Geomorphology 234 (2015) 151–160
Contents lists available at ScienceDirect
Geomorphology
journal homepage: www.elsevier.com/locate/geomorph
Erosion rates and landscape evolution of the lowlands of the Upper
Paraguay river basin (Brazil) from cosmogenic 10Be
Fabiano do Nascimento Pupim a,⁎, Paul R. Bierman b, Mario Luis Assine c, Dylan H. Rood d,e,
Aguinaldo Silva f, Eder Renato Merino a
a
Universidade Estadual Paulista (UNESP — IGCE), Programa de Pós-Graduação em Geociências e Meio Ambiente, Av. 24-A, 1515, Rio Claro, SP 13506-900, Brazil
Department of Geology and Rubenstein School of the Environment and Natural Resources, University of Vermont, Burlington, VT 05405, USA
Universidade Estadual Paulista (UNESP — IGCE), Departamento de Geologia Aplicada, Av. 24-A, 1515, Rio Claro, SP 13506-900, Brazil
d
Department of Earth Science and Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
e
Scottish Universities Environmental Research Centre, East Kilbride G75 0QF, UK
f
Universidade Federal de Mato Grosso do Sul — UFMS, Campus do Pantanal, Av. Rio Branco, 1270, Corumbá, MS 79304-020, Brazil
b
c
a r t i c l e
i n f o
Article history:
Received 10 March 2014
Received in revised form 5 January 2015
Accepted 18 January 2015
Available online 24 January 2015
Keywords:
Cosmogenic nuclides
Erosion rates
Relief evolution
Cuiabana lowlands
Brazil
a b s t r a c t
The importance of Earth's low sloping areas in regard to global erosion and sediment fluxes has been widely and
vigorously debated. It is a crucial area of research to elucidate geologically meaningful rates of land-surface
change and thus the speed of element cycling on Earth. However, there are large portions of Earth where erosion
rates have not been well or extensively measured, for example, the tropical lowlands. The Cuiabana lowlands are
an extensive low-altitude and low-relief dissected metamorphic terrain situated in the Upper Paraguay river
basin, central–west Brazil. Besides exposures of highly variable dissected metamorphic rocks, flat residual lateritic caps related to a Late Cenozoic planation surface dominate interfluves of the Cuiabana lowlands. The timescale over which the lowlands evolved and the planation surface developed, and the rate at which they have
been modified by erosion, are poorly known. Here, we present measurements of in situ produced cosmogenic
10
Be in outcropping metamorphic bedrock and clastic–lateritic caps to quantify rates of erosion of the surface
and associated landforms in order to better understand the Quaternary landscape evolution of these lowlands.
Overall, slow erosion rates (mean 10 m/Ma) suggest a stable tectonic environment in these lowlands. Erosion
rates vary widely between different lithologies (range 0.57 to 28.3 m/Ma) consistent with differential erosion
driving regional landform evolution. The lowest erosion rates are associated with the low-relief area (irregular
plains), where clastic–laterite (mean 0.67 m/Ma) and quartzite (mean 2.6 m/Ma) crop out, whereas the highest
erosion rates are associated with dissection of residual hills, dominated by metasandstone (mean 11.6 m/Ma)
and phyllite (mean 27.6 m/Ma). These data imply that the Cuiabana lowland is comprised of two dominant landform sets with distinct and different dynamics. Because the planation surface (mostly lowlands) is lowering and
losing mass more slowly than associated residual hills, regional relief is decreasing over time and the landscape is
not in steady state. The extremely slow erosion rates of the clastic–laterite are similar to the slowest outcrop
erosion rates reported worldwide. These slow rates are due to the material's properties and resistance, being
comprised of quartzite fragments cemented by an iron-rich crust, and reflecting long-term weathering with
iron chemical precipitation and ferricrete formation, at least since the Middle Pleistocene. The lateritic caprock
appears to be a key factor maintaining hilltop summits of the planation surface over long timescales.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
The Earth's continental surface is ~90% dominated by low relief and
gently sloping areas (Larsen et al., 2014). Understanding how important
⁎ Corresponding author at: Programa de Pós-Graduação em Geociências e Meio
Ambiente, Av. 24-A, 1515, Rio Claro, SP 13506-900, Brazil. Tel.: +55 19 3526 9321.
E-mail addresses: [email protected] (F.N. Pupim), [email protected]
(P.R. Bierman), [email protected] (M.L. Assine), [email protected] (D.H. Rood),
[email protected] (A. Silva), [email protected] (E.R. Merino).
http://dx.doi.org/10.1016/j.geomorph.2015.01.016
0169-555X/© 2015 Elsevier B.V. All rights reserved.
these flat landscapes are as controls on erosion rates, sediment fluxes
and biogeochemical cycling has been vigorously debated in the recent
literature (Willenbring et al., 2013, 2014; Larsen et al., 2014; Warrick
et al., 2014). Underlying much of this debate is the observation that
we know little about the erosion rate of large portions of Earth because
limited measurements have been made there (Portenga and Bierman,
2011). Specifically, the lowlands of Africa and South America are two
large landscapes where few erosion rate measurements exist. In these
continents, cosmogenic measurements of erosion are concentrated in
the Andean mountain belt (e.g. Safran et al., 2005; Placzek et al., 2010;
Bookhagen and Strecker, 2012), passive margin escarpments (e.g.
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F.N. Pupim et al. / Geomorphology 234 (2015) 151–160
Bierman and Caffee, 2002; Salgado et al., 2008, 2013; Cherem et al.,
2012; Scharf et al., 2013; Bierman et al., 2014) and large-scale Amazon
rivers (e.g. Wittmann et al., 2010).
Situated in central-west Brazil, the Upper Paraguay river drainage
basin is an inlier of Precambrian rocks exposed due to erosion of Phanerozoic rocks that crop out in its surroundings (Fig. 1). The Quaternary
Pantanal basin developed in the inner part of the inlier and is an extensive modern alluvial depositional tract (Assine and Soares, 2004), one of
the world's largest freshwater wetland ecosystems (Hamilton, 2002).
The inlier margin known as Upper Paraguay lowland (also called a
depression) is dominated by low-altitude and low-relief dissected
metamorphic terrains (Fig. 1A). The Upper Paraguay lowlands are comprised of three major geomorphological units; the Cuiabana lowlands
are the site we focus on in this paper (Fig. 1B). Besides exposures of
highly variable dissected metamorphic rocks, flat residual lateritic
caps related to a Late Cenozoic planation surface dominate interfluves
of the Cuiabana lowlands. The timescale over which the lowlands
evolved and the planation surface developed are uncertain, and the
rate at which they have been modified by geomorphic processes are
poorly quantified over long temporal scales. The planation surface age
was indirectly estimated as Plio-Pleistocene through geomorphological
methods and regional correlations (Ab'Saber, 1988).
Terrestrial cosmogenic nuclides have become invaluable tools
in geosciences because they allow quantitative evaluation of
geomorphologically relevant parameters, including exposure ages,
erosion rates, and burial ages (e.g., Gosse and Phillips, 2001; Granger
et al., 2013). In particular, beryllium-10 (10Be) is useful in the study of
landscape evolution over timescales of 103 to 106 years (e.g., Brown
et al., 1994; Bierman and Caffee, 2001; Jakica et al., 2011; Portenga
et al., 2013; Scharf et al., 2013). In Brazil, for example, Braucher et al.
(1998), based on cosmogenic 10Be analyses in one lateritic stone-line
(similar to samples we collected BRC 21 and 22, which are remnants
of the Cuiabana planation surface), proposed that this stone-line
resulted from depositional events occurred at least 500 ka ago.
In this paper, we present measurements of in situ produced cosmogenic 10Be in outcropping metamorphic bedrock and clastic–lateritic
caps to determine minimum exposure ages and maximum limiting erosion rates of the planation surface and associated landforms. The results
are discussed in the context of a general landscape evolution of the
Cuiabana lowlands, as well as in the context of the outcrop erosion
rates from around the globe. Our work begins to address the lack of erosion rate measurements in large, tropical, flat areas.
2. Regional setting
The plateaus surrounding our field area are dominated by planation
surfaces (Fig. 1B), the formation of which has been linked to erosion and
deposition during a long period of arid climate and relative tectonic
stability following the Cretaceous (Ab'Saber, 1988). Contemporary
Brazilian planation surfaces are thought to reflect inheritance of ancient
surfaces and their ages are constrained indirectly through stratigraphic
and geomorphologic correlation (e.g. King, 1956; Bigarella and Andrade,
1965; Braun, 1972; Ab'Sáber, 2000; Peulvast and Claudino Sales, 2004;
Valadão, 2009).
Cenozoic tectonic reactivation caused uplift of this portion of the
South America platform and raised the planation surfaces to high elevation on the Brazilian plateau (Tello Sáenz et al., 2003; Hackspacher et al.,
2004). Apatite fission track data from southeastern Brazil are consistent
with regional uplift events and consequently a response that includes
both significant denudation and deformation of ancient planation surfaces during the Cenozoic (Tello Sáenz et al., 2003; Hackspacher et al.,
2004). In the Quadrilátero Ferrífero, Brazil, 40Ar/39Ar dating of deep
weathering profiles indicated ages of 51–41 Ma for a higher surface at
2100 m of altitude (Spier et al., 2006) and 10–8 Ma for mid-level surface
at 1100 m (Carmo and Vasconcelos, 2004). These ages suggest long
weathering periods, low denudation rates, and the preservation of ancient landscapes (Carmo and Vasconcelos, 2004; Spier et al., 2006).
Fig. 1. Study area location and physiographic provinces (black square is the location area). (A) Study area location and regional topographic overview at the Upper Paraguay river
basin, central west region of Brazil (image from Shuttle Radar Topographic Mission — SRTM); (B) The main physiographic provinces of the Upper Paraguay river basin (regional lowlands:
1 — Cuiabana; 2 — Paraguay; 3 — Miranda).
F.N. Pupim et al. / Geomorphology 234 (2015) 151–160
High-level residual surfaces are supported by deep, iron-rich lateritic
formations (duricrusts) (Ab'Saber, 1988) formed following Cretaceous
times (Figs. 1B, 2A), and are thought to be regionally correlated to the
regional Sul-Americana surface (e.g. King, 1956; Ab'Saber, 1988). In
the Upper Paraguay river basin, although the surrounding plateaus are
extensively dissected, some areas of them are flat and the remnants of
the Sul-Americana planation surface can be found on their tops (900
to 1000 m) (Fig. 1).
153
Throughout the Cenozoic, a long period of fluvial degradation
formed the Upper Paraguay lowland within the inlier and the surrounding plateaus that are being dissected by headward erosion. The lowland
has residual hills, which have relatively high elevation (200 to
480 m asl) and relief (50 to 200 m) (Figs. 2B, 3A), but is characterized
by extensive low-lying (110 to 240 m) and low-relief (b 20 m) terrains
(Fig. 3B, C) carved into Neoproterozoic metamorphic rocks (Fig. 2A). At
the northern border of the Brazilian Pantanal basin, Precambrian
Fig. 2. Geological and geomorphological settings of the northern part of the Upper Paraguay river basin. (A) Geologic map (CPRM, 2004); (B) Geomorphic map (IBGE, 2009).
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F.N. Pupim et al. / Geomorphology 234 (2015) 151–160
Fig. 3. Digital elevation model and sampling sites. (A) The DEM was derived from Shuttle Radar Topographic Mission (SRTM) and depicts the main geomorphic zones of the Upper
Paraguay river basin: Província Serrana and Chapada dos Guimarães are the uplands, the first reflects an Appalachian style relief and the second is a planation surface that was preserved
under sedimentary layers; Cuiabana lowland is an intraplanation area that includes the irregular plains and residual hills, where the samples were collected; Pantanal is an extensive alluvial plain, dominated by large rivers and megafans. (B) Topographic swath profile from west to east, on which all samples are plotted schematically. (C) Topographic swath profile from
north to south, showing the conceptual Cuiabana planation surface (red line) and the laterite sample distribution.
metamorphic terrains dipping gently toward the south, covered by
clastic–lateritic caps, and still not systematically mapped, are remnants
of an ancient planation surface called the Cuiabana surface (Fig. 3C).
Some use this surface to surmise a pediplanation event during the
Plio-Pleistocene (Ab'Saber, 1988). However, Braucher et al. (1998)
proposed that the lateritic stone-line resulted from at least local
deposition N 500 ka ago.
The Cuiabana surface is clearly older than the modern Pantanal
alluvial plain, but there is no data about the relationship between
the lowland evolution and the initiation of subsidence in the
Pantanal basin. The Pantanal is an elliptically shaped, shallow
(~ 0.5 km), wide (~ 250 km), and seismically active basin, the origin
of which has been related to a lithospheric flexural forebulge east
of the central Andes mountain belt (Horton and DeCelles, 1997;
Ussami et al., 1999; Chase et al., 2009). The basin infilling was mainly
by alluvial sediments deposited by large rivers, megafans, and in
lakes (Assine and Soares, 2004). Sedimentary processes have been
active since the Pleistocene and strongly influenced by climate
changes, as evidenced by changes in channel pattern (Assine et al.,
2014) and palynological records (McGlue et al., 2011, 2012;
Whitney et al., 2011). Climate records indicate that significant
climate changes occurred in the late Quaternary, when semi-arid
Pleistocene conditions changed to a more humid Holocene climate
(Ledru et al., 2005; Whitney et al., 2011).
Today, climate in Upper Paraguay river basin is tropical-savannah,
Aw class in the Köppen–Geiger climate system; mean annual temperature is ~ 25 °C and mean annual precipitation is ~ 1350 mm (SEPLAN,
2001). Evaporation exceeds precipitation during most of the year and
F.N. Pupim et al. / Geomorphology 234 (2015) 151–160
the majority of precipitation falls during the wet season that lasts from
late January to March (SEPLAN, 2001).
3. Methods
3.1. 10Be cosmogenic nuclide analysis
In order to constrain the rate at which the landscape of the Cuiabana
lowland changes, ten exposed, outcropping rock samples were collected
from different geomorphological and lithological settings (Figs. 2, 3).
Sampling sites were on flat summit hilltops and samples were from
bare bedrock outcrops or boulders (see Fig. 4 and Table 1 for details).
Four different lithologies were sampled: laterite, phyllite, quartzite,
and metasandstone (Fig. 4). The two laterite samples BRC-21 and
BRC-22 were collected on the surface (Fig. 4A); they are thin sedimentary layers (1 to 3 m in thickness) comprised of quartzite fragments
cemented by iron-rich crust. Samples were collected using a hammer
and their locations and elevations were determined with a hand-held
Global Positioning System (GPS).
Isotope extraction procedures were performed using standard techniques at the University of Vermont cosmogenic laboratory. Samples
were crushed and sieved to the 0.25–0.85 mm fraction and this fraction
was magnetically separated. For quartz purification, ~ 50 g of sample
was ultrasonically etched in 6 N HCl followed by repeated etchings in
1% HF and HNO3 (Kohl and Nishiizumi, 1992). The concentration of Al
(b 150 ppm) and total cations (b 200–300 ppm) was verified by inductively coupled plasma emission spectrometry (ICP-OES). 10Be was
extracted from about 20 g of purified quartz, following procedures of
Corbett et al. (2011). Samples were dissolved in HF along with 250 mg
of Be carrier made from beryl and Al carrier to bring the total quantity
of 27Al to 2500 mg. Solutions were purified by anion and cation
155
exchange. Be and Al were separated by cation exchange, precipitated
as hydroxides, and burned to produce oxides. The oxides were mixed
with Nb and packed in copper cathodes.
10
Be/9Be ratios were measured by accelerator mass spectrometry
(AMS) at the Scottish Universities Environmental Research Centre
(SUERC) (Xu et al., 2010). All measurements were normalized to the
NIST standard with an assumed ratio of 2.79 × 10−11. A 10Be half-life
of 1.36 Ma (Nishiizumi et al., 2007) was assumed to calculate erosion
rates and exposure ages. The exposure age and erosion rates were calculated using CRONUS-Earth online calculators v. 2.2.1 (http://hess.ess.
washington.edu/) (Balco et al., 2008), assuming a rock density of
2.7 g/cm3, and using the Lal (1991) and Stone (2000) constant production rate and elevation and latitude scaling scheme for spallation. 26Al
was not measured.
3.2. Digital terrain analysis
Topographic data used for calculate morphometric parameters were
based on a 3 arc-second (nominally ~ 90 m) digital elevation model
(DEM). The DEM consists of Shuttle Radar Topography Mission
(SRTM) elevation data with a worldwide coverage, which has an absolute vertical accuracy of ± 9 m (Rodríguez et al., 2006). SRTM data
were acquired from Jarvis et al. (2008) and processed using ArcGIS software. Additional SRTM technical details can be found in van Zyl (2001)
and Farr et al. (2007).
Elevation is the primary variable and was obtained directly from the
original DEM (Fig. 3A). The topographic levels were enhanced through
color palette reclassification by histogram analysis. The original slope
angles were calculated using the average maximum technique and a
3 × 3 cell neighborhood operator. However, the DEM grid cell size significantly affects the slope values, being that slope angles decline
Fig. 4. Photographs showing the context of the rock outcrops sampled. (A) BRC-22 is a typical laterite site; (B) BRC-23 is a phyllite site; (C) BRC-25 shows a large quartzite outcrop; and
(D) BRC-31 is a metasandstone outcrop (arrows are indicating the sampling point).
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F.N. Pupim et al. / Geomorphology 234 (2015) 151–160
Table 1
Sampling site, isotopic and morphometric data.
Sample ID Long
Lat
UTM (WGS84)
BRC-21
BRC-22
BRC-23
BRC-24
BRC-25
BRC-26
BRC-28
BRC-29
BRC-30
BRC-31
560475
574957
549639
515782
527455
563762
593258
516387
530164
548901
8240570
8265400
8258300
8228650
8211630
8204140
8237530
8247460
8251690
8259410
Elevation
(m)
[10Be]
P 10Be
Minimum exposure age Maximum erosion rate
(m/Ma)
(105 atoms g−1) (atoms g−1 y−1)* (ka)
Lithology
204
237
235
188
190
139
146
331
452
285
21.79 ± 0.22
26.55 ± 0.24
1.20 ± 0.02
2.81 ± 0.05
5.22 ± 0.09
12.90 ± 0.15
14.30 ± 0.20
2.58 ± 0.05
1.42 ± 0.03
2.66 ± 0.06
Laterite
Laterite
Phyllite
Metasand
Quartzite
Quartzite
Quartzite
Metasand
Phyllite
Metasand
3.584
3.656
3.656
3.543
3.553
3.429
3.440
3.912
3.758
3.789
739 ± 78
918 ± 102
33.6 ± 3
82.1 ± 7.5
155 ± 14
423 ± 41
474 ± 47
68.1 ± 6.2
33.5 ± 3
72.7 ± 6.6
0.76 ± 0.09
0.57 ± 0.07
28.32 ± 1.91
10.51 ± 0.77
5.04 ± 0.40
1.53 ± 0.15
1.33 ± 0.14
12.64 ± 0.91
26.92 ± 1.87
11.85 ± 0.87
Slope (degree)
Local Mean 3 km
STD**
3.8
3.8
3.1
4.3
3.0
2.0
3.4
3.1
8.1
3.6
1.3
1.5
1.8
6.0
1.1
1.1
1.5
4.0
6.0
2.2
2.6
2.8
3.1
7.4
2.2
1.6
1.5
6.4
8.5
3.7
*Scalling scheme for spallation: Lal (1991) and Stone (2000); **abbreviation for standard deviation.
Sample thickness: 2 cm; sample density: 2.7 g/cm−2; shielding correction: 1; Be-AMS standard: NIST_27900 (NIST standard ratio of 2.79 × 10−11). Age exposure and erosion rates were
calculated using CRONUS-Earth online calculators v. 2.2.1 (Balco et al., 2008).
logarithmically as DEM grid increases (Zhang and Montgomery, 1994;
Larsen et al., 2014). Thus, to minimize the effect of grid size on our analysis, the slope angles were calibrated employing a correction factor of
~1.65, as proposed by Larsen et al. (2014).
The relationship between topography and the 10Be-derived metric
of maximum limiting erosion rate was calculated using the local value
(one cell value) for elevation, whereas the slope angle has determined
using local value and mean slope within a 3 km radius for each 10Be
sample. This approach allows us to test if there is a dependency between
erosion rates and topographic variables at two different spatial scales, as
well as to compare our results with cosmogenic measurements in the
other landscapes around the world.
4. Results
Samples have a large range in concentrations of 10Be (Table 1).
Concentrations of 10Be in lithified surface samples varied from 1.20 to
26.6 × 105 atoms g−1 (mean 8.61 × 105 atoms g−1), consistent with
previous measurements made in the Cuiabá region (Braucher et al.,
1998). High 10Be concentrations indicate long durations of surface exposure and low rates of erosion (Bierman and Caffee, 2001). The measured concentrations of in situ produced 10Be can be interpreted
either as minimum limiting exposure ages between 33 and 918 ka or
maximum limiting erosion rates that range from 0.57 to 28.3 m/Ma
with a mean and median of 10 and 7.8 m/Ma, respectively. The high
standard deviation of the erosion rate population (10 m/Ma, about
100%) suggests a heterogeneous spatial intensity of geomorphic process
over the timescale represented by our measured 10Be inventories (104
to 106 years).
The observation that mean annual precipitation is spatiality uniform
(SEPLAN, 2001) and that erosion rates are not, implies that geologic or
geomorphic factors control the spatial variability of erosion rates.
Thus, we tested the relationship between erosion rates and morphometric variables (elevation and slope) and lithology (Table 1).
Differences in erosion rates are well correlated with lithology
(Kruskal–Wallis p = 0.037; Fig. 5A). The laterite samples have the
Fig. 5. Statistical correlation between erosion rates, lithology and morphometric parameters. Erosion rates are (A) strongly correlated with lithology type (p = 0.037); (B) moderately
correlated with elevation; and weakly correlated with (C) local slope and (D) mean slope calculated within a 3 km radius by SRTM-DEM derivatives and GIS analysis.
F.N. Pupim et al. / Geomorphology 234 (2015) 151–160
slowest erosion rates (mean 0.67 ± 0.1 m/Ma, n = 2), followed by
quartzite (mean 2.6 ± 2.1 m/Ma, n = 3), metasandstone (mean
11.7 ± 1.1 m/Ma, n = 3) and phyllite (mean 27.6 ± 1 m/Ma, n = 2).
Likewise, minimum exposure ages have similar behavior to erosion
rates; laterite samples have the oldest minimum exposure age (mean
829 ± 126 ka), followed by quartzite (mean 351 ± 172 ka),
metasandstone (mean 74 ± 7 ka), and phyllite (mean 33 ± 1 ka).
Morphometric variables are not well correlated statistically with
erosion rates (Fig. 5A–C). Erosion rates are weakly to moderately correlated with elevation (r2 = 0.47; p = 0.03), whereas slope is not significantly correlated with erosion rates (p N 0.05). In addition, there is little
difference between the measurements of local slope angle (one cell
grid) and mean slope angle within 3 km radius (Table 1 and Fig. 5C, D).
5. Discussion
5.1. Erosion rates: local controls and global correlation
Erosion rates measured in samples collected from outcropping
bedrock in the Cuiabana lowlands are slow and consistent with those
measured from outcrops in tectonically quiescent landscapes around
the world (average of 12 m/Ma; Portenga and Bierman, 2011), such as
Appalachian Mountains (range 1.0 to 66 m/Ma; Portenga et al., 2013),
Darling Scarp in Australia (range 0.5 to 19.3 m/Ma; Jakica et al., 2011),
the Cape Mountains in South Africa (range 1.98 to 4.61 m/Ma; Scharf
et al., 2013), and Quadrilátero Ferrífero in Brazil (range 0.29 to
12.9 m/Ma; Salgado et al., 2008). Slow erosion rates suggest that tectonism does not appear to be the dominant factor driving the erosion rates
in the Cuiabana lowlands, even though the study area is situated around
the Pantanal basin (Fig. 1B), an active sedimentary basin. In the Pantanal
basin, seismic events usually do not exceed magnitude 3.0 mb, although
there are occasional events with magnitude 5.0 mb (Assumpção et al.,
2004). Evidence of structures that have been reactivated during the
Quaternary is restricted to the center of the basin (Assine and Soares,
2004). Thus, today's tectonic activity does not play a key role in the
landform evolution in the Cuiabana lowlands.
The wide range of erosion rates measured for bedrock outcrop
samples suggests that over time there have been significant differences
in the amount of erosion affecting Cuiabana lowland landforms. Elsewhere, spatial variations in erosion rates have been correlated with
landscape parameters. Some studies have proposed that tectonic
activity and climate are the main forces that control regional spatial variations in erosion rates (Montgomery et al., 2001; Burbank et al., 2003;
Henck et al., 2011; Decker et al., 2013) with the fastest erosion rates
documented in the steepest and tectonically most active mountain
belts and the slowest rates occurring in stable and extreme arid regions
of the world (Matmon et al., 2009). Therefore, the absence of spatial climatic variation and tectonic influence suggests that factors of lithology
and topography are most significant for explaining the spatial variation
of erosion rates at the local scale (Summerfield, 1991) or in low-relief
areas of the world, such as where data presented in this paper were
collected.
Our results show that lithology is strongly correlated with erosion
rates (Fig. 5A), whereas morphometric variables (elevation and slope;
Fig. 5) are not. Portenga and Bierman (2011), using a global-scale data
compilation, demonstrated that erosion rates from outcropping bedrock
are better correlated with lithology and climate than with tectonic and
topographic parameters. Regional geomorphological studies have also
argued that lithology is the main controlling factor on erosion rates
and landform development. For example, the maintenance of rugged topography and slow erosion rates on Cape Mountains interfluves in
southern Africa (~3.3 m/Ma) was attributed to the presence of physically robust and chemically inert quartzites that constitute the backbone of
the mountains (Scharf et al., 2013). The high relief in the Quadrilátero
Ferrífero landscape in southeast Brazil was also correlated with lithological differences, where the slow erosion rates from quartzite and
157
banded-quartz hematite – Itabirite rock – (range 0.3 to 2.4 m/Ma) maintain the high-level surfaces and schist-phyllites and granite-gneisses
erode more rapidly (range 8 to 13 m/Ma) (Salgado et al., 2008). Much
of the unexplained variability of erosion rates in Appalachian Mountain
outcrops is likely related to the specific rock properties of each outcrop
rather than environmental parameters (Portenga et al., 2013).
The relative resistance to erosion of our samples, in the sequence
laterite N quartzite N metasandstone N phyllite, appears to be influenced
by three main factors: quartz content, development of iron crust and
cement, and the presence of structures (foliation and jointing).
Neoproterozoic rocks in our field area underwent several deformational
events over time, resulting in fractures and folds that are clearly controlling the spatial distribution of rock units, fracture density and rock
strength and, consequently, the erosion rates.
In the Cuiabana lowlands, the lack of a significant statistical correlation between morphometric variables and erosion rates appears to reflect the strong geologic control on spatial variation of the erosion
rates. Willenbring et al. (2013, 2014) asserted that the global erosion
rate dataset supports a first-order topographic control on erosion. However, in tectonically stable areas, such as where we sampled, erosion
rates can be largely independent of topography. Together with others,
it is found that differences in rock resistance and chemical weathering
are the dominant factors controlling erosion rates (e.g., Salgado et al.,
2008; Chadwick et al., 2013; Bierman et al., 2014).
Despite our finding that sample site elevation and both local and regional slope angles are not strongly statistically correlated with erosion
rates, there is an interesting linkage between lithology, geomorphic
units, and erosion rates (Fig. 2). The metasandstone and phyllite samples (BRC 23, 24, 29, 30 and 31) that show relatively rapid erosion
rates are located in the residual hills, which are characterized by relatively high values of altitude and slope angle (Table 1). In contrast, the
quartzite and laterite samples (BRC 21, 22, 25, 26, 28) that show the
lowest erosion rates are situated in the irregular plain, for which the altitude and slope angles are lower than in the residual hills (Table 1).
Therefore, it is clear that geomorphic units and morphometric parameters are controlled by rock erodibility, which plays the most important
role in the spatial variation of the erosion rates in the Cuiabana
lowlands.
5.2. Implications for landscape evolution
The cosmogenic data, suggestive of spatially differential erosion
rates, imply that the Cuiabana lowland does not correspond to the remnants of a single planation surface with a simple history, but to at least
two sets of landforms with distinct and different erosional dynamics
(Fig. 6). Slow erosion rates for laterite and quartzite samples suggest
that the irregular plain has near-stable topography, changing only slowly since deposition of the lateritic deposits, at least since the Middle
Pleistocene. Because the lateritic deposits are so erosion resistant, they
support hilltop summits of the Cuiabana planation surface, slowing
the overall rate of denudation.
In contrast, relatively rapid erosion rates for metasandstone and
phyllite samples demonstrate that the residual hills have changed
more quickly than the irregular plain over the last 100 ka. Therefore,
the planation surface is lowering and losing mass twenty times slower
than associated residual hills. In a regional and long-term extrapolation,
if we consider the mean erosion rates of residual hills (~18 m/Ma) and
the mean of the laterites (~ 0.7 m/Ma) and a relief of 200 m between
them, in a tectonically stable environment and with erosion rates that
remain constant through time, it might take ~11.5 Ma to flatten the regional surface (Fig. 6B). This implies that the relief of Cuiabana lowlands
is slowly decreasing over time and that the overall landscape form is
long-lived.
Differential erosion is particularly evident on stratified and exhumed
landscapes, resulting in low-relief surfaces surrounded by abrupt and
steep scarps, such as planation surfaces in Africa (Coltorti et al., 2007),
158
F.N. Pupim et al. / Geomorphology 234 (2015) 151–160
Fig. 6. Schematic diagram showing the implications of the lithologic control on erosion rates and Cuiabana lowland evolution. (A) The present topography and erosion rates. Cuiabana
planation surface is related to the summit of the Pleistocene laterite cap (in red); (B) Cuiabana lowland evolution under the erosion rate effect through time. The future scenario was proposed to show how the lithology type distribution and its erosion rates can affect the tropical landscape evolution, in which the laterization process appears to be the key factor controlling
this evolution. Thus, we assumed that other environmental aspects, such as climate, tectonic and base level, will be constants through time. The neighborhood reliefs are not included in the
interpretation because we do not have data concerning them.
Australia (Twidale, 1994) and northeastern Brazil (Peulvast and
Claudino Sales, 2004). In the irregular lowland plain, the differential
erosion is driven by the contrast in resistance between laterite cropping
out on the hilltop summits and weak rocks (phyllite) in the valley bottom. We speculate that the Cuiabana lowlands may be an initial stage in
the development of the landscapes mentioned above.
Extremely slow erosion rates measured in laterite samples are similar to the slowest outcrop erosion rates reported worldwide, previously
obtained mostly in arid environments such as Negev Desert of southern
Israel — 0.25 to 0.8 m/Ma (Matmon et al., 2009), Allan Hills Antarctica —
0.24 to 1.31 m/Ma (Nishiizumi et al., 1991), and Atacama desert, Chile —
0.43 to 5.66 m/Ma (Placzek et al., 2010). Kober et al. (2007), using single
and paired cosmogenic nuclides (10Be, 26Al and 21Ne), reported that the
erosion rates on western central Andes (Chile) show a positive correlation with elevation and present-day rainfall. Erosion rates are slowest at
lower altitudes and in the hyperarid at the northern part of the Atacama
Desert (0.1–1 m/Ma), whereas erosion rates are up to 4.6 m/Ma on the
high altitude and semiarid Western Cordillera. Bierman and Caffee
(2002) found a similar low range of erosion rates in Eyre Peninsula,
south central, Australia (0.3 to 5.68 m/Ma), where N300 mm of precipitation falls annually on massive, resistant granite outcrops. Rapid runoff
and lack of soil make these outcrops quite dry for most of the year.
Those authors also suggested that surface stability is inversely related
to mean annual precipitation in Australian inselbergs, with rock
surfaces in more humid conditions having lower nuclide activities
and higher erosion rates than those in arid conditions. Similarly, in
South Africa, Bierman et al. (2014) found very low rates of erosion
(≪1 m/Ma) on quartzite pediments covered by silcrete. There is no evidence that our study area has experienced long periods of hyperaridity,
but more likely arid and humid oscillations predominated during the
Late Quaternary (Ledru et al., 2005; Whitney et al., 2011). Hence,
extremely slow erosion rates measured in the laterite samples are due
to the material resistance, not climate, similar to low rates of silcrete
erosion measured elsewhere by others (Bierman et al., 2014).
The laterite, comprised of quartzite fragments cemented by an ironrich crust, reflects long-term weathering with iron chemical precipitation and the formation of resistant layers (ferricrete), at least since the
Middle Pleistocene. Spier et al. (2006) studied deep-weathering profiles
in the Quadrilátero Ferrífero (Minas Gerais, Brazil) and suggested that
the nature of the porosity and permeability in iron-cemented profiles
may hinder the development of an effective internal drainage system,
slowing the erosive power of surface waters, enhancing chemical
erosion by the deeply penetrating weathering solutions, and slowing
physical erosion by diffusion or advection in slopes and streams. Over
long timescales (106 to 107 years), the lateritic deposit can become
even more resistant, due to slow erosion, deep weathering and thus increased cementation. The lateritic layers remain stable in a landscape
while the associated residual hills are lowered more quickly, creating
conditions amenable to a future relief inversion. Several examples of relief inversion supported by duricrust formation (ferricrete, calcrete or
silcrete) and differential erosion have been reported to Brazilian Highlands (e.g. King, 1956; Bigarella and Andrade, 1965; Ab'Sáber, 2000)
F.N. Pupim et al. / Geomorphology 234 (2015) 151–160
as well as worldwide (e.g. Goudie, 1973; Thomas, 1994; Coltorti et al.,
2007).
The Cuiabana lowland has lowered over time losing mass as the regional relief also decreased during the Quaternary. Differential erosion
rates appear to be driving the landscape evolution without a significant
tectonic input, indicating that the topography is not in steady state.
6. Conclusions
Measurements of in situ produced cosmogenic 10Be from bedrock
outcrops in the Cuiabana lowlands provide a means to understand
regional long-term landscape evolution. Slow erosion rates reflect tectonic stability and the variability of these rates is evidence for strong
lithologic control on topography and landform development.
Differential erosion appears to drive regional landform evolution.
The lowest erosion rates are associated with relative lowlands (irregular
plains with laterite and quartzite lithologies) and the highest rates are
associated with relatively high areas (residual hills with metasandstone
and phyllite lithologies). This implies that the Cuiabana lowland landscape does not correspond to remnants of a single planation surface
but two sets of landforms with distinct and different dynamics. Regarding regional landscape evolution, the irregular plains (including the
Cuiabana planation surface) are lowering and losing mass more slowly
than adjacent residual hills; thus, regional relief is decreasing over
time, indicating that the landscape is not in steady state.
The residual hills are controlled by the geologic framework (lithology and structure), whereas the laterization of the deposits rich in
quartzite clasts appears to be a key factor maintaining hilltop summits
of the planation surface over long timescales. Minimum limiting exposure ages indicate that laterized detrital deposits burying the Cuiabana
planation surface are at least Middle Pleistocene in age.
Acknowledgments
CAPES Foundation for scholarship grants at the “doctorate sandwich” (Process 12539/12-0). CNPq Foundation for scholarship grants,
PQ-research grants (305108/2009-3) and the Edital Universal grants
(484300/2011-3). São Paulo Research Foundation (FAPESP): 2014/
06889-2. Programa de Pós-Graduação em Geociências e Meio Ambiente
(UNESP, campus Rio Claro) for financing. Universidade Federal de Mato
Grosso do Sul (UFMS, campus do Pantanal) and SESC Pantanal for field
logistics. We thank Verónica Sosa-González (University of Vermont —
UVM) for support during 10Be isolation.
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