TOPOGRAPHIC FINGERPRINTS OF HILLSLOPE EROSION IN THE CENTRAL APPALACHIANS ...

159 Pages·2014·6.35 MB·English

The Pennsylvania State University

The Graduate School

College of Earth and Mineral Sciences
TOPOGRAPHIC FINGERPRINTS OF HILLSLOPE EROSION IN THE CENTRAL
APPALACHIANS AS REVEALED BY METEORIC 10BE: TOWARD UNDERSTANDING
THE EVOLUTION OF CRITICAL ZONE ARCHITECTURE
A Dissertation in

Geosciences

by

Nicole West
 2014 Nicole West
Submitted in Partial Fulfillment
of the Requirements
for the Degree of
Doctor of Philosophy

August 2014
The dissertation of Nicole West was reviewed and approved* by the following:

Eric Kirby
Associate Professor
R.S. Yeats Chair of Earthquake Geology and Active Tectonics
Oregon State University
Dissertation Advisor
Co-Chair of Committee

Rudy Slingerland
Professor of Geology
Pennsylvania State University
Co-Chair of Committee

Susan Brantley
Distinguished Professor of Geosciences
Pennsylvania State University

Doug Miller
Associate Professor of Geography
Pennsylvania State University

Demian Saffer
Professor of Geosciences
Associate Head for Graduate Program and Research in Geosciences
Pennsylvania State University

*Signatures are on file in the Graduate School
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ABSTRACT

Regolith-mantled hillslopes are ubiquitous features of most temperate landscapes, and
their morphology is a reflection of the climatically, biologically, and tectonically mediated
interplay between regolith production and downslope transport. Despite this understanding,
relatively few studies have been able to independently measure these processes and relate them to
mechanistic controls (e.g., bioturbation, freeze-thaw). This dissertation encompasses two field-
based studies and one remote data study that aim to quantify the rates of regolith production and
transport, define the physical rules that dictate the shape of hillslopes, and identify the
mechanisms by which topography adjusts. Here, I exploit the cosmogenic radionuclide meteoric
10Be to measure regolith residence times, erosion rates, and downslope regolith fluxes in the
Susquehanna Shale Hills Critical Zone Observatory (SSHO), located in the Valley and Ridge
Physiographic province of the central Appalachians. I compare these isotopic measurements with
measures of hillslope morphology taken from recently acquired high-resolution LiDAR derived
digital topography in order to test transport rules acting at SSHO and tease out mechanisms
responsible for regolith transport.
From this work I have determined that the Shale Hills Critical Zone Observatory is
approaching steady state, where the fluxes of regolith production at SSHO ridgetops are balanced
by regolith erosion, within uncertainty. Additionally, regolith fluxes along hillslopes in and
adjacent to SSHO suggest that the entire landscape is lowering at a rate of ~30 m/My. I have
found that this condition exists despite observed aspect-related asymmetry with respect to
hillslope gradient and regolith thickness. In fact, it is likely that the topographic asymmetry is a
result of the landscape adjusting its morphology in order to maintain a steady flux of material off
hillslopes receiving different amounts of solar radiation over geologic time. I contend that a
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transport mechanism that is both aspect-controlled and dependent on regolith depth is freeze-thaw
cycling, which appears to occur more frequently on sun-facing hillslopes at SSHO.
Along SSHO ridgetops, where regolith is uniformly thin and aspect-related differences
are minimal, regolith flux appears to be linearly dependent on local topographic gradient.
Ridgetop curvature values extracted from high-resolution digital topography accurately reflect
erosion rates and transport efficiency measured at SSHO, using meteoric 10Be. Applying this
understanding of landscape evolution from SSHO to first-order watersheds elsewhere in the
Susquehanna River Basin (n=27), reveals that, in the steadily lowering reaches of the basin,
transport efficiency is everywhere equal, despite differences in lithology, and proximity to the last
glacial margin. These results suggest that much of the topography of the central Appalachians
has adjusted to regional climatic and tectonic perturbations occurring over the Cenozoic.
v

TABLE OF CONTENTS
List of Figures .......................................................................................................................... vii
List of Tables ........................................................................................................................... xii
Acknowledgements .................................................................................................................. xiii

Chapter 1 Introduction ............................................................................................................. 2
1.1 Summary of Chapters ......................................................................................... 4
1.2 Conclusions ........................................................................................................ 9
References ................................................................................................................ 10
Chapter 2 Regolith production and transport at the Susquehanna Shale Hills Critical Zone
Observatory – Part 2: Insights from meteoric 10Be .......................................................... 15
Abstract .................................................................................................................... 15
2.1 Introduction ........................................................................................................ 16
2.2 Background ........................................................................................................ 18
2.3 Derivation of mass balance of meteoric 10Be on eroding hillslopes ................... 24
2.4 Methods .............................................................................................................. 31
2.5 Results ................................................................................................................ 35
2.6 Discussion .......................................................................................................... 41
2.7 Implications ........................................................................................................ 45
2.8 Conclusions ........................................................................................................ 49
Acknowledgements .................................................................................................. 50
References ................................................................................................................ 51
Chapter 3 Aspect-dependent variations in regolith creep revealed by meteoric 10Be .............. 77
Abstract .................................................................................................................... 77
3.1 Introduction ........................................................................................................ 78
3.2 Methods .............................................................................................................. 80
3.3 Results ................................................................................................................ 81
3.4 Discussion .......................................................................................................... 83
Acknowledgements .................................................................................................. 85
References ................................................................................................................ 86
Chapter 4 Using ridgetop curvature to test the influence of lithology and climate on the
evolution of first-order watersheds in the central Appalachians ...................................... 91
Abstract .................................................................................................................... 91
4.1 Introduction ........................................................................................................ 92
4.2 Background ........................................................................................................ 93
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4.3 Field Site ............................................................................................................ 98
4.4 Methods .............................................................................................................. 102
4.5 Results ................................................................................................................ 104
4.6 Discussion .......................................................................................................... 107
4.7 Conclusions ........................................................................................................ 110
References ................................................................................................................ 112
Appendix A Supplemental Stratigraphic Descriptions for Geoprobe Core used in
Chapter 2 .................................................................................................................. 125
Appendix B Supplemental Material for Chapter 3 .......................................................... 139
B.1 Sampling and Analytical Methods..................................................................... 139
References ................................................................................................................ 141
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LIST OF FIGURES
Figure 1-1. Location of Shavers Creek watershed within the greater Susquehanna River
Basin. The Shale Hills Critical Zone Observatory is located within the Shavers
Creek watershed. .............................................................................................................. 13
Figure 1-2. Shaded relief image of high resolution LiDAR-derived digital elevation data
for the Shavers Creek Watershed, collected as a collaborative effort between SSHO
researchers and the National Center for Airborne Laser Mapping. SSHO is outlined
in red. ............................................................................................................................... 14
Figure 2-1. Shaded relief perspective image of the Susquehanna Shale Hills Critical Zone
Observatory (SSHO). Image created from high resolution LiDAR digital elevation
data collected in concert with the National Center for Airborne Laser Mapping
(NCALM). The SSHO catchment exhibits relatively low relief and contains a subtle
fill terrace into which the present day channel is incised. ................................................ 59
Figure 2-2. a) Map view of the SSHO showing sample positions. White symbols indicate
sample locations from hand augers (RT – ridgetop; MS – midslope; VF – valley
floor), from a bedrock core (DC1), and from shallow subsurface cores along the
valley floor (GP). The black line corresponds to the topographic profile presented in
Figure 2-2c and Figure 2-5. b) Distribution of topographic gradient throughout the
SSHO. Boxes show regions of relatively planar sections of the north and south
hillslopes. c) Topographic profile across the SSHO. The dashed box shows the area
of Figure 2-5. d) Histograms showing the distributions of hillslope gradients from
regions in Figure 2-2b. These data suggest a dependence of mean hillslope gradients
on aspect. .......................................................................................................................... 60
Figure 2-3. Definition sketch for the derivation of mass balance of regolith and meteoric
10Be flux along a hillslope transect. D10Be is the delivery rate of meteoric 10Be (at
cm-2y-1), ρ and ρ are regolith and rock bulk densities, respectively (g cm-3), z and
re ro
z are the ground surface and bedrock surface elevations, respectively (cm), q is the
b
lateral volumetric flux of regolith (cm2 y-1), and U is local uplift rate (cm y-1). .............. 61
Figure 2-4. Detailed sampling and analysis strategy for the north and south hillslopes at
SSHO. White stars indicate locations of meteoric 10Be concentration depth profiles.
White squares indicate locations at which we measured a single depth-averaged
meteoric 10Be concentration. Partially filled squares indicate sample positions where
samples were amalgamated along contour lines to yield a single average measure of
meteoric 10Be concentration. ............................................................................................ 62
Figure 2-5. a, b) Topographic profiles along the north (a) and south (b) sample transects
(dashed lines). X-axes are flipped in Figures 5b and 5d to reflect the landscape
profile if viewing from the mouth of the watershed. Note that these transects are
traced along the 10Be profile transects on the north and south hillslopes, not the
transect shown in Figure 2-2c. Data show average depth of refusal for regolith on
the north and south hillslopes at SSHO (a and b). Error bars indicate 1 standard
deviation from the mean at each hillslope position. c, d) Local topographic gradient
along the north and south hillslope transects. Gradients are generally higher along
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the south hillslope than the north hillslope. The dark line represents a moving
average [of 3 pixels] of individual pixel-to-pixel gradients. ............................................ 63
Figure 2-6. Composite cross-section of valley floor stratigraphy inferred from shallow
coring. The cross section only represents the lower portions of the SSHO catchment
as shown in Figure 2-2c, and does not represent the entire landscape topographic
profile. Lithostratigraphic units are derived from core logs, and are divided into
three categories. Unit 1 represents alluvial deposits immediately adjacent to the
present day channel. Unit 2 represents poorly-stratified colluvial material,
interbedded with layers of more well sorted sands and silts. Unit 3 represents
massive, unstratified soil and mobile regolith on upland hillslopes. ................................ 64
Figure 2-7. a) Meteoric 10Be concentration profile of the deep bedrock core (DC) drilled
into the northern ridge. Open symbol represents uppermost sample from the DC1
location, interpreted to be regolith and not representative of bedrock 10Be
concentrations. Meteoric 10Be concentrations decline sharply with depth within
bedrock. b) Comparison of meteoric 10Be in core with concentrations at the adjacent
north planar ridgetop regolith sample sites (NPRT - Figure 2-4). 10Be concentrations
are one to two orders of magnitude lower in bedrock than in regolith. The
uppermost DC1 sample (open circle - 7a) exhibits similar concentrations to the
lowermost samples in the NPRT profile (open circle - 7c), and is interpreted to
reflect regolith atop the disturbed drill pad. c) Meteoric 10Be concentration profiles
measured along the north hillslope transect. d) Meteoric 10Be concentration profiles
measured along the south hillslope transect. All meteoric 10Be profiles are
consistent with addition at the surface and attenuation in regolith materials. .................. 65
Figure 2-8. Mean inventories of meteoric 10Be along the north and south hillslopes at
SSHO (a and b, respectively). The error bars indicate 1 standard deviation from the
inventory mean at each hillslope position. The SPVF sample location (gray) exhibits
a meteoric 10Be inventory considered not to be representative of the total inventory
at that location (see text for discussion). .......................................................................... 66
Figure 2-9. Mean volumetric flux of mobile regolith along the north and south hillslopes
(a and b, respectively) of SSHO inferred from meteoric 10Be concentrations
measured in hillslope regolith. Error bars reflect 1 standard deviation from the mean
regolith flux rate at each hillslope position. ..................................................................... 67
Figure 2-10. Comparison of regolith flux along the north and south hillslopes with local
topographic gradient (a and b, respectively) and with the product of regolith depth
and gradient (c and d, respectively). These results suggest that regolith transport is a
function of both depth and hillslope gradient. Fitting only the upper samples on the
north hillslope (dashed lines) shows that near the ridgetops, regolith flux is
proportional to gradient, and there is less aspect control on transport efficiency (K ). ... 68
1
Figure 2-11. Cartoon summarizing and comparing residence times and fluxes of materials
into and out of the north and south ridge tops at SSHO (a and b, respectively), using
U-series and meteoric 10Be. Regolith production rates were calculated using U-series
disequilibrium [Ma et al., 2010; 2013] and were multiplied by the distance between
the ridgecrest and the upper slope 10Be sampling positions to estimate the material
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flux across the bedrock-regolith interface on each ridge. Calculated bedrock-to-
regolith fluxes are compared to downslope transport rates inferred from meteoric
10Be. The flux of material from bedrock to mobile regolith calculated using U-series
and associated regolith residence times are noted in light gray. The downslope flux
rate of mobile regolith calculated using meteoric 10Be and associated regolith
residence times are noted in dark gray. ............................................................................ 69
Figure 3-1. A: Topographic gradient of watersheds in and adjacent to Shale Hills Critical
Zone Observatory (SSHO). NE-northeast; PA-Pennsylvania. B–D: Distribution of
gradient from the sampled regions of planar hillslopes (solid black boxes) along the
north-facing (orange) and south-facing (blue) hillslopes in NV1 (north valley) (B),
SSHO (C), and SV1 (south valley) (D). E,F: Depth to refusal for hand auger along
south-facing (E) and north-facing (F) hillslopes. Error bars represent one standard
deviation of the mean auger depth at each position. ........................................................ 89
Figure 3-2. A: Regolith flux with distance from ridge crest. Error bars represent one
standard deviation from the mean flux at each position. Regolith flux increases
linearly with position downslope in all three watersheds, and is similar between
north-facing (closed symbols) and south-facing (open symbols) hillslopes.
Regression line passes through zero at the ridgecrest where flux sums to zero. B–E:
Regolith flux with slope and the product of slope and transport profile for north-
facing (B,C) and south-facing hillslopes (D,E). A transport rule that considers
regolith depth is a better predictor of measured regolith flux on all hillslopes.
Variations in the slope of linear regressions show that transport efficiencies are
consistently a factor of 2 higher on south-facing hillslopes. ............................................ 90
Figure 4-1. Extent of the Susquehanna River Basin (thick black line) within the
Commonwealth of Pennsylvania (PA). Thin black lines delineate physiographic
provinces within PA. Dashed blue line represents the furthest extent of the
Laurentide Glacial Margin. Red dots mark locations of watersheds analyzed in the
Appalachian Plateau. Green and orange dots mark the locations of shale watersheds
and sandstone watersheds analyzed in the Valley and Ridge province, respectively.
Purple dots make the locations of watersheds analyzed in Piedmont schists. All
digital geographic and topographic data accessed via Pennsylvania Geospatial data
clearinghouse (pasda.psu.edu). ........................................................................................ 117
Figure 4-2. Slope area product versus curvature for the Shale Hills Critical Zone
Observatory. A distinct break in slope occurs at 1 m2, where we define the boundary
of ridgetops in this area. ................................................................................................... 118
Figure 4-3. Hillshade of SSHO created from high-resolution LiDAR data, collected in
collaboration with NSF and NCALM. Overlay shows curvature in the region of the
landscape corresponding to a slope-area product of < 1 m2. ............................................ 119
Figure 4-4. Slope area product versus curvature for A) shale watersheds in the Valley and
Ridge, B) sandstone watersheds in the Valley and Ridge, C) sandstone watersheds in
the Appalachian Plateau, D) schist watersheds in the Piedmont. Darker zones
indicate where regions of SA v C overlaps between watersheds in the same group.
x

Ridgetop delineations for all groups was set to 1 m2, generally consistent first break
in slope. ............................................................................................................................ 120
Figure 4-5. Calculated ridgetop curvature versus in situ 10Be derived basin average
erosion rate (Reuter, 2005) for watersheds formed in Valley and Ridge shale, Valley
and Ridge sandstone, and Piedmont Schist. Y error bars represent variance in
curvature. X error bars represent uncertainty reported in Reuter (2005). ....................... 121
Figure 4-6. Mean diffusivity versus minimum distance to last glacial margin for
watersheds formed in Valley and Ridge shale, Valley and Ridge sandstone, and
Piedmont Schist. Error bars represent propagated uncertainty from variance in
curvature and erosion rates............................................................................................... 122
Figure A-1. Geoprobe core across SSHO Valley Fill. ............................................................. 125
Figure A-2. Description for GP-10. ......................................................................................... 126
Figure A-3. Description for GP-11. ......................................................................................... 127
Figure A-4. Description for GP-12. ......................................................................................... 128
Figure A-5. Description for GP-13. ......................................................................................... 129
Figure A-6. Description for GP-14. ......................................................................................... 130
Figure A-7. Description for GP-15 .......................................................................................... 131
Figure A-8. Description for GP-09. ......................................................................................... 132
Figure A-9. Description for GP-08. ......................................................................................... 133
Figure A-10. Description for GP-07. ....................................................................................... 134
Figure A-11. Description for GP-05. ....................................................................................... 135
Figure A-12. Description for GP-04. ....................................................................................... 136
Figure A-13. Description for GP-03. ....................................................................................... 137
Figure A-14. Description for GP-02. ....................................................................................... 138
Figure B-1. Sample and amalgamation strategy for hillslopes adjacent to SSHO. Open
symbols indicate locations of samples collected in 10 cm intervals. 1/8 splits of
these aliquots were mixed for a depth-amalgamated sample at each location. Closed
symbols indicate samples that were mixed in the field; the entire augerable thickness
was collected as one sample for each location. 1/8 splits of samples within bounding
boxes were amalgamated to one analytical sample per box, resulting in 11 meteoric
10Be analyses per hillslope. .............................................................................................. 142