Differential control in the formation of river potholes on basalts of the Paraná Volcanic Province

Journal of South American Earth Sciences 59 (2015) 86e94

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Differential control in the formation of river potholes on basalts of the
Volcanic Province Parana Adalto Gonçalves Lima a, *, Andrey Luis Binda b a b

Departamento de Geografia, Universidade Estadual do Centro-Oeste, 85040-080, Guarapuava, PR, Brazil
, SC, Brazil Departamento de Geoci^ encias, Universidade Federal da Fronteira Sul, 89812-000, Chapeco

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 July 2014 Accepted 18 February 2015 Available online 26 February 2015

Variations in rock properties control geomorphic processes and thus landscape evolution. Potholes developed on basaltic riverbeds are generally associated with vesicular-amygdaloidal zones, although they also occur in massive basalts. Until now, this relationship has not been quantified, nor have the parameters controlling the development of these features in basalts been evaluated. Based on field data collected from 71 sites distributed in three rivers in the Paran
a Volcanic Province (PVP), southern Brazil, we investigated the relationship between the occurrence of potholes and features of basalt flows. Reaches were analyzed both in areas with potholes and in areas without these features. The data collected refer to the joint density, the intact rock strength measured with a Schmidt hammer and the typology of basaltic units in terms of vesicularity. It was found that potholes preferentially occur in vesicularamygdaloidal units (86%). This predominance is not associated with the joint density, which is the same in massive basalts (z5 m/m2); moreover, potholes occur in basalts with very different joint densities. The intact rock strength is lower in vesicular-amygdaloidal basalts (58) than in massive basalts (61) and does not explain fully the preferential abrasion in vesicular-amygdaloidal units because potholes occur with varying resistances. The basalt strength is a secondary variable. The controlling parameter seems to be vesicularity, which by producing irregularities in the bed and flow triggers the formation of potholes. In the massive basalts, irregularities are produced primarily by joints. In massive basalts there seems to be an upper threshold of stream power beyond which the formation of potholes is restricted (drainage area z700 km2; slope z0.06). A lower threshold in stream power could also exist to massive basalts of the study area and it is suggested by the inexistence of potholes in sites with drainage area less than 100 km2. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Bedrock river Pothole Fluvial erosion Basalt Paran
a Basin

1. Introduction Landscape evolution has been the subject of several studies since the renewed interest on bedrock rivers (Bishop, 2007; Tucker and Hancock, 2010; Whittaker, 2012, and references therein). In this way, many studies have shown the need for more information about fluvial erosion processes (e.g., Howard et al., 1994; Whipple et al., 2000a; Sklar and Dietrich, 2004) and that is true also to riverbeds in basalts (e.g., Seidl et al., 1994; Stock and Montgomery, 1999; Whipple and Tucker, 1999). Although abrasion is notably secondary in the sculpting of riverbeds in basalts, at least in large

* Corresponding author. Tel.: þ55 42 3629 8117; fax: þ55 42 3629 8100. E-mail address: [email protected] (A.G. Lima). http://dx.doi.org/10.1016/j.jsames.2015.02.004 0895-9811/© 2015 Elsevier Ltd. All rights reserved.

continental provinces, it is necessary to understand its role and check its constraints. Abrasion process, particularly by pothole generation, can be coupling with plucking, which is characterized by the entrainment of blocks by hydraulic action and is associated with jointed rocks (Hancock et al., 1998; Whipple et al., 2000a). This coupling can accelerate the incision of rivers in general (Whipple et al., 2000a; Chatanantavet and Parker, 2009) and on basalts (Baker and Kale, 1998; Sengupta and Kale, 2011), contributing significantly to the evolution of longitudinal profiles and landscapes. Potholes are features of abrasion (Wohl and Ikeda, 1997; Whipple et al., 2000a) that are commonly found in bedrock rivers and in various lithologies and channel types (see references in Wohl, 1998). In the geomorphological literature, the record of potholes in basalts is very limited. Kale and Joshi (2004) and Sengupta and Kale (2011) reported the existence of potholes

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developed on basalts, predominantly in amygdaloidal zones of flow lobes at the Indrayani River, Deccan Volcanic Province, India. Also related to the Deccan Volcanic Province, Baker and Kale (1998, 2005) have mentioned the existence of potholes in the Tapi River, with indications that these features occur in vesicularamygdaloidal zones. Field observations (this paper and others not
Volcanic Province (PVP), in published) we made on the Parana southern Brazil, have also revealed that potholes are common features, which are usually associated with vesicular-amygdaloidal zones of basaltic flows. The occurrence of potholes in vesicular-amygdaloidal zones suggests that the differential erosive response in relation to the zones of massive basalt may be related to one or more of the following factors, according to general concepts of the geomorphological literature: (1) lower density of fracturing (e.g., Hancock et al., 1998), (2) lower intact rock strength (e.g., Sklar and Dietrich, 2001) and (3) greater rock surface heterogeneity (e.g., Richardson and Carling, 2005). All three of these mechanisms are plausible explanations of the patterns observed in the PVP. Based on the theoretical assumption and empirical findings, several authors point out the reduction of abrasive processes in highly jointed rocks (e.g., Hancock et al., 1998; Whipple et al., 2000a; Dubinski and Wohl, 2013). In the PVP, fracturing in both types of basaltic zones was evaluated by Lima and Binda (2013) and showed no significant difference in their means (4.26 m/m2 for vesicular units and 4.42 m/ m2 massive units). Although the means are equal, the range of values varies from 1 to 10 m/m2 in both units (unpublished data surveyed by Lima and Binda, 2013). Thus, an open question remains about what joint density value is favorable for potholes formation. Conversely, the intact rock strength expresses the susceptibility to abrasion (e.g., Sklar and Dietrich, 2001; Stock et al., 2005; Hayakawa et al., 2008), and the strength of basalts is only generally known (e.g., Kahraman et al., 2002; Sumner and Nel, 2002; Dinçer et al., 2004; Grab et al., 2005). Could vesicularamygdaloidal basalts be significantly less resistant? With regards to heterogeneity, vesicles and amygdales provide irregularities that can catalyze vertical vortices responsible for potholes formation (Hancock et al., 1998). Until now, there have only been unquantified observations that relate the occurrence of potholes with vesicular-amygdaloidal basalts (e.g., Kale and Joshi, 2004; Sengupta and Kale, 2011). Because plucking is the more evident process in basalts due to jointing, little attention has been given to abrasive processes in these rocks. Simply considering the existence of different basaltic units is unlikely capture the true response of the long-term fluvial erosion, as required by landscape evolution models, with less than complete understanding of the erosion processes. The assumption of uniform behavior within a lithology was recently analyzed and considered risky in geomorphic modeling (Marshall and Roering, 2014). For the first time, this study presents research focusing on the relationship between pothole erosion and the differential features of basalts, which is also rarely studied from the geomorphic perspective. Data from an extensive field survey in PVP rivers were used. Density of joints, strength of rocks and lithological heterogeneity in basaltic flows were evaluated. Two objectives guided this research: (1) to quantify the relationship between potholes and different basaltic units; and (2) to verify the possible controlling parameters on the occurrence of potholes in basaltic riverbeds.

87


sedimentary basin (a) and the detailed Fig. 1. Location of the study area in the Parana geologic setting of the surveyed rivers (b). The geologic aspects in Fig. 1b are based on the unpublished map of RadamBrasil with scale of 1:250.000.

between 138 and 120 Ma (Renne et al., 1992, 1996; Turner et al., 1994) and is related to the South AmericaeAfrica breakup. Approximately 90% of the volcanic rocks of PVP are toleitic basalts, and the remainder is felsic in nature (Piccirillo et al., 1988). The felsic rocks are located, stratigraphically, above the mafic rocks (Nardy et al., 2008). In the present study area, no felsic rocks appear, except for some that are restricted to the top of interfluves. In the studied region, simple basaltic flow morphology (sense of Walker, 1971) predominates, with an average thickness of approximately 30 m (Nardy, 1995). Each unit of flow has several internal zones, distinguishable by jointing and/or by petrographic structure. In the specific sites of the study we can distinguish at least two zones, a thicker zone with a massive structure and another with a vesicular-amygdaloidal structure (Fig. 2). Zones of massive basalts have vertical to subvertical joints with straight or curved planes and correspond to colonnaded and entablature zones observed in other continental provinces (e.g., Bondre et al., 2004). However, in the areas analyzed in this study, the distinction between entablature and colonnaded zones is unclear, and

2. Study area
in southern Brazil The study area is located in the State of Parana where the volcanic rocks belong to the Serra Geral Formation,
Volcanic Province (Fig. 1). Based on 40Ar-39Ar forming the Parana dating, the continental volcanism generator of PVP was located

Fig. 2. Schematic illustration of the basaltic flow morphology present in the study area. The breccia zone is not always present and each zone may vary in the relative thickness.

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entablature zones with dense fracturing (centimeter scale) do not appear, although these features are present in other areas of the central region of PVP (Nardy, 1995). Vesicular-amygdaloidal zones also present vertical and subvertical jointing, with variable spacing. The thickness of these zones ranges from 0.3 to 10 m. In the riverbeds of the surveyed area, weathering crusts commonly present joints that are progressively open and filled with sediments. In the fresher rocks, most of the joints are tight with no fill. The analyzed sites are located in the Bananas, Das Pedras and Jord~ ao rivers, which form part of the Iguazú river middle basin (Fig. 1). The flow of these rivers is in the back region of the sandstoneebasalt escarpment (Serra Geral), which marks the eastern boundary of the occurrence of volcanic rocks in the Paran
a Basin in
state. The channel of the rivers on the plateau can be the Parana characterized as a mixed bedrock-alluvial, having sections with little alluvial cover (thickness <1 m), consisting of coarse sand to boulders, and sections of exposed bedrock (Lima and Binda, 2013). Climatically, the study area is situated in a region of subtropical climate. In terms of rainfall, there is a good distribution throughout
, 1998). During spring (October to December) and the year (Parana autumn (April to June), the average total is between 450 and 500 mm. In the winter months (July to September), the amount of precipitation decreases slightly to a average total between 300 and 350 mm. The most frequent and convective rainfall occurs during the summer, when the amount of rainfall can reach approximately 550 mm. Thus, the annual rainfall in the region ranges from 1700 to 1900 mm. Concentrated frontal rainfall (1.39 mm/h) occurs during incursions of cold air masses (Polar Atlantic Mass), especially during autumn, and has a greater probability of occurrence and a ~ o events (Lima, 2009). In contrast, higher magnitude during El Nin ~ a events tend to reduce the total annual precipitation, with La Nin severe droughts occurring in the winter months. River discharges follow the rainfall regime. The data collected from the Das Pedras River from 1985 to 2005 show a maximum discharge of 364 m3/s and a minimum discharge of 0.39 m3/s (Lima, ~ o and 2009); notably, the two extreme values coincide with El Nin ~ a events, respectively. La Nin 3. Methodology The Das Pedras River, with a length of 61 km and a drainage area of 330 km2, was analyzed in the field from the source to its mouth (Fig. 3). This detailed survey, which served to develop another study (Lima and Binda, 2013), also allowed us to identify several bedrock reaches with and without abrasion features. Aiming to expand the number of observations of bedrock reaches, especially under higher discharge, three reaches were subsequently analyzed

~o Rivers. The Fig. 4. Detailed location of the sites examined in the Bananas and Jorda sites are indicated by the combination of letters and numbers (italic and underlined); the other letters indicate the basaltic flow unit (M: massive; V: vesicular-amygdaloidal) and the riverbed morphology (k: knickpoint). Boxes indicate sites with potholes. In the site B1 there are two vesicular-amygdaloidal units (see text), but the picture shows only one.

~o River (drainage areas of 721 km2 and (Fig. 4): two in the Jorda 726 km2) and the other in the Bananas River (391 km2). To maintain the homogeneity of the climate and geological aspects, such as the type of basalt and morphology of lava flows, the analysis was restricted to the region near the Das Pedras river basin, which was analyzed in detail. The survey along the Das Pedras River occurred during a dry ~ a event in season, coincident with the strong expression of a La Nin the southern region of Brazil. Due to low water levels, much of the bed can be verified directly. The other sites were checked in the field in subsequent years. In all cases, data collected were about the basalt typology (vesicular or massive), joint density, intact rock strength, and only indication, without measurements, about presence of sculpted features. The basalt typology was evaluated macroscopically in the field by checking the presence and relative quantity of vesicles and/or amygdales. Some basalt zones presented few amygdales (<5) per square meter; in these cases, the rock was considered massive. The employed joint density data were the same as those used by Lima and Binda (2013). The density was

Fig. 3. Detailed location of the sites examined in the Das Pedras River. The numbers indicate how many stations were analyzed on each site and letters indicate the basaltic flow unit (M: massive; V: vesicular-amygdaloidal) and the riverbed morphology (k: knickpoint; r: riffle). Boxes indicate sites with potholes.

A.G. Lima, A.L. Binda / Journal of South American Earth Sciences 59 (2015) 86e94

obtained by the measurement of the cumulative joint length per sampling area based on the methodology described by Goldstein and Marshak (1988). The rock strength, as well as joint density, were measured directly from the river bed, taking advantage of the exposed portions. An N-model Schmidt hammer was used. The number of impacts made at each site was from 15 to 20, according to the recommended by Selby (1980). Sometimes, due to the lack of access to the bed, rocks on the channel walls were analyzed as close as possible to the water surface. Along the Das Pedras River, measurements were taken at 63 locations (Fig. 3). In other rivers, eight more locations were analyzed (Fig. 4). The strength values measured with the Schmidt hammer may have been affected by the presence of microfractures, the degree of weathering, and the rock moisture content (Day, 1980; Selby, 1980). Microfractures may underlie the tested surface and, therefore, be inevitable. A high possibility of occurrence of such microfractures is in association with a higher degree of chemical weathering of the rock. The transit of clasts during flooding causes impacts on the surfaces of the bed, and this can lead to microfractures (Hancock et al., 1998), especially where the rocks are more fragile by chemical weathering. The effect of microfractures can be minimized in the survey by avoiding areas with pronounced signs of altered rocks. Basalts are naturally jointed rocks and the Schmidt hammer application near joints can result in lower values (Day, 1980). Following the recommendations of the literature (e.g. Day, 1980; Selby, 1980), measurements were performed at least 6 cm from joints. Some potholes are located in more weathered rocks. The measurement of strength outside these areas would result in higher values associated with the potholes, both in some samples and in the population as a whole. However, weathered areas were excluded only in cases of obvious weathering (Schmidt hammer number < 40). Thus, the association between potholes and rock strength would only omit the low-resistance cases, which are the minority, producing an artificial increment in some higher resistance classes. To perform the analysis, the mean strength was first calculated at each sampling site. According to the rationale of Selby (1980) about the elimination of as much variability as possible, a slightly modifying the proposed protocol was introduced, so that the value with the highest deviation from the mean was eliminated and sample standard deviation was decreased. Furthermore, a new mean was calculated, this being the final one. The values were rounded to the first decimal place. Thus, in the frequency analysis, each class of values, although labeled with integer values, encompasses many fractional values. The remaining analyses were also performed considering the fractional values. 4. Results 4.1. Basaltic flow units and abrasion In the study area, features of abrasion occur mainly in riffles and waterfall zones (Figs. 3 and 4). It is not uncommon, however, for these features to occur in some pools in vesicular-amygdaloidal units. It was observed in the field that the bottoms of some of these pools have relatively smoothed surfaces - flat, corrugated or rough - without the sharp edges that are common in pools developed in massive basalts. The potholes found in the sites studied vary widely in size, but their diameters do not exceed 50 cm (Fig. 5a, b). The most common diameters are between 10 and 30 cm. In many places there is coalescence of potholes and in massive units a variety of sculpted forms is more common than in vesicular-amygdaloidal units (Fig. 5c, d). The most effective control over the maximum size of the potholes is apparently the joint density, regardless of the type of

89

Fig. 5. Examples of potholes found in the study area. Potholes in vesicularamygdaloidal basalt (a) and massive basalt (b). Coalescence of potholes in vesicularamygdaloidal basalt (c) and in massive basalt (d).

basaltic unit (vesicular or massive). When the excavation of a pothole reaches a plane of a joint, the probability that the block in which it is located will be upset by plucking increases (Fig. 5a). The turbulent flow inside the pothole can, in certain situations, force the displacement of the lateral blocks bounded by the joints (e.g., Springer et al., 2006). Coalescence of sculpted features is present in many field sites (Fig. 5c and d), thus the main target of the field survey was only registering the presence or absence of these features, regardless of the pothole size. A total of 71 sites were analyzed. Sculpted features were recorded in 21 sites (29%), 18 sites (86%) in vesicularamygdaloidal units and only three sites (14%) in massive basalts. Along the Das Pedras River, two sites with potholes in massive basalts were found (Fig. 3). In one of them, situated in the rapids area, plucking dominates the morphology of the riverbed and potholes are isolated occurrences. Elsewhere, at the base of a small waterfall, sculpted features (diameter of z15 cm) are more ~o River, at frequent and coalesced. On the examined site at the Jorda the beginning of a knickzone entirely carved into massive basalt (Fig. 4), abrasive features are frequent, much more developed in size (diameter of 30 cm) and sometimes coalesced. In the Bananas River a strong relationship between the erosive processes and basalt typology can be seen where the field checks were performed (B1 in Fig. 4). A knickzone, with a total height of approximately 10 m, is present on three basalt units: an upper and a lower one in the vesicular-amygdaloidal basalt and an intermediate one in the massive basalt. In the upper and lower units, potholes are developed, whereas in the massive unit, no such feature was found. Although plucking is the predominant erosion process in the three units and probably small abrasion features can occur in the massive unit, potholes are restricted to vesicular-amygdaloidal units. Not all sections formed in vesicular-amygdaloidal basalt exhibit potholes. Thirty-seven sites in this type of basaltic unit were

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examined, but only 18 showed features of abrasion. This means that approximately 50% of the sites, regardless of the identity of the lithologic structure, have no signs of such erosion process. 4.2. Joints and potholes The occurrence of potholes in vesicular-amygdaloidal basalts is predominant, but it is neither exclusive nor complete. Joint density is independent of the type of basaltic unit, having the same mean (z5 m/m2) in the massive and vesicular-amygdaloidal basalts (Lima and Binda, 2013). However, as this property is variable, was there a relationship between low density and the occurrence of potholes, as suggested by the general concept? To answer this question we compared the joint densities measured in the sites where there were potholes and in sites where there were no potholes. In the case of the first site studied in the Jord~ ao River (J1a, Fig. 4), where potholes of massive basalts occur, the joint density measured in the area where the abrasive features occur (5.5 m/m2) is equal to the pothole-free areas (5.6 m/m2) located farther downstream (J1b, Fig. 4). In the Das Pedras River, only one site among the two in massive basalts with abrasive features could be assessed in its joint density. At that site, the value found was 5.4 m/ ~o River, m2, which is very similar to those found in the Jorda although the number of sculpted features was significantly lower. For vesicular-amygdaloidal basalts the number of field sites with potholes is larger, allowing a more comprehensive analysis (Fig. 6). The average joint density of sites with potholes is statistically equal to the average of sites without these features. In addition, the occurrence of potholes is spatially associated but not necessarily correlated with a broad spectrum of joint densities. The center of the probability distribution situated at approximately 5 m/m2 is not too prominent, as evidenced by the kurtosis value of 0.6. In some cases, potholes were spatially related to joints, with the center positioned over the joint trace (e.g., Fig. 5a). However, in the most frequently observed relationship, the joints are only transient boundary surfaces to the potholes (Fig. 5a and b). These joint surfaces are exposed by plucking with the lateral progress of the abrasion originated away of the joints.

of this section is to analyze the strength difference in the basaltic units. The strengths of the rocks in the studied riverbeds vary from 45 to 68 (Schmidt hammer units), as shown in Fig. 7. However, the distribution of values in this interval is not uniform; a higher concentration of values is observed between 57 and 63. Within this subinterval, there is a noticeable decrease in classes 59 and 60. The two peaks of the distribution (class 58 and class 61) are related to vesicular-amygdaloidal basalt and massive basalt, respectively. The strengths are varied in both basaltic units. The dispersion is equal, as shown by the standard deviation (Fig. 7). The average strength of massive basalts is higher than that of vesicularamygdaloidal basalts at a statistically significant level (p ¼ 0.001). This justifies the separation of data in two groups as done in Fig. 7. Indeed, if the lithology for classes 57 and 58 are observed, 64% of the occurrences correspond to vesicular-amygdaloidal basalts. In turn, the examination of classes 61, 62 and 63 reveals that 71% of the basalts are massive. The frequency of massive basalts with strengths between 57 and 59 is not negligible, with at least six occurrences recorded in the field. In all studied locations the presence of horizontal joints was verified. These joints could explain the strength reduction in massive basalts to produce inconsistencies in the impact of the Schmidt hammer. However, in sites where measurements were taken, the thickness of the basalt blocks between horizontal planes was always larger than 10 cm, which is considered sufficient for the Schmidt hammer record to be reliable. In addition, the blocks were juxtaposed and firm. Therefore, there is a greater possibility that textural differences are responsible for decreasing the strength of massive basalts at these sites. Chemical weathering can significantly influence the resistance values by the appearance of microcracks and increased porosity as a result of the dissolution of minerals. Measurements made on intensely weathered crusts (Table 1) resulted in mean values below those obtained for bedrock in the same locations. Whereas rock weathering occurs in a progressive scale, it is possible that the range of resistance registered for rocks of the study area (45e68) include values influenced by weathering. Most likely, this effect was registered on the tail of the distribution that includes values near 45.

4.3. Intact rock strength and lithological differences 4.4. Intact rock strength and potholes It was noted in the introduction that the difference in strength between vesicular and massive basalts could be the cause of the differential occurrence of potholes in these rocks. Thus, the purpose

To investigate the relationship of rock strength with potholes in the study area, it is useful to compare the strengths of massive and

Fig. 6. Joint density of vesicular-amygdaloidal basalts with and without potholes (Pt). NF: normalized frequency; PDF: probability distribution function. VA: vesicular-amygdaloidal basalt; M: massive basalt.

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91

Fig. 7. Strength of basaltic rocks in the study area from the Schmidt hammer rebound number. NF: normalized frequency; PDF: probability distribution function. VA: vesicularamygdaloidal basalt; M: massive basalt.

5. Discussion

Table 1 Comparative strength of basalts in fresh rock and weathered rock. Siteb/River

Strengtha Fresh rock

Weathered rock

66/Das Pedras 56/Das Pedras 23/Das Pedras B1/Bananas

58.7 59.4 62.9 58.4

32.9 41 33.4 45.8

sR

s%

25.8 18.4 29.5 12.6

43.9 31.0 46.9 21.6

a

Compressive strength from Schmidt hammer rebound number (R). Numbers in Das Pedras River refers to field stations. See supplementary material. b

vesicular-amygdaloidal basalts that contain potholes (Table 2). As noted in section 4.1, the number of sites containing potholes is equal to 21, and only three are massive basalts. Despite the disparate number of locations in massive and vesicularamygdaloidal basalts, some aspects of the strength distribution are clear. At the three sites with massive basalts, the strength is approximately 61. Therefore, these values agree exactly with the mean strength of these unit types (Fig. 7); i.e., potholes in massive basalts are not related to low strengths. Conversely, the vesicular-amygdaloidal basalts have a very broad distribution (45.6e66.9) and same mean of the units without potholes (Table 2 and Fig. 8). It has been noted (Section 4.1) that not all field sites with vesicular-amygdaloidal basalt show potholes. Almost 50% of the analyzed field sites do not present evidences of sculpted features. In these sections, the strength varies between 49 and 62 (Fig. 8).

Table 2 Characteristics of strength of basalts with and without potholes. Statistics

n Min Max Mean Mode* Stand. deviation

With potholes

Without potholes

Massive

Vesicular-amygdaloidal

Vesicular-amygdaloidal

3 61.9 62.0 61.9 61 0.1

18 45.6 66.9 57.9 58 6.1

19 49.3 62.5 57.6 58 3.3

Based on classes rather than on individual values; each class comprises all decimal values 0e9 (e.g.: 50.0e50.9).

The occurrence of potholes in basalts is generally limited by the intense fracturing of these rocks. However, the few observations of potholes in vesicular-amygdaloidal basalts reported in the literature (e.g., Kale and Joshi, 2004; Sengupta and Kale, 2011) do not represent a sporadic relationship. As indicated by the data collected
Basin, there is a strong tendency in the field on basalts of the Parana for the abrasion features to occur in these different units of basaltic flows. However, the survey in the study area showed that in the vesicular-amygdaloidal basalts, there are sections both with and without potholes. This suggests the existence of some intrinsic control on these units (i.e., some property that is unique to these rocks but not constant). The low occurrence of potholes in massive basalts also suggests the existence of a controlling property, which may not be exclusive to vesicular-amygdaloidal basalts. 5.1. The role of joint density Joint density is a property that could control the existence of potholes regardless of the nature of the basaltic unit. Jointed rocks tend to favor plucking, which prevents the development of features carved by abrasion (Hancock et al., 1998; Tinkler and Parish, 1998; Whiplle et al., 2000a, b; Tooth and McCarthy, 2004). However, the joint density in the basalts is not a limiting factor on the occurrence of potholes; it only limits the dimensions of these features. In vesicular-amygdaloidal units, potholes can occur in very different fracturing conditions. The center of the probability distribution situated at approximately 5 m/m2 (Fig. 6) indicates that this value of joint density is more common in basalts of the study area (Lima and Binda, 2013). Furthermore, under the same fracturing conditions, potholes may or may not occur in both types of basaltic units. The predominance of potholes in vesicularamygdaloidal basalts cannot be explained by the analysis of fracturing, especially considering that the average joint density is equal to that of massive basalts (Lima and Binda, 2013). 5.2. The role of rock strength Unlike other studies (e.g., Selby, 1980; Kahraman et al., 2002; Sumner and Nel, 2002; Dinçer et al., 2004; Grab et al., 2005), the results of our survey show a clear distinction in strength between the vesicular-amygdaloidal and massive basalt units (Fig. 7). Although the mean strengths are different, the distribution of

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Fig. 8. Strength of vesicular-amygdaloidal basalts in relation to potholes occurrence.

values is relatively broad. Consequently, the massive and vesicularamygdaloidal basalts may have similar resistances in some cases. The distribution in Fig. 7 shows a tail with occurrences of low strength values near 45, likely associated with weathering of rocks. This is in agreement with the work of Dinçer et al. (2004), who reported values of 35e51 for basalts that exhibited considerable weathering. As opposed to compressive strength, which is studied herein, Sklar and Dietrich (2001) studied tensile strength and showed that the resistance to erosion by abrasion depends on the intact rock strength. Could the lower strength of vesicular-amygdaloidal units be the reason for the higher incidence of potholes in those zones of basaltic flows? The potholes in the basalts of the study area occur in a wide range of strengths and predominantly in vesicularamygdaloidal zones with strengths near of 58 (Fig. 8 and Table 2). The occurrences of massive basalts without potholes but with relatively low strengths (<61) similar to vesicular-amygdaloidal basalts indicates that intact rock strength is not the key variable in pothole formation in the study area. In addition, riverbeds in vesicular-amygdaloidal basalts, independently of their strength which varies between 49 and 62, can be free of potholes. The association of potholes with lower rock strengths is no stronger than the association with vesicular-amygdaloidal units. The strengths of the vesicular-amygdaloidal basalts likely depend on the tridimensional distribution and characteristics of the bubbles produced inside lava flows. In other words, the strength is a consequence of vesicularity, which is a primary variable that influences pothole formation (see Section 5.3). Wohl (1998) suggested that potholes would take longer to form at high rock strength, which was also observed by Ortega et al. (2014) in rivers on granites. The data presented here indicate a low occurrence of potholes in massive basalts, which have a statistically higher mean strength than vesicular-amygdaloidal basalts. This suggests that in addition to the absence of vesicles/ amygdales, high strength slows pothole formation. Plucking is likely faster and removes the signs of abrasion. 5.3. Differential combinations of variables The predominance of potholes in vesicular-amygdaloidal units may indicate that the rock structure, which is prone to form irregular surfaces, promotes the initiation of turbulent vortices. Minor irregularities are considered fundamental for sculpted forms to start and maintain their development (Springer and Wohl, 2002;

Richardson and Carling, 2005). In most cases, the vesicularamygdaloidal basalts in this study have amygdales formed by chalcedony. The plucking of these amygdales leaves millimeter to centimeter-scale cavities that can facilitate micro-turbulent flow and catalyze the formation of potholes. The same process occurs more easily in vesicular zones that lack amygdales. The fact that not all sections of the vesicular-amygdaloidal basalts showed pothole formation may results from the combination of intrinsic and extrinsic variables. The intrinsic variable would be vesicularity. The amount and characteristics of the vesicles and/or amygdales depend on several aspects related to the dynamics and rheology of lava and may vary in the internal profile of the flows (Cashman et al., 1994; Cashman and Kauahikaua, 1997). Increased vesicularity and hence increased irregularity of the riverbed could favor the development of potholes. The rock strength is likely controlled by the vesicularity and can be a secondary control of potholes formation. The association of higher occurrence of potholes and strength close to 58 suggests that the lower strengths than 58 favor plucking process. On the other hand, higher strengths with low vesicularity inhibit the development of potholes, unless an extrinsic variable control the process. The extrinsic variables refer to the flow conditions in the channel, primarily the stream power, which is roughly the product of discharge and slope and varies downstream and in the cross section of the channel. In the case of massive basalts, intrinsic irregularities are relatively few, and the beginning of pothole sculpting may be related to joints in the bed. Lorenc et al. (1994) and Ortega et al. (2014), who studied granites, and Springer et al. (2005, 2006), who studied quartzites and gneisses, attributed the formation of potholes to the existence of joints in the bed, which can initiate turbulent flow and differential erosion. In basalts, Sengupta and Kale (2011) also found a connection between potholes and joints. This can be seen in areas of the PVP, both in the vesicular-amygdaloidal units (Fig. 5a) and in the massive basalt units (Fig. 5b). Joint intersections are prone to plucking, due to the impacts of bed load that can produce some cracks (Whipple et al., 2000a); consequently, bed irregularities can initiate at joint intersections. Plucked surfaces are recognized as sites suitable for the initiation of sculpted forms by abrasion (Richardson and Carling, 2005). In areas near the margins, pothole formation may be more frequent than in the center of the channel, as observed in various locations of the studied area. The flow deflection in the irregularities of the margins, which results in turbulence associated with the decreased depth in those areas (Zen and Prestegaard, 1994), can

A.G. Lima, A.L. Binda / Journal of South American Earth Sciences 59 (2015) 86e94

facilitate erosion by abrasion (Richardson and Carling, 2005). Thereby, flow and sediment convergence may develop distinct erosion sites in the channel (Johnson and Whipple, 2007). This is an important way by which massive basalts could support pothole formation independent of the existence of intrinsic bed irregularities. However, the few potholes present in these basalts indicate that this mechanism has a secondary importance, at least in the studied area. Moreover, the low frequency of potholes in massive basalts may be related to specific situations that depend on a critical threshold related to the erosive capacity being reached. The formation of abrasive features such as potholes due to increases in the erosive capacity of rivers caused either by an increase in flow velocity or stream power is well documented for fractured rocks (e.g., Benito, 1997; Baker and Kale, 1998; Hancock et al., 1998). In the Das Pedras River, the two sites where potholes occur in massive basalts are characterized by significant increases in stream power; one site is located at the beginning of a rapid, and the other at the base of a ~o River (J1a in Fig. 4; drainage area of waterfall. At site 1 of the Jorda 721 km2), the reach with potholes has a slope between 0.0349 and 0.0626 m/m, and the location is the head of a knickzone. Channel zones with low relative depths and high velocities of flow are favorable locations for potholes, as reported by Zen and Prestegaard (1994) and Baker and Kale (1998). Approximately 70 m downstream from site 1 of the Jord~ ao River (J1b in Fig. 4), the slope increases to 0.1128 m/m, which corresponds to an almost 100% increase in stream power. The result is the complete absence of potholes and the predominance of plucking. Note that in the section from J1a to J1b, the observed flow, joint density, and intact rock strength are virtually constant. At site 2 of ~o River, which is situated 1.5 km downstream (J2 in Fig. 4; the Jorda drainage area 726 km2), abrasion features do not occur, although the slope is approximately the same as at site 1, and the discharge is greater. The absence of potholes at sites J1b and J2 indicates that the stream power is above a critical threshold, which prevents the formation of potholes in favor of plucking. The discharge and slope values that compose this threshold seem to be near those of site J1a. Features such as potholes may occur under conditions of higher discharge than that found at site J2; however, the slope needs to be smaller to provide a lower stream power. In the study area a lower threshold of stream power may exist, as suggested by the absence of potholes in the massive basalts at sites with drainage areas less than 100 km2 (Fig. 3) or bankfull discharges of approximately 50 m3/s (Lima and Amaral, 2013). It is possible that below the lower limit, there is not sufficient power to sustain strong abrasive interaction of sediments with the riverbed. Smooth surface and higher strength of the massive basalts probably reinforce this behavior. Another posterior and extreme threshold must exist that allows the formation of mega-potholes in combination with the plucking process. The increase in erosive capacity, which is associated with flood events of great magnitude, can lead to the development of potholes, even in heavily jointed rocks. The dimensions of these features can reach several tens of meters (Baker and Kale, 1998; Lamb and Fonstad, 2010). In these cases, as in those of smaller scales reported by Whipple et al. (2000a), it is quite likely that a combination of plucking and abrasion is responsible for pothole enlargement. In the rivers analyzed in the present study, the magnitude of stream flow does not reach this extreme threshold. The formation of potholes in vesicular-amygdaloidal basalts does not depend much on power thresholds, as is evident from its high frequency of occurrence in the rivers analyzed. Abrasive features in these basalts have a greater dependence on topographical irregularities generated by rock heterogeneity than the joints. In massive basalts, the dependence of joints results in few potholes

93

under conditions of relative low stream power. When stream power is increased above the likely upper limit, more features may be carved. However, a large increase in stream power favors plucking, which reduces the frequency and magnitude of abrasion features. 6. Conclusions In the studied rivers, potholes occur mostly (86%) in vesicularamygdaloidal units. However, not all sections (50%) of this type of unit exhibit abrasion features. By dismissing the importance of joint density and rock strength, our study suggests that the prevalence of potholes in vesicular-amygdaloidal units is due to the degree of vesicularity and variability of the hydraulic channel conditions (stream power). For the vesicular-amygdaloidal units, the average joint density of sites containing potholes is equal to the average of the sites without potholes. Moreover, abrasion features occur in basalts with very different joint densities. Taken together, these facts indicate that the joint density is not a key variable in controlling the occurrence of potholes in basaltic beds, especially in vesicularamygdaloidal units. Conversely, joints may provide the irregularities necessary to initiate the development of potholes in massive basalts, without discounting other variables such as stable flow eddies generated by irregularities of channel boundaries. The intact rock strengths of basalts in the study area are variable; the average strength value is 58 for vesicular-amygdaloidal units and 61 for massive basalts. The difference does not seem to explain the more frequent occurrence of abrasion features in vesicular-amygdaloidal units. Thus, it is considered that: (1) in some cases, massive basalts may have strengths equal to those of vesicular-amygdaloidal units but without the occurrence of potholes; and (2) these features occur in a wide range of basalt strengths. On the other hand, the predominance of potholes in rocks with strength values close to 58 suggests that the lower strength of the vesicular-amygdaloidal units may secondarily favor the abrasion features. In massive basalts, the occurrence of potholes depends primarily on channel hydraulic conditions. Apparently, there are upper and lower stream power thresholds between which potholes can occur in the analyzed rivers. Above the upper threshold, the plucking process is favored, while below the lower limit, there is not sufficient power to carve potholes, considering the relatively smooth surface and high strength of the massive basalts. However, these limits need further investigation. Acknowledgments This study was initially inspired by discussions with Gerusa Duarte of Universidade Federal de Santa Catarina. We thank Phairot Chatanantavet and others two anonymous reviewer which helped to improve the manuscript. We also thank Daniela Hort and Pedro Santos of Universidade Estadual do Centro Oeste for their valuable field assistance. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jsames.2015.02.004. References Baker, V.R., Kale, V.S., 1998. The Role of Extreme Floods in Shaping Bedrock Channels. In: Rivers Over Rock: Fluvial Processes in Bedrock Channels. American Geophysical Union, Washington D.C., pp. 153e164 Benito, G., 1997. Energy expenditure and geomorphic work of the cataclysmic Missoula flooding in the Columbia River Gorge, USA. Earth Surf. Process. Landf.

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