A cool-water carbonate seaway in an extensional setting : Oligo-Miocene sedimentology of the Nile Group and Paparoa Trough, western South Island, New Zealand.
Thesis DisciplineGeological Sciences
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This body of work endeavors to document the sedimentology and stratigraphy of the Nile Group, a widespread, carbonate-dominated Oligocene–earliest Miocene unit extending across much of the West Coast of South Island, New Zealand. Rock descriptions form the foundation of interpretive facies, stratigraphic, paragenetic, and palaeogeographic models of the Nile Group and the West Coast sedimentary basin – the Paparoa Trough. These models highlight important relationships between basement geology, basin evolution, basin fill and the role of cool-water carbonates along a locally active tectonic margin. The Paparoa Trough is an Eocene to earliest Miocene intracratonic extensional basin situated along the West Coast of South Island, New Zealand. The basin’s fill is capped by a relatively thin veneer of Oligo-Miocene bioclastic carbonates and siliciclastics of the Nile Group. The Nile Group is composed of a wide range of temperate carbonates rich in coralline algae, benthic foraminifera, echinoderms, bryozoans, planktic foraminifera and micrite, as well as quartz derived sandstones, granite derived conglomerates, and basement-derived breccia. The respective lithology fall into two broad categories – coarse, micrite-poor and bioclastic- and quartz-rich platform facies and finer grained, micrite-rich basin facies. These groupings are further subdivided into six facies associations: inner-shelf, mid-shelf, seaway, outer-shelf and slope, submarine fan, and condensed glauconitic associations. These facies are interpreted to have formed within a structurally-controlled and topographi- cally complex basin. The composition and texture of the majority of these lithofacies matches models for cool-water carbonate sedimentation along the inner and mid-shelf depths of pro- tected and energetic platform environments as well as deeper outer-shelf and slope settings. Interbedded siliciclastic breccia and impure carbonate wackestones, however, do not fit either of these models and are interpreted as a record of cool-water carbonate-dominated sedimen- tation proximal to a fault scarp. Recognition of cool-water carbonate deposition proximal to fault scarps has been observed locally elsewhere in New Zealand (c.f. Kamp and Nelson 1988; Tikorangi Limestone, Hood and Nelson 2012), but is relatively novel globally and worth including in future models of carbonate sedimentology and rift-related sedimentation. Diagenetic features within the Nile Group vary wildly. Although seafloor cements are scarce throughout, stylolites and burial cements are well developed in coarse grainstone facies. More micrite-rich lithologies contain progressively less cement and more weakly developed disso- lution seams. Wackestones lack either of these features entirely. This relationship can be extrapolated to New Zealand limestones at large, noting that grainy lithologies that are not well-cemented must not have been buried deeply, and well-cemented limestones were buried to significant depths (>500 m). Micrite-rich cool-water carbonates, being more diagenetically inert by nature, are more challenging to interpret. The geographic breadth of this study enables correlations to be made across much of the West Coast, as well as important steps towards a West Coast-wide Oligocene sequence stratigraphy and palaeogeography. The Nile Group contains three unconformity-bounded transgressive- regressive depositional sequences. Broadly speaking, the changes in lithofacies progress from biologically-controlled, bioturbated and muddier lithologies to more physically-controlled, cross-bedded and micrite-poor lithologies. There is also a trend towards diminishing ter- restrial input in the form of quartz sand and silt and leaves, as well as fewer shoreface facies. Lastly, breccia facies, associated with submarine fans and fault scarps, are found exclusively in the Early Oligocene succession (Whaingaroan and early Duntroonian). These trends are interpreted to reflect progressive drowning of terrestrial environments within and surround- ing the Paparoa Trough, with the possibility of complete submergence likely by the latest Oligocene and earliest Miocene (Waitakian). The increasing oceanographic currents sweeping through the basin are likely related to the progressive drowning and the development of a seaway system across the Paparoa Trough. Deep-water facies near Greymouth, Whitecliffs, and Little Wanganui are situated at the ends of seaway systems, with constricted flow in between. Finally, the disappearance of facies from the submarine fan facies association by the Waitakian is interpreted to record the disappearance of fault scarps through diminished fault movement and sufficient burial of these features on the seafloor to prevent their contribution to the sedimentary record of the basin. Lastly, the distribution and geometry of certain facies, stratigraphic, and palaeogeographic features within the Nile Group coincides with patterns in the underlying basement geology. In particular, the most obvious compositional and structural features of the Paparoa Meta- morphic Core Complex have unique Oligocene cover rocks and are interpreted herein to have continued influencing the palaeogeography of the West Coast into the Oligocene. Similar sedi- mentological trends extend further north of the Paparoa Metamorphic Core Complex, possibly indicating more structural complexity north of Westport than currently on the geologic maps. In summary, this thesis contains extensive description and interpretation of the Nile Group, the results of years of concentrated field and laboratory work. The resulting stratigraphic and palaeogeographic models built upon this data highlight intriguing relationships between the Nile Group, underlying and overlying strata and basement geology worth extrapolating offshore, into different periods of geologic time on the West Coast, and elsewhere in New Zealand and abroad.