Bulldog Shale Research Articles

Systematic reassessment of Edaphodon eyrensis Long, 1985 (Holocephali, Chimaeroidei) from the Early Cretaceous of South Australia

ABSTRACT A chimaeroid species, Edaphodon eyrensis Long, 1985 (Holocephali, Chimaeroidei), from the Lower Cretaceous Bulldog Shale of the Eromanga Basin, South Australia, is reassessed as Ptyktoptychion eyrensis (Long, 1985), comb. nov. This is the oldest representative of the endemic Australian chimaeroid genus Ptyktoptychion Lees, 1986. An ancestor of this genus could be the Early Cretaceous chimaeroid Ischyodus thurmanni Pictet and Campiche, 1858 from the northern hemisphere. Ptyktoptychion eyrensis survived in Australia in southern polar environment conditions.

Diverse provenance of the Lower Cretaceous sediments of the Eromanga Basin, South Australia: constraints on basin evolution

This study presents new geochemical and mineralogical data collected on 13 drill holes across the South Australian part of the Eromanga Basin, with a focus on the Lower Cretaceous Cadna-owie Formation, Bulldog Shale, Coorikiana Sandstone and Oodnadatta Formation. Mineralogical and geochemical data are used to determine the provenance of the sediments along with the paleoenvironments at the time of deposition. The dominant clay species across the formations are kaolinite and montmorillonite, although each drill hole shows individual mineralogical assemblages. The Cadna-owie Formation is mainly composed of quartz and kaolinite; montmorillonite is the dominant clay species in the other formations. Differences in major-element chemistry are best shown on the K–Na–Ca ternary diagram and display a strong relationship with mineralogy. Trace and rare earth elements are preserved at similar average concentrations between formations and are depleted relative to the upper continental crust. The signature of the sediments is interpreted to be derived from a mix of felsic and mafic igneous rocks with almost no sedimentary recycling. Sources of the sediments are likely to be from exposed basement packages surrounding the study area such as the Gawler Range Volcanics, the Musgrave Province, the Benagerie Volcanic Suite, the Mount Painter Inlier and the Arunta Block. The variations in dominant clay species between the formations could have been driven by an increase in temperature and a decrease in rainfall during the Early Cretaceous along with seasonal variations, and/or by the input in volcanogenic sediments from the active volcanoes located along the eastern margin of Australia. KEY POINTS Lower Cretaceous formations in the South Australian Eromanga Basin show limited regional lateral and vertical geochemical and mineralogical variations. Sediments are a mix of mafic through felsic igneous rocks derived from basement terranes exposed in the Early Cretaceous. The paleoclimate during deposition evolved from cold and humid to warm and arid.

Mineralogy as a proxy to characterise geochemical dispersion processes: A study from the Eromanga Basin over the Prominent Hill IOCG deposit, South Australia

The Early Cretaceous interbedded sandstone, siltstone and mudstone of the Cadna-owie Formation and the reduced mudstone of the Bulldog Shale overlying the Prominent Hill IOCG deposit in South Australia (total tonnage: 140 Mt. @ 1.2% Cu and 0.5 g/t Au; OZ Minerals Limited 2017) are shown to preserve Cu concentrations up to 5043 ppm Cu. These cover sequence materials are approximately 100 m thick over the basement rocks that host the deposit. This study investigates the source of elevated Cu in the cover material by assessing the Cu deportment in these formations and focuses on understanding element migration processes resulting in development of elevated Cu concentrations. Petrographic and QEMSCAN® analysis show that Cu resides within sulphides including high-temperature Cu-sulphides (e.g. bornite) in the Cadna-owie Formation. The Cu-sulphides are associated with Fe-oxides (magnetite) and minerals including pyroxene, amphibole, garnet, chlorite and plagioclase that are interpreted to be detrital and are likely to be of metamorphic origin. Based on these observations, lateral physical processes driven by sedimentary dynamics are interpreted to be the main processes responsible for elevated Cu concentrations in the Cadna-owie Formation in the Prominent Hill area. Chemical migration processes likely had only localised influence and resulted in minor Cu redistribution. QEMSCAN® data collected on samples from the weathering profile of the Bulldog Shale shows that weathering processes (e.g. pyrite oxidation, serpentinisation and feldspar hydrolysis) mostly affect the upper Bulldog Shale and that Eh-pH conditions gradually change with depth from the oxidised and acidic upper Bulldog Shale to the reduced and slightly acidic lower Bulldog Shale. Copper speciation and Eh-pH conditions are used to interpret downward chemical migration of Cu due to weathering as the main process for development of elevated Cu concentrations within the weathered Bulldog Shale in the Prominent Hill area. Upward chemical and lateral physical migration processes may have influenced Cu concentrations within the unweathered Bulldog Shale, however the degree of influence is yet to be determined. Overall this study shows that elevated Cu concentrations in the Early Cretaceous cover sequences overlying the Prominent Hill deposit can be linked back to mineralisation itself and that processes resulting in elevated Cu concentrations include chemical and physical migration.

Evaluation of cover sequence geochemical exploration sample media through assessment of element migration processes

Cover sequence materials of the Bulldog Shale and underlying Cadna-owie Formation within the Eromanga Basin (South Australia) that overlie the Prominent Hill iron oxide-copper-gold (IOCG) deposit are evaluated for their potential to host a geochemical expression of the underlying mineral system and thereby as geochemical exploration sampling media. The Bulldog Shale is clay-rich and has elevated concentrations of trace elements, however, its major and trace element (Cu, Mo, Ni, Pb, U, Zn and REE) chemical characteristics are significantly influenced by intense surface weathering to a depth of ∼50 m from the landsurface. The Cadna-owie Formation is sand-dominated with lesser clay-rich horizons and has elevated concentrations of multiple trace elements (Cu, U, Mo, Ni and REE). A positive correlation between trace and Al2O3 suggests these elements are concentrated within clay-rich horizons. A comparison with background element concentrations within the Cadna-owie Formation in the broader Eromanga Basin, however, shows introduction of additional elements via upward element migration from the underlying mineralised basement rocks and/or downward element migration from the Bulldog Shale. Once weathering influences and natural background concentrations are accounted for, elevated Cu concentrations in the Bulldog Shale and trace elements in the Cadna-owie Formation appear to be spatially associated with the Prominent Hill deposit and surrounding prospects and can then be useful sampling media for mineral exploration in the Prominent Hill region.

Groundwater flow underneath mound spring tufas from geophysical surveys in the southwestern Great Artesian Basin, Australia

ABSTRACTArtesian springs, which form carbonate mounds at the surface, occur in groups along the southwestern edge of the Great Artesian Basin (GAB), in northern South Australia. Their underground structure and relationship to faulting are not well understood. This study investigated four mound spring groups (Beresford Spring, Warburton Spring, the Bubbler Spring group, and Freeling Springs), which have different geological settings, using a range of geophysical techniques: self-potential (SP), magnetotellurics (MT) and time-domain electromagnetics (TEM). The results confirmed that despite generally similar mound morphologies at the surface, spring vents in different groups have different mechanisms for sourcing water from the GAB aquifer. SP measurements effectively showed where most of the vertical flow occurs at all groups, while MT and TEM models identified the location of faults, other geological structures and the thickness of the Bulldog Shale aquitard. Beresford and Warburton springs are similar in that the spring locations at both are directly related to points on a regional-scale fault, which appears to provide a low permeability pathway to the surface. At the Bubbler Springs group, a regionally aligned fault identified in MT data seems to be bounding an area where the aquifer is much closer to the surface (and the overlying aquitard has been thinned) where the spring abundance is highest. SP responses indicate that shallow flow paths feeding individual vents are unrelated to the fault. At Freeling Springs, the SP suggests that groundwater is flowing up within shallow aquifers, and that spring location is offset from this upward flow, instead located on range-front faults that provide the final conduit to the surface.

Electrical geophysics of carbonate mound spring complexes of the South- Western Great Artesian Basin

SUMMARY Artesian mound springs occur along the south-western edge of the Great Artesian Basin, in northern South Australia, but their underground structure and relationship to faulting is not well understood. We have performed geophysical surveys over three different systems using a range of techniques: early-time TEM, self-potential, and magnetotellurics. The self-potential data contains a local response due to specific spring vents, and also a broader stronger response due to laterally extensive upwelling in the lower part of the Bulldog Shale, at depths of approximately 100 m. Modelling of TEM and magnetotelluric data show that the confining Bulldog Shale, which is generally very conductive, contains resistive areas underneath springs and spring complexes which are believed to be related to spring-related carbonate deposition. Magnetotelluric modelling in particular indicates that anisotropic resistivity in the form of vertical sheets at a depth of 100 to 200 m, can explain the observations more readily than a conductive 2D feature, suggesting that the structures underlying the springs are sets of closely-spaced faults. The orientation of this anisotropy matches the regional NNW/SSE orientation of spring complexes.

Analysis of subsurface mound spring connectivity in shale of the western margin of the Great Artesian Basin, South Australia

Mound springs provide the primary discharge mechanism for waters of the western margin of the Great Artesian Basin (GAB), Australia. Though these springs are an important resource in an arid environment, their hydraulics as they discharge from shale are poorly defined. The springs can include extensive spring tails (groundwater-dependent wetlands) and hundreds of springs in a given spring complex. Electrical resistivity imaging (ERI) was used to evaluate spring subsurface hydraulic-connectivity characteristics at three spring complexes discharging through the Bulldog Shale. The results demonstrate that fresher GAB water appears as resistors in the subsurface at these sites, which are characterized by high-salinity conditions in the shallow subsurface. Using an empirical method developed for this work, the ERI data indicate that the spring complexes have multiple subsurface connections that are not always easily observed at the surface. The connections are focused along structural deformation in the shale allowing fluids to migrate through the confining unit. The ERI data suggest the carbonate deposits that the springs generate are deposited on top of the confining unit, not precipitated in the conduit. The data also suggest that spring-tail ecosystems are not the result of a single discharge point, but include secondary discharge points along the tail.

Barremian–Cenomanian palaeotemperatures for Australian seas based on new oxygen-isotope data from belemnite rostra

Oxygen‐isotope ratio (δ18O) data are presented for 55 belemnite rostra specimens of late Aptian–Albian age from the Great Artesian Basin of eastern Australia and of late Barremian–early Cenomanian age from the Carnarvon Basin of Western Australia. The belemnites were carefully screened for diagenesis by means of catholuminescence and trace element geochemistry. The δ18O record for the Carnarvon Basin (palaeolatitude ~40°S) is interpreted to show a long‐term warming trend from ~10°C in the Aptian to a maximum of ~15°C in the Cenomanian. Shorter-term fluctuations in the mid-Albian are also apparent. Data from the Great Artesian Basin (palaeolatitude 40°–55°S) show a wider δ18O range which is attributed to the epeiric sea context of this sample set. The larger range in δ18O is interpreted to reflect differing water mass characteristics, due possibly to a combination of variable freshwater dilution, seasonal temperature fluctuations and isotopic exchange between seawater and pyroclastic sediment within the epeiric sea. Nevertheless, the Aptian–Albian δ18O trends are still comparable to those obtained for the Carnarvon Basin. The inferred palaeotemperature trends for the Australian sedimentary basins are consistent with records from other regions and demonstrate that broadly synchronous palaeotemperature variations can be recognised on a global scale during the Cretaceous. Although our data do indeed show cool temperatures within the Aptian (and the Albian) reaching a low of ~10°C, these estimates are somewhat warmer than those based on glendonite genesis. However, the exact position of the glendonite-bearing and dropstone-rich parts of the Bulldog Shale is poorly constrained with respect to our isotope data, and the warmest temperatures were of late Albian and early Cenomanian age. However, to conform to greenhouse warmth and equability, and to achieve parity with other palaeotemperature records, the δ18Oseawater estimate of −1.0‰ that is generally used for Cretaceous oceans in an ice-free world needs to be adjusted to a slightly higher value.

Opalized archosaur remains from the Bulldog Shale (Aptian: Lower Cretaceous) of South Australia

Terrestrial reptile remains are very rare in the Lower Cretaceous of South Australia, but include the holotype of the small theropod Kakuru. Here, we review this taxon and other archosaur specimens collected from the Bulldog Shale (Aptian) of Andamooka and Coober Pedy. Kakuru possesses no unique characters or character state combinations and is regarded as a nomen dubium, representing an indeterminate tetanuran theropod. Two other specimens (a left metatarsal and astragalus) can be referred to Dinosauria, but the identity of several other specimens (phalanges and a centrum) can only be resolved to the level of an indeterminate archosaur.

Luminescence dating of spring mound deposits in the southwestern Great Artesian Basin, northern South Australia

Artesian spring mound deposits in the southwestern part of the Great Artesian Basin arise from groundwater discharge from flowing springs in the southern and western margins which reaches the surface through faults and weaknesses in thin confining beds overlying the Mesozoic artesian aquifers. Carbonate in solution in the artesian groundwater is deposited by many springs as tufa, building ‘spring mounds.’ Active flowing and dry inactive spring mounds occur in a variety of sizes and shapes. The ages of a representative selection of spring mound deposits have been found by luminescence dating of quartz sand grains that have been incorporated in the mound deposits. The spring deposits of the active flowing springs, Big Bubbler, Blanche Cup and Beresford Spring have ages of 15.1 ± 2.2, 10.9 ± 1.5 and 13.9 ± 1.0 ka, respectively. Spring complexes with both active flowing and dry extinct mounds, Strangways and Beresford Springs, have ages of 60 ± 8 and 219 ± 35 ka, respectively. Spring deposits of springs which ceased flowing and are overlying pedestals of Cretaceous Bulldog Shale and of substantial height (up to >45 m above the surrounding plain) of Beresford Hill and Kewson Hill gave ages of 128 ± 33 ka and 400 ± 100 ka, respectively. Elizabeth Springs is a large spring mound, and parts contain active flowing springs; a sample from a dry inactive part gave an age of 740 ± 120 ka. The analyses for radioactive content, necessary for the estimation of dose rates for luminescence dating, were unusual because the uranium radioactive decay chain is in disequilibrium, with the parent uranium patently unable to support the lower part of the chain. We interpret this as being due to radium being carried up with the spring water from the confined Mesozoic aquifer, which overlies Proterozoic basement rocks. Calculations of dose rates take account of detailed measurements on individual nuclides and differ from site to site.

A JUVENILE PLIOSAUROID PLESIOSAUR (REPTILIA: SAUROPTERYGIA) FROM THE LOWER CRETACEOUS OF SOUTH AUSTRALIA

A partial skeleton tentatively referred to the Lower Cretaceous pliosauroid genus Leptocleidus is described from predominantly Aptian (Lower Cretaceous) opal-bearing deposits of the Bulldog Shale near Andamooka in South Australia. Small size coupled with incomplete fusion of the basicranial elements, vertebral centra, neural arches and cervical ribs, and incomplete ossification of the articular surfaces on the propodials and distal limb bones, indicate that the specimen was at an early stage in its ontogeny. Comparison with more mature specimens attributed to Leptocleidus spp. highlights several growth-related differences, particularly a marked disparity in proportions of the humerus and femur relative to the estimated maximum body length. Changes in the cranial skeleton during growth are harder to document because of the fragmentary nature of the skull. Nevertheless, fusion of some cranial sutures and well-developed cranial joint surfaces suggest that extensive ossification may have taken place in parts of the skull during early ontogeny. The potential implications of these findings for feeding, locomotion, and behavior in juvenile plesiosaurs are discussed.

Chlorine stable isotope studies of old groundwater, southwestern Great Artesian Basin, Australia

Stable Cl isotope ratios ( 37Cl/ 35Cl) were measured in groundwater samples from the southwestern flow system of the Great Artesian Basin, Australia to gain a better understanding of the Cl − sources and transport mechanisms. δ 37Cl values range from 0‰ to −2.5‰ (SMOC), and are inversely correlated with Cl − concentration along the inferred flow direction. The Cl isotopic compositions, in conjunction with other geochemical parameters, suggest that Cl − in groundwaters is not derived from salt dissolution. Mixing of the recharge water with saline groundwater cannot explain the relationship between δ 37Cl and Cl − concentration measured. Marine aerosols deposited via rainfall and subsequent evapotranspiration appear to be responsible for the Cl − concentrations observed in wells that are close to the recharge area, and in groundwaters sampled along the southern transect. δ 37Cl values measured in the leachate of the Bulldog shale suggest that the aquitard is the subsurface source of Cl − for the majority of groundwater samples studied. Diffusion is likely the mechanism through which Cl − is transported from the pore water of the Bulldog shale to the aquifer. However, a more detailed study of the aquitard rocks is required to verify this hypothesis.

Marine reptiles from the Lower Cretaceous of South Australia: elements of a high‐latitude cold‐water assemblage

Abstract: The Lower Cretaceous rocks of South Australia have yielded a diverse marine reptile assemblage of up to five families of plesiosaur (including a new cryptoclidid or cimoliasaurid, indeterminate elasmosaurids, a possible polycotylid, rhomaleosaurids, and pliosaurid) and one family of ichthyosaur (ophthalmosaurid). Other common associated vertebrates include chimaerids and osteichthyans. Sharks, dipnoans and dinosaurs are uncommon and marine turtles are notably absent. The main fossil‐producing strata belong to the Lower Aptian–Lower Albian Bulldog Shale although the Upper Albian Oodnadatta Formation has produced isolated elements. Both these units comprise finely laminated shaly mudstones and claystones deposited in a transgressive shallow coastal, epicontinental marine environment. Estimates of palaeolatitude place South Australia between 60° and 70°S, in the late Early Cretaceous. Sedimentary structures (including lonestone boulders and glendonites), fossils, isotope data and climatic modelling also indicate that seasonally cool–cold conditions (possibly with winter freezing) prevailed during deposition of the Bulldog Shale. This contrasts markedly with climate regimes typically tolerated by modern aquatic reptiles but suggests that some of the South Australian Mesozoic taxa may have possessed adaptations (including elevated metabolic levels and/or annual migration) to cope with low temperatures. A high proportion of juvenile plesiosaur remains in the Bulldog Shale might also indicate that nutrient‐rich cold‐water coastal habitats functioned as both ‘safe calving grounds’ and refuges for young animals prior to their entering the open sea as adults. The occurrence of plesiosaurs and ichthyosaurs in the high‐latitude Lower Cretaceous of southern Australia, along with plesiosaurs and mosasaurs in the Upper Cretaceous of South America, Antarctica, New Zealand and the Chatham Islands, demonstrates that Mesozoic marine reptiles utilized southern high‐latitude environments over a considerable period of time, and that these records do not represent casual occupation by isolated taxa.

Glendonites as a paleoenvironmental tool: implications for early Cretaceous high latitude climates in Australia

Glendonites, calcite pseudomorphs after the metastable mineral ikaite (CaCO 3 · 6H 2O), occur in the Late Aptian interval of the Bulldog Shale in the Eromanga Basin, Australia and in other Early Cretaceous basins at high paleolatitudes. Ikaite precipitation in the marine environment requires near-freezing temperatures (not higher than 4°C), high alkalinity, increased levels of orthophosphate, and high P CO2. The rapid and complete transformation of ikaite to calcite at temperatures between 5 and 8°C provides an upper limit on the oxygen isotopic composition of the pore waters: −2.6 <δ w <−3.4‰SMOW. If it is assumed that these pore waters are representative of the shallow Eromanga Basin, the calculated δ w can be used to reassess belemnite fossil oxygen isotopic paleotemperatures—temperature recorded by fauna living in the basin at the time of ikaite precipitation. Data previously reported as 11 to 16°C (assuming δ w = 0.0‰SMOW) yield paleotemperatures ranging from −1 to 5°C, squarely in the range of ikaite stability. The low δ w indicates hyposaline conditions, most likely caused by mixing high latitude meteoric waters with seawater. The 18O depleted, low temperature waters suggest that the region was at least seasonally colder than previously accepted.

Isotopic constraints on the origin of salts in Australian playas. 1. Sulphur

The δ 34S values of a variety of gypsum types from the moderm playa lakes of southern Australia show little relationship with underlying bedrock types or the δ 34S values of the bedrock sulphur. On a regional scale, in both Western Australia (Yilgarn Block) and South Australia, the δ 34S values of surficial gypmsum have a regular pattern over distances of 500 to 1000 km with highest values ( ≈ +21%‰) near coastlines decreasing to δ 34S values of ∼ 14% further inland. Sea-salt sulphate is the dominant source of sulphur to the lakes investigated, although the proportion decreases from ∼ 100% near coastlines to ∼ 55% in some inland areas. A secondary source of sulphate is also airborne, but derives from volatile biogenic sulphur compounds of largely marine origin. Sulphur derived from rock weathering is a minor component, except in areas where the bedrock contains abundant sulphur. Thus Lake Cadibarrawirracanna receives ∼ 10% of its sulphur from weathering of the pyritic Bulldog Shale (Cretaceous), and Lake Amadeus, possibly up to one-third of its sulphate from evaporites of the Bitter Springs Formation (Late Proterozoic). Our δ 34S measurements are the first tracers to directly establish the marine origin of components in Australian surface brines. The δ 34S analyses indicate the accession of sulphur from both sea-salt and marine biogenic sulphur and clearly support delivery of salts to the Australian landscape as aerosols following established wind patterns. The data negate the possibility of derivation of the lacustrine “seawater-like” brine chemistry from either marine transgression or weathering of connate salts from marine strata alone. The occurrence of non-marine evaporites exhibiting largely marine-like chemistry and isotopic signatures is probably common to low-latitude tectonically stable areas and will make the distinction between some marine and non-marine evaporites from the geological record more difficult. The regular pattern of δ 34S values of surficial sulphate in lakes and groundwaters in southern Australia provides an ideal baseline against which to search for anomalous δ 34S values associated with base-metal or gold mineralization.

CRETACEOUS OF THE EROMANGA BASIN — IMPLICATIONS FOR HYDROCARBON EXPLORATION

The Cretaceous sequence in the Eromanga Basin covers nearly one-fifth of the Australian mainland and has a maximum thickness of about 2000 m. The lowermost prospective unit is the Neocomian Cadna-owie Formation, which commonly contains hydrocarbon shows and recently yielded oil in Merrimelia 15. Likely source rocks include organic-rich sediments at the base of the overlying Bulldog Shale and Wallumbilla Formation. Future commercial production from the Cadna-owie Formation will be hampered by fine grain size and carbonate cements, resulting in low porosity and permeability. In southwestern Queensland, the Cadna-owie Formation appears to have been locally dissected by prominent channels. If sand-infilled, these channels will represent an interesting stratigraphic play, although they are likely to be mudstone-and diamictite-infilled submarine canyons, similar to those in the Tertiary Latrobe Group, Gippsland Basin. As such, they represent one component of a complex hydrocarbon play that has yet to be evaluated.A third, stratigraphically higher exploration target is the middle Albian Coorikiana Sandstone. Although the marine mudstones which encase the Coorikiana Sandstone are only poor to fair source rocks, hydrocarbon shows are common in the unit, which yielded 9900 m3 of gas per day when tested in Strzelecki 8. The potential of the unit is limited by poor reservoir quality and marginal oil maturity.The Toolebuc Formation, of middle-late Albian age, is the fourth promising target for significant hydrocarbons. The formation consists of organic-rich mudstone which is oil-shale bearing over large parts of Queensland. Minor oil and gas shows, attributed to early generation from a very rich source rock, suggest that oil production from the Toolebuc Formation may be possible if naturally fractured shale reservoirs can be located. Overall, the Cretaceous sequence in the southwestern Eromanga Basin is considered to have a modest oil and gas potential which should be evaluated in the course of drilling for deeper targets.It is emphasised that knowledge of Cretaceous and Cainozoic stratigraphy is essential in understanding the evolution of the Eromanga Basin. Rapid deposition of up to 2000 m of Cretaceous sediments enabled generation of hydrocarbons in the Jurassic sequence, beginning in the latest Cretaceous and continuing today. Although early, syn-sedimentary structuring and mild epeirogeny are evident in Cretaceous sediments, of far greater significance is a major phase of Early Tertiary folding, due probably to the effects of continental breakup and collision. This greatly enhanced pre-existing structures, produced some very large anticlines, and caused up to 800 m of the Cenomanian Winton Formation to be eroded from elevated areas. A complex interplay between timing of structural growth and maturation of the sequence is thus considered to be a major control on the present distribution of hydrocarbons in the Eromanga Basin.

Basal Jurassic-Cretaceous Rocks of Western Great Artesian Basin, South Australia: Stratigraphy and Environment

Basal sedimentary rocks of the western Great Artesian basin record the change from the continental regime in Late Jurassic time to the epicontinental marine environment in the Early Cretaceous. The sequence consists of the Algebuckina Sandstone, the Cadna-owie Formation, and the Mt. Anna Sandstone Member. These units are overlain by the uniform marine Bulldog Shale. The Algebuckina Sandstone consists of lower, ill-sorted kaolinitic sandstone beds with typical angular current bedding, and well-sorted, concavely current-bedded upper sandstone beds. The Cadna-owie Formation disconformably overlies the Algebuckina Sandstone, and consists of feldspathic sandstone, micaceous and carbonaceous siltstone, Fontainbleu-type and oolitic sandstones, and lenticular boulder beds. The well-sorted Mt. Anna Sandstone Member is in the upper part of the Cadna-owie Formation and is characterized by an abundance of red, porphyritic rhyolite pebbles. The distribution of these units was controlled by topographic features and syngenetic tectonic events. The lower kaolinitic beds of the Algebuckina Sandstone were deposited on a stable, senile land surface under low-gradient fluviatile conditions with episodic sheet floods. A seasonally arid climate is suggested by the presence of dreikanters. The upper Algebuckina Sandstone is thought to have formed under conditions of depositional regression, and, as indicated by the fossil flora, a moist subtropical climate. The Cadna-owie Formation represents the sedimentary record of the Early Cretaceous marine transgression. The sediments of this formation, with the exception of those of the Mt. Anna Sandstone Member, were deposited under shallow-water, marginal-marine conditions, and several specialized environments are documented. The boulder beds of the Cadna-owie Formation, previously cited as evidence of Cretaceous glaciation, are the product of sediment creep in the vicinity of basement highs. The Mt. Anna Sandstone Member was deposited in a fluviatile-deltaic environment. The abundance of red porphyritic rhyolite pebbles within this sedimentary unit is evidence of a major uplift of the porphyry provenance area at the southwestern basin margin in Early Cretaceous time, which resulted in the for ation of the Gawler Range Massif. Correlation of these basal beds with stratigraphic units recognized in the deeper parts and along the southern and northwestern margins of the Great Artesian basin is discussed.