| Summary: | The Paparoa Group of the Greymouth Coalfield, (West Coast, South Island, New Zealand) comprises a wholly terrestrial sequence of alternating fluvial and lacustrine strata deposited in a Late Cretaceous to Early Tertiary rift basin (the Paparoa Basin). Stratigraphic and tectonic evolution of the Paparoa Basin was determined from the lithologic record of 200 drillholes, and chronostratigraphic control was obtained from palynological identification of the Cretaceous-Tertiary Boundary (KTB). Basin fill comprises three lithosomes, which were distinguished using lithofacies and geophysical log character. Coal measure lithosomes comprise pebble-cobble conglomerate, poorly sorted carbonaceous sandstone, siltstone, carbonaceous mudstone and coal, and represent fluvial and mire environments. Mudstone lithosomes comprise massive silty mudstone with rare plant fragments and freshwater bivalves, and represent lacustrine environments. Transitional lithosomes comprise coarsening upwards packets of moderate-well sorted fine to medium non-carbonaceous sandstone and rare conglomerates interbedded with massive mudstone. Lithofacies and unit geometry indicated these strata were deposited as progradational deltas in shallow lakes.
Lithostratigraphic unit definitions were revised in accordance with the lithesome framework. Five Formations and five members were recognised within the reinstated
Paparoa Group: Jay Fm.; Ford Fm. (including the Ford Transitional Member); Rewanui
Fm. (comprising Rewanui Coal Measure Member, Waiomo Mudstone Member and
Morgan Coal Measure Member); Goldlight Fm. (including the Goldlight Transitional Member) and Dunollie Fm. The Ford and Goldlight Transitional Members are newly established to incorporate transitional lithosomes within the Ford Fm. and Goldlight Fm. Correlation of all units below the Rewanui Coal Measure Member (CMM) was revised, with Morgan CMM and W aiomo MM in the west of Greymouth Coalfield being transferred to Jay Fm. and Ford Fm. respectively.
Revised isopach models for all units (excluding Dunollie Fm.) were constructed using manual and numerical modelling techniques. Maximum present-day thickness of the Paparoa Group (excluding Dunollie Fm.) is 758.6m, and average thickness is 290m. Successively younger units onlap greater areas of basement, and basin geometry becomes simpler with time. Unit thicknesses were decompacted to account for
differential burial of 1.75km (west coast) to 4.75km (eastern basin margin). Average
thickness reduction during burial was 27%, and there was up to 290m of compaction.
Compaction from cover strata deposition was greater than syndepositional compaction within the Paparoa Group.
The KTB was identified in 16 drillholes in the western Greymouth Coalfield. Key floral events were the extinction of Tricolpites lilliei and other rare large and/or ornamented angiosperm pollen taxa, and the rise in abundance of Triorites minor. The occurrence of diagnostic taxa was controlled by lithofacies and floral diversity in addition to sample age. Total spore proportions of palynofloras declined across the KTB, though dominant conifer and spore taxa were unaffected. There was a brief decline in angiosperm and conifer diversity c.4m above the KTB. Average palynoflora diversity was unchanged across the KTB, and no mass extinction event was recognised.
The interval between the KTB and the upper contact of Rewanui CMM varied throughout the western Greymouth Coalfield, and was controlled by syndepositional faulting. The position of the KTB was predicted in all western drillholes, and major coal seams were found to occur at predictable intervals beneath the estimated KTB position. At the time of the KTB, the western Paparoa Basin was occupied by a floodplain and mire-dominated environment intersected by rivers and associated crevasse splays.
Two principal sediment sources fed into the Paparoa Basin. The northeastern axial sandy fluvial system carried granite-derived detritus whereas the northwestern basin margin supplied sand and gravel derived from Paleozoic Greenland Group basement.
Sediment flux from both sources declined during lacustrine deposition, however elongate to lobate deltas marked persistent sources of coarse clastic influx into the lake basins. During Goldlight Fm. deposition, sediment transport in the south of the basin was reversed. Minor sediment entered the Paparoa Basin from the southwest, but the eastern basin margin was not a significant sediment source.
Tectonic history of the Paparoa Basin was reconstructed from decompacted isopach patterns. Persistent sediment entry points were interpreted as transfer zones between normal faults. Unit thickness patterns were controlled by two orientations of normal faults: WNW/ESE and NNE/SSW. The steep eastern basin margin comprised a
NNE/SSW aligned segmented fault zone, whereas there was block faulting along the western basin margin. The southwestern basin margin was flexural. Initial extension was oblique, resulting in reactivation of older (WNW/ESE) structures. Increasing dominance of the NNE/SSW faults during Paparoa Group deposition represented slight anticlockwise rotation of the extension direction.
Basin fill style was determined by the interaction of the two sediment sources and the two prominent fault orientations. Structural constraint of sediment entry points, and small scale normal faults within the basin, determined the position of fluvial systems within the basin during coal measure deposition, and the location and geometry of deltaic deposits during lacustrine episodes.
The Paparoa Basin is the largest basin within the West Coast Rift System, which comprises a colinear segmented rift system of extensional basins and rift gaps.
Extension was initiated in the latest Cretaceous, at c.70Ma, approximately 10Ma after commencement of opening of the Tasman Sea, and related basin formation at Ohai Coalfield and in Taranaki. The West Coast Rift System may therefore contain the record of a previously unrecognised latest Cretaceous tectonic event.
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