Greymouth Coalfield, New Zealand: vertical coal rank gradients, depths of burial and palaeotemperatures

Abstract

Vertical rank gradients are inferred from lateral variations of Rank(S r ) in successive coal horizons in the Paparoa Group and Brunner Coal Measures in Greymouth Coalfield. The gradients are lowest, c. 3 Rank(S r )/km, in the centre of the coalfield where the lateral rank pattern is simple. In the west, vertical gradients ranging from B4 to 8 Rank(S r )/km result from contrasting Upper Rewanui and Brunner lateral rank patterns. There, it is inferred that warm fluids migrated from more deeply buried coal measures further east and have enhanced Upper Rewanui ranks. For calculating Brunner burial depths, the high western vertical gradients are rejected and a gradient of 4.0 Rank(S r )/km is adopted, as it is also east of the Mt Davy Fault Zone where the gradient is unknown. Brunner burial depths increased from 1.8 km in the far west to 4.3 km in the Mt Davy area, and palaeotemperatures increased eastwards from 90Á958C to 150Á155 8C.

Late Cretaceous and Paleogene stratigraphy and structure linking offshore with onshore north Westland, New Zealand

“…In drillhole 653 in the west of Greymouth Coalfield (Figs. 1, 8), Cretaceous oil associated with a fault was attributed to migration from more deeply buried Paparoa Group coal measures to the east (Suggate & Boyd 2012). In the middle of the coalfield, oil in these coal measures was documented by Young (1967).…”

Extensional basin development and subsequent inversion in Greymouth Coalfield

“…In the west of the coalfield, the c. 550 m thick Rapahoe sequence northwest of Runanga thickens eastwards to a maximum west of the developing westerly downthrown Mount Davy Fault Zone; the trough margin, however, was at the Montgomerie Fault. Suggate & Boyd (2012) inferred that at Blackwater Creek 1.5 km northeast of Mount Davy, c. 0.5 km of westerly dowmthrow took place at the Mount Davy Fault Zone during Rapahoe Group deposition, and that syndepositional faulting during Rapahoe Group deposition was also indicated by differences in both coal rank and Island Sandstone thickness across the Dobson Fault in the south of the coalfield. The Mount Davy Fault Zone is here regarded as a feature that was initiated within the Paparoa Trough, which was a fault-angle basin bounded on the east by the active Montgomerie Fault.…”

“…In the west at drillhole d653, 1.5 km northwest of Runanga and 9 km west of Mount Davy (Fig. 1), the Brunner was buried to a depth of 1.8 km (Suggate & Boyd 2012) and is now at 0.4 km below sea level, having been uplifted 1.4 km, 4.1 km less than at Mount Davy. Rates of this late Quaternary deformation led to estimates of the approximate duration of uplift across the coalfield.…”

“…North of Dobson in the centre south of the coalfield (Fig. 1), where Suggate & Boyd (2012) estimated a maximum depth of burial of the Brunner Coal Measures of 3 km, the thickness of the Rapahoe Group is c. 2200 m (Suggate & Waight 1999, fig. 4.7).…”