The facies architecture and geochronology of the subaerial to subaqueous Palaeoproterozoic El Sherana Group, Northern Territory, and the submarine Early Devonian Crudine Group, New South Wales: implications for eruption and depositional processes.

2019-02-06T04:50:39Z (GMT) by Elizabeth Anne Jagodzinski
The El Sherana Group is a succession of fluvial and lacustrine sedimentary rocks and bimodal volcanics outcropping in a NW-trending faulted belt in the South Alligator Valley area of the Proterozoic Pine Creek Inlier, Northern Territory, Australia. The El Sherana Group comprises, in ascending stratigraphic order, the Scinto Breccia, Coronation Sandstone, Pu! Pu! Rhyolite and Big Sunday Formation. The Pu! Pu! Rhyolite represents a major period of silicic volcanism during deposition of the El Sherana Group. It contains an association of rhyolitic volcanic, volcaniclastic and intrusive facies, dominated by welded ignimbrites. The volcanic succession is thickest (over I km thick) around the southern margin of the Malone Creek Granite, thinning to under 200m to the northwest, where it outcrops discontinuously throughout the South Alligator Valley Mineral Field, and around the base of the Mt Callanan Basin. In the southeast, where the formation is thickest, the Pul Pul Rhyolite is disconformably overlain by the Big Sunday Formation, a succession of volcaniclastic turbidites interpreted to represent a local lacustrine environment in the predominantly subaerial palaeo-landscape.
The thick volcanic succession south of the Malone Creek Granite is interpreted to represent the preserved remnant of an ancient caldera structure that developed incrementally through partial collapse in the early stages of eruption, followed by downsagging in the later stages. The thinner succession to the northwest is interpreted to represent the associated outflow (i.e. extracaldera) deposits. The position of the caldera margin between the proximal and distal deposits is marked by a series of faults, including an early collapse scarp interpreted as a preserved remnant of the early caldera margin, which correspond spatially to a significant
change in the thickness of the ignimbrite succession. sparsely porphyritic quartz monzonite porphyries at this locality may correspond to a ring dyke associated with this fracture system. Because the Big Sunday Formation overlies the intracaldera facies, it is interpreted to have accumulated within the topographic depression remaining after caldera collapse. The spatial relationship between the Pu! Pu! Rhyolite and the central Malone Creek Granite pluton suggests that late-stage resurgence of the volcanic pile accompanied emplacement of the granite.[...]