Geomorphological evidence of neotectonic ...

Geomorphology 228 (2015) 579–596

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Geomorphological evidence of neotectonic deformation in the Carnarvon Basin, Western Australia Beau B. Whitney ⁎, James V. Hengesh The Centre for Offshore Foundation Systems, The University of Western Australia, M053, 35 Stirling Highway, Crawley, Perth, Western Australia 6009, Australia

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Article history: Received 16 May 2014 Received in revised form 1 October 2014 Accepted 13 October 2014 Available online 23 October 2014 Keywords: Fluvial geomorphology Avulsion SCR Neotectonic Western Australia

a b s t r a c t This study examines channel-scale morphodynamics of ephemeral streams in the onshore Carnarvon basin in arid west-central Western Australia. The rivers in this region have low gradients, the landscape has low relief, and the rates of climatically and tectonically driven geomorphic processes also are low. As a result, the rivers in the Carnarvon alluvial plain are highly sensitive to minor perturbations in base level, channel slope, and fluvial energy. We use channel planform adjustments, stream gradient changes, and floodplain profiles across multiple ephemeral streams within a variety of catchments and flow regimes to determine if tectonically driven land level changes are affecting channel form and fluvial processes. Growth of individual fold segments is shown to have altered stream and floodplain gradients and triggered repeated avulsions at structurally controlled nodes. Aligned perturbations in channel form across multiple channel-fold intersections provide systematic geomorphic evidence for the location and orientation of neotectonic structures in the region. These features occur as a belt of low relief anticlines in the Carnarvon alluvial plain. © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction This study assesses the fluvial geomorphology of the Gascoyne and Lyndon rivers and adjacent streams within the Carnarvon basin in west-central Western Australia (Fig. 1). The region herein referred to as the Carnarvon alluvial plain is arid with ephemeral low gradient alluvial rivers and includes the Minilya plain of Hocking et al. (1985a, b) in the north and the Gascoyne alluvial plain or old delta system of Passmore (1968) in the south. We use channel planform characteristics and avulsion histories to assess whether previously unidentified tectonic deformation is evident in the fluvial geomorphological record. Channel characteristics are mapped and evaluated using digital elevation data on a number of channels that range in size from the 865-km-long Gascoyne River, Western Australia's longest river, to unnamed streams just a few kilometers long. Channel response to gradient changes observed across this range of scales varies systematically in relation to stream characteristics (e.g., discharge, flow frequency) and variations in fold amplitudes. River channels and bedforms are highly sensitive to changes in fluvial gradients (Holbrook and Schumm, 1999) and arguably the lower a river's gradient the more sensitive it is to these changes. Ephemeral dryland rivers may not adjust their gradients in response to allogenic controls as rapidly as perennial streams (Schumm et al., 2000), and as a consequence,

evidence of channel gradient changes can have greater longevity in the landscape (Bull and Kirkby, 2002; Nanson et al., 2002). Tectonic influences have been identified as amongst the most significant allogenic control at the field and at the laboratory scale (Ouchi, 1985). Digital elevation models (DEMs) have been used extensively to identify and assess tectonic control on alluvial systems (e.g., Timár, 2003; Timár et al., 2005; El Hamdouni et al., 2008; Petrovski and Timár, 2010; Zámolyi et al., 2010; Özkaymak and Sözbilir, 2012; Bagha et al., 2014). We utilize the approaches of Willemin and Knuepfer (1994) and Pearce et al. (2004) applied to digital elevation data. We first mapped the fluvial geomorphological characteristics in the vicinity of a known Quaternary active fold and document the fluvial response. We then use the results to identify similar stream channel responses elsewhere in the Carnarvon alluvial plain and test whether these channel responses are related to underlying folds. Our investigation builds on the approach presented by Pearce et al. (2004) for analyzing ephemeral stream response to active tectonics. We demonstrate the use of remotely sensed data in recognizing neotectonic structures in arid land settings and document a regional-scale pattern of neotectonic deformation in this part of Western Australia. 2. Regional setting 2.1. Geology

⁎ Corresponding author. Tel.: +61 4 1221 4726. E-mail addresses: [email protected], [email protected] (B.B. Whitney), [email protected] (J.V. Hengesh).

The Carnarvon basin contains over 13 km of Palaeozoic and Mesozoic marine sedimentary rocks overlying crystalline basement (Playford and

http://dx.doi.org/10.1016/j.geomorph.2014.10.020 0169-555X/© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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Johnstone, 1959; Hocking et al., 1987; Hocking, 1990). Near the lower Gascoyne River, the Mesozoic package is about 450 m thick (Allen, 1972b). The Cenozoic section consists of over 60 m of marine carbonates unconformably overlain by terrigenous floodplain deposits (Allen, 1972b). The Carnarvon alluvial plain is an aggrading surface with many tens of meters of Quaternary alluvium overlying Cenozoic and Cretaceous bedrock (Baxter, 1966; Allen, 1972a,1972b). The entire sedimentary package thickens and dips gently to the west. The Carnarvon alluvial plain is a low-relief feature, dropping ~1 m per 1000 m from the base of the Kennedy Range escarpment to the coast (Fig. 1). The Kennedy Range escarpment rises abruptly 100 m in elevation and demarcates where west-flowing drainages transition from bedrock to alluvial systems. The Lyndon, Minilya, and Gascoyne rivers flow from east to west across the Carnarvon alluvial plain. The rivers locally have incised into older floodplain deposits, but underlying Cenozoic or Cretaceous bedrock outcrops are rare. Near the coast, Pleistocene and Holocene nearshore deposits (beaches and dunes) are preserved. The most conspicuous surface features on the Carnarvon alluvial plain are NW–SE oriented longitudinal dunes that overlie aggraded alluvial floodplain deposits. The dunes commonly are over 100 km long and 10 m high with a ridge-form pattern. The interdune troughs commonly are a few hundred meters wide. The troughs locally channelize surface flow and are dotted with clay pans.

The western margin of the Australian continent was extended and thinned during the Late Triassic to Early Cretaceous rifting and fragmentation of Gondwana (AGSO North West Shelf Study Group, 1994). Reactivation of Gondwana era rift-related structures is well documented in the Carnarvon basin (cf, Clark et al., 2012). The most recent structural reactivation is widely attributed to the reorganization of the northern Australian plate boundary that initiated during the Neogene (Densley et al., 2000; Kaiko and Tait, 2001; Audley-Charles, 2004, 2011; Cathro and Karner, 2006; Keep et al., 2007; Hengesh et al., 2011) when the horizontal stress field in the Carnarvon basin realigned to east–west compression (Hillis and Reynolds, 2000, 2003; Hillis et al., 2008; Müller et al., 2012). Old rift-related normal structures act as zones of weakness and are being preferentially exploited to accommodate crustal shortening (Sykes, 1978; Crone et al., 1997; Cathro and Karner, 2006; Revets et al., 2009; Keep et al., 2012; Müller et al., 2012; McPherson et al., 2013). Former rift-related extensional structures have undergone transform and contractional reactivation leading to structural inversion of basin sequences (Densley et al., 2000; King et al., 2010). A system of late Neogene to Quaternary anticlines occurs onshore and offshore. The anticlines are commonly developed as fault propagation folds above blind oblique reverse faults (McWhae et al., 1956; Boutakoff, 1963; Hocking, 1988; Hillis et al., 2008). Many of the reactivated faults show little evidence for net-reverse slip motion (Iasky and Mory, 1999), indicating either limited reverse slip or recent onset of inversion. The most topographically expressed folds in the Carnarvon basin occur in the Cape region and include the Cape Range, Rough Range, Cape Cuvier, Giralia, and Minilya anticlines (Fig. 2). A number of researchers have documented the tectonic deformation of the Cape region and suggest this deformation may be ongoing (Raggatt, 1936; Clarke, 1938; Teichert, 1948; Condon et al., 1955, 1956; Raynor and Condon, 1964; Logan et al., 1970; Hocking et al., 1987; Kendrick et al., 1991; Clark et al., 2012). Folding of the last interglacial shoreline deposits demonstrates growth of these anticlines through late Quaternary time (Denman and van de Graaff, 1977; Veeh et al., 1979; Clark et al., 2012; McPherson et al., 2013). The Rocky Pool anticline located southeast of the Cape region is the most prominent structure in the Carnarvon alluvial plain, and late Quaternary deformation has been documented (Allen, 1972a). 2.3. Climate The Carnarvon alluvial plain is an arid region with ~220 mm average annual precipitation and 2900 mm average annual evaporation (Dodson, 2009). Rainfall generally is attributed to tropical depressions and cyclones and falls in abrupt high-intensity summer downpours (Wyrwoll et al., 2000). As a consequence, precipitation varies considerably from year to year. On average, the Gascoyne River flows for only 2 to 4 months 1.5 times per year; two years in a decade it fails to flow altogether (Allen, 1972b). Average annual discharge is between ~ 450 and 864 × 10 6 m3. However, during major floods it may exceed 3700 × 106 m3 (Allen, 1972b; Dodson, 2009). Typically discharge decreases downstream from flow loss through infiltration (Dodson, 2009). The sporadic and dynamic nature of the Gascoyne River is illustrated by rainfall data collected over the past century. At Gascoyne Junction (Fig. 1) the average annual rainfall over the past century is 214 mm, whereas the highest recorded daily rainfall during the same period is 293 mm (Dodson, 2009). 2.4. Fluvial geomorphology The Gascoyne River is the longest river in Western Australia and has a drainage basin area of ~ 79,000 km2 (Dodson, 2009). The Gascoyne River is an ephemeral dryland river and has a dendritic channel form

B.B. Whitney, J.V. Hengesh / Geomorphology 228 (2015) 579–596

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or debris flow surge; (iii) sediment often is deposited within channels; (iv) antecedent channel conditions direct the flow front or bore; (v) discharge in the main channel may decrease downstream from flow loss through transmission (infiltration) into channel bed sands and overflow into distributary channels; and (vi) hydraulic gradient may exceed stream channel gradient during extreme precipitation events.

3. Data and research methodology

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as it drains the southwestern corner of the Pilbara craton and adjacent Gascoyne Province. Within the Pilbara craton and Gascoyne Province, the river's course is confined within a low relief bedrock valley with a floodplain that is up to 6 km wide; it has the channel morphology of an underfit stream. The Gascoyne and Lyndon rivers transition from bedrock to alluvial channels where they exit the Kennedy Range. This transition delimits the deeply weathered and erosional terrain to the east, from the coastal depositional and alluvial terrain to the west. Downstream of the Kennedy Range, the rivers are unconfined alluvial systems with avulsion and distributary channels (Fig. 3). Across the Carnarvon alluvial plain rivers are sand-dominated and meet the general descriptions of types 2 and 4 anabranching rivers of Nanson and Knighton (1996); rivers generally are straight with cohesive banks, vegetated elongate mid-channel bars, and localized in-channel aggradation. The Gascoyne and Lyndon distributary systems are positive relief features with architecture similar to alluvial fan and deltaic systems. Channels primarily flow in alluvium and are weakly confined by aggraded flood levees. Cross sections normal to the river are convex up and semiconical in form, unlike the trough-like cross-sectional form typical of valley-confined river systems. Thus, steeper gradients exist locally on the margins of flood levees compared with downstream or longitudinal gradients. Key hydrological and geomorphological characteristics of the Gascoyne and Lyndon rivers and adjacent smaller fluvial systems are: (i) channels are dry or contain disconnected ponds prior to seasonal flow; (ii) flow is ephemeral and episodic and may initiate with a bore

We utilize both forward and inverse approaches (Willemin and Knuepfer, 1994) to investigate fluvial response to active tectonics on the Carnarvon alluvial plain. A previously identified tectonic structure, the Rocky Pool anticline, crosses the Gascoyne River and affects its channel morphology. Yet elsewhere in the study region channel anomalies and avulsions occur where tectonic features previously have not been recognized. Therefore, we systematically analyze the channel response to a neotectonic structure (the forward approach) and make observations of similar responses elsewhere within the river systems on the Carnarvon alluvial plain (the inverse approach). To complete this analysis we: • analyze remotely sensed data and completed field reconnaissance mapping over approximately 25,000 km2 of the onshore Carnarvon basin • use SRTM 3 arc second (10 m) digital elevation data to determine channel architecture and to conduct tectonic and fluvial dimensional analysis; • derive channel cross sections, channel gradients, braiding index, and sinuosity parameters, and develop topographic profiles utilizing survey controlled topography and DEMs; and • compile and review engineering geology reports prepared for abandoned dam projects on the Gascoyne River. We utilize data collected by the Geological Survey of Western Australia in the 1960s to investigate dam sites on the Gascoyne River (Gordon, 1964; Baxter, 1966, 1967a,1967b,1967c, 1968; Wyatt, 1967; Passmore, 1968; Hancock, 1969a,1969b; Allen, 1972a,1972b). The data include survey-controlled longitudinal and cross channel profiles, borehole logs, and geological maps. We integrate these legacy data to complement our mapping and descriptions of the surficial geology. We investigate seven locations where fluvial response to tectonic activity is evident at the channel scale. Channels are divided into 3 to 5 reaches based on their planform characteristics. Channel width, sinuosity, and braiding index were measured in each reach where data have adequate resolution. We determine braiding parameters following the methods of Brice (1964) and sinuosity parameters after Friend and Sinha (1997), as modified from Leopold and Wolman (1957). Channel reaches with braiding indexes (BI) N1.5 are considered braided. Channel reaches with sinuosity (P) b1.05 are considered straight, and P = 1.05–1.3 are described as sinuous (Brice, 1964). Channel width is delimited by a sharp contact between the dry channel and green vegetated channel banks and mid-channel bars on the imagery; a marked contrast exists between brown channel belts and green vegetated channel thalwegs. Sinuosity is calculated based on the length of the thalweg divided by the length of the channel belt for individual reaches. The braiding index can be difficult to determine using imagery because of the ephemeral nature of the streams. For example, the number of observable mid-channel bars varies depending on the flow stage captured in the image. Most often, the channel belt is dry, and individual braids cannot be distinguished. An example of this is shown in Fig. 7 where the bottom portion of the image was captured during lower flows than the top portion. Nonetheless, where the braiding index is determined it provides a tool to compare the planform of successive channel reaches.

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Fig. 3. Map of the southern part of the Carnarvon alluvial plain showing some of the structures discussed in text. RP — Rocky Pool; RPA — Rocky Pool anticline; BHA — Brick House anticline; QA — Quarry anticline; YA — Yandoo anticline; MA — Mundary anticline. Blue shows watercourses and Lake MacLeod. Dark gray lines indicate longitudinal dune crests. Numbers within polygons indicate corresponding detailed figures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table 1 Detailed descriptions of geomorphic units shown in Fig. 4. (1) Young avulsion: The most recent avulsion and distributary channel of the Gascoyne River system is just north and upstream of Rocky Pool. A 23-km-long distributary channel flows northwest from Rocky Pool and floods-out prior to reaching the coast or Lake MacLeod. Avulsion occurs when discharge through the main channel becomes constricted at Rocky Pool and overflows through the young avulsion channel. The channel has a similar expression as the channels on the young floodplain (unit 2) with the exception that the Gascoyne has not constructed a levee at its spillover. (2) Young floodplain: The young floodplain covers ~1000 km2. Runoff across the floodplain initiated from the Gascoyne River at a number of spillover locations. The Gascoyne River has constructed flood levees at the heads of the spillover locations and no longer flows into the channels. These relic channels carry water during precipitation events but flow is generated locally. (3a) Quarry channel: The Quarry channel is a 10-km-long avulsion channel that flowed northwest from the Boodalia channel. The avulsion node is at an unnamed sand and gravel quarry ~15 km east of Carnarvon. The avulsion channel has a meandering morphology prior to terminating at flood-outs east of Brown Range. (3b) Gascoyne terrace: The Gascoyne Terrace extends ~18 km along the Gascoyne River across the Rocky Pool anticline. The terrace may have developed during episodic avulsions from the Boodalia channel. Once the Gascoyne became the main river channel, flow incised through the terrace deposits. (4) Boodalia channel and delta: The Boodalia channel is a 58-km-long southwest trending paleochannel that flowed to the Boodalia delta. The channel initiates at Rocky Pool where it diverges southwest from the main stem of the Gascoyne River. The main channel is relatively straight with a meandering thalweg, similar to the modern Gascoyne River. Multiple avulsions and multiple distributary channels flow from the Boodalia channel and most are weakly developed, short, flood-levee overflows. The Gascoyne channel was a successful avulsion event from the Boodalia channel. The Boodalia delta is Pleistocene age, tentatively correlated to the Marine Oxygen Isotope stage 5e (MIS) sea level highstand that occurred ~125 ka. (5) Intermediate floodplain: The intermediate floodplain is a 400-km2 feature on the north side of the Gascoyne River adjacent to Rocky Pool. The surface is internally drained and contains a number of disconnected clay pans. (6) Brown channel and delta: The Brown channel is an 80-km-long southwest trending paleochannel that flowed to the Brown delta. The Brown channel diverges from the main stem of the Gascoyne River at two locations ~20 and 40 km upstream of Rocky Pool. The channels are anabranches of the same system and rejoin downstream. The channel morphology of the anabranches is similar to the old floodway (8a). It has short discontinuous channels and numerous clay pans within a floodway that traverses and dissects the longitudinal dune field. The channel and delta aggraded over 6 km from the Brown Range. Although the channel has incised through the Brown Range, it no longer reaches the coast. It is a single channel, positive relief feature with bars and levees perched above the adjacent floodplain downstream from where its anabranches merge. The Brown delta is Pleistocene age, tentatively correlated to the MIS 7 sea level high stand (Hocking et al., 1987). (7) Brown Range: The Brown Range comprises a series of beach ridges that parallel the coast just east of the town of Carnarvon. The ridges are up to 20 m high and represent former coastal dune fields. The Brown Range is interpreted as representing MIS 7–240 ka interglacial beach complex (Hocking et al., 1987). (8a) Old floodway: The old floodway is a poorly developed channel system that flows from where the river exits the Kennedy Range to the shore of Lake MacLeod. The old floodway consists of a poorly developed distributary network with two main anabranches. The anabranches traverse and dissect the dune field (Fig. 4) and are characterized by numerous clay pans and short discontinuous channels. (8b) Old floodplain: The old floodplain is adjacent to the middle and lower reaches of the old floodway. The floodplain contains the footprints of eroded longitudinal dunes. Local clay pans and intermittent ponds are numerous. (9) Longitudinal dunes: Longitudinal dunes cover the oldest floodplain. Dunes are generally oriented northwest-southeast (Fig. 4). Dune crests have an anastomosing pattern with intersecting crests and are often continuous with lengths up to 120 km. The dunes are up to 15 m high and fixed with low scrub and spinifex. Locally interdune troughs have directed and channelized sheetwash surface flow. Numerous clay pans and intermittent ponds dot the floodplain within interdune troughs. Clay pans are more abundant near active channels. In some locations, the anastomosing pattern of the dunes has created small dune-bounded basins containing clay pans. (10) Oldest floodplain: The oldest floodplain slopes gently westward dropping ~1 m per 1 km from the Kennedy Range to the coast and locally is weakly incised by the modern Gascoyne River. This surface is the top of the Gascoyne alluvium or Pleistocene flood deposits of Allen (1972a). It contains poorly developed and discontinuous east–west oriented distributary drainage networks. Portions of the floodplain surface are internally drained and numerous channels start and stop abruptly. The channel network is a relic feature locally buried by longitudinal dunes.

4. Results 4.1. Surficial geology of the Carnarvon alluvial plain Three principal fluvial stratigraphic complexes are recognized on the Carnarvon alluvial plain: active channels, older channels, and floodplains. Active channels include the main stem of the Minilya, Lyndon, and Gascoyne rivers and the young avulsion channel that leaves the Gascoyne River to the north at Rocky Pool; Rocky Pool is a perennial water hole just downstream of the Rocky Pool anticline on the Gascoyne River. Older channels include the old floodway, Brown, Boodalia, and Quarry channels (Fig. 4). The older channels act as local catchments and carry flow during storm events, but they are relic features. Floodplains are broad alluvial surfaces where sheetwash is the dominant surface flow process and that contain localized depressions that pond water and form clay pans. The three principal fluvial stratigraphic complexes (active channels, older channels, and floodplains) are subdivided into 10 distinct time-stratigraphic geomorphic units (Fig. 4, Table 1). The units represent four temporally distinct drainage configurations that existed on the Carnarvon alluvial plain. These are the old floodway and the Brown, Boodalia, and Gascoyne river systems. 4.2. Neotectonic structures Seven locations with evidence of neotectonic activity have been identified (Figs. 2 and 3) and are discussed below.

4.2.1. Rocky Pool anticline The Rocky Pool anticline, as mapped by Allen (1972a), is a faultcored fold that strikes northeast (N. 20o E.) transverse to the east– west orientation of the Gascoyne River (Fig. 5). The anticline is the most prominent topographic feature that crosses the Carnarvon alluvial plain. It is cored by Cretaceous calcilutite, which is exposed in the Gascoyne River channel (Fig. 6). The fold is asymmetrical with a steeper dip on the eastern limb (Fig. 7) (Baxter, 1967b). The calcilutite is overlain by Cenozoic siltstone, Cenozoic–Quaternary sandstone, and Quaternary alluvium (Baxter, 1966) (Figs. 6 and 7). Borehole data indicate the depth to the top of the Cretaceous abruptly increases upstream and downstream of Rocky Pool (Baxter, 1967a). Quaternary alluvium on both sides of the structure is about 45 m thick, whereas alluvium overlying the structure is 3 m or less (Allen, 1972a). Hancock (1969b) and Allen (1972b) surmise that the Rocky Pool anticline is an actively growing structure in light of progressive deformation of the stratigraphy. The Cenozoic–Quaternary sandstone has a maximum dip of 12°E on the eastern limb (Fig. 7) and is less folded than the Cenozoic siltstone, which has a maximum dip of 24°E on the eastern limb (Hancock, 1969a). Laterization of the siltstone pre-dates the onset of folding as indicated by deformation within the laterized unit (e.g., boudins) (Hancock, 1969a). In addition, the Quaternary alluvium overlying the structure appears slightly folded (Allen, 1972a). This suggests at least three episodes of deformation: one after laterization of the Cenozoic siltstone yet prior to deposition of the sandstone; one after deposition of the sandstone yet prior to deposition of the alluvium; and one after deposition of the alluvium. The Gascoyne River channel characteristics change across the Quaternary Rocky Pool anticline. At Rocky Pool the channel narrows


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dramatically and incises through the anticline (profiles 21 to 24 in Fig. 8). The channel is sinuous both upstream and downstream of Rocky Pool and is straight across the anticline axis (Table 2). The channel gradient decreases across the anticline; however, the adjacent floodplain surface gradient increases from east to west across the fold axis (Table 2). Repeated avulsions have occurred near Rocky Pool. Approximately 3 km upstream from Rocky Pool, the Gascoyne River diverges from the abandoned 2-km-wide Boodalia channel (Fig. 6). The Boodalia channel was formerly the main river channel on the Carnarvon alluvial plain (unit 3, Fig. 4). Less than 1 km upstream of Rocky Pool on the north bank of the river, a young avulsion channel (unit 1, Fig. 4) cuts into bedrock near the crest of the anticline (Fig. 6 and 7). The elevations of the young avulsion spillover and the Boodalia channel spillover are at elevations of ~ 44 and 46 m, respectively; the elevation of Rocky Pool is 35 m. This indicates overflow from the Gascoyne River into the young avulsion channel will occur at lower flows than overflow into the Boodalia channel. 4.2.2. Rocky Pool anticline (north) The Rocky Pool anticline north of Rocky Pool is expressed at the surface as a series of low-relief Cenozoic siltstone hills (unit III, Fig. 4) that project through the old floodway (unit 8a, Fig. 4). A 1-kmwide band of aligned siltstone ridges outcrop over 30 km in a northeast direction then step to the west and continue north for another 20 km (Fig. 5). The elevations of the outcrops increase away from the river from ~ 55 m adjacent to Rocky Pool to 70 m near the left stepover (Figs. 6C and 9C). Likewise, the heights of their crests above the alluvial

surface increase from 5 m near Rocky Pool to 10 m at the stepover. North of the left stepover, the topography of the northern segment of the structure is more subdued and is partially covered by longitudinal dunes (Fig. 5). The Rocky Pool anticline is crossed by wind and water gaps. The old floodway (unit 8a, Fig. 4) crosses the Rocky Pool anticline at a number of locations. At each location the floodway narrows across the anticline axis then widens (fans out) downstream. The northernmost exposure of the main fold segment is partially dissected by westward- and eastward-flowing consequent streams on either side of the fold axis (Fig. 9B). The quantity, maturity, and density of these streams increase along strike from south to north away from the Gascoyne River. Supercedent streams (reflecting combined superposition and antecedence) are deflected along the base of the fold, then flow across the fold through water gaps, narrowing through the fold axis. At some locations ponded water is damned upstream of the fold. Clay pans are numerous upstream (east) of the Rocky Pool anticline where the gently westward sloping surface gradient of the old floodplain and old floodway have ponded sheetflow along the eastern limb of the fold. 4.2.3. Brick House anticline The Brick House anticline is located 15 km east of the town of Carnarvon (Fig. 3). It is expressed as a subtle ridgeline that trends north–south for ~25 km north of the Gascoyne River. The ridgeline is a remnant of the oldest floodplain (unit 10, Fig. 4) that is preserved at a higher elevation than the adjacent young floodplain (unit 2, Fig. 4). A seismic survey conducted in 1964 (Raitt and Turpie, 1965) traversed a structure we herein refer to as the Brick House anticline.


E114o 10’

A

E114o 10’

Axis of anticline

C” C

Sand ridge crests

B’ S24o 45’

A

C’ B

A

Boodalia Channel

YA RP

S24o 45’

RP

B

Boodalia Channel

60 50 10000

5000

Elevation (meters)

A’

Anticline crest

70

Siltstone

B Floodplain 54

Boodalia Channel

Flood surfaces/Terraces

Terraces Gascoyne Channel

Floodplain

B’

50

5000

2500

D

Distance (meters) 100x V:H

Channel Clay pan

B

5 km

Calcilutite

C

yne River Gas co

B 5 km

A

B’

A’

A

Elevation (meters)

585


Fig. 6. Annotated aerial photograph, geomorphic sketch map, and topographic profiles near Rocky Pool. A) Annotated air photo of a portion of Rocky Pool anticline and Gascoyne River showing the location of topographic profiles A–A′ and B–B′ across the topographic high and channel; RP — Rocky Pool; YA — young avulsion. B) Geomorphic map of Rocky Pool anticline. C) Topographic profile A–A′ across the ridge crest and adjacent slopes. D) Topographic profile B–B′ transverse to the Gascoyne River showing inset terraces.

The seismic transect (Raitt and Turpie, 1965, Plate 2) shows a gentle fold with an axis that is aligned with the ridgeline. The dip angles of seismic horizons change from ~1° west to 5° east across the Brick House anticline (Raitt and Turpie, 1965). Folded strata are evident in the seismic data, but poor data resolution prevents interpretation of the shallow section. Channel gradient and planform adjust across the ridgeline (Fig. 10; Table 2). Upstream of the Gascoyne River bridge the main Gascoyne River channel widens, aggrades, and becomes braided (BI = 2.5) before abruptly narrowing along an irrigated stretch of farmland (reach A) (profiles 2 to 4 on Fig. 8; and Fig. 10). The river channel narrows, straightens (P = 1.04), and decreases gradient across the Brick House anticline (reach B) compared to upstream (P = 1.06) and downstream (P = 1.09) reaches (Table 2). A 5-km-wide abandoned meander terrace is preserved along the south bank of the Gascoyne River upstream of the fold axis. Channel characteristics reflect aggradation upstream (reach

4.2.4. Yandoo anticline The Yandoo anticline forms the northernmost left step over from the Rocky Pool anticline (Fig. 3). It is expressed as a subtle 17-km-long axial ridgeline that trends in a north-northeast direction. The ridgeline is within the dune-covered oldest floodplain (unit 10, Fig. 4). The axial ridgeline affects the course and planform of Yandoo Creek.

A’

12 o o

24

Schematic geologic cross-section A-A’

Alluvium (Quaternary)

RP

Avulsion sill

n sion Cha n vul

e

ou

~15 m

A

ng A

ls

A

A) and downstream of the axis (reach C) and incision through the anticline (reach B). Several distributary channels that drain local runoff incise through the axial ridgeline (unit 2, Fig. 4). Upstream of the fold axis, the chaotic channel network on the floodplain is dendritic and consolidates to four main antecedent channels that cross the ridgeline. The four channels straighten and narrow across the axis and fan out downstream.

Y

A’

Rocky Pool anticline

F Zone of aggradation

Sandstone (Cenozoic) Unconformity Siltstone (Cenozoic) Calcilutite (Cretaceous) Fig. 7. Schematic geologic cross section across the Rocky Pool anticline. Oblique aerial Google Earth® image of Rocky Pool. View is downstream, approximately to the west– northwest. RP — Rocky Pool; F — small tributary channel flows east during low flows. River is 70 m wide at RP.

586

B.B. Whitney, J.V. Hengesh / Geomorphology 228 (2015) 579–596 Youn g

3.57 1

Avul sio n

l oo yP

e

lin

tic

an

62.41 25

ck

Ro

9.76

ne River scoy Ga

2

64.07 26

annel Ch alia od o B

16.19 3

Brick House anticline

66.01 27

5 km 22.82 4

28

67.46

Rocky Pool

15 Brick House anticline

29.07 5

34.44 6

12

30 21

5

Doorawarrah

35

Quarry anticline

3

29

68.83

25

30

71.19 35.62 7

19

55.26

45.44 13

20m

73.08 56.85

37.26 8

47.36

14

0 200m Profile scale

Fishy Pool 57.19 21 Rocky Pool

39.00 9

48.93

15

58.09 22 40.61 10

50.41 16

43.68

58.85

52.04 17

42.21 11

12

Quarry anticline

53.65

18

60.26

31

20

23

24

74.44

32

76.12

33

78.39 34

79.77 35

Fig. 8. Cross sections along the lower 70 km of the Gascoyne River (redrawn from Allen, 1972b). Bolder numbers are the profile numbers. Numbers on the lower left indicate distance in kilometers from the river mouth. Gray polygons are channel sands (mobile). Ticks along river trace correspond to specific cross sections. Relative fold axis locations delineated with anticline symbols on the river trace and within the cross sections. Inset DEM is a portion of Fig. 5 showing the young avulsion channels and the Boodalia channel heading from the avulsion node at Rocky Pool.

Yandoo Creek is one of a number of ephemeral systems that drain the western escarpment of the Kennedy Range (Fig. 3). The main stem of the creek is ~100 km long and its upper catchment has a well-developed dendritic drainage pattern. Upon exiting the Kennedy Range and entering the Carnarvon alluvial plain, Yandoo Creek flows northwest and is channelized within an interdune trough (Fig. 11). The channel turns abruptly (N90°) to the south-southwest and flows transverse to the longitudinal grain of the dune field for ~ 8 km. It then turns abruptly (~ 90°) to the northwest. At this bend the channel cuts obliquely across the dune field for 8 km until it again becomes confined within an interdune trough. The end of the channel floods-out within the dune field (Fig. 11). The conspicuous bends in Yandoo Creek are aligned with the left step over north of the Rocky Pool anticline, 15 km north of the northernmost siltstone outcrop. The Yandoo anticline deflects the creek channel at this location. The Yandoo Creek channel is straight (P = 1.03) and confined within an interdune trough upstream of the Yandoo anticline (reach A) (Table 2). It widens, decreases in gradient, and becomes braided (BI = 2) with increased sinuosity (P = 1.13) along the base of the anticline (reach B). The creek has a single relatively straight channel (P = 1.06) around the fold axis (reach C). Downstream along reach D the channel gradient increases and it has a distributary network. The oldest floodplain (unit 10, Fig. 4) slopes gently from east to west except where interrupted by the Yandoo anticline. Clay pans and ephemeral ponds are concentrated along the eastern base of the fold. 4.2.5. Mundary anticline The Mundary anticline is the easternmost topographically expressed fold in the Carnarvon alluvial plain (Fig. 12). It trends northeast for ~ 30 km as a subtle ridge in the old floodplain north of the Gascoyne River (Fig. 3). The axial ridgeline affects the channel planform of Mundary Creek. Mundary Creek is one of the main drainages along the western escarpment of the Kennedy Range (Fig. 3). Its main tributaries have

well-developed dendritic drainage networks in the upper parts of their catchments. The north fork of Mundary Creek flows from north to south along the eastern base of the axial ridgeline and has an asymmetric drainage pattern with all of its major tributaries entering it from the east (Fig. 12). Individual tributaries of Mundary Creek have distributary channels that fan out across 30 km2 forming a zone of aggradation upstream of the anticline (Fig. 12B). Downstream of this zone of aggradation, the forks join and flow west through a 1400-m-wide braided channel network. The planform of the channel's lowest 23 km (reaches A, B, and C; Fig. 12) systematically changes as it flows across the fold axis. Upstream of the fold, the channel is steep, narrow, and sinuous (P = 1.14) (reach A) (Table 2). The channel gradient decreases and is braided (BI = 2) where it crosses the fold (reach B). The creek is a single straight channel (P = 1.01) for 2 km downstream of the fold (reach C), then distributes into three floodout channels in the dune field (reach D). The oldest floodplain (unit 10, Fig. 4) slopes gently from east to west except where interrupted by the Mundary anticline (profile F–F′, Fig. 12C). Clay pans are concentrated along the base of both fold limbs. The channel planform of the old floodway between the Gascoyne River and Mundary Creek also shows a pattern of channel widening and aggradation upstream and downstream of the fold and channel narrowing and incision through the fold axis (Fig. 12A). 4.2.6. Quarry anticline The Quarry anticline is located between the Brick House and Rocky Pool anticlines and affects the channel planform of the Gascoyne River and Boodalia channel (Fig. 3). The anticline is ~12 km long and trends in a north-northeast direction. Its length and location are inferred from aligned channel planform changes that follow the regional structural trend. The Quarry avulsion formed three channels that exit the west side of the Boodalia channel at an unnamed sand and gravel quarry (Fig. 13). The three anabranching channels are weakly incised (b 1 m) through


587

Table 2 Channel parameters near anticlines.

East

West

Anticlines

Rocky Pool anticline Gascoyne River

Reach A

Reach B

Reach C

Gradient

0.0010

–

0.0008

width (m)

1200

70

300

P

1.09

–

1.15

B.I.

2.5

0

2.3

–

–

–

0.0010

–

–

–

–

Reach C´

Reach C´´

Boodalia channel Gradient Young avulsion channel Gradient

0.0012

Intermediate floodplain Gradient

0.0006

–

0.0008

Gascoyne River

Reach A

Reach B

Reach C

Gradient

0.0012

0.0008

0.0008

Width (m)

915

200

400

P

1.06

1.04

1.09

B.I.

2.5

0

0

Brick House anticline

Yandoo anticline Yandoo Creek

Reach A

Reach B

Reach C

Reach D

Gradient

0.0011

0.0009

0.0011

0.0012

Width (m)

210

1000

200

dist.

P

1.03

1.13

1.06

–

–

2

0

–

Mundary Creek

Reach A

Reach B

Reach C

Gradient

0.0020

0.0014

0.0023

–

90

1410

160

dist.

1.14

1.10

1.01

–

–

2

0

–

Reach A

Reach B

Reach C

Reach D

Reach E

–

–

–

–

–

Width (m)

300

1000

900

500

110

P

1.17

1.14

1.02

1.20

–

B.I.

2.1

4

4.2

0

–

Reach A

Reach B

Reach C

Reach D

B.I. Mundary anticline

Width (m) P B.I.

Reach D

Quarry anticline Boodalia – Quarry channels Gradient

Gascoyne River Gradient

–

–

515

820

580

610

P

nd

1.13

1.03

1.05

B.I.

2.2

3.6

0

–

Lyndon River

Reach A

Reach B

Reach C

Reach D

Gradient

0.0012

0.0005

0.0016

–

Width (m)

160

70

180

dist.

P

1.09

1.05

1.06

–

–

–

–

–

Width (m)

–

Lyndon anticline

B.I.

B.I. is braiding index. P is sinuosity. Braiding index “0” indicates no mid-channel bars were observed in the reach. (–) indicates either no data were available or no measurements were made. Dist. indicates reach is distributary. Gray and italicized cells indicate fold axis locations.

the adjacent floodplain. A topographic profile adjacent to the Quarry and Boodalia channels shows a subtle convexity (topographic high) coincident with the avulsion location (Quarry channel reach C) (Fig. 13). Along reach A, the Boodalia channel is 300 m wide and sinuous (P = 1.17) upstream of the Quarry avulsion (Table 1). The channel widens at the Quarry, becomes braided (BI = 4), and remains sinuous (P = 1.14) (reach B). The three anabranching avulsion channels are straight (P = 1.02) for ~2 km (reach C) then join together into a sinuous channel (P = 1.20) downstream (reach D). The Quarry channel has a distributary network and floods-out east of the Brown Range (reach E) (Fig. 13).

The Boodalia channel is deflected south along the eastern limb of the fold at the Quarry (Fig. 13). The thalweg along this part of the channel is less sinuous than elsewhere and is confined along the eastern channel bank—deflected away from the fold. The northern end of the Quarry anticline intersects the Gascoyne River (reach B). Across the fold axis in Reach B the Gascoyne River widens and becomes braided (BI = 3.6; P = 1.13) (Table 2). Downstream of the fold axis in reach C, the river narrows and straightens (P = 1.03). In reach D, the channel widens and sinuosity increases (P =1.20). Reach D in part could be responding to the change

588


S24o 34’

E114o 18’

E114o 18’

S24o 34’

C C’

C Ol d

Flo o

B

dw ay

C’

Axis of anticline Dune crests

A

B

2 km

Elevation (meters)

C

Clay pan

Siltstone

Channel (dashed where discontinuous)

C’

Anticline crest

C

2 km

75 70 65 60 0

1000

2000

3000

4000

5000

6000


Fig. 9. Annotated aerial photograph, geomorphic sketch map, and topographic profiles along the northern end of the Rocky Pool anticline. A) Annotated air photo of a portion of Rocky Pool anticline showing the location of topographic profile C–C′ across the topographic high. B) Geomorphic map of the northern extent of the Rocky Pool anticline. Note the consequent streams on the fold limbs. C) Topographic profile C–C′ across the ridge crest and adjacent slopes.

in gradient associated with the downstream Brick House anticline (profiles 10 to 12 in Fig. 8). 4.2.7. Lyndon anticline The Lyndon anticline is located in the northern Carnarvon alluvial plain (Fig. 2). Folded Miocene Trealla limestone is exposed near the fold axis along the river bank (Fig. 15). The Lyndon anticline overlies the steeply westward-dipping Marilla fault (Fig. 2) (Crostella, 1995). The anticline has formed an 18-km-long by 5-km-wide ridgeline that is oriented north–south, perpendicular to the surface gradient of the Carnarvon alluvial plain and the flow direction of the Lyndon River (Hocking et al., 1985a). The Lyndon River flows west across the Lyndon anticline and discharges into the northern end of Lake MacLeod (Fig. 2). The river flows for over 100 km through bedrock highlands and across the alluvial plain then desiccates via an alluvial fan distributary channel network. It is discontinuous across a 2-km-long channel gap between its upstream and downstream channelized reaches. Near this location, Cardabia Creek flows into the Lyndon River from the north (Fig. 14). A zone of aggradation occurs at the confluence of Cardabia Creek and the Lyndon River. The Lyndon River narrows, straightens (P = 1.05), and decreases channel gradient across the fold axis compared with upstream and downstream reaches (P = 1.06 and P = 1.09, respectively) (Table 2). The channel form is distributary with a down gradient elongated conical fan shape downstream of the fold (reach C). In reaches A and B, the river has incised through a terrace. Channel characteristics reflect aggradation followed by incision upstream of the fold axis. The Lyndon River has a conspicuous S-shaped curve near its mouth where it bends around the tips of two topographically prominent

anticlines (two of the Minilya folds) (Fig. 2). The Lyndon River is antecedent through the Lyndon anticline, yet downstream it is supercedent around the tips of the Minilya folds (Figs. 2 and 15). Prominent differences in channel responses are observed at these two locations as the Minilya folds are more topographically expressed than the Lyndon anticline. Approximately 4 m of relief is measured across the Lyndon anticline adjacent to the river, whereas 6 and 15 m of relief are measured across the tips of the Minilya folds (Fig. 14). 5. Discussion The response of the rivers to tectonic deformation has a major role in the overall late Neogene to Quaternary evolution of the Carnarvon alluvial plain. Fluvial geomorphology of major streams and rivers demonstrate late Neogene to Quaternary active folding. Six tectonic structures that have influenced fluvial dynamics are identified including: Rocky Pool, Lyndon River, Mundary, Yandoo, Quarry, and Brick House anticlines. Each of these structures has affected channel morphology, gradient, sediment carrying capacity, and avulsion histories of the respective streams. The folds have caused river avulsions that led to large-scale channel abandonment and channel migration across the plain. 5.1. Response of ephemeral river channels to active tectonics A pattern of geomorphological responses to changes in fluvial dynamics across neotectonic folds is recognized in the Carnarvon alluvial plain. These geomorphological responses were initially validated at the Rocky Pool anticline and subsequently used to identify other neotectonic


589

E113o 50’

E113o 50’

wy

5 km

lH

5 km

ta as

Co

D

S24o 42’

D S24o 42’

Seismic transect

D’ Seismic transect

o

o

1 5

D’

Bo oda

ne h an C a li

C

l

Brick House (approx.)

B

Elevation (meters)

D

Channel reach Axis of ridge crest Clay pan Sand ridge crests Channel

B Old floodplain (remnant)

40

A

A

e River oyn Gasc

A

o 1o 5

Terrace

D’

Ridge crest

30 20 10

1o

0

C

6000

5o

Dip of horizons on seismic transect (Raitt and Turpie, 1965)

12000

18000

24000

30000


Fig. 10. Annotated aerial photograph, geomorphic sketch map, and topographic profiles along the Brick House anticline. A) Annotated air photo of a portion of Brick House anticline and Gascoyne River showing the location of topographic profile D–D′ across the topographic high and seismic transect from Raitt and Trupie (1965) showing dip angles. B) Geomorphic map of Brick House anticline showing the three stream reaches (A–C) discussed in text. C) Topographic profile D–D′ across the ridge crest and adjacent slopes.

structures in the area. Most structures are traversed by multiple drainages, and channel response on any given feature is a function of the drainage characteristics and the underlying fold. Aligned changes in channel planform characteristics across multiple channel-fold intersections provide geomorphological evidence for the location and orientation of neotectonic structures. Pearce et al. (2004) provided a conceptual model for how ephemeral channel patterns adjust as a function of tectonic and fluvial controls. We expand on this model to conceptually illustrate a wider range of channel responses. We present seven idealized channel responses to uplift along discrete neotectonic structures oriented transverse to flow direction (Fig. 15). Channel responses are a function of the tectonic deformation rate and the fluvial rate. The tectonic deformation rate in this area is expressed as growth of a fold, while fluvial rate is a function of flow frequency, discharge, and stream power. Seven stages are presented that illustrate the variability in the spectrum of response from complete tectonic control (Fig. 15A), in which flow is halted and a lake or pond is formed upstream of the fold, to complete fluvial control (Fig. 15G) in which the channel is not affected by the fold. Observed channel responses that correspond to the seven-stage model are summarized below: • Rocky Pool — antecedent channel aggrades and channel widens upstream of the fold and incises across the fold axis (Fig. 15D) • Rocky Pool north — supercedent channels divert around fold segments (Fig. 15B) • Brick House — channel widens and aggrades upstream of the fold axis, and channel narrows across the fold; smaller channels fan out and distribute flow downstream of the fold axis (Fig. 15D, E) • Yandoo Creek — channel aggrades upstream of fold axis, locally ponds, and diverts around and aggrades downstream of the fold (Fig. 15A, B, E)

• Mundary Creek — channel aggrades upstream of fold axis, diverts around the fold, widens, braids, and aggrades across fold axis and distributes downstream of the fold (Fig. 15B, C, E); • Quarry — Boodalia channel widens upstream and is deflected by the fold (Fig. 15B); Gascoyne channel widens and aggrades across the fold axis (Fig. 15C); and, • Lyndon River — channel narrows and incises across the fold axis and uplift causes terrace formation (Fig.15F); channel fans out and distributes downstream of the fold axis (Fig. 15E). The fluvial responses to folds on the Carnarvon alluvial plain indicate that the rate of fold growth is sufficient to affect the planform of the ephemeral rivers and creeks. A similar example of dryland river response to active tectonics occurs in the region at Lake Wooleen on the Roderick and Murchison Rivers (Clark, 2004; Hengesh et al., 2011; Whitney and Hengesh, 2013). Lake Wooleen is an example of a river ponding upstream of a fold, and tectonically influenced drainages in the area exhibit a spectrum of stream responses as illustrated in Fig. 15A–F. 5.2. Avulsion Avulsion has played a major role in the fluvial geomorphic development of the Carnarvon alluvial plain. Avulsion is a response of fluvial channels to growth of folds as the stream gradient lowers and deposition chokes the channel (Fisk, 1952; Allen, 1965). Repeated avulsion events can occur at nodes where aggradation in the channel raises base level to a sill, which the river eventually overflows as a new avulsion channel. Channel aggradation and constriction upstream of the folds on the Gascoyne River were sufficient to trigger avulsion events.

590


E114o 18’

E114o 18’ 5 km

S24 14’

S24o 14’

5 km

o

E

C

B

A

E oC r e ek

Flow d

nd Ya

Ya nd o

D

E’

oo

Cr

e ek

E’

ire ct i

on

Axis of ridge crest Clay pan Dune crests

A

B

A

Channel reach

Channel

Elevation (meters)

Ridge crest

Yandoo Creek E

E’

80 76 72 1000

C

2000

3000

4000

5000

6000

7000

8000

8000


Fig. 11. Annotated aerial photograph, geomorphic sketch map, and topographic profiles along the Yandoo anticline. A) Annotated air photo of a portion of Yandoo Creek showing the location of topographic profile E–E′ across the topographic high. B) Geomorphic map of Yandoo Creek showing the four stream reaches (A–D) discussed in text and the location of topographic profile E–E′ across the topographic high. C) Topographic profile E–E′ across the ridge crest and adjacent slopes.

The locations of avulsions include: • Gascoyne avulsion from Boodalia channel: upstream from the Rocky Pool anticline the Gascoyne River avulsed from the Boodalia channel and trends 295°. • Young avulsion at Rocky Pool: two anabranching distributary channels avulse from the Gascoyne River upstream of Rocky Pool and trend 300°. • Quarry channel avulsion from the Boodalia channel: at the Quarry, three anabranching channels avulse from the Boodalia channel and trend 265°. Avulsion by progradation (Slingerland and Smith, 2004) is the dominant avulsion style in the study region. Once the avulsion channel is diverted from the parent channel onto an adjacent floodplain, sediment carrying capacity is dramatically reduced: deposition is rapid, and a conical or fan-shaped sediment wedge progrades down-gradient from the avulsion overflow. Ultimately, this can progress to the complete abandonment of the parent channel, as occurred at the Boodalia channel, and development of a new channel network downstream of the avulsion overflow (e.g., the Gascoyne River avulsion at Rocky Pool). Fig. 16 illustrates our four-phase model that describes the process of avulsion observed on the Carnarvon alluvial plain. This four-phase process is described below: Phase I channel aggradation. Phase I is initiated by an event (e.g., folding) that reduces channel gradient and triggers aggradation. As shown in Figs. 6 and 13, this occurred upstream of Rocky Pool and Quarry anticlines. Phase II the avulsion event. A channel bank is breached, and overflow of the river is diverted onto the floodplain such as where the Quarry channel avulsed from the Boodalia channel or where the Gascoyne avulsed from the Boodalia channel.

Phase III incision of the avulsion sill following overflow. Incision can be abrupt and occurs during Phase II, or can be gradual and occurs episodically where subsequent spillovers at the avulsion overflow incrementally lower the sill elevation. An example of this is seen at the young avulsion overflow at Rocky Pool. Phase IV formation of a new channel. A new channel is established downstream of the avulsion overflow, such as the Gascoyne channel avulsion from the Boodalia channel. Fig. 7 shows an example of the failed avulsions of Guccione et al. (1999) and Makaske (2001) at Rocky Pool where avulsion occurred, incised a new channel, and deposited sediments on the downstream floodplain, but the original channel was not abandoned. In this instance the failed avulsion channel may act as a distributary channel during future high discharge events, but it has not become the primary channel. This is phase III in our process model. The onset of phase IV occurs when the avulsion channel becomes the primary channel and the old channel becomes the subordinate channel, such as the Gascoyne avulsion from the Boodalia channel. Phase II occurs instantaneously, whereas phases I, III, and IV require aggradation, erosion, and new channel development and old channel abandonment, respectively. In ephemeral systems, this provides a context to establish relative age control of avulsion features where they are numerous and interfingered, as observed on the lower Gascoyne River. In ephemeral systems, multiple flood (or flow) events often are required to work through three of these four phases of the avulsion cycle, and therefore the term failed avulsion may be misleading. If phase III is repeated through multiple flooding events, then each subsequent episode of incision brings the avulsion closer to reaching phase IV, resulting in final channel abandonment. Therefore, some failed avulsions may more accurately be considered incipient phase III avulsions (Fig. 16).


5.3. Neotectonic deformation of the Carnarvon alluvial plain

While the Boodalia River was the dominant channel on the Carnarvon alluvial plain, it developed a sinuous thalweg and delta system. An avulsion occurred along the Boodalia channel at the Quarry site. This avulsion again occurred off the north side of the channel, and the avulsion channel again began to flow to the west. The Quarry channel is locked in phase III of the avulsion cycle since the Boodalia channel became abandoned upstream. The Gascoyne River formed as a result of an avulsion that occurred off the north side of the Boodalia channel at Rocky Pool. The Boodalia– Gascoyne avulsion was successful; it is in phase IV of the avulsion cycle. The Gascoyne channel established itself with a more westward azimuth (260°) than the southwest-directed (240°) Boodalia channel. The most recent avulsion event also occurred on the north side of the Gascoyne channel at Rocky Pool. The avulsion channel trends to 285° and is directed toward Lake MacLeod. This recent avulsion is in phase III of the avulsion cycle. The smaller structures such as the Yandoo and Lyndon anticlines also have made an important contribution to the overall fluviomorphological development of the Carnarvon alluvial plain. All of the smaller channels that are diverted around the anticlines revert to a west or northwest direction toward Lake MacLeod after circumventing the fold. Fluvial responses enable recognition of anticlines affecting fluvial geometry on the Carnarvon alluvial plain. Fold axial locations are interpreted from multiple lines of geomorphic evidence including topographic ridgelines on the alluvial plain, stream gradient changes, abrupt changes in stream flow direction, systematic changes in channel planform, and nodal avulsion. Tectonic control on geomorphic processes is demonstrated by the on-trend continuity of these phenomena across multiple geomorphic settings. In isolation, the geomorphic criteria presented here could be the result of independent in-channel or climatological controls such as sediment load, precipitation, and sea level adjustments. However, continuity of geomorphic responses in myriad flow settings across a broad alluvial plain indicates a tectonic influence.

Based on the relative positions of the paleochannel complexes and observed processes, we suggest the following model for evolution of the Carnarvon alluvial plain. The old floodway (unit 8a of Fig. 4) is the oldest primary paleochannel. This channel flowed in a northwest direction (310°) toward Lake MacLeod; a fundamentally different direction than the southwest-flowing Brown, Boodalia, and Gascoyne channels. We suggest that the flows in the old floodway were diverted to the southwest by growth of the north-northeast trending Rocky Pool anticline across the Carnarvon alluvial plain. An avulsion formed the Brown River and flowed subparallel to the strike direction of the Rocky Pool anticline. The eastward-dipping limb of the anticline and southward plunge imposed a structural control on the direction of flow and deflected the Brown River, similar to model (b) in Fig. 15. The avulsion event that formed the Brown paleochannel occurred 20 to 35 km upstream from the Rocky Pool anticline, and therefore most likely occurred at a lower stand of sea level when the river was graded to a lower base level and the structural relief of the Rocky Pool anticline was more pronounced. As sea level rose, the fluvial rate will have decreased with reduction in gradient of the Brown River. This may have caused a decrease in sediment carrying capacity and channel aggradation, especially near the Doorawarrah homestead (Fig. 4). The river then completed an avulsion on the north side of the Brown River channel forming the Boodalia channel. The new ~125-ka Boodalia channel (Hocking et al., 1985b) extended from Doorawarrah homestead downstream 30 km to Rocky Pool in a westward direction, but once encountering the Rocky Pool anticline again deflected southward. The new Boodalia channel turned southwest and roughly followed the axial trend of the Rocky Pool anticline. Channel aggradation at a higher sea level stand would have attenuated the structural relief of the anticline. This is consistent with the Brown-to-Boodalia avulsion occurring at a higher sea level stand than the old floodway-to-Brown avulsion.

S24o 42’

C r ee

k

A C

D

B

N or t

S24o 42’

F’

h For k

F

Flow d

? O

G ascoyn

ld

A

F floodw ay

Ri

r ve

?

Elevation (meters)

B

5 km

Ridge crest

F

on

Channel reach Axis of ridge crest Dune crests Clay pan Channel (dashed where discontinuous)

e

A

ire c ti

Zone of aggradation

F’

5 km

ry M u nda

E114o 42’

E114o 42’

M u n d ary

591

F’

134 130 126 122 0

C

1000

2000

3000

4000

5000

6000

7000

8000

9000


Fig. 12. Annotated aerial photograph, geomorphic sketch map, and topographic profiles along the Mundary anticline. A) Annotated air photo of a portion of Mundary Creek and Gascoyne River showing the location of topographic profile F–F′ across the topographic high. Note the clay pans along the base of the topographic ridge. B) Geomorphic map of Mundary Creek showing the three stream reaches (A–C) discussed in text and the location of topographic profile F–F′ across the topographic high. C) Topographic profile F–F′ across the ridge crest and adjacent slopes.

592


Tectonic deformation on individual structures alters stream gradients that localize aggradation and incision. In some locations, aggradation subsequently triggers stream avulsion due to channel constriction. Ongoing tectonic processes have led to repeated avulsions at the same structurally controlled nodes (e.g., Rocky Pool and Quarry anticlines). In addition to geomorphological data, geological and geophysical data indicate late Neogene to Quaternary active tectonic structures exist in the region. Borehole logs and seismic data demonstrate the presence of anticlinal structures at Rocky Pool and Brick House. Development of the Carnarvon alluvial plain appears to be the result of a complex interplay of fluvial responses to changing sea levels, or base levels, growth of tectonic structures, and aggradation and degradation of channels across, through, and around folds. River channel orientations are most likely controlled by the orientations of fold axes, especially at lower sea levels when river base levels are many tens to over a hundred meters lower than today. As sea levels rise, channel aggradation occurs and avulsion events are more likely. As almost all avulsion events have occurred on the north sides of the river channels and the river channels are progressively taking a more westward to northwestward direction, the entire river system may be attempting to reestablish northwest-oriented flow (the old floodway system) and connect to the Lake MacLeod depocenter. 5.4. Regional neotectonic significance A system of neotectonic folds exists in the Cape region northwest of the Carnarvon alluvial plain (cf. Clark et al., 2012). Reactivation of structures preferentially oriented to a recently established neotectonic stress regime (e.g., Cathro and Karner, 2006) is occurring in the Cape

region. This study documents the lateral continuation of this neotectonic deformation in the Western Australia shear zone (WASZ). Within this belt of deformation, structures to the southeast are less topographically developed than structures to the northwest, indicating a decreasing strain gradient across this zone (McPherson et al., 2013). The pattern of folding above reactivated bedrock structures in the Carnarvon alluvial plain is consistent with Quaternary tectonic deformation in the broader WASZ. Geomorphological observations suggest fold growth along the structural trend in the region is occurring per the segment linkage model for fault growth of Segall and Pollard (1980). In this model, structural growth is step-like. An array of en echelon or aligned fold segments link through time along strike as individual segments lengthen and connect (Fig. 17). An en echelon series of normal fault segments strike approximately north–south along the eastern margin of the southern Carnarvon basin (Hocking et al., 1985a,1985b; Crostella, 1995). Fault segments within this system are reactivated with an oblique-reverse (transpressional) sense of motion (Keep et al., 2012). As the reactivated segments grow, the segments lengthen and connect. The younger folds of the Carnarvon alluvial plain plot in stage I of Fig. 17 and are isolated, whereas the Minilya folds plot in stage II as overlapping structures, and the Cape region structures plot in stage III illustrating through-going segment linkage. The observation that tectonomorphogenic structures have greater topographic expression in the Cape region (northwest) compared with structures that cross the Carnarvon alluvial plain (southeast) means: i) reactivation commenced earlier in the northwest compared with structures to the southeast; ii) rates of neotectonic deformation are higher in the northwest than in the southeast; or iii), both. E113o 57’

E113o 57’

A B C

Ri yne sco a G G’ G’ D

a

el

Ch

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y Quarr

an Ch

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ia

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E

el

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A

B

5 km

A

B

C

Topographic high

el

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v

nn

e G ascoy n

er

D

Ch

a

Quarry

Clay pan

Axis of ridge crest Channel Dune crests

Channel reach

G’

45 35 25

C

10000

20000


Fig. 13. Annotated aerial photograph, geomorphic sketch map, and topographic profiles along the Quarry anticline. A) Annotated air photo of a portion of the Quarry anticline and Gascoyne River showing the location of topographic profile G–G′ across the topographic high. B) Geomorphic map of Quarry anticline showing a number of stream reaches on the Gascoyne, Boodalia, and Quarry channels discussed in text. C) Topographic profile G–G′ across the topographic high and adjacent slopes.


E114o 10’ MF2

E114o 10’ 5 km

A

B

5 km

CC

H’

CC

TOC

Lyndon River

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TOC

593

o

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B

H

H on

MF1

ve

r

Lyndon Anticline

A

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r

Lyn

d Lyn

er

Ri

Lyndon Anticline

Lyn d

Marilla fault

Limestone

Elevation (meters)

H

Midchannel fan (older)

B

Terrace

Midchannel fan

H’ Lyndon River anticline

Minilya folds

A

60

Channel reach Axis of ridge crest

50 40 MF1

Dune crests Channel (dashed where discontinuous) Clay pan

MF2

15000

C

30000

45000


Fig. 14. Annotated aerial photograph, geomorphic sketch map, and topographic profiles along the Lyndon anticline. A) Annotated air photo of a portion of Lyndon River showing the location of topographic profile H–H′ across the topographic high. B) Geomorphic map of Lyndon anticline showing the three stream reaches (A–C) discussed in text. C) Topographic profile H–H′ across the anticline and adjacent slopes.

5.5. Uncertainties Allen (1972a) alternatively considered that the geomorphology of Rock Pool could be related to exhumation of an older structure. However, he favored a recent tectonic genesis rather than exhumation for a number of reasons, including folding of alluvium, similarities in discontinuous alluvium suggesting displacement, and river downcutting through the structure despite the general raising of river base level since the last

glacial period. This relatively high base level is unlikely to coincide with incision and downcutting at Rocky Pool. Most importantly, tens of meters of sediment have accumulated through fluvial aggradation on the Carnarvon alluvial plain. The thickness of sediments increases from east to west, indicating activity on the Rocky Pool anticline initiated prior to alluvial deposition on the lower reaches of the Gascoyne River and aggradation is continuing under the current fluvial and tectonic environments.

ponding

aggradation and braiding upstream of fold

diversion (supercedence)

uplift

A uplift

uplift

antecedence and terrace formation

D

aggradation and braiding across fold

uplift

C

uplift

uplift

Tectonic rate

B

antecedence

F

E

uplift

aggradation and braiding upstream and aggradation downstream of fold

G Fluvial rate Fig. 15. Block diagrams illustrating channel responses to stream gradient adjustments as a function of localized tectonic uplift and fluvial energy. Block diagrams are illustrated with respect to relative tectonic uplift rate (on the y-axis) and fluvial rate (on the x-axis): (A) stream is ponded upstream of the fold; (B) stream is diverted around fold; (C) stream widens, aggrades, and braids along uplift axis; (D) stream widens and aggrades upstream of uplift axis; (E) stream aggrades upstream of uplift, narrows and incises across uplift, fans-out, and aggrades downstream of uplift; (F) stream maintains course and incises through uplift, uplifted terraces are formed; and (G) stream retains its channel characteristics unaltered by uplift.

Channel 1 Channel 2

Phase II

Degradation


Aggradation

594

Phase I

Primary channel Autogenic or switch Allogenic event

Phase III

Phase IV

Phase I

time Fig. 16. Diagrams showing avulsion chronology. Gray horizontal bars indicate when each channel is active. Phase IV begins at the time in which channel 1 is no longer the dominant channel although it might be reoccupied during subsequent high flows.

Hocking et al. (1987) tentatively correlated the Brown and Boodalia systems to Pleistocene sea level high stands (240 and 125 ka, respectively), but the features have not been directly dated. Research on the evolution of Lake MacLeod (Logan, 1987) indicated that considerable nearshore dune and beach development occurred between the Gascoyne River and Lake MacLeod during the Holocene. Therefore, the Brown and Boodalia systems may prove to be younger than currently thought. This study identifies neotectonic structures and suggests a relative age framework; future work should focus on developing a numerical chronology. 6. Conclusions This paper presents examples of fluvial geomorphological responses to neotectonic activity on the Carnarvon alluvial plain in west-central Western Australia. The response of streams to tectonic deformation is demonstrated by channel deflections, recurrent channel avulsions at structurally controlled nodes, as well as changes in channel form and gradient within structurally controlled reaches. The geomorphological characteristics are consistent with the observations of Pearce et al. (2004), where channel aggradation and degradation are the dominant channel response to growing folds. Rates of tectonic deformation on individual structures in the region are low (e.g., b0.5 mm/y) although the rate of deformation across the extended margin is several millimeters per year (Hengesh et al., 2011; Whitney and Hengesh, 2013). In addition, rates of climatically driven landscape denudation within this arid land setting also are low. This results in the condition where slow rates of tectonic deformation are geomorphically preserved in the landscape as subtle folds that alter stream gradients and affect fluvial architecture at the channel scale. Fluvial response to individual tectonic structures also helps to characterize the overall style of deformation occurring along the former rifted continental margin. Variations of stream response in a number of different ephemeral drainages indicate variable rates of deformation and timing for onset of deformation of individual structures. Antecedent river channel patterns change across fold axes, at times avulsing upstream of the folds. Where rates of uplift slightly outpace fluvial response, streams are unable to maintain their course and are deflected

around the growing nose of folds. In numerous locations sheetwash flow is dammed and ponded upstream of fold axes. Variability of fold limb dissection along strike indicates progressive deformation and along strike growth of structures. Progressive deformation of older underlying stratigraphy coupled with fluvial-geomorphic indicators suggests ongoing tectonic activity. A tendency for neotectonic faults to be blind and expressed as shallow folding in the Carnarvon basin could be related to the youthful nature of the WASZ and indicative of an early stage of fault growth through segment linkage per Segall and Pollard's (1980) model. Previous studies of the Carnarvon basin have been extensive. They have focused on hydrocarbons, mineral exploration, and groundwater resources and have documented the structural setting of old basin rift structures, including the anticline at Rocky Pool. In the course of our work, a considerable volume of geologic and geophysical information collected to support these earlier investigations in the region has been examined. Each of these disciplines offers a perspective and data that collectively provide a more complete picture of the regional neotectonic setting. This study strives to illuminate the regional significance of neotectonics in shaping the landscape in areas considered Stable Continental Regions (SCRs) (Johnston, 1994). Reverse reactivation of Mesozoic rift structures in the region has been recognized for decades, but prior to this study no investigation into the relation between neotectonic deformation and fluvial response has been conducted. This provides a promising approach for recognizing other neotectonic structures in arid Western Australia where stratigraphic evidence of recent deformation is lacking, but where fluvial geomorphology may provide a window into these processes. Acknowledgments We gratefully acknowledge the generous financial support provided by the Chevron Australia Business Unit (Project PDEP AES 12-P1ABU-82) for activities related to this project and the genuine interest and support for this research shown by Dr. Dan Gillam. We thank Dr. Karl-Heinz Wyrwoll for introducing us to Rocky Pool. We thank Dr. Stephen White and the Geological Survey of Western Australia who provided access to their aerial photograph collection. We thank Martha Whitney, two anonymous reviewers, and Dr. Richard Marston for thorough and thoughtful reviews of drafts of this manuscript. All of the reviews helped to improve this manuscript. This work forms part of the activities of the Centre for Offshore Foundation Systems (COFS), currently supported as a node of the Australian Research Council Centre of Excellence for Geotechnical Engineering and as a Centre of Excellence by the Lloyd's Register Foundation. The Lloyd's Register Foundation invests in science, engineering, and technology for public benefit, worldwide. The lead author is the recipient of a SIRF scholarship from The University of Western Australia. References

A (I)

B

(II)

(III)

Fig. 17. Fold growth by segment linkage model: (A) map view and (B) profile view. (I) isolated folds; (II) overlapping folds; (III) through-going fold linkage redrawn from Cartwright et al., 1995, Fig. 4.

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