LATE PLEISTOCENE BEDROCK CHANNEL INCISION OF THE SUSQUEHANNA RIVER, HOLTWOOD GORGE, PENNSYLVANIA A...
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LATE PLEISTOCENE BEDROCK CHANNEL LATE PLEISTOCENE BEDROCK CHANNEL INCISION OF THE SUSQUEHANNA RIVER, INCISION OF THE SUSQUEHANNA RIVER,
HOLTWOOD GORGE, PENNSYLVANIAHOLTWOOD GORGE, PENNSYLVANIA
A Masters Thesis Presented
by
Lucas Jonathan Reusser
to
The Faculty of the Geology Department
of
The University of Vermont
Presentation OutlinePresentation Outline
• Problem statement and background
• Dating river incision with 10Be
• Holtwood Gorge field area
• Timing and style of bedrock channel incision
• Potential drivers of incision
• Conclusions
• Future avenues of research
Susquehanna Basin
Holtwood Gorge
Late WisconsinanGlacial Limit
Mather GorgeMather Gorge
Potomac Potomac BasinBasin
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
Heavily Weathered HighHeavily Weathered HighPointsPoints
Bedrock Terrace LevelsBedrock Terrace Levels
Level 1 StrathLevel 1 Strath
11 22
Level 2 TerraceLevel 2 Terrace
33
Level 3 TerraceLevel 3 Terrace
44
Level 4:Level 4:Heavily WeatheredHeavily Weathered
High PointHigh Point
Nested Sampling StrategyNested Sampling Strategy
Lab Work and Nuclide MeasurementLab Work and Nuclide Measurement
• Quartz Purification
• Chemical Isolation of 10Be (Jen’s Magic)
• Accelerator Mass Spectrometer Measurement at the Lawrence Livermore National Laboratory
(From Piles of Rocks to Piles of Numbers)
Spatial PatterningSpatial Patterningof Erosion in Holtwood Gorgeof Erosion in Holtwood Gorge
Does the Livermore Does the Livermore Accelerator really work??Accelerator really work??
LR-04c 14.0 +/- 1.5 ka
LR-04cX 14.2 +/- 1.6 ka
+/- 1.0 %+/- 1.0 %
LR-37 17.4 +/- 1.9 ka
LR-37X 17.9 +/- 1.9 ka
+/- 1.9 %+/- 1.9 %
Laboratory and Measurement Replication
Does One Sample Represent the AgeOf An Entire Bedrock Surface??
+/- 3.8%+/- 3.8%(geomorphically Identical)
26.8 +/- 2.9 ka
26.0 +/- 2.8 ka
25.0 +/- 2.7 ka
Level 3 small-scale variance
Level 2 small-scale variance
17.5 +/- 1.9 ka18.6 +/- 2.0 ka
18.2 +/- 2.0 ka
+/- 3.3%+/- 3.3%(geomorphically Identical)
Level 1 small-scale variance(lowest bedrock terrace)
13.9 +/- 1.5 ka
16.4 +/- 1.8 ka
14.1 +/- 1.6 ka
+/- 8.5%+/- 8.5%(Much more variable(Much more variablethan surfaces higherthan surfaces higher
in the gorge)in the gorge)
Terrace Level 3:Terrace Level 3:Longitudinal Incision RateLongitudinal Incision Rate
Highest, Well PreservedHighest, Well PreservedStrath TerraceStrath Terrace
14 SamplesOver
4.5 km
35
40
45
50
0 2 4 6
Distance Downstream (km)
Elevation (m)
Inferred River Gradient = 2.0 m/km R2 = 0.9
20
30
40
50
0 2 4 6
Distance Downstream (km)
Model Age (ky)
No Relationship BetweenDistance and Model Age
Terrace Level 2:Terrace Level 2:Longitudinal Incision RateLongitudinal Incision Rate
Mid-LevelMid-LevelStrath TerraceStrath Terrace
10
20
30
0 2 4 6
Distance Downstream (km)
Model Age (ky)
20 SamplesOver
~5 km
30
35
40
45
0 2 4 6
Distance Downstream (km)
Elevation (m)
Inferred River Gradient = 1.5 m/km R2 = 0.9
Significant Age Gradient 1.4 ky/km upstreamOlder Ages Downstream
Knickpoint Retreat?
Terrace Level 1:Terrace Level 1:Longitudinal Incision RateLongitudinal Incision Rate
Lowest LevelLowest LevelModern StrathModern Strath
10 SamplesOver
~2.5 km
32
34
36
38
0 1 2 3
Distance Downstream (km)
Elevation (m)
Inferred River Gradient
~1.5 m/kmR2 = 0.9
12
14
16
18
0 1 2 3
Distance Downstream (km)
Model Age (ky)
No RelationshipBetween DistanceAnd Model Age
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?
Model Age (ka)
Hei
ght
abov
e riv
e r b
e d (
m)
Middle Gorge Cross-Section
n=22Incision Rate = Incision Rate = 0.52 m/ka0.52 m/ka
R2=0.72
Are Rates of Downcutting At Specific Are Rates of Downcutting At Specific Locations Similar to Gorge Wide Averages?Locations Similar to Gorge Wide Averages?
Model Age (ka)
Hei
ght
abov
e riv
e r b
e d (
m)
Upper Gorge Cross-Section
n=13Incision Rate = Incision Rate = 0.60 m/ka0.60 m/ka
R2=0.96
Long- vs. Short-Term Rates of Incision Long- vs. Short-Term Rates of Incision along the Susquehanna Riveralong the Susquehanna River
Piedmont
~15 My
Long-Term Rate
~12 m/My
GORGE
LatePleistocene
Rate
20 X - 60 X20 X - 60 XFasterFaster
Episodic IncisionEpisodic Incision
Rates in active regions measured Rates in active regions measured with cosmogenic isotopes:with cosmogenic isotopes:
(Indus River, Himalayas)(Indus River, Himalayas)
1 to 12 m/ka1 to 12 m/kaDriven primarily by upliftDriven primarily by uplift
An alternative approach to considering the An alternative approach to considering the timing of terrace abandonment:timing of terrace abandonment:
(Probability Modeling: (Probability Modeling: Balco Balco et alet al., 2002., 2002))
Model Age (ka)
Rel
ativ
e P
rob
abil
ity
Level 1 Terrace
LR-5417.0 +/- 0.7 ka
An alternative approach to considering the An alternative approach to considering the timing of terrace abandonment:timing of terrace abandonment:
(Probability Modeling: (Probability Modeling: Balco Balco et alet al., 2002., 2002))C
um
ula
tive
Pro
bab
ilit
y
Model Age (ka)
Level 1 Terrace
Su
mm
ed P
rob
abil
ity
An alternative approach to considering the An alternative approach to considering the timing of terrace abandonment:timing of terrace abandonment:
(Probability Modeling: (Probability Modeling: Balco Balco et alet al., 2002., 2002))
Model Age (ka)
No
rmal
ize d
Cu
mu
l at i
v e P
rob
abi l
i ty
Mean = 14.4 kaPeak = ~14 ka
Mean = 19.8 kaPeak = ~18 ka
Mean = 36.1 kaPeaks at
26, 32 and 45 ka
Rapid Incision From~50 ka to ~10 ka
No
r ma l
ized
Su
mm
e d P
rob
abil
i ty
Potential Drivers of Rapid Potential Drivers of Rapid Late Pleistocene IncisionLate Pleistocene Incision
Holtwood Gorge
Mather GorgeMather Gorge
Similar Incision HistoriesE
lev
ati
on
Thousands of Years Before PresentThousands of Years Before Present
SusquehannaSusquehannaRiverRiver
Holtwood GorgeHoltwood Gorge
Ele
va
tio
n
PotomacPotomacRiverRiver
Mather GorgeMather Gorge
Rapid IncisionRapid IncisionFromFrom
~50 to 10 ka~50 to 10 kaAtAt
0.4 to 0.6 m/ka0.4 to 0.6 m/ka
Rapid IncisionRapid IncisionFromFrom
37 to 13 ka37 to 13 kaAtAt
0.5 to 0.8 m/ka0.5 to 0.8 m/ka
What Caused this Pulse ofWhat Caused this Pulse ofLate Pleistocene Incision?Late Pleistocene Incision?
• Glaciated Susquehanna BasinGlaciated Susquehanna Basin
• Unglaciated Potomac BasinUnglaciated Potomac Basin
• Similar TimingSimilar Timing
• Regional Forcings:Regional Forcings:
• Land-LevelLand-Level• Base-LevelBase-Level
• Global and Regional ClimateGlobal and Regional Climate
• Similar RateSimilar Rate
Land Level Change…Land Level Change…(The Growing Glacial Forebulge)(The Growing Glacial Forebulge)
Modeling by:Modeling by:Jon PelletierJon Pelletier
• Crustal Response to mantleCrustal Response to mantle displacement by advancing displacement by advancing ice loadice load
ICEICE
Tens of m of upliftTens of m of uplift
Right PlaceRight Place
~Location of~Location ofOur GorgesOur Gorges
Timing and extent uncertainTiming and extent uncertain
Where was the ice front? Where was the ice front?
Probably didn’t initiate Incision, Probably didn’t initiate Incision, But likely helped maintain itBut likely helped maintain it
Increased River GradientsIncreased River GradientsUniversityUniversityof Arizonaof Arizona
Thousands of Years Before Thousands of Years Before PresentPresent
Base Level Change…Base Level Change…(Global Sea-Level Through The Last Glacial Cycle)(Global Sea-Level Through The Last Glacial Cycle)
Mean Global Mean Global Sea-LevelSea-Level
(Huon Peninsula)(Huon Peninsula)
MetersBelow
Present
Elevation
GlaciatedGlaciatedSusquehannaSusquehanna
RiverRiverHoltwood GorgeHoltwood Gorge
Elevation
Mather GorgeMather Gorge
UnglaciatedUnglaciatedPotomacPotomac
RiverRiver
Timing of Sea-Level FallTiming of Sea-Level FallUncertaintiesUncertainties
Exposure Age ModelingExposure Age ModelingUncertaintiesUncertainties
Rate and Process of Rate and Process of Upstream TranslationUpstream Translation
Through BedrockThrough BedrockNot KnowNot Know
Thousands of Years Before Thousands of Years Before PresentPresent
Base Level Change…Base Level Change…(Global Sea-Level Through The Last Glacial Cycle)(Global Sea-Level Through The Last Glacial Cycle)
Mean Global Mean Global Sea-LevelSea-Level
(Huon Peninsula)(Huon Peninsula)
MetersBelow
Present
Paleo-ShorelinesPaleo-Shorelines~70 mbpAt 50 ka
HoltwoodGorge
~150 mbpAt ~20 ka
Continental shelf edge
Upstream effect uncertain
Deg
rees
CD
egre
es C PaleotemperaturePaleotemperature
GISP2GISP2(Cuffey et al., 1997)(Cuffey et al., 1997)
0 25 50 75 100
Global Climate Change…Global Climate Change…(GISP2 ice core records)(GISP2 ice core records)
Temperature trends correlate Temperature trends correlate To Florida (Grimm et al.), To Florida (Grimm et al.),
And Blue Ridge And Blue Ridge (Litwin et al., 2004)(Litwin et al., 2004)
s.s.
Na
pp
bs.
s. N
a p
pb PaleostorminessPaleostorminess
GISP2 sea saltGISP2 sea salt(Mayewski et al., 1997)(Mayewski et al., 1997)
Storminess in NortheastStorminess in NortheastThrough HoloceneThrough Holocene(Noren et al., 2003)(Noren et al., 2003)
Susquehanna RiverSusquehanna River
Potomac RiverPotomac River
Thousands of Years Before Present
Terrace formationand rate increase
coincident with glacial Max in PA
What Do Temperature and Storminess What Do Temperature and Storminess Have to do with Bedrock Incision??Have to do with Bedrock Incision??
FloodsFloodsIncision Threshold…Critical Shear StressIncision Threshold…Critical Shear Stress
Frozen Ground…More Runoff??Frozen Ground…More Runoff??
Discharge Concentration:Discharge Concentration:•Snow Melt FloodsSnow Melt Floods
•Rain on Snow EventsRain on Snow Events•Stormier ClimateStormier Climate
Hurricane IsabelHurricane IsabelOctober, 2003October, 2003
ConclusionsConclusions
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
-Joanna Reuter-Joanna Reuter-Jen Larsen-Jen Larsen
-UVM Geology Department-UVM Geology Department-Paul Bierman-Paul Bierman
-Staff at Holtwood Dam-Staff at Holtwood Dam-George and Melody-George and Melody
Hecras Modeling:Hecras Modeling:
Hecras Modeling:Hecras Modeling:
Hecras Modeling:Hecras Modeling:
<10% averageadjustment for water shielding
Hecras Modeling:Hecras Modeling:
Modeling explains~32% of the model
Age variance betweenLevel 1 samples
Cosmic ray Cosmic ray bombardment bombardment in situin situ
Cosmic ray Cosmic ray bombardment bombardment in situin situ
OO
SiSi
Ca, K, ClCa, K, Cl
36Cl, 3He
26Al, 21Ne, 3He
10Be, 14C, 3He
Paleo River Gradients
30
40
50
0 1 2 3 4 5 6
Distance Downstream (km)
Elevation (m)
Inferred Paleo River GradientsInferred Paleo River Gradients
Level 1 Gradient = 1.5 m/km
Level 1 Gradient = 1.5 m/km
Level 2 Gradient = 1.5 m/km
Level 2 Gradient = 1.5 m/km
Level 3 Gradient = 2.0 m/km
Level 3 Gradient = 2.0 m/km
Residual Analysis:Residual Analysis:
Boulders:Boulders:
Boulder:~24 ka
Surface:~15 ka
Boulders:Boulders:
Boulder:~15.5 ka
Surface:~17.9 ka
• Late Pleistocene origin• Ages may incorporate prior Periods of burial and exposure• Two boulders alone don’t tell us much.• Flood origin??
TitleTitle