Daly Basin,Northern Territory: lithostratigraphic revision resolves context of incongruous Ordovician fossils

A local succession of interbedded dolostone, limestone and glauconitic sandstone in the central Daly Basin of the Northern Territory, dated as Early Ordovician, has for many years appeared incongruous in terms of lithology and age relative to mapped formations of the Cambrian Daly River Group. Geological mapping and stratigraphic drilling have now shown that this interval, recently named the Florina Formation and described here, unconformably overlies the karstified surface of the uppermost formation of the Daly River Group, the Oolloo Dolostone. It is the youngest formation of the Daly Basin succession, but due to the lengthy hiatus between it and the underlying units, it is not included within the Daly River Group. It comprises three intervals of carbonate rocks alternating with thicker intervals of siliciclastic rocks. The latter are viewed as forming under dominantly moderate energy, shallow marine conditions with sediments derived from a distant terrigenous source. The carbonate rocks were dominantly subtidal, too far offshore to receive significant amounts of terrigenous material but shallow enough to be above storm wave base. The Oolloo Dolostone is formally divided here into two members, the lower Briggs Member and the upper King Member. Both consist largely of dolostone, but the Briggs Member is typically well bedded, contains ooids and has a minor component of quartz sand. It accumulated mainly as ooid shoals seaward of tidal flats. The overlying King Member is massive to coarsely bedded with only traces of terrigenous sediment and was deposited in deeper water seaward of the Briggs Member.