Bitter Springs Formation Research Articles

Microbial fossils from the Kheinjua Formation, Middle Proterozoic Semri Group (Lower Vindhyan) Son Valley area, central India

Abundant coccoid and filamentous microfossils are found in petrographic thin sections of stratiform stromatolitic cherts from the ≈ 1200 Ma-old Fawn Limestone of the Kheinjua Formation, lower Vindhyan Supergroup, central India. The assemblages is dominated by coccoid forms of probable chroococcacean and entophysalidacean affinities; most filaments closely resemble the oscillatoriacean Gunflintia minuta. Unlike other Middle to Late Proterozoic stromatolitic microbiotas, the Kheinjua is dominated by microfossils < 10 μm in diameter. Another notable feature of the assemblage is that its microfossils are found in extensive sheets of amorphous organic matrix, which also contains rare bizarre morphs unique to this formation. Taxonomically, the Kheinjua microbiota most closely resembles Proterozoic microbiotas described from the Belcher Islands (Kasegalik Formation), McArthur Group, Bitter Springs Formation, and the Yudoma Suite. The following taxa are formally described: Chroococcaceae — Myxococcoides minor Schopf, Eosynechococcus isolatus n.sp., Tetraphycus congregatus n.sp., Kheinjuasphaera vulgaris n.gen., n.sp., Glenobotrydion aenigmatis Schopf, Melasmatosphaera media Hofmann; Entophysalidaceae — Eoentophysalis belcherensis Hofmann, E. magna n.sp.; Oscillatoriaceae — cf. Gunflinta minuta Barghoorn, Eomycetopsis? siberiensis Lo.

Thermal Maturation Model of Late Proterozoic-Paleozoic Amadeus Basin, Central Australia: ABSTRACT

The thermal maturation of the Amadeus basin, central Australia, was modeled using a modified version of Lopatin's time-temperature index. The late Proterozoic-Paleozoic basin presented numerous difficulties: (1) the absence of vitrinite in pre-Silurian strata (about 75% of the total section), (2) two major orogenic events that markedly deformed the basin, (3) moderate to extreme (25,000 ft, 7,620 m) amounts of surface erosion since the Carboniferous, and (4) a paucity of data (27 wells in an area equal in size to Oklahoma). The presence of possible giant oil and gas fields (from Ordovician shales as source) and significant gas discoveries in Proterozoic rocks made the study of special interest. Assuming a constant geothermal gradient of 1.65°F/100 ft to be representative of the basin, the amount of surface erosion was found to be the most significant factor controlling distribution of thermal facies. Interval-transit times in shales and palinspastic reconstructions of sedimentary thicknesses were used to estimate amounts of surface erosion and missing section associated with unconformities. An estimated 25,000 ft (7,620 m) of section has been removed along the northern margin of the basin, with approximately 8,000 to 9,000 ft (2,438 to 2.743 m) absent in the vicinity of the major oil and gas fields. Surficial erosion in both areas is the result of uplift associated with the Alice Springs orogeny. The Amadeus basin is an excellent example of a generation/migration/accumulation system favorable for commercial reserves of petroleum. The formation and accentuation of large anticlinal traps in Cambro-Ordovician sandstones slightly predated deep burial and strong oil generation in the Lower Ordovician Horn Valley Siltstone (Early to Late Carboniferous). The late Proterozoic Bitter Springs Formation, another possible source, was generative in the late Proterozoic to Early Ordovician. Suitable traps were also extant at this time. The thermal history of the Amadeus also demonstrates that maturation essentially halts if burial temperatures are substantially decreased by erosional unloading. In this manner sediments may remain deeply buried for long periods without undergoing substantial thermal alteration. The Amadeus is a very mature basin, with 47% of the strata being overmature (prospective for gas only). Hydrocarbon liquids are most likely preserved in the north-central part of the basin, as shown on a depth to liquid-limits map. End_of_Article - Last_Page 441------------

Length of day and obliquity of the ecliptic 850 MA ago: Preliminary results of a stromatolite growth model

A sinusoidal growth pattern in a columnar stromatolite from the approximately 850 million year‐old Bitter Springs Formation suggests a method for deducing length of day and other geophysical parameters. Phototropic microorganisms following the seasonal inclination of the Sun are proposed as responsible for the estimated 410 or more laminae (daily) per sine wave (annual). A preliminary paleomagnetic analysis yields results consistent with the proposed model.

Salt-Induced Growth Structures and Subsequent Overthrusts, Northeastern Amadeus Basin, Central Australia: ABSTRACT

Thicknesses of the late Proterozoic-Early Cambrian Arumbera Sandstone and Middle Cambrian Giles Creek Formation show initiation of growth of the Brumby anticline during the Middle Cambrian. Growth probably resulted from flowage of bedded salt in the late Proterozoic Bitter Springs Formation, possibly due to uneven deposition of fluvial sediments of the Arumbera. Such early growth structures are more likely to be oil prone than anticlines that formed later, during the Alice Springs orogeny (Late Devonian-Early Carboniferous), because adjacent source rocks were not buried as deeply. Positive source-rock analyses are known from several late Proterozoic units in the Northeast Amadeus basin. Four major tectonic elements formed during the Alice Springs orogeny: (1) the N'Dahla nappe in the north, (2) the Phillipson nappe in the center, (3) tectonic windows (autochthon?) southward in the Phillipson nappe, and (4) the Camel Flat nappe in the south. Isopach maps drawn for each tectonic element suggest a minimum of 10 to 20 km of southward movement for each of the two northern nappes. A strike-slip fault along the east side of Todd River anticline, with 1 to 1.5 km of left-lateral offset, forms a prominent north-trending lineament across the dominant east-northeast trend of most of the folds in the Northeast Amadeus basin. It is paralleled on both sides by closed, doubly plunging anticlines. Major strike-slip faults and north-trending anticlines were not widely recognized prev ously in the Amadeus basin. End_of_Article - Last_Page 614------------

Anatomy and taphonomy of a precambrian algal stromatolite

Abstract A single specimen of silicified flat algal stromatolite from the late Precambrian Bitter Springs Formation, Australia, contains three distinct microbial communities: (1) an entophysalidacean mat-building community comparable to both modern members of the genus Entophysalis and 2000 Ma old algae from the Belcher Islands, Canada; (2) a filamentous assemblage composed entirely of the Phormidium or Lyngbya-like cyanophyte Eomycetopsis; and (3) a non-stromatolitic assemblage of chroococcoid unicells that may have been associated with small, quickly evaporating ponds or puddles of water. Within each community, fossil preservation varies as a function of preservational microenvironments. Recognition of degradation sequences permits an assessment of taphonomic alterations of the original biological patterns. This, in turn, allows reconstruction of community associations and diversity patterns. The paleoenvironment of this particular Bitter Springs stromatolite is considered to be the lower part of the intertidal zone bordering a shallow, perhaps somewhat restricted sea. Post-mortem degradation has acted to increase apparent diversity through the creation of diagenetic variations in morphology and to decrease the same measure by the selective destruction of less resistant taxa. Three new species are described: Eoentophysalis cumulus, Eosynnechococcus amadeus, and Gloeodiniopsis gregaria.

Algal Fossils from a Late Precambrian, Hypersaline Lagoon

Organically preserved algal microfossils from the Ringwood evaporite deposit in the Gillen Member of the Bitter Springs Formation (late Precambrian of central Australia) are of small size, low diversity, and probable prokaryotic affinities. These rather primitive characteristics appear to reflect the stressful conditions that prevailed in a periodically stagnant, hypersaline lagoon. This assemblage (especially in comparison with the much more diverse assemblages preserved in the Loves Creek Member of the same formation) illustrates the potential utility of Proterozoic microbiotas for basin analysis and local stratigraphic correlation and demonstrates the need to base evolutionary considerations and Precambrian intercontinental biostratigraphy on biotas that inhabited less restricted environments.

A barred‐basin marine evaporite in the Upper Proterozoic of the Amadeus Basin, central Australia

ABSTRACTThe Ringwood evaporite is part of the 900 m.y. old Bitter Springs Formation, a warm‐water shallow‐marine sequence of stromatolitic dolomite and limestone, microfossiliferous chert, red beds, quartzite, and evaporites. The evaporite at Ringwood comprises two parts: (i) a lower 127 m characterized by brecciated pyritic bituminous dolomite, together with smaller amounts of dolomite‐gypsum breccia, friable chloritic dololutite, coarsely crystalline anhydrite, and satin‐spar gypsum; and (ii) an upper 133 m which is similar except that bituminous dolomite forms only one bed, and the characteristic rock‐type is dolomite‐gypsum breccia. The evaporite is overlain by limestone breccia and massive stromatolitic limestone, interpreted as an algal reef. Gypsum is secondary after anhydrite, and the ratio of gypsum to anhydrite increases upwards. The evaporite shows none of the features of a sabkha or desiccated deep ocean basin deposit, and instead is interpreted as the filling of a barred basin which was cut off from the ocean by growth of an algal barrier reef. As circulation became restricted, bituminous dolomite deposited in the lagoon behind the reef, together with pyrite from the destruction by anaerobic bacteria of algal debris derived from the reef. With continued evaporation, brine concentration increased and gypsum precipitated. Occasional dust storms contributed wind‐blown clay to the deposit. The barrier reef transgressed diachronously across the evaporite lagoon, and was eventually drowned when normal marine conditions became established. Burial of the evaporite to about 7000 m beneath the succeeding sediments of the Amadeus Basin converted gypsum to anhydrite, and formed chlorite by reaction of clay with dolomite. Late Palaeozoic tectonism folded and brecciated the rocks, and was followed by erosion which eventually exposed the evaporite to ingress of meteoric water. Hydration of anhydrite to gypsum ensued, the reaction becoming less complete with increasing depth from the ground surface.

Stromatolites from Adelaidean (Late Proterozoic) sequences in central and South Australia

New forms of late Precambrian stromatolites are described from the Amadeus and Georgian Basins and additional information is given on Minjaria pontifera Walter from the Bitter Springs Formation. Tesca stewartii gr. et f. nov. is described from the Boord Formation, Elleria minuta gr. et f. nov. from the Pioneer Sandstone, and Tungussia julia f. nov. from the Julie Formation. The latter is compared with Tungussia cf. T. julia from the Wonoka Formation, South Australia. Acaciella australica Walter has been recognised in the Yackah Beds of the Georgina Basin, supporting a correlation with the Bitter Springs Formation. The stromatolites described were selected because of their potential regional and global stratigraphic significance.

On the significance of tetrahedral tetrads of Precambrian algal cells

Tetrahedral tetrads of coccoid cells are known from two Precambrian localities; one example has been reported from the late Proterozoic Bitter Springs Formation and a second example, morphologically different from the first, is here described from the middle Proterozoic Amelia Dolomite. Among modern algae, sheath- or wall-enclosed tetrahedral tetrads are regularly produced only by eukaryotes, either during meiotic or mitotic reproduction. In evaluating the significance of Precambrian tetrads, consideration should be given to the following: whether the tetrad geometry is truly tetrahedral; if so, whether the tetrahedral configuration is original or artifactual; and if original, whether the tetrad was produced during meiotic or mitotic reproduction. We suggest several kinds of evidence which may assist in making these determinations. Considering the Amelia Dolomite specimen, we conclude that it most closely resembles a mitotically produced green algal autospore tetrad, although this interpretation cannot yet be confirmed because of the difficulty in ascertaining whether the tetrahedral configuration is original. Considering the Bitter Springs specimen, we conclude that the organism probably was eukaryotic but that its mode of origin (whether through meiosis or mitosis) cannot be unequivocally ascertained from data presently available.

Basement and cover relations on the northern margin of the Amadeus Basin, central Australia

A major, linear, west-trending deformed zone (The Redbank Zone), 350 km long and up to 20 km wide, can be identified in the Arunta Block immediately north of the Amadeus Basin. The marked linearity of this zone and of the coincident gravity anomaly probably result from thrust-fault movement during the Carboniferous Alice Springs Orogeny. However, in the Ormiston area, there is evidence that the zone originated prior to 1070 m.y. and acted as a major crustal feature controlling the later orogenic event. The Alice Springs Orogeny affected the overlying Proterozoic and Lower Palaeozoic cover rocks as well as the Arunta Block basement. During the orogeny, steep north-dipping thrusts within the Redbank Zone were reactivated causing uplift to the north. These faults penetrated the Heavitree Quartzite—the basal unit of the cover sequence—to drive wedges of basement, with attached veneers of Heavitree Quartzite, for up to 20 km southward within the overlying Bitter Springs Formation. The nappes did not reach the surface or penetrate formations above the Bitter Springs. Accompanying nappe emplacement the Basin to the south rapidly deepened to receive a thick wedge of synorogenic molasse sediments. Gravity, sedimentary and structural features combine to suggest that the Alice Springs orogeny movements reached their maximum on the central part of the northern margin of the Amadeus Basin, in the Ormiston area.

Precambrian Eukaryotic Organisms: A Reassessment of the Evidence

Comparison of partially degraded unialgal cultures of Chroococcus turgidus with coccoid microfossils from the Late Precambrian Bitter Springs formation, Australia, suggests that the Precambrian fossil record has been seriously misinterpreted. Use of degradational features as taxonomic characters has resulted in unrealistically high estimates of Precambrian algal diversity. There is at present no compelling evidence for the presence of eukaryotic microfossils in rocks from the Bitter Springs formation or any older sedimentary sequences.

Stromatolitic Microfabric: Petrographic Model: ABSTRACT

Serial, stained peels, at 10-20 µ intervals, show that a combination of organic and inorganic mineral stains distinguishes between original framework and diagenetic product in a suite of carbonate stromatolites from the Bitter Springs Formation of central Australia. Five laminar microfabrics are recognized: (1) particulate (125-400 µ diameters); (2) wispy laminations (0.5-1.0 mm thick); (3) grumous aggregates with knobby surfaces; (4) mixed calcite and dolomite brick-and-mortar texture; (5) saccharoidal dolomite. These original textures are exaggerated by diagenesis and are comparable with recent Bahamian and Floridian microfabrics of organo-sedimentary origin. They recur both vertically and laterally within a single columnar stromatolite and between parallel ntervals in adjacent columns. A calcite particle (125-400 µ) which preferentially absorbs organic and bacterial stains forms the basic stromatolitic unit structure. It is commonly encompassed by 10-µ dolomitic halo, and is more distinct in bedding peels than in vertical sections where it tends to aggregate with its overlying neighbor. The loose packing and aggregational tendency suggest an unconsolidated origin comparable with Holocene particulate matter of debated sedimentary/fecal/bacterial origin. Grain-supporting algae are not found. However 2 sizes of dichotomizing transgressive dolomitic tubules (20 µ in diameter), which are reminiscent of voids left by bunches of filaments and meiobenthonic worms, commonly form the basis of the mortar fabric. Intrastromatolitic unconformities abound and the contact with the interstromatolitic dolomite, which comprises 400-µ detrital particles with overgrowths, is generally sharp. Periodic wispy laminated drapes outline the original stromatolite's amplitude, and an incorporated dolomitized brachiopod suggests that the interstromatolitic dolomite may have been contemporaneous shell hash. Diagenetic rhomboidal carbonate distributions are attributed to contemporaneous migrant Mg-rich brines which are size specific and give lit par lit replacements which can be traced into intrastromatolitic faults with 2-mm displacements. End_of_Article - Last_Page 630------------

The Arltunga Nappe Complex, macdonnell ranges, Northern Territory, Australia∗

Abstract The Arltunga Nappe Complex trends east‐west along the northeastern margin of the Amadeus Basin, approximately in the centre of the Australian continent. The nappes formed in a stratigraphic sequence consisting of the crystalline Precambrian Arunta Complex overlain unconformably by Upper Proterozoic Heavitree Quartzite and carbonate rocks of the Bitter Springs Formation. The uppermost and largest nappe was transported at least 24 km across strike and developed by a combination of recumbent folding and overthrusting. The lower nappes formed by overthrusting alone. The nappes root to the north in a belt of crystalline rocks of the Arunta Complex, originally in the amphibolite facies, that are retrograded to the greenschist facies. North of the retrograded zone a belt of rocks belonging to the granulite facies and amphibolite facies is exposed. There is a sharp discontinuity in the style and pattern of deformation along the thrust fault surface that defines one of the lower thrust nappes. The autocht...

Potassium‐argon dates from the Arltunga Nappe complex, northern territory

Abstract Twenty‐four mineral separates from the Arunta Complex, four from the metamorphosed Heavitree Quartzite (White Range Quartzite), and one whole rock sample of metamorphosed Bitter Springs Formation, all from the western part of the White Range Nappe of the Arltunga Nappe Complex, and two samples from the autochthonous basement west of the nappe have been dated by the K‐Ar method. The samples from the basement rocks form two groups. Those in the southern or frontal part of the nappe are of Middle Proterozoic (Carpentarian) age (1660–1368 m.y.), determined on hornblende, biotite, and muscovite. In the northern or rear part of the nappe, all but one of the muscovite samples and two biotites are of Middle Silurian to Early Carboniferous age (431–345 m.y.); the remainder of the biotite dates range from 1775 to 548 m.y. (including the two samples from the autochthon), and two hornblendes gave dates of 1639 and 2132 m.y. respectively. All the muscovite samples from the Heavitree Quartzite, and the whole r...

Extractable organic matter in Precambrian cherts

The concentrations of hydrocarbons and fatty acids, and the ratios of the stable isotopes of carbon and sulfur were determined for Precambrian cherts from the Gunflint Iron-Formation, the Paradise Creek Formation, and the Bitter Springs Formation. All three cherts are known to contain organically preserved microfossils. For comparison, studies were conducted on two fossiliferous Phanerozoic cherts (from the Rhynie Chert Beds and Serian Volcanic Formation) of comparable origin and geologic history. The highest concentrations of n-alkanes, pristane and phytane, and saturated and unsaturated fatty acids were generally recovered from the untreated surfaces of the samples; these compounds are primarily, and probably entirely, of recent origin. Extremely small concentrations (a few ppb) of similar compounds were extracted from interior portions of the Precambrian samples; although in part apparently indigenous to the sediments, these compounds are not demonstrably syngenetic with original sedimentation, and the major portion of these extracts also appears to be of relatively recent origin. Permeability and porosity measurements conducted on separate rock samples from the collection on which the organic studies were made, showed the presence of microfractures that could allow the passage of ground water under a pressure gradient. In the absence of chemical criteria firmly establishing the syngenetic nature of extracted organic constituents, such studies of Precambrian sediments may only provide ambiguous evidence of early biochemical processes.

Precambrian (Aphebian) microfossils from Belcher Islands, Hudson Bay

A small assemblage of primitive microscopic fossils is here reported for the first time from the lower part of the Belcher Group in Hudson Bay. The microbiota includes chains and clumps of bacteria, and filamentous and spheroidal structures of probable algal or fungal affinities. In addition, a variety of isolated and clustered spheroids and other structures of probable biologic origin is present. The structures are more than 1600 m.y. old, and occur in a black chert associated with thick dolomite beds considered to be of Aphebian (Early Proterozoic) age. Morphologically comparable Precambrian microfossils occur in the Gunflint Formation (Early Proterozoic, Ontario) and the Bitter Springs Formation (Late Proterozoic, central Australia).

Amino acids in Precambrian sediments: an assay.

phytes.9, 10 Investigations of hydrocarbons,ll-13 porphyrins,l2 fatty acids,14 and carbon isotopic ratios4' 15 in ancient sediments suggest basic biochemical similarities between Precambrian and extant organisms. To extend these studies, we have analyzed three fossiliferous Precambrian black cherts, ranging from about I to more than 3 billion years in age, for the presence of fossil amino acids. Our choice of amino acids for this preliminary survey is based on the following considerations: (1) Polypeptides, composed of amino acids, perform the indispensable role of mediating biochemical reactions in all known organisms; (2) analyses of Phanerozoic sediments and fossils16 17 and laboratory studies of reaction kinetics'6-18 have established that many amino acids are geochemically stable; (3) sensitive methods of amino acid analysis by cation-exchange chromatography19 and by gas chromatography20 have been developed. Materials and Mlethods.-Samples: Black cherts from fossiliferous localities of the Fig Tree Series (Early Precambrian, South Africa), the Gunflint Iron Formation (Middle Precambrian, Ontario, Canada), anld the Bitter Springs Formation (Late Precambrian, Northern Territory, Australia) were analyzed for free and combined amino acids. All three cherts are dense, black, waxy, and somewhat lustrous on freshly broken conchoidal faces, and finely and rather irregularly laminated. They are composed of about 98% cryptocrystalline quartz, and contain 0.3-0.8% organic matter; they appear to be chemical sediments of primary, rather than secondary, origin. Each of the deposits has had a mild thermal history. Bacteriumlike microfossils1 and algalike organic spheroids2 in the Fig Tree cherts, having a minimum radiogenic age of 3.1 billion years,21 constitute the oldest structurally preserved evidence of life now known from the geologic record. Pristane, phytane, and n-alkanes have been reported from the Fig Tree sediments,2 22 and the C13:C12 ratio of the indigenous organic matter, as well as the geology and mineralogy of the sedimentary sequence,', 10 suggests the presence of photosynthetic organisms. The Fig Tree chert analyzed for amino acids was collected at the same locality and horizon from which microfossils have been reported.23 The Gunflint Iron Formation has a radiogenic age of approximately 1.9 billion years.4 The fossiliferous chert analyzed in the present study was collected near Schreiber, Ontario,24 at the type-locality for many of the twelve species of microscopic plants recognized in the well-documented Gunflint microbiota.3-7 Pristane, phytane, and normal paraffins have been reported from these cherts,l3 as have C::C12 ratios for both organic and inorganic carbon.4 Black cherts of the Bitter Springs Formation, from the Amadeus Basin of central Australia, contain at least 30 species of organically preserved microorganisms;-10 chert from this fossiliferous locality25 was investigated for amino acids. Radiogenic age determinations indicate that these sediments are approximately 1 billion years old.' Reagents and analytical controls: The commercially prepared chemicals used were of reagent grade; prior to use, all reagents were tested for amino acid content. Reagent 639

An interpretation of the structure of the Blatherskite Nappe, Alice springs, Northern Territory

Abstract The basal unit of the Amadeus Basin sequence is the Heavitree Quartzite, and this formation usually forms a single east‐west ridge along the northern side of the MacDonnell Ranges. However, at Alice Springs there are two such ridges. Basement rocks crop out on the northern side of each ridge, and dolomite and shale of the Bitter Springs Formation crop out on their southern sides. The northern outcrop of dolomite and shale is tightly folded, and is separated from the southern outcrop of basement by a major fault. The bedding of the sediments, the axial plane of the fold, and the fault all dip south at about 45°. Inverted facings on parasitic folds indicate that the northern outcrop of quartzite and dolomite plus shale is an antiform in inverted rocks. Hence the southern outcrop of basement and quartzite is synformal, and is interpreted as the frontal part of a fold nappe. The nappe started as a recumbent anticline whose middle limb of quartzite sheared out as the anticline travelled several kilome...

General Geology and Hydrocarbons of the Northern Amadeus Basin, Australia: ABSTRACT

The Amadeus basin is a structural depression extending easterly from the Canning basin in Western Australia to the Great Artesian basin in Queensland. It covers about 80,000 square miles and contains up to 30,000 feet of late Proterozoic and early Paleozoic marine and continental sediments. The marine cycle of deposition started in late Proterozoic time and terminated in late Ordovician. No unconformity is present at the base of the Paleozoic sequence and both Proterozoic and Paleozoic beds are unaltered except along the northern margin of the basin where low grade metamorphic facies were developed in End_Page 349------------------------------ the sediments during the Pertnjara orogeny in Devonian time. The marine sequence consists of clastics, carbonates, and evaporites. They were deposited in shallow water, shelf environments as is shown by algal growths, biohermal and biostromal carbonates, abundant ripple marks and abundant cross laminations in sandstones, and by widespread coquinoid facies in Ordovician shales. Silled or barred basins with restricted circulation of marine waters existed from time to time during the marine cycle as is shown by salt deposits in the upper Proterozoic and Cambrian sections and by thick accumulations of dark shales with abundant pyrite in late Proterozoic and Ordovician sediments. The marine cycle of deposition was terminated by the Pertnjara orogeny. This orogenic episode created a welt north of the Amadeus basin and a bordering foredeep whose depoaxis follows the present northern margin of the basin. Marine sediments were stripped from the rising welt, transported southward, and dumped into the subsiding foredeep where they now form a thick apron of poorly sorted, coarse clastic deposits. Salt tectonics has played an important role in the growth of structures in the northern Amadeus basin. Thick salt deposits in the Bitter Springs formation of late Proterozoic age constituted a semi-plastic layer near the base of the Proterozoic sequence. Sedimentary loading on this layer produced flowage and initiated salt anticlines and salt domes. These structures grew during late Proterozoic and early Paleozoic deposition as is shown by crestal stratigraphic convergence and local unconformities confined to one structure. The evaporite layer also provided a lubricated zone along which slippage was localized during the Pertnjara orogeny, and it may be responsible in part for the large nappes and overthrusts along the northern margin of the basin. The anticlines and salt domes initiated by salt flowage were formed early in the history of deposition and thus constituted potential traps for hydrocarbons long before the Pertnjara orogeny. However, folding during the Pertnjara orogeny greatly increased structural relief on the anticlines and thereby created traps having large volumetric capacities. Two of these large structures have been tested with encouraging results. The Exoil-Magellan-United Canso groups have discovered a large wet-gas accumulation, possibly with an appreciable oil leg, in Ordovician reservoirs on the Mereenie anticline in the western part of the Amadeus basin. They also discovered a non-commercial gas accumulation in Proterozoic sediments on the Ooraminna anticline in the eastern part of the Amadeus basin. A third test of a small structure near the Ooraminna anticline encountered non-commercial oil shows in Cambrian sediments. In addition, the Australian Bureau of Mineral Resources discovered oil-saturated sands in Ordovician sediments penetrated by a well being drilled as a test for phosphates. With the exception of shallow water bores, no other wells have been drilled to date in this basin. End_of_Article - Last_Page 350------------