Fluvial sedimentology and revised stratigraphy of the Triassic Flagstone Bench Formation, northern Prince Charles Mountains, East Antarctica

Abstract

The Flagstone Bench Formation ranges in age from earliest Triassic to Norian (Late Triassic) and is exposed in the Beaver Lake area of the northern Prince Charles Mountains. This sandstone-dominated formation rests conformably on the Bainmedart Coal Measures and represents the upper part of the Permian-Triassic Amery Group. It is divisible into three members: the Ritchie, Jetty and McKelvey members (in ascending order). Nine sedimentary facies assignable to three facies associations (major channel, crevasse/fan and flood-basin deposits) are recognized within the formation. Ritchie Member sedimentation took place during a transition from consistently hygric to seasonally dry conditions and the unit comprises sandstone-dominated, sheet-like channel deposits interspersed with few, thin, mottled, haematite-rich floodbasin siltstones. Deposition took place under fluctuating discharge conditions chiefly within the channel tracts of axially (northwesterly/northeasterly) flowing, low-sinuosity braided rivers. The Jetty Member shows a gross upward-fining profile dominated in the lower part by poorly sorted pebbly sandstones and in the upper part by ferruginous mudcracked siltstones, mottled palaeosols, calcrete and thin massive sandstone sheets. This unit reflects deposition of easterly directed alluvial fans and extensive flood-basin silt under a semi-arid climatic regime. The Upper Triassic sandstone-dominated McKelvey Member shows a return to axial drainage along the Lambert Graben with sedimentation occurring primarily within lowsinuosity braided channel tracts under wetter climatic conditions.

Sedimentology of the fluvial and fluvio-marine facies of the Bahariya Formation (Early Cenomanian), Bahariya Oasis, Western Desert, Egypt

“…This lithofacies corresponds in part to the massive and laminated siltstones and mudstones (facies Fm, Fl) of Miall (1978), and it is interpreted to reflect deposition in floodplain areas during periods of high water table (McLoughlin and Drinnan, 1997). The presence of ferruginous concretions and plant remains further supports a depositional model that accounts for relatively high sedimentation rates in a low-energy environment where fluvial accommodation was available for significant periods of time thus allowing for the preservation of organic matter in a relatively reducing environment.…”

“…The thin sheet-like geometry in association with small-scale sedimentary structures and fine-grained lithology indicate deposition as bar-top sand sheets or as in-channel deposits during the terminal waning stage of seasonal flood events (McLoughlin and Drinnan, 1997). Miall (1977) and Rust (1978) described thin veneers of similar facies on bar tops and as in-fills of minor channel and scour hollows during low discharge phases in modern braided rivers.…”

“…Low angle crossbeds may be deposited under high-energy conditions in particular channel locations such as at the base of longitudinal bars or at the intersection of channels (McLoughlin and Drinnan, 1997). These sandstones have relatively flat bases and tops, and high angle foresets approaching 35°.…”

The Cambrian Araba Formation in northeastern Egypt: Facies and depositional environments

“…The violet and reddish colours of the mudstone suggests subaerial exposure when the iron-forming minerals were oxidized between tidal cycles. Desiccation cracks and the mottled colouring indicate fluctuation from wetdry conditions, and intense oxidation of the sediment surface (McLoughlin and Drinnan, 1997). Strong mottling of the sediments near the top of mudstone may be caused by extensive mobilization of iron, manganese, calcium and other ions due to frequent fluctuation in groundwater levels (Turner, 1993).…”

Limnology and Sedimentary Record of Radok Lake, Amery Oasis, East Antarctica

“…The overdeepening of Radok Lake likely occurred along a tectonically weak zone, the Amery Fault, which runs along the long axis of the lake. The Amery Fault separates Precambrian metamorphic and intrusive rocks to the west from Permian Radok Conglomerate to the south and Permo-Triassic sandstones to the east of the lake (McLoughlin and Drinnan 1997;Mikhalsky et al 2001).…”