Anhydrite cements after dolomitization of shallow marine Silurian carbonates of the Gascoyne Platform, Southern Carnarvon Basin, Western Australia

Sedimentary Geology 164 (2004) 75 – 87 www.elsevier.com/locate/sedgeo Anhydrite cements after dolomitization of shallow marine Silurian carbonates of the Gascoyne Platform, Southern Carnarvon Basin, Western Australia Mohamed El-Tabakh a,*, Arthur Mory b, B. Charlotte Schreiber c, Raza Yasin d a Queens College, School of Earth and Environmental Sciences, Flushing, NY 11367, USA b Geological Survey of Western Australia, East Perth Western Australia 6004, Australia c Department of Geology, Appalachian State University, 118 Rankin, Science Building, Boone, NC 28608, USA d Saudi Aramco, Dharan 31311, Saudi Arabia Received 21 March 2001; received in revised form 25 February 2003; accepted 5 September 2003 Abstract Carbonates and evaporites in the Dirk Hartog Group were deposited in subtidal, peritidal and shallow-marine evaporitic mudflat environments across the Gascoyne Platform within the Southern Carnarvon Basin, Western Australia. The carbonates are composed of ooids, peloids and minor bioclastic fragments, with early cements. They have been extensively dolomitized and replaced, in part, by late anhydrite sparry cements. Four types of dolomite which range from early to burial types are identified, incorporating re-equilibration or re-crystallization from basin brines. Accordingly, they exhibit distinctive petrographic features and isotopic signatures of carbon and oxygen. Evaporites deposited are present as discrete beds and displacive nodules in carbonate and siliciclastic beds, and show d34SCDT values and 87Sr/86Sr ratios indicative of variable marine and non-marine conditions. Late dissolution of evaporites has produced satin spar gypsum veins in the shallowest section of the platform, whereas blocky and sparry anhydrite cements and void fillings formed deeper within the platform. Dissolution of the evaporites and formation of anhydrite cements post-date dolomitization. D 2003 Elsevier B.V. All rights reserved. Keywords: Carnarvon basin; Western Australia; Dolomites; Evaporites; Sulphur and strontium isotopes 1. Introduction Diagenetic stabilization after multiple periods of dolomitization generally produces complex dolomite textures and chemical compositions (Gao and Land, 1991; Feng and Meyers, 1998; Lumsden and Caudle, * Corresponding author. E-mail address: eltabakh@hotmail.com (M. El-Tabakh). 0037-0738/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2003.09.003 2001; Gregg et al., 2001). If associated with evaporites, extensive dolomitization can be genetically linked to saline formation fluids during the precipitation or dissolution of such evaporites (e.g., Gill et al., 1995; Meyers et al., 1997; Feng and Meyers, 1998). Gypsum and anhydrite are common by products of carbonate dolomitization associated with the evaporation of seawater and the dissolution of sulfate (Melim and Scholle, 2002; Qing et al., 2001). Few of the published dolomitization studies, however, have 76 M. El-Tabakh et al. / Sedimentary Geology 164 (2004) 75–87 addressed the formation of calcium sulfate during seawater dolomitization (Wilson et al., 2001). There are no previous studies of carbonate and evaporite associations and their diagenesis across the Gascoyne Platform in the mostly Paleozoic Southern Carnarvon Basin of Western Australia, despite its regional significance for hydrocarbon exploration. The platform deposits are virtually undeformed and for the most part have been subjected to only shallow burial (Mory et al., 1998; Ghori, 1999). Several types of dolomite are evident in cores from the carbonate and evaporite succession of the platform but no one single mechanism can explain their origin. Coarse and sparry anhydrite cements are pervasive in deeper parts of the succession and are associated with the dolomitized strata. This study focuses on the petrography and carbon, oxygen and strontium isotope geochemistry of the diagenetic dolomite and anhydrite of the Gascoyne Platform and, when combined with the sedimentology of the succession, the carbonate and evaporite diagenesis can be related to the structural evolution of the platform. the analytical procedure and methods used for the 87 Sr/86Sr analysis are given in El-Tabakh et al. (1998). Dolomite fabrics and textures are identified by using Sibley and Gregg’s (1987) and Gregg and Sibley’s (1984) classification. 3. Regional geology The Southern Carnarvon Basin covers an area of 115,000 km2 in Western Australia (Fig. 1) and is largely confined to onshore areas with a relatively thin Mesozoic cover over a Paleozoic succession. The Northern Carnarvon Basin, by comparison, lies predominantly offshore and contains a thick Mesozoic succession (Hocking, 1988). The Gascoyne Platform is so named as it was structurally elevated between deposition of Ordovician –Lower Carboniferous strata 2. Methods This study is based on material from the continuously cored Coburn 1 borehole, archived at the Geological Survey of Western Australia (GSWA). One hundred samples were collected for conventional petrography and cathodoluminescent microscopy from the Silurian section between 974 and 505 m. Thirty-four dolomite samples were analyzed for their d18OPDB and d13CPDB isotopic values at the Commonwealth Scientific and Industrial Research Organization (CSIRO) laboratory in Perth, Western Australia, using the method first outlined by Craig (1957). Samples were obtained from polished sample cuts using 1.0- and 0.1-mm-diameter rotary drills. A Vacuum Generator SIRA-9 was used for the isotopic analysis. Sixteen samples of different anhydrite types, including nodules, beds and cements, were analyzed for both d34SCDT and 87Sr/86Sr isotope ratios. The d34SCDT isotope values were obtained at the CSIRO laboratory of South Australia using the procedures of Holt and Engelkemeir (1970). The 87Sr/86Sr ratios were analyzed following the methods of Holser and Kaplan (1966) and Hart and Brooks (1976). Details of Fig. 1. Locality map of the Gascoyne Platform and adjacent divisions of the Southern Carnarvon Basin, Western Australia, with the location of wells mentioned in text. M. El-Tabakh et al. / Sedimentary Geology 164 (2004) 75–87 and the thin cover of marine Cretaceous sedimentary rocks. The Silurian shallow marine carbonates of the Dirk Hartog Group represent a relatively quiescent period between the more rapid deposition of Ordovician and Lower Devonian red-bed siliciclastics. Deposition of limestone and evaporitic shoaling-up cycles on the Gascoyne Platform took place throughout most of the Silurian, as indicated by conodont fauna (Mory et al., 1998), and possibly extended east into the Merlinleigh Sub-basin, which only became a separate entity in the mid-Carboniferous (Fig. 1). The platform contains up to 5000 m of Ordovician –Devonian limestone, evaporite and siliciclastic strata. On the platform, the record of sedimentation starts with Ordovician fluvial to coastal red beds up to 3500 m (Tumblagooda Sandstone, Fig. 2), the only Paleozoic unit to outcrop within the platform. During the Silurian, marine to restricted marine limestone and evaporites accumulated (Dirk Hartog Group, comprising the Ajana, Yaringa and Coburn formations, in ascending order; Gorter et al., 1994; Mory et al., 1998). Silurian paleogeographic reconstructions show reasonable agreement in the position of Australia indicating that the Carnarvon Basin lay between 10jS and 20jS during the Silurian, within an arid belt (Hocking et al., 1987, 1994). Following an earliest Devonian hiatus, shallow marine carbonates and fluvial sands (Faure Formation, Kopke Sandstone and Sweeney Mia Formation) were deposited in the early to mid-Devonian (Fig. 2). After another hiatus in the Middle Devonian, basal transgressive sandstone and shelf carbonates accumulated during the Givetian to Frasnian. In the mid-Carboniferous, a period of uplift attributed to the collision of Gondwana and Laurussia was followed by deposition of glacially influenced sediments (Lyons Group), mainly in the Merlinleigh and Byro Sub-basins to the east, whereas these strata are restricted to the north of the Gascoyne Platform. Subsequently, the platform was a relatively positive feature during most of the Permian to late Jurassic. Thin marine Cretaceous sedimentary rocks overlie the entire section. 77 the Gascoyne Platform, of which Coburn 1 (Yasin and Mory, 1999) is the only complete, fully cored section through the Silurian. Ages within the group, which spans the late Llandovery to latest Pridoli, are based on conodont faunas (Mory et al., 1998) from all three formations (Ajana, Yaringa and Coburn, in ascending order). The low-diversity conodont faunas, as well as rare brachiopod and ostracod fragments 4. Sedimentology and stratigraphy of the Dirk Hartog Group The wells shown in Fig. 1 were used to determine the lithostratigraphy of the lower Paleozoic strata on Fig. 2. Lithostratigraphy of the Lower Paleozoic sedimentary section, Gascoyne Platform Western Australia as shown by Coburn 1, the only fully cored Silurian section. 78 M. El-Tabakh et al. / Sedimentary Geology 164 (2004) 75–87 in the core, indicate restricted circulation of open marine waters across the platform throughout the Silurian. The lowermost unit, Ajana Formation, is 125 m thick in Coburn 1 (977 – 853 m), and conformably overlies the Tumblagooda Sandstone. The formation consists of gray sandy mudstone, laminated dolomudstone and minor wackestone. Sandstone beds show current and wave ripples and minor anhydrite nodules. Deposition of the Ajana Formation probably commenced in playa lakes, with occasional marine incursions (Yasin and Mory, 1999). The Yaringa Formation (853 – 784 m in Coburn 1), conformably above the Ajana Formation, consists of lime mudstone, sandstone, wackestone and minor grainstone. Oolitic grainstone beds, marked by erosional bases, characterize this formation. Evaporites are common, especially as isolated nodules within carbonate grainstone beds and as layers up to 30 cm thick. In many of the anhydrite layers, such nodules grade between vertically elongate and equidimensional shapes. These observations suggest precursor primary gypsum to the anhydrite. North of Coburn 1, this formation also contains thick beds of halite with minor anhydrite (Gorter et al., 1994), indicating that deposition most likely took place over a shallow marine platform, under an arid climate when the basin was partially isolated from the sea. The Coburn Formation (784 –571.5 m in Coburn 1) contains oolitic grainstone, wackestone, packstone and minor fine-grained sandstone beds. Overall, the formation coarsens—upwards, but displays depositional cycles 1 –20 m thick. Typically, the cycles have sharp bases marked by oolitic grainstone beds up to 25 cm thick, which grade upward into oolitic wackestone and/or packstone beds. Thin oolitic grainstone beds become thicker and more common, with increasing numbers of oolite beds towards the top of the formation. Minor ribbon carbonate beds are present in the section. The unit may disconformably overlie the Yaringa Formation as the Coburn Formation directly overlies the basal part of the Ajana Formation along the eastern margin of the Gascoyne Platform (Mory et al., 1998). Evaporites occur as isolated nodular anhydrite in a mixed carbonate and fine-grained siliciclastic matrix, massive beds of nodular mosaic anhydrite, and minor halite beds. Anhydrite is present as laminated, nodu- lar, and bedded forms within grainestone layers. Anhydrite nodules average 3 cm in diameter, and beds range from 2 to 10 cm thick. Secondary fibrous satin spar-filled veins are found throughout the upper hundred meters of the succession where anhydrite beds and nodules have been leached, thereby producing void spaces and dissolution collapse breccias in both carbonate and siliciclastic layers. Carbonate and evaporite lithologies and sedimentary structures within the Dirk Hartog Group indicate deposition in subtidal, peritidal and associated evaporitic mudflat environments. The ooid sand and supratidal evaporite facies are typical of shoaling— Table 1 Values of d18OPDB and d13CPDB for different types of dolomites of the Dirk Hartog Group in Coburn 1 Sample depth (m) Dolomite type 579 579 585 585 585 585 586 587 589 594 596 596 596 597 600 601 601 603 611 611 612 613 620 639 639 745 655 666 668 685 692 765 766 909 Replacement dolomite Coarse-filling dolomite Early-fine dolomite Early-fine dolomite Replacement dolomite Rim dolomite Rim dolomite Rim dolomite Rim dolomite Replacement dolomite Early-fine dolomite Replacement dolomite Replacement dolomite Replacement dolomite Coarse-filling dolomite Replacement dolomite Replacement dolomite Early-fine dolomite Early-fine dolomite Early-fine dolomite Rim dolomite Replacement dolomite Early-fine dolomite Early-fine dolomite Early-fine dolomite Coarse-filling dolomite Coarse-filling dolomite Coarse-filling dolomite Rim dolomite Replacement dolomite Replacement dolomite Replacement dolomite Rim dolomite Coarse-filling dolomite d18O 8.2 13.2 6.8 7.6 6.3 8.9 9.4 10.3 10.9 4.9 5.4 5.2 7.4 8.6 11.7 10.3 9.2 6.2 6.9 3.4 8.7 4.8 5.8 5.9 6.1 14.5 10.9 11.1 12.4 9.7 7.6 9.9 10.2 11.2 d13C 0.1 1.2 + 1.1 + 0.6 0.8 + 0.6 + 0.8 + 0.3 + 1.2 0.6 + 0.6 + 1.0 + 0.9 + 1.2 1.4 + 1.3 + 0.4 + 0.8 + 1.2 + 0.1 + 1.3 + 1.4 + 1.1 + 0.7 + 0.9 0.7 0.3 0.9 + 0.5 0.9 + 0.4 + 1.5 + 0.7 + 0.6 M. El-Tabakh et al. / Sedimentary Geology 164 (2004) 75–87 upwards, open marine to shallow ramp conditions. The paucity and low diversity of carbonate bioclasts imply adverse ecological conditions such as elevated salinity. Occasionally, the shallow ramp was drowned after deposition of evaporites, and the platform was covered by deeper, low-energy lime mudstone beds, conditions similar to those described by Read (1982, 1985). 5. Dolomite: petrology and geochemistry On the basis of petrographic characteristics, four types of dolomite have been identified in the Dirk Hartog Group: (a) early fine-crystalline dolomite, (b) replacement dolomite, (c) rim dolomite, and (d) voidfilling coarse dolomite. The isotopic values of 34 dolomite samples were analysed in terms of their d18OPDB and d13CPDB values to discriminate between these dolomites (Table 1; Fig. 3). 5.1. Early fine-crystalline dolomite This dark gray to red dolomite forms anhedral and fine-grained unimodal crystals, a few microns across in fine laminae mixed with dark residues of clayey material. The laminae are locally disturbed by the growth of anhydrite nodules, particularly in dark gray and reddish, lime-mudstone beds, which contain minor 79 mudcracks. Oolitic limestone layers are interbedded with the laminated dolomite layers and truncate the tops of laminated dolomite. Under cathodoluminescence, this dolomite is mottled, reddish, and lacks zonation. The eight samples of fine dolomite analyzed for d18OPDB and d13CPDB yielded d18OPDB values from 5.4xto 7.6x, and d13CPDB values from + 0.6xto + 1.2x(Fig. 3). 5.1.1. Discussion Laminated, finely crystalline dolomite layers interbedded with limestone and evaporite deposits commonly form from evaporitic brines in sabkha-type environments similar to the Quaternary sabkha in the Persian Gulf (e.g., Pursar, 1973; Chafetz et al., 1999; Chafetz and Rush, 1994). Such layers may develop penecontemporaneously within the carbonate platform or ramp from brine reflux related to the sinking of saline brine from sabkha mudflats. Evidence for this type of early diagenetic dolomite includes: (a) fine-grained anhedral crystals; (b) disrupted dolomite laminae associated with anhydrite nodules; and (c) laminations that are commonly reddish and contain mudcracks. The negative values of d18O PDB of this dolomite type, however, may reflect late interaction with basin fluids in the burial environment. Such penecontemporaneous dolomite is commonly recrystallized because it originally forms as metastable dolomite (Mazzullo, 2000). Fig. 3. Carbon/oxygen isotope plot of different types of dolomite. Values are displayed in standard delta notation relative to the PDB standard. 80 M. El-Tabakh et al. / Sedimentary Geology 164 (2004) 75–87 5.2. Replacement dolomite Crystals of replacement dolomite have a size range of 40– 150 Am, and are the most abundant type of dolomite in the Dirk Hartog Group. They are commonly present as mosaics of subhedral crystals that almost completely obliterate precursor limestone textures (Fig. 4), leaving only ghosts of the original allochems. In plain light, the replacement dolomite crystals have a cloudy appearance and straight to weak undulose extinction (classified as xenotopic by Gregg and Sibley, 1984). They are found in massive, interlocking crystal forms (Fig. 4A), and where replacement is incomplete, isolated dolomite crystals are found in the calcite matrix. Replacement dolomite is usually recognized where the fabric of original allochems, such as peloids and ooids, is not completely destroyed (Fig. 4B). In beds composed of finegrained siliciclastic grains interbedded with the limestone layers, dolomite masses replace original calcite cements and include some angular quartz grains (Fig. 4C). Generally, the dolomite crystals are nonferroan, and have dull to mottled orange luminescence. Replacement dolomite exhibits a wide range of d18OPDB values from 3.4xto 10.3x. Corresponding 0.1xto + 1.5x d13CPDB values range from (Fig. 3). 5.2.1. Discussion Replacement dolomite is pervasive throughout the platform succession and commonly forms massive crystals, which replace early fabrics. This dolomite type is produced by burial of limestone during a prolonged period of dissolution and reprecipitation, or dolomite neomorphism, in the burial environment, as invoked by Mountjoy and Amthor (1994) and by Machel and Hubscher (2000) as reflux dolomite that results from recrystallization that is governed by heterogeneities in mineralogy and permeability. In the Dirk Hartog Group, this interpretation seems valid, based on (a) the cloudy appearance of the dolomite crystals due to the presence of relics of original mineral inclusions, suggesting incomplete dolomitization of the precursor limestone; (b) brown to dark red to mottled orange luminescence with bright luminescence areas, which indicate compositional contrasts within the precursor limestone; (c) the presence of crystal mosaics showing a wide range of Fig. 4. Photomicrographs of replacement dolomite. (A) Mosaic of subhedral and replacement dolomite. Dolomite crystal size is variable within the same sample, suggesting recrystallization of dolomite from small to larger crystals. Scale bar is 0.5 mm. (B) Ooids that are entirely dolomitized and compacted. Scale bar is 0.5 mm. (C) Fine-grained crystalline dolomite within lime mudstone. Scale bar is 0.5 mm. crystal sizes; and (d) the wide range of d18OPDB values, representative of varied formative conditions. Replacement dolomite commonly reduces most of earlier porosity and permeability in the platform, M. El-Tabakh et al. / Sedimentary Geology 164 (2004) 75–87 81 and is produced by saturated fluids with a high brine/ evaporative reflux (Saller and Henderson, 2001; Lucia, 2002). type of dolomite analyzed show d18OPDB values ranging from 8.9xto 10.9xand d13CPDB values ranging from 0.9xto + 1.3x(Fig. 3). 5.3. Rim dolomite 5.3.1. Discussion Rim dolomite is a late phase, with euhedral crystals that typically grow into open pore spaces leached from within skeletal grains or earlier dolomite and calcite cements. Low d18OPDB values indicate that dolomite formed during burial, most likely at elevated temperatures or from fluids with a low d18OPDB value. Rim dolomite is volumetrically minor and forms continuous layers or coats of euhedral to subhedral dolomite crystals on former limestone grains, such as ooids and peloids (Fig. 5A), or as growths of individual dolomite crystals (Fig. 5B). These crystals are clear and free of inclusions, have a size range of 50– 100 Am, and are found in all types of void spaces where they may be associated with coarse anhydrite cements (Figs. 5A and 5C) or with coarse void-filling dolomite (Fig. 5D). Under cathodoluminescence, they have bright yellow rims and dark red interiors. The seven samples of this 5.4. Coarse void-filling dolomite Coarse void-filling dolomite is volumetrically important and forms medium to very coarse crystals that range from 40 to 150 Am. These dolomites are found Fig. 5. Photomicrographs of overgrowth and dolomitic void-fillings. (A) A continuous overgrowth of dolomite rime on an ooid (at arrow). (B) Euhedral dolomite crystals on ooid filling a void space (at large arrow). Scale bar is 0.5 mm. (C) Euhedral dolomite that grew from massive and fine-grained crystalline dolomite into a void, filled with anhydrite. Scale bar is 0.5 mm. (D) Dolomite entirely filling all space between ooids. Scale bar is 0.5 mm. 82 M. El-Tabakh et al. / Sedimentary Geology 164 (2004) 75–87 as clear void fillings and replacing earlier cements in voids between grains (indicated as cd in Fig. 5A and D). They predate sparry anhydrite cement. Under cathodoluminescence, the coarse dolomite crystals are zoned, with an early dull band near void walls and a bright luminescent rim in the middle. Towards the middle of such voids, the dolomite crystals exhibit zoned and dull luminescence. Isotopically, their d18OPDB values range from 10.9xto 13.2x 1.4xto + 0.6x and d13CPDB values range from (Fig. 3). 5.4.1. Discussion A late origin for this dolomite is evident because (a) it post-dates fine-grained and replacement dolomite in pore fillings; (b) it occurs in all types of pore and void spaces; (c) the size of crystals increases towards the center of the pore space suggesting growth of dolomite crystals into free space; and (d) samples exhibit wide variations in d18OPDB values and the most depleted values compared to other dolomite types 6. Sulfate: petrology and isotope geochemistry The origin and conditions of formation of the anhydrite and satin spar gypsum were deduced from their petrography, as well as d34SCDT values and 87 Sr/86Sr ratios (Table 2 and Fig. 6). Table 2 Values of d34S and 6.1. Petrology Anhydrite is present in the Dirk Hartog Group as beds, nodules, and cements. In beds and nodules, anhydrite forms fine-grained anhedral crystals as well as very fine-grained dense mosaics of elongate crystals up to 50 Am in length (Fig. 7A and B). Where limestone grains are partially dissolved and deformed, anhydrite is found as pervasive microcrystalline or coarse crystalline fillings (sparry) up to 2 mm across (Fig. 7B and C). Anhydrite cements are present as blocky laths and elongate fiber-like forms, displaying random orientations. These cements are found in intercrystalline, interparticle, and mouldic pores, are free of inclusions, and may include dolomite in poikilotopic masses. Sparry anhydrite replaces ooids and peloids and fills pores between dolomite crystals. The anhydrite cements also replace some dolomite crystals. Pore-filling anhydrite cement is by far the most prevalent cement type within the Dirk Hartog Group and includes blocky cement, found within interparticle and moldic pore spaces in ooids, and bladed cement in large and vuggy pore spaces. Pyrite is present as individual euhedral crystals, and clusters within anhydrite masses (Fig. 7D). Satin spar gypsum veins are common in the upper 100 m of the section, in the Lower Devonian Faure Formation. Veins average 5 cm in thickness and are composed of fibrous gypsum crystals perpendicular to bedding, on both sides 87 Sr/86Sr isotopic values for anhydrite and gypsum found in beds and cements from Coburn 1 Sample no. Core Depth (m) Description d34S 87 Sr/86Sr 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Coburn Coburn Coburn Coburn Coburn Coburn Coburn Coburn Coburn Coburn Coburn Coburn Coburn Coburn Coburn Coburn 966 964 934 801.2 752 700 655 572 552.4 540.5 521.8 571 572 751 802 822 Anhydrite nodule in red clastics Anhydrite nodule in red clastics Anhydrite nodule in red clastics Anhydrite nodule in grey lime mud Anhydrite nodule in grey lime mud Anhydrite nodule in grey lime mud Anhydrite nodule in grainstone Bedded anhydrite Satinspar Satinspar satinspar Anhydrite sparry cement Anhydrite sparry cement Anhydrite sparry cement Anhydrite sparry cement Anhydrite sparry cement 23.3 23.4 23.0 24.9 25.2 32.2 32.1 30.4 23.3 23.8 24.5 24.2 26.4 25.9 26.3 26.9 0.709321 0.709181 0.70921 0.708378 0.708367 0.708465 0.708403 0.708603 0.708682 0.708654 0.708942 0.708972 0.709551 0.709431 0.709392 0.708981 M. El-Tabakh et al. / Sedimentary Geology 164 (2004) 75–87 83 Fig. 6. A plot of d34S and 87Sr/86Sr for evaporites, satin spar and anhydrite cements from the Dirk Hartog Group. Anhydrite nodules and beds collected form red and gray carbonate and siliciclastic cements have characteristic d34S and 87Sr/86Sr isotopic compositions. Gypsum in satin spar veins and anhydrite in cements have intermediate d34S values between anhydrite in red and gray lithologies, suggesting that the source of their sulfate is from either types. The anhydrite cements, however, have 87Sr/86Sr ratios that are more radiogenic than the sedimentary anhydrite and than gypsum in the satin spar veins which suggest that influence of rock – water reactions involving the siliciclastics at elevated temperatures. of the central parting. Gypsum crystals usually display optical continuity. 6.2. d34S values and 87 Sr/86Sr ratios The d34SCDT values of the anhydrite samples analyses are between 23.3xand 32.2x(Table 2; Fig. 7) and vary according to the type of the anhydrite and its host sediment. Anhydrite samples (1– 3; Table 2) collected from nodules within red and fine-grained clastics of the Ajana Formation have an average d34SCDT value of 23.2xand an average 87Sr/86Sr ratio of 0.7092. Displacive nodular anhydrite samples (4 and 5; Table 2) from within gray lime mudstone have an average d34SCDT value of 24.6xand an average 87 Sr/86Sr ratio of 0.7084. Samples 6 – 8 from anhydrite nodules and beds within oolitic grainstone beds have an average d34S value of 31.5xand an average 87 Sr/86Sr ratio of 0.7088. Samples 9 – 11, selected from satin spar veins from the Faure Formation, have d34SCDT values averaging 23.8xand an average 87 Sr/86Sr ratio of 0.7088. Anhydrite sparry cement samples (12 – 16; Table 2) have an average d34S value of 25xand an average 87Sr/86Sr ratio of 0.7093. 6.3. Discussion The d34S values correspond well with the types of anhydrite identified. These include: anhydrite nodules and beds in both red limestone and siliciclastic lithologies (light sulfur isotopic values; samples 1– 3) and 84 M. El-Tabakh et al. / Sedimentary Geology 164 (2004) 75–87 Fig. 7. Photomicrographs of anhydrite in limestone. (A) Anhydrite (A) replacing ooids. Scale bar is 1 mm. (B and C) Sparry anhydrite filling space between ooids and replacing dolomite (D) and earlier cements. Scale bar is 0.5 mm. (D) Fine-grained crystalline anhydrite replacing all older cements and partially replacing ooids. The letter p indicates cubes of pyrite within the anhydrite (A). Scale bar is 1.2 mm. gray limestone layers (heavier sulfur isotopic values; samples 4 – 8; Fig. 7). The d34S values, however, are within the range of composition of the Silurian marine evaporites, determined by Holser and Kaplan (1966) and Claypool et al. (1980), as listed in Strauss (1997). The light sulfur isotopic values in samples 1 –5 are still within the marine values of the Silurian seawater. These values suggest that the sulfur was possibly derived from mixed marine sources such as organic sources or from sulfate-reducing bacteria, whereas the remaining samples (samples 6 – 8) reflect an origin within marine waters. The Sr isotopic ratios are used to finally determine the origin of sulfate in different evaporites. Carbonate 87Sr/86Sr ratios are reliable indicators for dolomite origins and for interpreting diagenetic fluid directions (Buschkuehle and Machel, 2002), where water mixing and rock –water interactions are deciphered. These geochemical approaches can be similarly extended to studies of calcium sulfate cements as described below. The 87Sr/86Sr isotopic ratios for anhydrite sampled from the red limestone and siliciclastic deposits (samples 1– 3) within the lowermost Silurian strata are more radiogenic than those for anhydrite in gray limestone layers in the overlying beds. Radiogenic 87 Sr/86Sr isotopic ratios for samples 1 –3 most likely reflect incorporation of strontium from clastic sources during deposition, and hence imply non-marine input of Sr into the restricted marine environment during deposition of the sulfate. Gypsum in satin spar veins (samples 9 – 11) and anhydrite in sparry cements (samples 12 – 16) have similar d34SCDT values to anhydrite in red and gray lithologies, suggesting that the original sulfate could be from either type. Gypsum samples 9 –11, however, were collected from satin spar veins in the lower Devonian Faure Formation. M. El-Tabakh et al. / Sedimentary Geology 164 (2004) 75–87 This formation lacks evaporites and calcium sulfate in the satin spar veins most likely were derived from dissolution and remobilization of the evaporites from the underlying Dirk Hartog Group during uplift and exhumation of the platform. The most significant source of diagenetic calcium sulfate in the Dirk Hartog Group is from the dissolution of original evaporites, probably during periods of non-deposition (in the earliest and mid-Devonian) and mostly during the uplift and exhumation of the platform (during most of the Triassic and Jurassic). The anhydrite cements have strontium that is more radiogenic than in the sedimentary anhydrite, suggesting the influence of rock –water reactions involving diagenetic fluids reacting with siliciclastics and possibly granitic basement. Anhydrite cements within carbonate of the Dirk Hartog Group indicate that the platform was exposed to fluids saturated with calcium sulfate. These late-stage brine movement precipitated anhydrite cement in most available voids and pore spaces. During the early Mesozoic, tectonic uplift of the platform allowed less saline water to react with evaporites and some of the carbonate grains to extensively alter the evaporites in the Dirk Hartog Group. These late diagenetic conditions released brines rich in calcium sulfate. The heavy brines most likely moved into deeper parts of the sedimentary section, forming individual crystals of anhydrite that commonly display interlocking and polygonal mosaic textures. These anhydrite cements filled pores between dolomite crystals and in part replaced some of the limestone layers. Coarse and sparry anhydrite commonly form under elevated burial temperatures, such as those described from the carbonates of the Nisku Formation by Machel (1986), or recrystallization of fine crystalline anhydrite in evaporitic sequences in the burial environment (Kasprzyk and Orti, 1998), or during hot fluid –rock reactions in evaporitic rocks (Spotel et al., 1998). 7. Dolomitization and anhydrite cements, mechanism and time of formation In modern environments, the problem of determining the source of dolomitizing fluids and the mechanism of dolomitization is not always straightforward (Chafetz et al., 1999). In ancient examples, this 85 problem is much more complex and interpretations commonly utilize both petrology and isotope chemistry. The most likely sources of magnesium for dolomitization of carbonates in the Dirk Hartog Group include (1) recrystallization of earlier dolomite and magnesium derived from evaporative brines, and (2) magnesium released by compaction in the burial environment. Progressive burial and accumulation of marine sediments across the platform continued into the early – middle Devonian. The early dolomitization included fine crystalline and laminated dolomite, suggesting a reflux stage from evaporative brines, most likely in a sabkha environment. This early diagenetic dolomite exhibits negative d18OPDB values, which suggests interaction with hot fluids during burial. With burial and accumulation of Devonian marine sediments (Kopke and Sweeny Mia formations), connate seawater released from sediment across the platform caused widespread dolomitization of limestone. The late burial processes formed different types of dolomite, and facilitated replacement and cementation of carbonate grains by dolomite. The wide range of d18O isotopic values in the different types of late dolomite suggests re-crystallization and formation of the dolomite from basin brines during burial. Such successive and complex diagenetic interactions of dolomite with waters are commonly generated during compaction of limestone (Vahrenkamp and Stewart, 1994) and cause progressive recrystallization of dolomite even at early stages of diagenesis (Mountjoy and Amthor, 1994). Overall, the porosity and permeability of the Dirk Hartog Group is fairly low, but reaches up to 26%, and 619 md (Yasin and Mory, 1999). These measurements indicate that fluids could have easily moved through the rocks, and that diagenesis has probably enhanced porosity and permeability. The process of dolomitization can create high porosity in carbonate rocks that can facilitate pathways for later diagenetic fluids. The Gascoyne Platform was a positive feature from the Permian to the Late Jurassic due to the uplift and exhumation during the collision of Gondwana and Laurussia, when about 1000– 1500 m of section was eroded (Iasky et al., 1998). The magnitude of this exposure is manifested in the dissolution of carbonate components in ooids and early cements and the 86 M. El-Tabakh et al. / Sedimentary Geology 164 (2004) 75–87 creation of oolitic and intercrystalline porosity. Exhumation resulted in dissolution of near-surface evaporites, and the formation of satin spar veins in this section. In deeper parts of the basin, sulfate was precipitated from descending high-density brines to form anhydrite cement in the pores of the dolomitized carbonate. These sulfate-rich brines most likely moved through pores created by dolomitization, in addition to pores produced by dissolution of evaporites. Tectonics, uplift and generation of fractures and joints in the succession further facilitated the movement of late diagenetic fluids. Sparry anhydrite diagenetic cements post-date dolomitization and are most common in oolitic grainstone facies. Such cements are widely distributed in all types of pores, post-date void-filling dolomite of burial origin, and are free of inclusions except where dolomite crystals are present. These late anhydrite cements locally replace dolomite and are considered the last major diagenetic event in the platform succession. Dissolution of anhydrite beds and nodules is probably the main source of late anhydrite cement in the carbonates. However, the 87Sr/86Sr ratios in the anhydrite cements are more radiogenic than the anhydrite in the beds and nodules, suggesting input of radiogenic strontium during reactions of sulfate-rich fluids with the basin siliciclastics or granitic basement. In summary, despite some cases of extensive dolomitization, dolomite formation is commonly linked to formation of saline fluids formed from dissolution of evaporites (Meyers et al., 1997), but this link is not obvious in the case of the Dirk Hartog dolomites and evaporites. Based on petrographic and isotopic data, we suggest that dolomitization and recrystallization of dolomite commenced during early diagenesis and continued in the burial environment. The formation of massive anhydrite cements, however, post-dates the formation of dolomites—a widespread phenomenon dependent on the availability of seawater or chemically modified seawater (Mountjoy et al., 1999). The major evaporite dissolution is inferred to coincide with tectonic uplift and exhumation of the platform, and subsequent movement of sulfate-rich brines into deeper and warmer parts of the platform. These cements filled pore spaces within dolomitized layers, as well as pores created during the dissolution of carbonate grains and some of the earlier calcite cements. Acknowledgements We sincerely thank JM Gregg (University of Missouri-Rolla) for his comments, which greatly improved the manuscript. Sedimentary Geology Editor Bruce Sellwood is thanked for his constructive reviews. This research was conducted while ME was an ARC (Australian Research Council) research fellow at Curtin University of Australia. AJM publishes with the permission of the Director of the Geological Survey of Western Australia. References Buschkuehle, B.E., Machel, H.G., 2002. Diagenesis and paleofluid flow in the Devonian Southesk – Cairn carbonate complex in Alberta, Canada. Mar. Pet. Geol. 19, 219 – 227. Chafetz, H.S., Rush, P.F., 1994. Diagenetically altered sabkha-type Pleistocene dolomite from the Arabian Gulf. Sedimentology 41, 409 – 421. Chafetz, H.S., Alicia, A., Imerito-Tetzlaff, A., Zhang, J., 1999. Stable-isotope and elemental trends in Pleistocene sabkha dolomites: descending meteoric water vs. sulfate reduction. J. Sediment. Res. 69, 256 – 266. Claypool, G.E., Holser, W.T., Kaplan, I.R., Sakai, H., Zak, I., 1980. The age curves of sulphur and oxygen isotopes in marine sulfate and their mutual interpretation. Chem. Geol. 28, 199 – 260. Craig, H., 1957. Isotopic standards for carbon and oxygen correction factors for mass spectrometric analysis of carbon dioxide. Geochim. Cosmochim. Acta 12, 133 – 149. El-Tabakh, M., Schreiber, B.C., Warren, J.K., 1998. Origin of fibrous gypsum in the Newark rift basin, Eastern North America. J. Sediment. Res. 68, 88 – 99. Feng, H.Lu, Meyers, W.J., 1998. Massive dolomitization of a late Miocene carbonate platform: a case of mixed evaporative brines with meteoric water, Nijar, Spain. Sedimentology 45, 263 – 277. Gao, G., Land, L.S., 1991. Early Ordovician cook creek dolomite, Middle Arbuckle Group, Slick Hills, SW Oklahoma, USA: origin and modification. J. Sediment. Petrol. 61, 161 – 173. Ghori, K.A.R., 1999. Silurian – Devonian petroleum source – rock potential and thermal history, Carnarvon Basin. Report, vol. 72. Geological Survey of Western Australia, Western Australia. 88 pp. Gill, I., Moore, C., Aharon, P., 1995. Evaporitic mixed-water dolomitization on St. Croix, U.S.V.I. J. Sediment. Res., A 65, 591 – 604. Gorter, J.D., Nicoll, R.S., Foster, C.N., 1994. Lower paleozoic facies in the Carnarvon Basin, Western Australia: stratigraphy and hydrocarbon prospectivity. In: Purcell, P.G., Purcell, R.R. (Eds.), Proceedings of Petroleum Exploration Society of Australia, Perth. The Sedimentary Basins of Western Australia, vol. 2, pp. 373 – 396. Gregg, J.M., Sibley, D.F., 1984. Epidiagenetic dolomitization and M. El-Tabakh et al. / Sedimentary Geology 164 (2004) 75–87 the origin of xenotopic dolomite texture. J. Sediment. Petrol. 54, 908 – 931. Gregg, J.M., Shelton, K.L., Johnson, A.W., Somerville, I.S., Wright, W.R., 2001. Dolomitization of the Waulsortian limestone (lower carboniferous) in the Irish midlands. Sedimentology 48, 745 – 766. Hart, S.R., Brooks, C., 1976. Clinopyroxene – matrix partioning of K, Rb, Cs, Sr, and Ba. Geochim. Cosmochim. Acta 38, 179 – 1806. Hocking, R.M., 1988. Regional geology of the Northern Carnarvon Basin. In: Purcell, P.G. (Ed.), The North West Shelf, Australia, Proceedings of Petroleum Exploration Society of Australia Symposium. Perth, Western Australia, pp. 97 – 114. Hocking, R.M., Moors, H.T., van De Graaff, J.E., 1987. Geology of the Carnarvon Basin, Western Australia. Geological Survey of Western Australia Bulletin. Perth, Western Australia 133 (289 p.). Hocking, R.M., Mory, A.J., Williams, I.R., 1994. An atlas of Neoproterozoic and Phanerozoic basins of Western Australia. In: Purcell, P.G., Purcell, P.R. (Eds.), The Sedimentary Basins of Western Australia, Proceedings of Petroleum Exploration Society of Australia Symposium. Perth, Western Australia, pp. 21 – 43. Holser, W.T., Kaplan, I.R., 1966. Isotope geochemistry of sedimentary sulfates. Chem. Geol., 93 – 135. Holt, B.D., Engelkemeir, A.G., 1970. Thermal decomposition of barium sulfate to sulphur dioxide for mass spectrometric analysis. Anal. Chem. 42, 1451 – 1453. Iasky, R.P., Mory, A.J., Shevchenko, S., 1998. A structural interpretation of the Gascoyne Platform, Southern Carnarvon Basin, W.A.. In: Purcell, P.G., Purcell, R.R. (Eds.), Petroleum Exploration Society of Australia Symposium, Perth, W.A. The Sedimentary Basins of Western Australia, vol. 2. Perth, Western Australia, pp. 589 – 598. Kasprzyk, A., Orti, F., 1998. Palaeogeographic and burial controls on anhydrite genesis: the Badenian basin in the Carpathian Foredeep (Southern Poland, Western Ukraine). Sedimentology 45, 889 – 907. Lucia, F.J., 2002. Origin and petrophysics of dolostone pore space. In: Rizzi, G., Darke, G., Braithwaite, C. (Eds.), The Geometry and Petrogenesis of Dolomite Hydrocarbon Reservoirs, Final Programme and Abstracts. Geological Society Petroleum Group, Burlington House-Picadilly, London, 3 – 4th December. (1 p.). Lumsden, D.N., Caudle, G.C., 2001. Origin of massive dolostone: the Upper Knox model. J. Sediment. Res. 71, 400 – 409. Machel, H.-G., 1986. Early lithification, dolomitization, and anhydritization of Upper Devonian Nisku buildups, subsurface of Alberta, Canada, in Reef Diagenesis. In: Schroede, J.H., Purser, B.H. (Eds.), Springer-Verlag, New York, pp. 336 – 356. Machel, H.-G., Hubscher, H., 2000. The Devonian Grosmont heavy oil reservoir in Alberta, Canada. Zentralblatt fur Geologie und Palaontologie, Teil I, Heft 1/2, 55 – 84. Mazzullo, S.J., 2000. Organogenic dolomitization in peritidal to deepsea sediments. J. Sediment. Res. 70, 10 – 23. Melim, L.A., Scholle, P.A., 2002. Dolomitization of the Capitan Formation forereef facies (Permian, West Texas and New Mexico): seepage reflux revisited. Sedimentology 49, 1207 – 1227. 87 Meyers, W.J., Lu, F.H., Zackariah, J., 1997. Dolomitization by mixed evaporative brines and freshwater, late Miocene carbonates, Nijar, Spain. J. Sediment. Petrol. 67, 898 – 912. Mory, A.J., Nicoll, R.S., Gorter, J.D., 1998. Lower Palaeozoic correlations and maturity, Carnarvon Basin, WA. In: Purcell, P.G., Purcell, R.R. (Eds.), Proceedings of Petroleum Exploration Society of Australia, PerthThe Sedimentary Basins of Western Australia, vol. 2, pp. 599 – 622. Mountjoy, E.W., Amthor, J.E., 1994. Has burial dolomitization come of age? Some answers from the Western Canadian Sedimentary Basin. In: Purser, B., Tucker, M., Zenger, D. (Eds.), Dolomites. A Volume in Honour of Dolomieu. International Association of Sedimentologists, Special Publication, vol. 21, pp. 203 – 229. Mountjoy, E.W., Macheal, H.G., Green, D., Duggan, J., WilliamsJones, A.E., 1999. Devonian matrix dolomites and deep burial carbonate cements: a comparison between the Brimbey-Meadowbrook reef trend and the deep basin of west-central Alberta. Bull. Can. Pet. Geol. 47, 487 – 509. Pursar, B.H., 1973. The Persian Gulf: Holocene Carbonate Sedimentation and Diagenesis in a Shallow Epicontinental Sea Springer-Verlag, Berlin, p. 471. Qing, H., Bosence, D.W.J., Rose, P.F., 2001. Dolomitization by penesaline seawater in early Jurassic peritidal platform carbonates, Gibraltar, western Mediterranian. Sedimentology 48, 153 – 163. Read, J.F., 1982. Carbonate platforms of passive (extensional) continental margins: types, characteristics and evolution. Tectonophysics 81, 195 – 212. Read, J.F., 1985. Carbonate platform facies models. Bull. Am. Assoc. Pet. Geol. 66, 860 – 878. Saller, A.H., Henderson, N., 2001. Distribution of porosity and permeability in platform dolomites: insight from the Permian of west Texas: reply. American Association of Petroleum Geologists 17, 249 – 252. Sibley, D., Gregg, J.M., 1987. Classification of dolomite rock textures. J. Sediment. Petrol. 57, 967 – 975. Spotel, C., Longstaffe, F.J., Ramseyer, K., Kunk, M.J., Wiesheu, R., 1998. Fluid-rock reactions in an evaporitic melange, Permian Haselgebirge, Austrian Alps. Sedimentology 45, 1019 – 1044. Strauss, H., 1997. The isotopic composition of sedimentary sulfur through time. Palaeogeogr. Palaeoclimatol. Palaeoecol. 132, 97 – 118. Vahrenkamp, V.C., Stewart, P.K., 1994. Late Cenozoic dolomites of the Bahamas: metastable analogues for the genesis of ancient platform dolomite. In: Purser, B., Tucker, M., Zenger, D. (Eds.), Dolomites. A Volume in Honour of Dolomieu. International Association of Sedimentologists, Special Publication, vol. 21, pp. 133 – 153. Wilson, A.M., Sanford, W.E., Whitaker, F.F., Smart, P.L., 2001. Spatial patterns of diagenesis during geothermal circulation in carbonate platforms. Am. J. Sci. 301, 727 – 752. Yasin, A.R., Mory, A.J., 1999. Coburn 1 well completion report, Gascoyne Platform, Southern Carnarvon Basin, Western Australia. Geological Survey of Western Australia Record 1999/5, 99 pp.