Megaripples from the Mesoproterozoic of the Kimberley Region, Northwestern Australia and Its Geological Implications

Journal of Palaeogeography 2012, 1(1): 15−25 DOI: 10.3724/SP.J.1261.2012.00003

Lithofacies palaeogeography and sedimentology

Megaripples from the Mesoproterozoic of the Kimberley region, northwestern Australia and its geological implications

Lan Zhongwu1, 2, 3, , Zhong-Qiang Chen2, 4, *

1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China 2. School of Earth and Environment, The University of Western Australia, Australia 3. Key Lab of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China 4. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan), Wuhan 430074, China

Abstract Large ripples are described from the Mesoproterozoic Hilfordy Formation in the Kimberley region, northwestern Australia. Both ripple index (RI) and ripple symmetry index (RSI) suggest the Kimberley ripples were likely generated by storm waves. Their wave height is up to 15−23 cm and wave length is up to 70−90 cm. These features, incorporated with other morphological characteristics such as symmetry, steepness, ripple spacing, and compositions, agree well with the megaripples previously reported from the intertidal-nearshore settings of modern seas and the geological past. The Mesoproterozoic ripples were likely generated by the storm-induced flows. Literature survey of the global record of megaripples reveals that such structures have occurred through the geological past from the Archean to present day. They were particularly common in the Neoproterozoic and had the largest ripple length and ripple height among the modern and geological records. This is probably because extreme storms prevailed at that time. Their frequent occurrence in present day beach is probably due to the prevalence of extreme storms caused by the monsoon or tsunami/earthquake influenced climatic regimes. Key words megaripples, palaeohydrodynamics, extreme storms, Mesoproterozoic, Hilfordy Formation, Kimberley, northwestern Australia

1 Introduction megaripples were broadened to include the wavy structures possessing wavelengths of less than 100 cm but Megaripples were originally referred to as bedforms more than 60 cm (Bhattacharyya et al., 1980; Eriksson that have a wave length of 100−500 cm and a height of and Fedo, 1994; Mukhopadhyay et al., 2006; Chak- 10−50 cm and are commonly present in the natural surf raborty et al., 2009). These large wavy structures are very zone along sandy coasts (Clifton et al., 1971). Later, common not only on present day beaches but also in the

geological past (Hyde, 1980; Cotter, 1985; Eriksson and * Corresponding author: Professor. Fedo, 1994; Ainsworth and Crowley, 1994; Allen and Email: [email protected]. Hoffman, 2005; Mukhopadhyay et al., 2006; Whitelaw et First author: Postdoctor, Email: [email protected]. Received: 2011-12-20 Accepted: 2012-02-14 al., 2007; Chakraborty et al., 2009). In particular,

16 JOURNAL OF PALAEOGEOGRAPHY July 2012 megaripples have been frequently reported from the late study are found along the slope of an unnamed hill Neoproterozoic (Table 1), a period that witnessed cli- (S18˚20΄32.3˝, E126˚46΄07.6˝), about 10 km from the matic extremes such as the Snowball Earth event and southern margin of the Goat Paddock crater in the super large storms and the rise of the metazoans in late Louisa Downs areas of the Kimberley region (Harms et al., 1980; Figs. 1C, 2A). In the Louisa Downs areas, the Ediacaran (Allen and Hoffman, 2005). Growing evidence Precambrian successions consist of the Palaeoprotero- shows that megaripples are a powerful tool in recon- zoic, the Mesoproterozoic and the Neoproterozoic (Ta- structing palaeohydrodynamic conditions, analyzing pa- ble 1). Of these, the Palaeoproterozoic successions laeoflow directions, and revealing extreme climates were assigned to the Kimberley Group, which com- (Larcombe, 1992; Larcombe and Ridd, 1995; Larcombe prises four units: the Warton Sandstone, Teronis Mem- and Jago, 1996; Gallagher et al., 1998; Sarkar et al., 2002; ber, Elgee Siltstone and Pentecost Sandstone (Roberts Allen and Hoffman, 2005). et al., 1968; Table 1). The oldest rock of the Pentecost

Table 1 Stratigraphical framework of the Louisa Downs Sandstone is conformably overlain by the Hilfordy areas, Kimberley region, northwestern Australia Formation of the Crowhurst Group (Roberts et al., 1968; Table 1). The Mesoproterozoic rocks were referred to Stratigraphic unit Age as the Crowhurst Group, which is composed of the Quaternary Cenozoic Hilfordy Formation, Liga Shale and Collett Siltstone Tertiary (Table 1). It underlies conformably the Liga Shale For- Yurabi Formation mation of the same group (Roberts et al., 1968; Tyler et Louisa Downs Group Neoproterozoic Egan Formation al., 1998; Table 1). These Palaeoproterozoic to Meso- Collett Siltstone proterozoic Formations are occasionally overlain by Crowhurst Group Liga Shale Mesoproterozoic Neoproterozoic and Cenozoic deposits. The Neopro- Hilfordy Formation terozoic Louisa Downs Group consists of five forma- Pentecost Sandstone tions, but only the Egan and Yurabi Formations are exposed in Louisa Downs (Roberts et al., 1968). Elgee Siltstone Kimberley Group Palaeoproterozoic Megaripples documented here are preserved on the Teronis Member bedding panes of coarse sandstone of the Hilfordy For- Warton Sandstone mation (Fig. 1C). In the study area, the Hilfordy Forma- Recently, we have discovered megaripples from the tion is characterized by thick-bedded (25−70 cm) coarse Mesoproterozoic Hilfordy Formation in the Kimberley sandstone (Fig. 2B). Tabular cross-bedding and large- region, northwestern Australia. Their morphological angle cross-bedding are occasionally present (Fig. 2C, 2D). characteristics (i.e. symmetry, steepness, ripple spacing, Megaripples occur solely on bedding planes of reddish and ripple height) and compositions agree with previously reported examples of megaripples from the inter- coarse quartz sandstone (Fig. 3A−3G). tidal-nearshore settings. Here, we document the Meso- proterozoic examples from the Kimberley region, NW 3 Petrographic description Australia and discuss their geological implications in a broad context. Palaeogeographic reconstruction of the The reddish quartz arenite has a grain-supported texture study areas is beyond this report since other sedimentary in thin section, and is composed predominantly of structures such as gutter and flute casts are absent. Rather, monocrystalline quartz, with minor amounts of feldspar we attempt to conduct a preliminary investigation on grains. Quartz grains are remarkably uniform in size, their palaeohydrodynamics based on previous studies on well-sorted and relatively rounded (Fig. 2E). Quartz grains their modern and ancient counterparts. range from 2 mm to 4 mm in size, with a mean value of 3 mm; the grain sizes between 2.8 mm to 3.8 mm are the 2 Geological setting most frequently present (Fig. 4). Thin sections show occa- sional alignment of opaque minerals, angular quartz and The Kimberley region is situated in the northwestern Australia (Fig. 1A). The Proterozoic sedimentary rocks kaolinite. Mica comprises mainly muscovite grains, which are exposed in the Kimberley region. The distribution are in elongate lath form and commonly present in associa- of the Proterozoic sedimentary rocks is controlled tion with opaque materials. Muscovite laths are often par- mainly by the King Leopold and Halls Creek Orogens tially altered to kaolinite. Detrital clay minerals are com- (Fig. 1B). The megaripples that are the focus of this monly compacted into intergranular pore space. Lan Zhongwu et al.: Megaripples from the Mesoproterozoic of the Kimberley region, north- Vel. 1 No. 1 western Australia and its geological implications 17

123"00'

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Fig, 1 Location of the Kimberley region, northwestern Australia A-Landscape map showing the location of the Kimberley region; B-Topographic map showing the study area adjacent to the King Leopold and Halls Creek Orogens; C-Detailed geological setting at the study location (base map after Tyler et al., 1998). The reddish quartz arenite is mainly cemented by quartz to describe ripple parameters are followed herein. They overgrowths. Grain contacts are chiefly concave-convex, are measured and calculated as below: ripple spacing A. = indicating the rock has been subjected to corresponding 76 em (Fig. 5); ripple height '1 = 15 em; vertical form diagenetic compaction. index (ripple index) )J'1 = 5.1; ripple steepness 11/A.= 0.2; 4 Ripple morphological parameters and average symmetry index RSI = 2.98; average symmetry factor SF = 0.72. In addition, Tanner (1971) has also comparison proposed the following empirical formula to describe The ripples are characterized by broad, flattened and measure ripples: H= 38.52 + 1.89). - 7.lllnD, where crests inlayed by relatively narrower troughs which are, H is wave height in centimeters, ). is ripple spacing in at some sites, branching (Fig. 3). Owing to limited out centimeters and D is grain size in microns. The wave crop and preservation, two sets of ripple wave length and height for the Hilfordy Formation ripples therefore is ripple height were selected to be measured. The strike of calculated to be 125.23 em. The values ofH,). and D are ripple crests is approximately 282°. 125.23 em, 76 em and 3,000 microns, respectively. The standard terms proposed by Aspler et al., (1994) Accordingly, the Hilfordy Formation wavy structures are 18 JOURNAL OF PALAEOGEOGRAPHY July 2012

Fig. 2 Photos of the coarse sandstone and physical sedimentary structures in the study area A-Aerial photo showing the Goat Paddock crater near which megaripples of the Hilfordy Formation were found; B-Field photo showing medium- to thick-bedded sandstone of the Hilfordy Formation; c-Field photo showing low-angle tabular cross-stratified sandstone of the Hilfordy Formation; D-Field photo showing hummocky cross-bedding in sandstone of the Hilfordy Formation; E Microphotograph showing the grain-supported texture of the megaripple-bearing coarse sandstone of the Hilfordy Formation. Note the coarse sandstone is mainly composed of well-sorted and relatively rounded monocrystalline quartz. Lan Zhongwu et al. : Megaripples from the Mesoproterozoic of the Kimberley region, north- Val. 1 No. 1 western Australia and its geological implications 19

Fig. 3 Planar views showing megaripples preserved on bedding planes of the massive sandstone of the Hilfordy Formation A-Megaripples branched at ripple crest; B-Asymmetrical, branching megaripples; C-Megaripples with flattened ripple crests and narrow ripple troughs; D-Asymmetrical, flattened megaripples; E-Near symmetrical. flattened megaripples; F-Irregular megaripples occasionally with branching crests; G-Well preserved near symmetrical megaripples from which measurements of ripple height and ripple length were taken; H -occurrence of symmetrical and asymmetrical ripples. 20 JOURNAL OF PALAEOGEOGRAPHY July 2012

12 grain-supporting texture, comprise coarse, nearly equally Mean size is 3.0 sized quartz grains and lack any involvement of mud. In 10 N = 60 contrast, aeolian ripples are characterized by coarse 8 grained crests and fine-grained troughs (Anderson and i)' Bunas, 1993). Thus, the Mesoproterozoic ripples are eas .,d 6 ::l ily distinguished from the aeolian ripples. Moreover, .,...0" r,... 4 Tanner (1967) defined a ripple index (RJ: ratio of length to height) and an average ripple symmetry index (RSJ) 2 and developed the RI vs. RSI plots to classify ripples of

0 various origin. The Kimberley ripples have a RI of 5.1 1.8 2.2 2.6 3.0 3.4 3.8 4.2 4.6 and a RSJ of 2.98. Both values suggest that Kimberley Size (mrn) ripples belong to wave ripples in view of the empirical Fig. 4 Frequency of grain sizes in coarse quartz sandstone of statistics compiled by Tanner (1967). the megaripple-bearing Hilfordy Formation The Kimberley megaripples exhibit either nearly par allel crests (Fig. 3C, 3D, 3E, 3G, 3H) or irregular, branch ing crests (Fig. 3A, 3B, 3F). These features indicate mi gration of palaeowaves at different sites where differen tial migration and erosion resulted in branching or coa lescence of megaripples (Krassay, 1994). The relatively high RSI value (2.98) indicates that unidirectional cur rents prevailed, excluding the possibility that they have been formed by bidirectional oscillatory flow. They were also unlikely generated by combined flow since no diag nostic interference or linguoid ripples occur on bedding planes of the Hilfordy Formation sandstones (Dyson, 1994, 1995; Krassay, 1994). These Kimberley megarip ples are therefore interpreted to have been caused by asymmetric flows during the unidirectional propagation of waves, which are common in modem intertidal-nearshore settings (Datta et a/., 1999). The well-sorted, large-sized grains with a mean diameter value of 3 mm require strong turbulent waves rather than weak laminar flow to transport them (Komar and Miller, 1973). The Fig. 5 Measurements of megaripples from the Hilfordy medium grain-size versus mean current velocity rela Formation tionship indicate that velocities less than 0.7 m/s can S = Stoss; L = Lee; A. = Ripple spacing; '1 = Ripple height; }J'l = Ripple index; Tf/A. = Ripple steepness; Ripple symmetry index= suspend and bedload fine-grained sand, whereas veloci Stoss projection/Lee projection; Ripple symmetry factor= Lee ties greater than 0.7 m/s suspend all fine sand and trans projection/A. port medium sand by both mechanisms (Sundborg, 1967). Thus, an intense hydrodynamic wave flow with the ve- assigned to megaripples in view of several parameters such locities far greater than 0.7 m/s is inferred to have win- as ripple height and wave length, and wave height (Galla- nowed and transported the very coarse quartz grains. This gher eta/., 1998; Vmcent eta/., 1999; Gallagher, 2003). inference is reinforced by the well-sorted and rounded quartz grains distributed evenly in the rocks in megarip 5 Discussion ple crests and troughs. The ripple height of 0.15 m is consistent with megaripple migration rates of 20-1 00 5.1 Palaeohydrodynamic implications mm/min, with an average of about 55 mm/min (Lar Aeolian ripples are usually very large and they are combe and Ridd, 1995). The large megaripple height easily confused with wavy megaripples. However, sand value could positively reflect a corresponding volume stones from subaqueous ripple crests and troughs have a transport of sediments (Kennedy, 1969), although this can Lan Zhongwu et al. : Megaripples from the Mesoproterozoic of the Kimberley region, north- Vol . 1 No. 1 western Australia and its geological implications 21 not be quantified with available data. modern Sandy Duck nearshore environment (Gallagher, Short term events such as tsunami or hurricanes could 2003), on Sable Island Bank, Scotian Shelf (Amos et al., not have given rise to such megaripples, as these events 1996), in mesotidal mangrove creeks, Townsville, Aus- generally resulted in the formation of a poorly sorted tralia (Larcombe and Ridd, 1995), in the Mawddach Es- sandstone sheet with some mudstone clasts, gravels or peat tuary, North Wales, Australia (Larcombe and Jago, 1996), and organic matter included (Bourgeois et al., 1988; Du- and in the mid-Cretaceous epeiric seaway of northern rand et al., 1998; Choowong et al., 2007; Bondevik, 2008). Australia (Krassay, 1994). The dynamics that are responsible for the formation of As outlined in Table 2, megaripples have been com- megaripples could have been generated by storm-induced monly reported from modern beaches. Their ancient sheet flows, as suggested by Li and Amos (1999). Some counterparts can be traced back to the Archean, about 3.0 sedimentary features, diagnostic of the storm events, such billion years ago. A few examples have been reported as hummocky cross-stratification, quasi-planar lamination from the Mesoproterozoic and they were also relatively and lenticular micro-hummocky sand beds (Krassay, 1994; common in the Neoproterozoic and also present from the Dyson, 1994, 1995) could have developed in coarse quartz Paleozoic to the Cenozoic (Table 2). sandstones despite limited access to the bedding planes of Ripple length versus ripple height plots show that the the Hilfordy Formation sandstone. Neoproterozoic megaripples generally have much broader sizes than those of any other ages, having ripple lengths of 5.2 Comparison with megaripples reported 60−540 cm and ripple heights of 10−43 cm (Fig. 6). The from modern beaches and geological past majority of the reported megaripples (60%) from the Neo- Apart from the relatively smaller ripple length and proterozoic have ripple lengths of 270−580 cm and 70% of height, the Kimberley megaripples are remarkably com- the megaripples have ripple heights of more than 35 cm parable in other aspects with their counterparts in the (Fig. 6) and represent the largest known megaripples in

Table 2 Selected megaripples from the geological past and modern seas

Ripple Ripple Megaripples spacing λ height η Location Stratigraphic unit Environment Age References (cm) (cm) Orange Grove Quartzite, Eriksson and Fedo 1 65 15 South Africa Witwatersrand Tidal Archean (1994) Supergroup Northeastern 2 192 32 Timiskaming Group Braided channel Archean Hyde (1980) Ontariao, Canada Singhora Group, Chakraborty et al. 3 75 18 Central India Shallow marine Mesoproterozoic Chattisgarh Supergroup (2009) Mungra Sandstone, Mukhopadhyay et al. 4 96 22 South India Craton Deep marine Mesoproterozoic Kolhan Group (2006) Kimberley region, 5 76 15 Hilfordy Formation Shallow marine Mesoproterozoic This paper Western Australia Sepotuba Formation, 6 160 32 Paraguay belt, Brazil Shallowmarine Neoproterozoic Bandeira et al. (2012) Alto Paraguai Group Lower Bhander Lilji Nala, Madhya Bhattacharyya et al. 7 60 15 Sandstone, Tidal Neoproterozoic Pradesh, India (1980) Vindhyan Supergroup Mackenzie Ravensthroat Formation, Neoproterozoic Allen and Hoffman 8 350 20 Shallow marine Mountains, Canada Windermere Group (635 Ma) (2005) Mackenzie Ravensthroat Formation, Neoproterozoic Allen and Hoffman 9 400 40 Shallow marine Mountains, Canada Windermere Group (635 Ma) (2005) Mackenzie Ravensthroat Formation, Neoproterozoic Allen and Hoffman 10 150 40 Shallow marine Mountains, Canada Windermere Group (635 Ma) (2005) Mackenzie Ravensthroat Formation, Neoproterozoic Allen and Hoffman 11 150 40 Shallow marine Mountains, Canada Windermere Group (635 Ma) (2005) Lower Dracoisen Forma- Neoproterozoic Allen and Hoffman 12 540 38 Svalbard Shallow marine tion, Polarisbreen Goup (635 Ma) (2005) Lower Dracoisen Forma- Neoproterozoic Allen and Hoffman 13 300 37 Svalbard Shallow marine tion, Polarisbreen Goup (635 Ma) (2005) Lower Dracoisen Forma- Neoproterozoic Allen and Hoffman 14 450 40 Svalbard Shallow marine tion, Polarisbreen Goup (635 Ma) (2005) 15 260 43 India Kansapathar Formation Shallow marine Neoproterozoic Datta et al. (1999) Taoudeni Basin, Benan and Deynoux 16 100 10 Agueni Formation Shallow marine Neoproterozoic West Africa (1998) (To be continued) 22 JOURNAL OF PALAEOGEOGRAPHY July 2012

(Continued)

Ripple Ripple Megaripples spacing λ height η Location Stratigraphic unit Environment Age References (cm) (cm) Unicoi County, 17 115 14 Hampton Formation Shallow marine Early Cambrian Whitelaw et al. (2007) Tennessee, USA Unicoi County, 18 113 15 Hampton Formation Shallow marine Early Cambrian Whitelaw et al. (2007) Tennessee, USA Unicoi County, 19 114 15 Hampton Formation Shallow marine Early Cambrian Whitelaw et al. (2007) Tennessee, USA Unicoi County, 20 100 14 Hampton Formation Shallow marine Early Cambrian Whitelaw et al. (2007) Tennessee, USA Unicoi County, 21 120 12 Hampton Formation Shallow marine Early Cambrian Whitelaw et al. (2007) Tennessee, USA Unicoi County, 22 112 11 Hampton Formation Shallow marine Early Cambrian Whitelaw et al. (2007) Tennessee, USA Unicoi County, 23 100 13 Hampton Formation Shallow marine Early Cambrian Whitelaw et al. (2007) Tennessee, USA Allen and Underhill 24 145 35 Dorset, UK Bencliff Grit Nearshore Late Jurassic (1989) 25 150 38 Southern Ireland Kinsale Formation Shelf Early Cretaceous Cotter (1985) 26 144 32 Northern Australia Mullaman Beds Epeiric seaway Middle Cretaceous Krassay (1994) McCrory and Walker 27 160 45 Alberta Milk River Formation Shoreface Late Cretaceous (1986) Ainsworth and Crowley 28 162 56 NE England Great Limestone Shoreface Late Cretaceous (1994) 29 270 28 USA Sandy coasts Surf zone Modern Gallagher et al. (1998) Gordon Creek, Mesotidal Larcombe and Ridd 30 169 8.1 Intertidal Modern Townsville, Australia mangrove creek (1995) Gordon Creek, Mesotidal Larcombe and Ridd 31 217 7.8 Intertidal Modern Townsville, Australia mangrove creek (1995) Gordon Creek, Mesotidal Larcombe and Ridd 32 193 5 Intertidal Modern Townsville, Australia mangrove creek (1995) Gordon Creek, Mesotidal Larcombe and Ridd 33 169 9.9 Intertidal Modern Townsville, Australia mangrove creek (1995) Gordon Creek, Mesotidal Larcombe and Ridd 34 192 12.7 Intertidal Modern Townsville, Australia mangrove creek (1995) Gordon Creek, Mesotidal Larcombe and Ridd 35 202 14.9 Intertidal Modern Townsville, Australia mangrove creek (1995) Gordon Creek, Mesotidal Larcombe and Ridd 36 168 13.5 Intertidal Modern Townsville, Australia mangrove creek (1995) Gordon Creek, Mesotidal Larcombe and Ridd 37 282 15.9 Intertidal Modern Townsville, Australia mangrove creek (1995) Gordon Creek, Mesotidal Larcombe and Ridd 38 292 17.4 Intertidal Modern Townsville, Australia mangrove creek (1995) Mawddach Estuary, Larcombe and Jago 39 180 25 Mawddach Estuary Intertidal Modern North Wales (1996) Atlantic insular shelf 40 250 40 Atlantic Shelf Modern Durand et al. (1998) of Martinique Phuket and Phangga, 41 160 30 Tsunami Ocean Modern Choowong et al. (2007) Thailand Sable Island Bank, 42 150 35 Sable Island Bank Shelf Modern Amos et al. (1996) Scotian Shelf Southern coast of 43 250 42 Southern coast of Oregon Nearshore Modern Clifton et al. (1971) Oregon the geological past. Some of the Neoproerozoic me- relatively small and have ripple lengths of 65−192 cm garipples also possess ripple lengths of 60−100 cm and and ripple heights of 15−32 cm. ripple heights of 10−15 cm and represent the smallest Both the Mesozoic and the Cenozoic ripples have an in- megaripples among the reported examples. Both the Pa- termediate size in ripple length and ripple height. Of these, leozoic and Mesoproterozoic megaripples show uniform the Mesozoic megaripples generally have relatively small and considerably small ripple lengths and ripple heights. ripple lengths, ranging from 144−162 cm, whereas the Ce- The formers have ripple lengths and heights of 100−120 nozoic megaripples possess ripple lengths of 150 cm to 292 cm and 11−15 cm, respectively, while the latter have cm. In contrast, the Mesozoic megaripples usually have ripple lengths and heights of 75−96 cm and 15−22 cm, relatively larger ripple heights, ranging from 32 cm to 48 respectively (Fig. 6). The Archean examples are also cm, than their Cenozoic counterparts. The latter possess rip- Lan Zhongwu et al. : Megaripples from the Mesoproterozoic of the Kimberley region, north- Vol . 1 No. 1 western Australia and its geological implications 23

Fig. 6 Wave length versus wave height plot of global megaripples through the geological past to present day Note the Neoproterozoic megaripples exhibit a much larger ripple length and ripple height than the same structures from other ages. ple heights from 5 cm to 43 cm, but the majority of the wave length, 70−90 cm, coupled with other morphologi- Cenozoic megaripples (~65%) have ripple heights cal features, agree well with the megaripples reported smaller than 18 cm (Fig. 6). elsewhere. The Mesoproterozoic megaripples were likely It should be noted that ripple size variations (Fig. 6) generated by the storm-induced flows. Global records can indicate broad environmental and climatic extremes indicate that megaripples have been present through the through the geological past, although the incomplete geological past from the Archean to present day. Their geological record may bias the analysis to some extent. frequent occurrence in the Neoproterozoic is due to the For example, megaripples were frequently present in the prevalence of extreme storms resulted from multiple glo- Neoproterozoic and they possess a very broad ripple size bal glaciations during that time. Their common presence range. This is, in part, because the Neoproterozoic is a in modern-day oceans is probably because of the pre- critical period when the Earth was repeatedly glaciated, dominance of monsoon regime-related extreme storms. namely the Sturtian, Marinoan and Gaskiers glaciations coupled with a series of geological events such as Gond- Acknowledgements wana assembly, true polar wander and eruption of flood basalts (Hoffman et al., 1998; Evans, 2000; Hoffman and Lan Zhongwu’s Ph.D. study is supported by a joint scholarship from China Scholarship Council and the Schrag, 2002; Hoffmann et al., 2004; McCall, 2006; University of Western Australia. Financial grant for this Fairchild and Kennedy, 2007; Hoffman and Li, 2009). research was provided by an Australian Research Council Extremely large storms often prevailed after the glaci- Discovery Grant (DP07752298 to ZQC). ations (Allen and Hoffman, 2005). The frequent occurrence of megaripples in modern-day oceans is probably References due to the prevalence of extreme storms caused by monsoon-dominated climatic regimes. Several Mesoprotero- Ainsworth, P. B., Crowley, S. F., 1994. Wave-dominated nearshore zoic examples of megaripples indicate that strong pa- sedimentation and ‘forced’ regression: Post-abandonment facies, laeodynamics, possibly caused by storm-induced flows, Great Limestone Cyclothem, Stainmore, UK. Journal of the may have prevailed in that time, as suggested by Sarkar Geological Society,151: 681–695. et al., (2002). Allen, P. A., Hoffman, P. F., 2005. Extreme winds and waves in the aftermath of a Neoproterozoic glaciation. Nature, 433: 123–127. Allen, P. A., Underhill, J. R., 1989. Swaley cross-stratification pro- 6 Conclusions duced by unidirectional flows, Bencliff Grit (Upper Jurassic), Dorset, UK. Journal of Geological Society, 146: 241–252. The Mesoproterozoic Hilfordy Formation from the Amos, C. L., Li, M. Z., Choung, K. S., 1996. Storm-generated, Kimberley region, northwestern Australia yields ex- hummocky stratification on the outer-Scotian Shelf. Geo-Marine tremely large ripples. Both ripple index and ripple sym- Letters, 16: 85–94. metry index suggest that these ripples were likely gener- Anderson, R. S., Bunas, K. L., 1993. Grain size segregation and ated by storm waves. Their wave height, 15−23 cm, and stratigraphy in Aeolian ripples modeled with a cellular automa- 24 JOURNAL OF PALAEOGEOGRAPHY July 2012

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