late pleistocene bedrock channel incision of the susquehanna river, holtwood gorge, pennsylvania a...

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??

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