EVIDENCE FOR SYNDEPOSITIONAL DEFORMATION FROM THE LATE DEVONIAN OF THE MANSFIELD BASIN, EAST-CENTRAL VICTORIA

The Late Devonian-Early Carboniferous Mansfield Basin is the northernm ost structural sub-basin of the Mt Howitt Province of east-central Vic toria. It is comprised predominantly of continental elastic sedimentar y rocks, and is superimposed upon deformed Cambrian to Early Devonian marine sequences of the Palaeozoic Lachlan Fold Belt. This paper docum ents evidence for synsedimentary deformation during the early history of the Mansfield Basin, via sedimentological, structural and stratigra phic investigations. Repeating episodes of folding, erosion and sedime ntation are demonstrated along the preserved western margins of Mansfi eld Basin, where fold structures within the lower sequences are trunca ted by intrabasinal syntectonic unconformities. A convergent successor basin setting (an intermontane setting adjacent to, or between major fault zones) is suggested for initial phases of basin deposition, with synsedimentary reverse faulting being responsible for source uplift a nd subsequent basin deformation. Palaeocurrents within conglomerate un its indicate derivation from the west and are consistent with episodic thrusting along basin margin faults providing elevated source regions . Periods of tectonic quiescence are represented by finer grained mean dering fluvial facies (indicative of lower regional topographic gradie nts) which display drainage patterns that appear not to have been infl uenced by bounding faults to the west. An up-sequence increase in the textural and compositional maturity of basin sandstones and conglomera tes is proposed to be a result of the incorporation of basin fill into ongoing basin deformation, with unstable metapelitic rocks being prog ressively winnowed from clast populations. Rather than resulting from Carboniferous (Kanimblan) reactivation of extensional structures, as i s generally assumed, the deformation observed within the lower units o f the Mansfield Basin is suggested here to be essentially syndepositio nal and at least Late Devonian in age.