The sedimentology and diagenesis of an Oligocene carbonate sequence, (Nile group), Westport to Punakaiki

Abstract

The Nile Group sediments in the Westport to Punakaiki area consist of a wide range of carbonate lithotypes including rhodoliths, terrigenous rich lime mudstones and wackestones, and algal, echinoid and bryozoan wackestones, packstones, and grainstones. The limestones are subdivided intothe Waitakere, Tiropahi, Potikohua and Cobden Limestone Formations within the Nile Group.The sediments range in age from Runangan to Waitakian. In the north of the field area much of the sequence has been eroded but in the south the Nile Group is conformably overlainby the Welsh formation. Deposition occurred in a subsiding basin, the axis of which ran approximately ten kilometres inland and parallel to the present day coastline. From the Runangan to the Duntroonian a low land mass may have existed to the west of the present coastline, but by the Waitakian little evidence of its existence remains. At Cape Foulwind, Nile Group sedimentation began in the Runangan with the deposition of algal bioherms over the shallow marine sandstones of the Little Totara Sand Formation. Periferal to the bioherms mixed clastic and algal lithologies were deposited in shallow shelfenvironments. In the south, at Punakaiki, a more abrupt change occurred from the shallow marine Island Sandstone Formation to the middle shelf echinoid and bryozoan grainstones of the Potikohua Limestone and only a thin sequence of shallow shelf algal grainstone is preserved. The transgression continued until the Waitakian with deposition of middle and outer shelf carbonate facies. The transgression occurred more rapidly in the south and some middle shelffacies observed in the north are missing. The water depths in which the sediments were deposited ranged from 10 to 250 metres. Hardground formation, seafloor erosion and phosphatization of carbonate lithoclasts occured in some of the middle and outer shelf grainstones. Sediment lithotype, early seafloor cementation and burial history are the most important controls on the late stage diagenesis of the sediments. In the rhodolith lithotype early seafloor cementation imparts a rigid framework to the sediment and most of the depositional texture is preserved. Hardground formation in the grainstone lithofacies produced an open textured, acicular spar cemented rock, which does not have the flaggy character of most packstones and grainstones in outcrop. In the fine grained carbonate lithotype little interparticle cement is deposited and most porosity is destroyed by mechanical compaction. Chemical dissolution becomes more important with increasing burial and pore fluids enriched in Ca and Mg ions are expelled from the sediment. In the grainstone lithotype about 48% of the original porosity is destroyed by mechanical compaction. With increasing burial, first grain to grain pressure solution occurs, followed by dissolution at discrete levels in the rock. Several parameters control the location of this dissolution and primary sedimentary bedding is not important in most cases. The bulk of cementation occurred at depths of 100 metres or more. Allochems and calcite cements in some grainstones are replaced by late stage dolomite. Migration of brines from adjacent terrigenous rich lime mudstones and wackestones undergoing compaction provided the magnesium source.