The influence of half-graben syn-depositional tilting on thickness variation and seam splitting in the Brunner Coal Measures, New Zealand

Abstract

The Brunner Coal Measures (Buller Coalfield, New Zealand) comprise a fluviatile sequence of Eocene age that is generally 30–100 m thick. The sequence rests on basement and is dominated by coarse to granular—very coarse sandstone. The measures contain one mineable coal seam (the Buller Seam Member) that is laterally extensive and up to 20 m thick. It is characterized by simple splitting with a thin upper split, a thick lower split and a split separation of up to 30—50 m. Locally, the Buller Seam Member exhibits splitting to form numerous thin seams. Areas of shale-out comprise thin linear belts between areas of thick coal with a low ash yield. The coincidence of faults and maximum thickness of seam isopachs indicates that the thickness and splitting characteristics of the Buller Seam Member were largely controlled by syn-depositional faulting and tilting of small half-graben blocks. Thickness variation in basal sequences is primarily due to syn-depositional faulting. Syn-depositional tilting of fault blocks also influenced the migration of river channels and hence the stacking and distribution of sandstone and locally thick mudstone units overlying coal.