The Bendigo Goldfield lies in a 9 km wide NNW trending block of Lower Ordovician turbidites bounded by the regional Whitelaw and Sebastian Faults (thrust faults). The productive portion of the goldfield lies in a zone 15 km long by 5 km wide central to the fault-bounded block.StratigraphyThe bedrock of the Bendigo area is composed of a very uniform sequence of turbiditic sandstones, siltstones and mudstones interbedded with hemipelagic mudstone and minor ‘cone-in-cone’ limestones. Sedimentary structures are perfectly preserved and are only locally modified by deformation. Graded bedding, cross bedding and other sedimentary structures provide evidence for the younging direction of beds. Graptolites mainly within black mudstones define the age of the sediments as Lower Ordovician (Lancefieldian to Castlemainian stages).Whilst it is difficult to identify mappable lithological units, approximate stratigraphic position can be determined from weathering colours, sand to mud ratios, and the overall upward fining of the sequence. Higher stratigraphic levels are characterised by more complex bed thickness variations often forming recognisable cycles 15 to 50 m thick.Sandstone beds are often grouped in 10 to 20 m thick packages able to be traced continuously along strike for up to several kilometres. However rapid facies changes and lensing of units over short distances along strike is also common making correlation across anticlines and even between adjacent mine levels uncertain.
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