channel incision around the globe
The episode of incision measured in Holtwood GorgeThe episode of incision measured in Holtwood Gorgerepresents one pulse in an ongoing period of river adjustmentrepresents one pulse in an ongoing period of river adjustment
operating over geologic time scales operating over geologic time scales
This study just scratches the surface: This study just scratches the surface:
• Similar approach on other major Appalachian drainages Similar approach on other major Appalachian drainages
• Tributary response to changing boundary conditionsTributary response to changing boundary conditionsand landscape conectivity and landscape conectivity
• Many questions regarding climate driven process rates Many questions regarding climate driven process rates still remain still remain
Acknowledgements:Acknowledgements:
Funding:Funding: -NSF-NSF
-D.O.E.-D.O.E.
Field Assistance etc.:Field Assistance etc.: -Eric Butler-Eric Butler