Geochemical Journal, Vol. 50, pp. 197 to 210, 2016 doi:10.2343/geochemj.2.0405 Geochemistry of Late Cambrian-Early Ordovician fluvial to shallow marine sandstones, western Tasmania, Australia: Implications for provenance, weathering, tectonic settings, and chemostratigraphy S. A. MAHMUD,1* S. NASEEM,2 M. HALL1 and KHALID A. ALMALKI3 1School of Earth, Atmosphere and Environment, Monash University, Clayton, VIC 3800, Australia 2Department of Geology, University of Karachi, Pakistan 3King Abdulaziz City for Science and Technology, Saudi Arabia (Received March 3, 2015; Accepted November 5, 2015) A geochemical study of the Late Cambrian-Early Ordovician sandstones exposed in the West Coast Range, Tasmania, Australia, was carried out to develop an understanding of the provenance and tectonic settings. The average composition of these sandstones displayed high SiO2 (92.72%), moderate Al2O3 (3.34%) and Fe2O3 (1.71%), low K2O (0.90%) and MgO (0.15%), and very low CaO and Na2O (<0.01%) concentrations. The sandstones were mainly classified as quartzarenite, and some samples were classified as sublitharenite. Tectonic discrimination diagrams based on major and trace elements suggest passive margin settings. Provenance diagram (Al2O3 vs. TiO2) revealed that the Owen Group was derived from a silica-rich source. The average chemical index of alteration (CIA) was 78.45, indicating that the source area suffered severe weathering due to persistent warm and humid climate. High amounts of rare earth elements (REE) and strong negative anomalies on the chondrite-normalized REE pattern indicate an oxidizing deposition environment. The trace element chemostratigraphy reflects sharp contrasts in concentrations, distinguishing the unconformity between the lower and upper sequences and also shows the effect of alteration assemblages. Keywords: geochemistry, sandstones, provenance, weathering, Tasmania between 494 Ma and 462 Ma. INTRODUCTION The Late Cambrian-Early Ordovician siliciclastics in The Late Cambrian-Early Ordovician syn-rift western Tasmania have a varied nomenclature history. siliciclastic depositional system (Owen Group) is mainly Initially described Owen Conglomerate by Officer and comprised of conglomerate, sandstone, and shale Hogg (1985), it was later referred West Coast Range Con- lithofacies. The Owen Group is widely distributed in glomerate by Conolly (1947). Subsequently it was ranked western and northern Tasmania (Fig. 1), where the main as a Group (Corbett, 1990), equivalent to the Denison exposures lie in an arcuate belt extending north-south Group, by Corbett et al. (1993) and Corbett and Turner along the West Coast Range before swinging to the north- (1989). The most widely accepted scheme for the inter- east, east-west, and eventually southeast at Mount Roland nal classification was proposed by Wade and Solomon and the Gog Range (Berry and Harley, 1983; Burns, 1964; (1958) and followed by Corbett (2001, 2004, 2014) and Corbett, 1975). The sequence overlies middle to Late Noll and Hall (2003, 2005, 2006) who classified the group Cambrian Mount Read Volcanics (MRV), which also ac- into: (1) Lower Owen Conglomerate, (2) Middle Owen cumulated in a narrow arcuate zone around the present- Sandstone, (3) Middle Owen Conglomerate, and (4) Up- day western and northern margins of the Tyennan Block, per Owen Sandstone. This is generally regarded as the adjacent to the West Coast Range. Based on well-con- classic Owen Group stratigraphy and has been applied in strained dates from the underlying MRV (McNeill et al., the West Coast Range both north and south of the type 2012; McClenaghan et al., 2008; Perkins and Walshe, area at Mt. Owen, near Queenstown in western Tasma- 1993) and overlying Gordon Group (Berry, 1994; Ross nia. and Ross, 2007), the estimated age of the Owen Group is Recent studies by the authors (Driscoll et al., 2013; Mahmud et al., 2013) in the West Coast Range immedi- ately south of the King River have established stratigraphy markedly different from that in the type area (Mt. Owen) *Corresponding author (e-mail: [email protected]) and one that varies dramatically between different Copyright © 2016 by The Geochemical Society of Japan. depocenters controlled by syn-depositional faulting in 197 Fig. 1. Geological map of western Tasmania, showing the distribution of Late Cambrian-Early Ordovician sediments (after Brown et al., 2001). 198 S. A. Mahmud et al. Geochemistry of Owen Group Siliciclastics, Tasmania, Australia Tasmania, Geochemistry ofOwenGroupSiliciclastics, 199 Fig. 2. Geological map of the study area. western Tasmania. The Owen Group exposed in the Mt. Jukes-Mt. Darwin area has been classified into two fin- ing up sequences, separated by a prominent unconformity (Fig. 2). However, in the Mt. Sorell-Mt. Strahan area, it is only comprised of a monotonous sandstone-conglom- erate sequence. Chemical composition (major and trace elements) of clastic sediments is widely used to evaluate provenance, tectonic settings, weathering, and climatic conditions (Banerjee and Banerjee, 2010; Caracciolo et al., 2011; Perri, 2014; Perri et al., 2012b, 2013, 2015a, 2015b, 2015c). Immobile trace elements such as Ti, Zr, Hf, Y, Sc, Th, Cr, and Co and rare earth elements (REE) are also good indicators of geological processes, provenance, and tectonic settings (Liu et al., 2007; Parisi et al., 2011; Perri et al., 2011a, 2011b, 2012a). Chemostratigraphy is a stratigraphic method based on variations in whole-rock geochemistry and is an effective tool for geochemical fin- gerprinting used for zonation and stratigraphic correla- tions (Weissert et al., 2008; Perri et al., 2015b). This study presents the chemical composition, includ- ing major, trace, and rare earth elements, of the Owen Group in the Mt. Jukes-Mt. Darwin and Mt. Sorell areas. The paper discusses various processes associated with the source rock, depositional settings, weathering, and diagenesis that might have affected Owen Group sediments. It also briefly presents trace element and REE chemostratigraphy of the Owen Group sediments for the Proprietary Peak section (Fig. 3). REGIONAL GEOLOGY The Tyennan Orogeny in the Middle Cambrian was followed by the accumulation of MRV in much of west- ern Tasmania. Continuing extension and related uplift over a wider area lead to the formation of north-south trending troughs that were filled with debris and sediments de- rived from the adjacent Proterozoic basement (Berry and Bull, 2012; Seymour et al., 2007). The Owen Group in the West Coast Range is overlaid by Middle Ordovician Pioneer Sandstone and Gordon Limestone that were de- posited during a regional transgression that marked the onset of marine conditions following the end of rifting (Noll and Hall, 2003). The Owen Group sediments were deformed during the Middle to Late Devonian Tabberabberan Orogeny; D1 compressional deformation caused the formation of re- gional, upright, open north-south trending folds F1, mainly as inversion of structures associated with the re- versal of late Cambrian normal faults (Noll and Hall, 2005, 2006). In the East Jukes area, Noll and Hall (2005) describe a west-dipping reverse fault, along the western margin of Lake Burbury, as a major basin boundary fault Fig. 3. Proprietary Peak stratigraphic section showing sam- that now juxtaposes an originally down thrown Owen ple locations. 200 S. A. Mahmud et al. Fig. 4. Plots of trace elements of sandstone samples on a ro- tated space diagram (Principle Component Analysis; PCA). Group and MRV against Silurian and Devonian sediments. The younger D2 compressional deformation refolded and reoriented many D1 structures, forming north-north- west to northwest trending upright folds (F2). Major D2 folding is also widespread throughout western Tasmania and is accompanied by northwest striking reverse faults and thrusts (Cox, 1981; Noll and Hall, 2005). The over- printing of first-generation structures has resulted in a locally complex structural geometry in the West Coast Range, including dome and basin fold interference pat- terns (Noll and Hall, 2005, 2006). MATERIALS AND METHOD Sample selection Fig. 5. Classification of Owen Group sandstones: (a) log(Na O/ Twenty-four (24) representative samples from the 2 K2O) vs. log(SiO2/Al2O3) diagram (after Pettijohn et al., 1972), Owen Group were selected in and around the study area, and (b) log(Fe2O3/K2O) vs. log (SiO2/Al2O3) diagram (after with one complete succession of 13 samples selected from Herron, 1988). a measured section at Proprietary Peak, just north of Mt. Jukes, for chemostratigraphic evaluation. Methodology 1996). Pure element oxide mixes in pure silica, along with Analysis of major elements was performed at the International and Tasmanian reference rocks, were used School of Earth Sciences - CODES, University of Tas- with numerous checks of reference rocks and pure silica mania. A half-gram (0.5 g) sample was mixed with 4.5 g blanks were run with each program. Corrections for mass flux (Lithium Tetraborate-Metaborate mix) for decompo- absorption were calculated using PANalytical Super-Q sition in a platinum crucible. Sulphide bearing samples software with its classic calibration model and alpha co- usually have a different mix with more LiNO3 as an oxi- efficients. dizing agent, the mix was pre-ignited at 700∞C for 10 min. Rare earth and selected trace element analysis was Iodine vapor was used as a releasing agent to remove discs performed at Monash University’s School of Earth, At- from the mold. Major elements were measured using mosphere