Chemical Weathering Across A Passive Margin Escarpment, southeastern Australia

Chemical Weathering Across APassive Margin Escarpment,

southeastern Australia

Benjamin Burke, Arjun M. HeimsathDartmouth College Earth Sciences Hanover, New Hampshire

Pardee Keynote Symposium 2004 GSA National Convention, Denver

2004 GSA Pardee Symposium, Denver2004 GSA Pardee Symposium, Denver

IntroductionIntroduction Hypothesis:Hypothesis: As a landscape is unzipped by a As a landscape is unzipped by a

propagating escarpment, chemical weatheringpropagating escarpment, chemical weatheringsignatures above and below the escarpmentsignatures above and below the escarpmentshould be different: more mature and moreshould be different: more mature and moreweathered above, less mature and lessweathered above, less mature and lessweathered below.weathered below.

Data from soil pits and borings.Data from soil pits and borings. Weathering above and below escarpmentWeathering above and below escarpment

Strain, mineral depletion, bulk weathering intensitiesStrain, mineral depletion, bulk weathering intensitiesand rates.and rates.


LocationsLocations The Great Escarpment ofThe Great Escarpment of

southeastern Australia.southeastern Australia. 50 km50 km fr fr Pacific coast:Pacific coast: 50 km/100Ma = 500 m/Ma50 km/100Ma = 500 m/Ma Escarpment retreat rateEscarpment retreat rate……?!?! No PleistoceneNo Pleistocene glaciation glaciation No No aolian aolian depositiondeposition Minimal vegetation changesMinimal vegetation changes

““Characteristic formCharacteristic form”” of linear of lineartransport lawtransport law……

3 sites - Frog3 sites - Frog’’s Hollow (FH),s Hollow (FH),Nunnock River (NR), andNunnock River (NR), andSnugburra Snugburra (SN)(SN)


LocationsLocationsAbove escarpment (FH)Above escarpment (FH) 500-700 mm rain annually500-700 mm rain annually Lithology:Lithology: granodiorite granodiorite.. Many tors present.Many tors present. Periglacial Periglacial effects present.effects present. FrogFrog’’s Hollow granite (s Hollow granite (AnemboAnembo

granodioritegranodiorite) ) ““texturally homogenous,texturally homogenous,””but but ““xenoliths up to 20 sq. cm.xenoliths up to 20 sq. cm.commoncommon””. (Black, 1967). (Black, 1967)

Below escarpment (NR and SN)Below escarpment (NR and SN) 800-1200 mm rain annually800-1200 mm rain annually Lithology: Lithology: granodioritegranodiorite Nunnock River granite characterized asNunnock River granite characterized as

““very homogenous,very homogenous,”” coarse grained, no coarse grained, noinclusions. (Chappell and Williams,inclusions. (Chappell and Williams,1991)1991)


Framework for SoilFramework for SoilProductionProduction

Conceptual framework

Mass Balance Model•P = soil production•Q = transport flux

dz/dt = soil production= divergence of flux (in steady-state)


Soil Production Across EscarpmentSoil Production Across Escarpment Determined by Heimsath etDetermined by Heimsath et

al. (1999, 2000,2004)al. (1999, 2000,2004) Lowland (NR)Lowland (NR) ε(ε(HH) = 53±3) = 53±3ee-(0.020±0.001)*H-(0.020±0.001)*H

Highland (FH)Highland (FH)ε(ε(HH) = 65±15) = 65±15ee-(0.020±0.003)*H-(0.020±0.003)*H

Different tectonic andDifferent tectonic andclimate histories, yetclimate histories, yetoverlapping soil productionoverlapping soil productionfunctionsfunctions


FieldworkFieldwork


DataData

Frog’s Hollow pit. View of S wall.

Nunnock River pit. View of W wall.

NRFH



Measuring ChemicalMeasuring ChemicalWeatheringWeathering

Two types of data:Two types of data:spatial from soil pitsspatial from soil pitsand depth from drilland depth from drillcores.cores.

Spatial data for CIA:Spatial data for CIA:

Intensity measured by ChemicalIntensity measured by ChemicalIndex of Alteration (Kirkwood &Index of Alteration (Kirkwood &Nesbitt, 1982, 1991):Nesbitt, 1982, 1991):

(Al(Al22OO33 / [Al / [Al22OO33 + Na + Na22O + O + CaO CaO ++KK22O])*100 = CIAO])*100 = CIA

Chemical weathering rateChemical weathering ratecalculation (calculation (Brimhall Brimhall et al, 1992,et al, 1992,Riebe Riebe et al., 2001, 2003):et al., 2001, 2003):

ρρsaprolitesaprolite*SPR*(1-(*SPR*(1-(ZrZrrockrock / Zr / Zr saprolitesaprolite))0.010.04

N = 25CV

0.82.7STDEV68.868.5MeanNRFH


StrainStrain

Strain - SN B2

0.00

200.00

400.00

600.00

800.00

1000.00

1200.00

1400.00

1600.00

1800.00

2000.00

-1.50 -0.50 0.50 1.50 2.50 3.50 4.50

Volume change, ! j,w

Dep

th (

cm)

Zr

Ce

Th

Strain - SN B1

0.00

100.00

200.00

300.00

400.00

500.00

600.00

700.00

800.00

900.00

1000.00

-1.50 -0.50 0.50 1.50 2.50 3.50 4.50


Dep

th (

cm)

Zr

Ce

Th

Strain - NR B1

0.00

200.00

400.00

600.00

800.00

1000.00

1200.00

1400.00

1600.00

1800.00

2000.00

-1.50 -0.50 0.50 1.50 2.50 3.50 4.50


Dep

th (

cm)

Zr

Ti

Ce

Th

Strain - FH B1

0.00

200.00

400.00

600.00

800.00

1000.00

1200.00

1400.00

-1.50 -0.50 0.50 1.50 2.50 3.50 4.50


Dep

th (

cm)

Zr

Ti

Ce

Th


Elemental LossElemental Loss

Elemental Loss, NR B1

0.000

200.000

400.000

600.000

800.000

1000.000

1200.000

1400.000

1600.000

1800.000

2000.000

-1.000 -0.750 -0.500 -0.250 0.000 0.250 0.500 0.750 1.000

tau

Dep

th (

cm)

MgO tau

CaO tau

Na2O

K2O

Elemental Loss, NR B1

0.000

200.000

400.000

600.000

800.000

1000.000

1200.000

1400.000

1600.000

1800.000

2000.000

-1.000 0.000 1.000

Tau

Dep

th (

cm)

SiO2

Al2O3

Fe2O3

Elemental Loss, FH B1

0.000

200.000

400.000

600.000

800.000

1000.000

1200.000

1400.000

-1.000 -0.500 0.000 0.500 1.000

Tau

Dep

th (

cm)

SiO2 tau

Al2O3 tau

Fe2O3

Elemental Loss, FH B1

0.000

200.000

400.000

600.000

800.000

1000.000

1200.000

1400.000

-1.000 -0.500 0.000 0.500 1.000

tau

Dep

th (

cm)

MgO tau

CaO tau

Na2O

K2O


Chemical weathering ratesChemical weathering rates

• Mass balance weathering rate calculated using Zr, sampled from top ofsaprolite.• CW rates from 2 meter pits at FH and NR were both 20.6 m/M. (n = 25)

SPR and CWR at Oz Sites

1

10

100

0 20 40 60 80 100

Sample Depth (cm)

Rate

(m

/M

y)

NRFHSNNR CWRFH CWR

SPR and CWR- Zr at Point Reyes, CA

1

10

100

0 20 40 60 80 100

Soil Column Thickness

Rate

(m

/M

y)

CWR Zr

SPR


Intensity of WeatheringIntensity of WeatheringCIA with Depth at FH

50

250

450

650

850

1050

1250

1450

1650

58.00 60.00 62.00 64.00 66.00 68.00 70.00 72.00 74.00

CIA

Dep

th (

cm) B1

B2

B3

B4

B5

CIA with Depth at NR

50

250

450

650

850

1050

1250

1450

1650

1850

58 60 62 64 66 68 70 72 74

CIAD

ep

th (

cm)

B1

• X error bars = 1 STDEV• FH shows lower cw intensitythan NR.• FH - statistical significanceIn trends of individual borings, little in mean profile. NR - no trend. Higher cw intensity.


Weathering and FracturesWeathering and Fractures Twidale Twidale (1982) notes the(1982) notes the

importance of fractureimportance of fracturesystems in systems in tor tor creation.creation.

Areas with tors at theAreas with tors at thesurface, therefore havesurface, therefore have““channellizedchannellized””groundwater flow -->groundwater flow -->lower bulk weatheringlower bulk weatheringintensities away fromintensities away fromthose fractures.those fractures.

We believe this is theWe believe this is thecase at FHcase at FH

Not the case at NRNot the case at NR


DataData 124

130

104

Zr (ppm)

80

144

123 150

130

90

SiOSiO22 (%) (%)

70.6 68.6 69.3

69.9 68.1 70.9

72.1 71.9 70.4

Frog’s Hollow 2MP floor (Anembo granodiorite)


Data - SiOData - SiO22 Values Values

W50

W100

W150

SW

NW

N50

N100

N150

NE60

65

70

75

80

SiO2 (Wt. %)

West Wall

North Wall

Contour Map of SiO2 values at NR-2MP

75-8070-7565-7060-65

0

50

100

150

2000

50

100

150

200

60

65

70

75

80

SiO2 (Wt. %)

North Wall

West Wall

Contour Map of SiO2 values at FH-2MP

75-8070-7565-7060-65


Data - CIAData - CIA

0

50

100

150

200

NW

N50

N100

N150

NE

64.0

65.0

66.0

67.0

68.0

69.0

70.0

71.0

72.0

73.0

CIA

West Wall

North Wall

Contour Display of Chemical Index of Alteration at NR-2MP

72.0-73.071.0-72.070.0-71.069.0-70.068.0-69.067.0-68.066.0-67.065.0-66.064.0-65.0

• Homogeneity of CIA at NR versus FH, cv at FH = 0.04, NR = 0.01

0

50

100

150

200

NW

N50

N100

N150

NE

64

65

66

67

68

69

70

71

72

73

CIA

West Wall

North Wall

Contour Display of Chemical Index of Alteration at FH-2MP

72-7371-7270-7169-7068-6967-6866-6765-6664-65


ConclusionsConclusions New relationships found between cosmogenic age datesNew relationships found between cosmogenic age dates

above and below escarpment.above and below escarpment. Mineral depletion: Mineral depletion: ““MatureMature”” weathering signature weathering signature

established at FH, and not at NR. SN is a middle case.established at FH, and not at NR. SN is a middle case. Strain values are expected, including NR erroneousStrain values are expected, including NR erroneous

strain below the strain below the watertablewatertable.. Weathering intensity: signature at FH, not at NR.Weathering intensity: signature at FH, not at NR. Relation to physical weathering:Relation to physical weathering:

Chemical weathering signature differences despite lack ofChemical weathering signature differences despite lack ofvariation in soil production functions at the hillslope scale.variation in soil production functions at the hillslope scale.

Higher Higher exumation exumation rates at NR may influence higher chemicalrates at NR may influence higher chemicalweathering intensities.weathering intensities.

2004 GSA Pardee Symposium,Denver

Acknowledgements

• Dartmouth College Earth Science Department• Australian National University

– Damien Kelleher, John Chappell• National Science Foundation• Geological Society of America

Documents

Chemical Weathering Across A Passive Margin Escarpment, southeastern Australia