A major tectonic crustal feature, defined as the Polda Graben, beneath the Polda Basin and Lock Coal Deposit, is interpreted, based on magnetic basement depths determined from poor quality regional aeromagnetic analog data, flown in 1955. This EW graben is truncated by numerous NNE and NE fractures and reaches depths between 1 to 1.5 kilometres. Some deeper sections may be present. The grabens basement is possibly granite and / or granite gneiss. Minor discontinuous intraformational magnetic units are present in the sedimentary pile. The Lock reflection seismic cross section, correlated with Lock 1 indicates 5 possible stratigraphic sequences within the graben, including suspected Cambrian evaporitic beds. (Observatory Hill Beds) at depth. Recommendations are made for future study to evaluate the petroleum and sodic evaporite potential of the graben.