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 in- dex (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 gen- erated 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 struc- tures 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 ex- posed 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 previ- ously 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' i NORTIIERN' i TERRITORY '. I' I QUEENSLAND WESTERN '·~------ - ----- - -·--' - -~ AUSTRALIA I SOUTII i AUSTRALIA ~ - ·- - - - -·-·- ,·-· f NEWSOUTII ! WALES k I\ ' ' !VICTORIA 600km 126"43' 126"44' 126"45' 126"46' 126"47' D Quaternary D Tertiary CJ Neoproterozoic Strata CJ Mcsoproterozoic Strata 18"20' CJ Hilfordy Formation CJ Palaeoproterozoic Strata 18"21' ~ fault -$- Meteorite impact structure 18"22' * Materiallocation 18"23' lr-----=2::..:..:;·~--=km=-_, 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