The Arumbera Sandstone forms distinctive strike ridges with dark reddish slopes and orange-white cliffs throughout most of the northeastern Amadeus Basin. It is divisible into four readily mappable informal units. The ridge-forming units, 2 and 4, are divided into three and two subunits, respectively. Unit 1, Subunit 2b, Unit 3 and Subunit 4b are generally comprised of recessive, pale-red to grayish-red, medium- to thin-bedded, fineto medium-grained arkose with major proportions of siltstone and mudshale. These sedimentary bodies are interpreted as a complex system of coastal to nearshore-marine environments including tidal flats, tidal channels, estuaries and beaches. Evidence includes: (1) predominance of alpha-, beta, and cross-stratification with common herringbone laminae, truncated wave-ripple laminae, and flaser bedding; (2) bimodal paleocurrents; (3) records of intermittent, subaerial exposure (desiccation mudcracks, raindrop prints, and casts of gypsum and halite crystals); and (4) rare to abundant trace fossils of probable marine origin. Subunits 2a and 4a are comprised of cliff-forming, white to pinkish-gray, thick-bedded, fine- to medium-grained lithic arkose and arkose. Subunit 2c is also resistant, and is comprised of "maroon" to moderate-red, thick-bedded conglomerate and conglomeratic sandstone with pebbles and small cobbles of chert, quartzite, and vein quartz. These three subunits are interpreted as fluvial sheet sandstones on the basis of: (1) predominance of pi-, omikron-, and lambda cross-stratification; (2) thick bedding and paucity of mudrocks; (3) unimodal, northeastward-oriented paleocurrents with a crude radiating pattern; (4) abundant shale pebbles and wedging channelsand bodies; (5) absence or rarity of trace fossils in the subunits; and (6) sheet-like geometry. The Arumbera Sandstone was probably deposited in a coastal environment perhaps analogous to the delta of modern Godavari River of India. Evidence includes: (1) a pronounced depocenter for the unit in the central part of the study area (1123 m relative to 216 m in the southwest); (2) unidirectional paleocurrents from fluvial sheet sands that radiate to the N, NE, E, and SE; (3) fluvial and coastal deposits in vertical, cyclic succession; and (4) east- and northeast-trending zones of thicker deposits within fluvial sheet sands, which may be distributary lobes. The Arumbera is considered part of the molasse sequence associated with the Late Proterozoic and Early Cambrian Petermann Ranges orogeny. The uplifted Petermann Ranges shed sediment from sedimentary, metamorphic, and plutonic rocks. Terrigenous material was probably transported to the coastal environment of the northeastern Amadeus Basin by braided streams in an environment devoid of vascular terrestrial vegetation. Grain mineralogy and weathering characteristics suggest a hot, semiarid to humid climate throughout this region. Detailed petrographic study of ten thin sections demonstrates the following average sandstone composition: quartz (54%), orthoclase (25%), chert (6%), plagioclase (5%), lithics (4%), microcline (2%), and minor zircon, tourmaline, rutile, magnetite, muscovite, and biotite. Common cementing agents are syntaxial quartz and feldspar overgrowths, chert, hematite, kaolinite, and carbonate. The inferred diagentic sequence is : Eogenetic: (1) mechanical compaction and (2) formation of "dust rims"; Mesogenetic: (3) syntaxial feldspar overgrowths, (4) syntaxial quartz overgrowths, (5) calcite cement, (6) organic maturation(?) and creation of secondary porosity, and (7) pyrite crystals; T elogenetic: (8) kaolinite, and (9) chert. The Arumbera is regarded as of possible Late Proterozoic and probable Early Cambrian age based on the presence of the trace fossils Rangea cf. longea and Phycodes antecedens in Unit 1, Arumberia banksi in Subunit 2b, and Bergauria, Diplichnites, Laevicyclus, Phycodes pedum, Plagiogmus, Psammichnites, Rusophycus and Skolithos in Units 3 and 4. The Arumbera is a potential petroleum reservoir. Suitable source rocks, sealing mechanisms, reservoir porosity and permeability, and stratigraphic and structural traps are present in the northeastern Amadeus Basin. The close association of organic maturation with generation of secondary porosity and tectonic fracturing, both in time and space, also favors the accumulation of petroleum in the Arumbera.

Throughout the James Ranges and Gardiner Range the Arumbera Sandstone forms prominent strike ridges with distinctive dark reddish slopes and pale red to orange-white cliffs. Because of their lithologic and stratigraphic similarities, the names Eninta and ''Quandong" for these units should be suppressed in favor of the name of Arumbera Sandstone, which has precedence. The stratigraphic and lithologic differences observed between the Quandong Conglomerate in the type locality and the Arumbera Sandstone in the study area suggest that these units are not equivalent. Similarites with the Areyonga Formation suggest the Quandong Conglomerate could be part of the Areyonga Formation. Lithofacies la, ld, and 2b, and Unit 3 of the Arumbera and its equivalents are typically recessive arkoses, subarkose, and mudrocks. They are interpreted as nearshore-marine to coastal deltaic deposits which include intertonguing tidal-flat, tidal-channel, and beach sediments. Lithofacies 1b and 2a consist of cliff-forming arkoses, subarkoses, and lithic arkoses. Lithofacies 2c is also resistant, and consists of orthoconglomerates and conglomeratic sandstones. Lithofacies 1e is moderately resistant, and consists of paraconglomerates, conglomeratic sandstones, and mudrocks. It and lithofacies 2c contain pebbles and small cobbles of chert, quartzite, vein quartz, silicified ooids, and limestone, dolostone, shale, and sandstone. These four lithofacies are interpreted as braidplain and fluvial sheet sands. In the east-central part of the Amadeus Basin the Arumbera Sandstone probably was deposited in a coastal environment as a sequence of deltaic sediments that was dominated by fluvial processes. The Arumbera Sandstone appears to be the molasse derived from the Late Proterozoic and Early Cambrian Petermann Ranges orogeny. Source rocks include sedimentary, low- to middle-rank metamorphic, and plutonic granites. Grain mineralogy and weathering characteristics suggest a hot, semiarid climate during deposition of the Arumbera. The Arumbera Sandstone and Quandong Conglomerate contain fair to good porosity and permeability, and petrographic evidence shows mesogenetic generation of secondary porosity. Previous and present burial depths are adequate for the generation of petroleum. The presence of suitable underlying .source rocks, overlying salt of the Chandler for a seal, and stratigraphic and structural traps suggest a good potential for petroleum. Production of dry gas from the lower part of the Arumbera at Dingo field, north of Deep Well Homestead, confirms the petroleum potential of this formation.

The Arumbera Sandstone consists of mappable informal units which are repeated in a vertical, cyclic succession. Sandstones of fluvial origin form resistant strike ridges separated by strike valleys, which consist of recessive sandstones and mudrocks of marine origin. Lithofacies 1a, 2b, and 3a are probably of marine origin in intertidal environments. Trace fossil assemblages in lithofacies 3a suggest Skolithos and Cruziana inchnofacies were present. Lithofacies 1e, 2a, 2c, 3b, and 4a are probably of fluvial origin, as the result of coalescing braided stream deposits. The Arumbera Sandstone probably was deposited in a deltaic environment characterized by low wave energy, a micro tidal range, and high input of sand-sized sediment br braided streams. In the western MacDonnell Ranges, the Arumbera overlies the Julie or Pertatataka formations along a sharp but conformable contact. The present upper contact is a low-angle regional unconformity which contains paleotopographic elements that resemble pediments, stripped structural plains, and steep erosional scarps. These paleotopographic surfaces are overlain from east to west by the Chandler, hugh River, and Cleland formations in an onlap relationship. The Arumbera Sandstone is considered part of a molasse sequence associated with the Late Proterozoic and Early Cambrian Petermann Ranges orogeny, which occurred along the present southern and southwestern margin of the Amadeus Basin. The uplifted Petermann Ranges shed detritus from metamorphic, sedimentary, and minor amounts of plutonic rocks. Paleocurrents suggest most terrigenous material was derived from the southwestern margin of the basin. The composition of detrital grains and lack of weathering features in labile detrital grains suggest a hot, semiarid to arid climate in the source area and in the basin of deposition. Sandstone samples examined petrographically primarily are subphyllarenites, subarkoses, arkoses, feldspathic litharenites, and lithic arkoses. The inferred paragenetic sequence is: Eogenetic: (1) mechanical compaction, (2) "dust rims" of hematite, illite, and chlorite, and (3) hematite cement; Mesogenetic: (4) syntaxial feldspar overgrowths, (5) syntaxial quartz overgrowths, (6) hematite cement, (7) carbonate cement, (8) kaolinite replacement, (9) formation of secondary porosity; Telogenetic: (10) chert cement and (11) gibbsite or hematite cement.