title: Petrology of the Arumbera Sandstone, Late Proterozoic(?) - Early Cambrian, Northeastern Amadeus Basin, Central Australia
Creators: name: Conrad, Keith T.
Publisher: name: Utah State University
Year: 1981

description: 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.