title: The Petrology of the Early Middle Cambrian Giles Creek and Upper Chandler Formations, Northeastern Amadeus Basin, Central Australia
Creators: name: Deckelman, James A.
Publisher: name: Utah State University
Year: 1985

description: The Giles Creek and upper Chandler formations crop out in the northeastern Amadeus Basin from the north flank of Ross River syncline south to the Pillar Range, and from the nose of Ooraminna anticline east to the Simpson Desert. 'Twenty-four sections of the Giles Creek and nineteen sections of the upper Chandler were measured by the author in this area. The Giles Creek lies disconformably above the upper Chandler Formation and conformably below the Shannon Formation. The upper Chandler is conformably underlain by the lower Chandler throughout the area except at Ross River Gorge and Wallaby No. 1 well. There the upper Chandler overlies the Todd River Dolomite.
The Giles Creek and upper Chandler consist of interbedded carbonates, terrigenous-rich carbonates, and mudrocks. Terrigenous-rich carbonates and mudrocks comprise over half of the volume of the Giles Creek at most locations in the area.
Lime mudstones and cryptalgalaminated boundstones with dorral stromatolites are common in the Giles Creek. The Phillipson and Northern Facies of the Giles Creek are locally fossiliferous at the base. Ooids are present at the top of the Southern and Phillipson Facies of the Giles Creek at most locations. Anhydrite is present in the carbonates and mudrocks of the Giles Creek at Dingo No. 1 and Wallaby No. 1 wells. Gypsum is present in dolostones of the Giles Creek at Wallaby No. 1 well. Oncolite grainstones and boundstones, crystalgalaminated boundstones, and birdseye-rich lime mudstones are common in the upper Chandler.
The mudrocks of the Giles Creek and upper Chandler are composed of quartz, K-feldspar, illite, muscovite, biotite, kaolinite, smectite, plagioclase, and anhydrite, with minor amounts of limonite, hematite, vermiculite, chlorite, and zircon. Calcite and dolamite cement the mudrocks. Acid-insoluble residues of the carbonates are comprised of the above noncarbonate minerals, organic matter, pyrophyllite, and witherite.
The size and amount of terrigenous material in the Giles Creek increases to the west-southwest, which indicates that the terrigenous sediments were derived from a source area in that direction.
Sediments of the Giles Creek and upper Chandler were dolomitized by seepage-reflux of a hypersaline brine. Lateral and vertical variations in the amount of dolanite are inversely related to the amount of terrigenous material, and indicate that permeability of the sediments was a controlling factor in the distribution of dolomite in the Giles Creek.
Sediments of the Giles Creek and upper Chandler accumulated on shoals, in shoal-margin lagoons, on tidal flats, and in intracoastal lagoons. Shallow, open-shelf deposits are also present at the base of the Giles Creek at Ross River Gorge. Cyclicity in the sediments of the Giles Creek was caused by lateral shifts in the position of the tidal flats and intracoastal lagoons during continual subsidence of the basin. Both the Giles Creek and upper Chandler -were deposited during major regressions of the sea. Lateral relations of lithofacies in the Giles Creek indicate that the area was bounded by deeper water to the north and south during the Middle Cambrian.
Differential subsidence of the basin resulted in deposition of greater thicknesses of Giles Creek sediments in the Phillipson Pound and Ross River-Fergusson syncline areas. Differential subsidence in the Phillipson Pound area was partially offset by salt-induced growth of Ooraminna, Tood River-Windmill, Brumby, and Teresa anticlines.
Facies relations and lateral variations in thickness of the Giles Creek suggest that the amount of offset on the Rodinga and Carrel Flat faults is minor, perhaps on the order of 1 to 2 kilometers at most.
Initial carbonate sediments of the Giles Creek and upper Chandler were altered by syngenetic inversion of aragonite to calcite, recrystallization of calcite, precipitation of pyrite, and replacer-rent of calcite by dolomite; anagenetic silicification, compaction, and fracturing; and epigenetic oxidation, precipitation of calcite, dedolomitization, and silicification.
Post-depositional changes in the terrigenous sediments include syngenetic oxidation, alteration of clay minerals, precipitation of silica, and dolomitization.