The eruptive history and depositional environment of the Devonian extrusive rocks, Eden, New South Wales

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Abstract
The Devonian, terrestrial Eden Rhyolite attains a thickness of 156 m and in stratigraphic order consists of the following units: Flow Two followed Flow One rapidly enough to form one cooling unit and these two flows represent the bulk of the Eden Rhyolite sequence. The sources of these acidic eruptions were probably north-south trending fissures formed in an elongated, terrestrial foredeep in front of an orogen toward the west. Post-Eden Rhyolite deformation is thought to have been restricted to faulting resulting in the development of an asymmetrical graben which cuts across the north-south orogenic trend of the underlying (?) Ordovician Mallacoota Beds and the depositional trend of the Eden Rhyolite. The southern, east-west to west-northwest trending boundary fault of this graben (the Twofold Bay Fault) is delineated by the contact between the Mallacoota Beds and the Devonian rocks near Eden. The Twofold Bay Fault may be the Devonian termination of the Berridale Wrench Fault. The development of this asymmetrical graben resulted in a northerly dipping fault scarp, and at its foot the northern down-thrown block provided the depositional area for the alluvial fan of the Late Devonian Lochiel Formation, which unconformably overlies the Eden Rhyolite. This fault was active during Lochiel time when deposition was accompanied by basaltic extrusion. The Lochiel alluvial fan, which buried at least one major point source of basaltic extrusive rocks, extends northward for 25 km from the fault scarp and attains a maximum thickness of 420 m, but thins rapidly towards its lateral margin within the map area. At Cusack Creek the Lochiel Formation, measuring 60 m, consists of a basal conglomerate termed the Cusack Creek Member, and three basalt flows of decreasing thickness averaging 7.2 m, which are interbedded with clastic red beds of the Lochiel alluvial fan. The main point source of the basic extrusive rocks may have been near Lochiel.

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