Sedimentology and structure of the Malvern Hills Coal Mine, Canterbury, New Zealand

Abstract

The Malvern Hills Coal Mine provides an opportunity to gain insight into the stratigraphy and sedimentology of the Broken River Formation while addressing the complex deformation of range-front tectonics. An up-to-date geological map, stratigraphic columns and cross-sections have led to a greater understanding of both the mine and regional geology. The stratigraphy reflects a transgressive sequence: the Broken River Formation comprises fining-upwards cycles of flaser- and lenticular-bedded quartz sandstones and carbonaceous mudstones and coal seams representing a coastal swamp environment. Cross-bedded marine quartz sandstones of the overlying Conway Formation are typical of bar or dune settings, and lenses of tidal sedimentation reflect a lower intertidal–subtidal environment. NW–SE compression resulted in D1 tilting and block-faulting along range-front structures. Subsequently, D2 conjugate strike-slip shears bisect the mine forming a deformation wedge. Inside this, D3 folding about NE–SW hinges and D4 listric break-out faulting of the folds has resulted in sheared anticline-syncline pairs.