The Early Devonian Tangerang Formation consists of a sequence of shallow marine conglomerate, limestone and shale interbedded with shallow marine and terrestrial volcaniclastic arenite, tuff and dacitic lava flows. Three distinctive members have been recognized within the formation, namely the Windellama Limestone Member, the Carne Dacite Member and the Aloes Tuff Member. Limestone is confined to the lower part of the formation in the south where it is overlain by an entirely shallow marine sequence of volcaniclastic arenite. The proportion of volcanic material increases northwards and includes fiamme and accretionary lapilli which are considered to represent ashflow and ashfall tuffs that accumulated, together with thick lava flows, on the flanks of subaerial dacitic volcanoes. These volcanoes were situated at the northern end of the Aloes Tuff Member and towards the top of the formation in the Marulan South area. In the south the Tangerang Formation unconformably overlies Ordovician strata and the presence of Lochkovian faunas within the lower part of the formation suggests a lateral interdigitation with the upper part of the Bungonia Limestone. The dacitic lavas and associated volcaniclastic rocks in the upper part of the formation are petrographically identical to rocks in the southern part of the Bindook Volcanic Complex. Thus the Tangerang Formation, which accumulated on an uplifted horst of Late Ordovician flysch on the eastern margin of the Wollondilly Tract, is considered to represent the basal unit of the Bindook Volcanic Complex in the Marulan‐Windellama area. The progressive filling of the Wollondilly Tract to the north of Marulan with volcanic ejecta is reflected in the change in palaeocurrent direction from a dominant northward longshore system to an E‐W onshore‐offshore system during the deposition of the Tangerang Formation.