If you haven't read the 24 May transcript yet, please start here then you can move on to the other 6 or so posts that i've added this week, in Archives for May 2014.
The Continent of New Zealand
- A SUBMERGED CONTINENT
The islands of New Zealand have a total area of about 268,000 km2 and are the emergent parts of an extensive, mainly submerged continental landmass with a total areaof more than 6 million km2. Northwest, south and east of New Zealand are large areas of relatively shallow sea underlain by plateaux and ridges that border deep ocean basins in the Pacific Ocean and Tasman Sea.
New Zealand established an Exclusive Economic Zone (EEZ) in 1977, defined by a line 200 nautical miles from the New Zealand coastline and, in 2004 established maritime boundaries with Australia.
New Zealand’s entitlement to an Extended Continental Shelf (ECS) beyond the EEZ, as defined in the United Nations Convention on the Law of the Sea (UNCLOS), was confirmed by the United Nations in August 2008.
The outer limits of the ECS north of New Zealand are currently subject to delimitation with Fiji, Tonga and France in respect of New Caledonia.
New Zealand now has sovereign rights over more than 5.7 million km2 of seabed.
This is an area more than 21 times greater than its land area and is equivalent in size to the European Union, the North Sea, and a quarter of the Mediterranean combined.
New Zealand’s sedimentary basins
A large area of New Zealand’s offshore territory is covered only by reconnaissance surveys, however the available data suggest large sedimentary basins that may host oil and gas cover about 20% of New Zealand’s territory – more than 1 million km2. For some of the basins, present understanding is based on modern, industry-standard seismic surveys, eg: Deepwater Taranaki 2001–2009, Raukumara 2005–2007, Reinga 2009 and Pegasus 2010.
For other basins there is a range of seismic data acquired from the 1970s onward, of variable quality. For remote areas there are limited seismic surveys completed for the 2008 New Zealand continental shelf submission to the United Nations. Additional information from gravity and magnetic surveys and satellite data help define little known basins.
The basin boundaries shown on the map (on page 14) are mainly determined by major geological structures
or seafloor physiography. In general, regions with stratigraphic continuity and a common geological history are included within a single basin. In places, the basin limits are set at a minimum sediment thickness.
For some, sub-basins and provinces can be differentiated on geological or geographical criteria respectively.
Modern setting New Zealand’s land area straddles the active boundary between the Australian and Pacific tectonic plates, and is above sea level mainly because of deformation and uplift in the last 20 million years.
The North Island is a part of the Australian Plate and the South Island is mainly on the Pacific Plate. In the east, the Pacific Plate is moving southwest and downward beneath the North Island but southwest of the South Island the polarity is the reverse; the Australian Plate is being forced eastward and down beneath the South Island. These opposing subduction systems (represented by the Hikurangi Trough and Puysegur Trench) are connected by the Alpine Fault, a major strike-slip zone that borders the Southern Alps. Close to the active plate boundary, deformation is moderate to intense but, away from it, seismic surveys show that in many of the basins the thick successions of sedimentary rocks are little disrupted.
Mesozoic bas in initiation
In Paleozoic and early Mesozoic time, the basement rocks of New Zealand’s islands and offshore plateaux were part of the Pacific margin of the Gondwana supercontinent, adjacent to the continental hinterlands of Australia and Antarctica. Triassic and Jurassic sedimentation in back-arc settings close to the Gondwana margin formed the thick marine and non-marine, structurally simple, low metamorphic grade Murihiku rocks of western and southern New Zealand, traditionally considered as economic basement.
Evidence from radiometric dating indicates that subduction persisted at the active continental margin until Early Cretaceous time (about 100 to 120 million years ago). After subduction ceased, the subsequent Gondwana continental break-up was preceded by a period of extensional tectonics, including rifting, with one marginparallel rift basin becoming the Tasman Sea.
Basins initiated during this period contain both marine and terrestrial sediments, including coals. Although many onshore outcrops exhibit low-grade metamorphism, samples from offshore wells remain un-metamorphosed and immature for petroleum generation. Although no petroleum accumulations have yet been geochemically typed to source rocks older than Late Cretaceous age, where carbon-rich facies are present under the right conditions, they may be effective source rocks, particularly in the deep-water frontier basins.
Rifts and a passive margin
In onshore and near-shore New Zealand, an unconformity representing Early Cretaceous uplift and erosion
separates the Cretaceous and Cenozoic sedimentary rocks from underlying basement. A large part of the
New Zealand region was land in Early Cretaceous time. Early basin-fill sedimentary rocks remain poorly dated. By the late Early Cretaceous, non-marine clastic sediments were accumulating in fault-controlled basins.
The oldest basin-fill rocks are typically coarse-grained alluvial fan, fluviatile and lesser lacustrine facies,
restricted to grabens and half-grabens. Deposition of thick passive margin marine sequences also began in
the New Zealand region of the Gondwana margin in the late Early Cretaceous. Magnetic anomalies show seafloor spreading in the Tasman Sea was well established in the Late Cretaceous (83-79 million years ago), by which time most of the major seafloor physiographic features of the New Zealand region had been formed.
Large river systems developed and thick accumulations of non-marine and paralic sediments, including coal measures, accumulated in the valleys and extensive coastal plains. A thick progradational sequence that is present offshore from Taranaki represents the delta of a major river, built out into the accommodation space of a failed rift basin. It is capped by Late Cretaceous coal measures. In Taranaki, these coal measures became the source rocks for a large proportion of the oil discovered to date. Late Cretaceous marine shelf and slope sediments accumulated adjacent to the paleo-Pacific continental margin and in the more restricted seaways elsewhere.
Quiescence and transgression
Active seafloor spreading in the Tasman Sea and southern Pacific Ocean prevailed in Late Cretaceous, Paleocene and earliest Eocene time, when the New Zealand region was tectonically stable. With post-rift thermal subsidence and associated marine transgression, early-formed rift basins were progressively inundated. By Paleocene time, rift sedimentation was confined to small sub-basins while, at the margins of the land, coastal plain, marginal marine and shelf deposits accumulated. They include thick units of coal measures that are source rocks for oil and gas accumulations. Eocene deposits represent late-rift and post-rift transgressive sequences and, by Middle Eocene time, the reduced landmass was surrounded, in the west
and south, by extensive coastal plains. Fine-grained clastic sediments and carbonates accumulated in marine settings distal from land areas.
Plate boundary propagation and inundation
Seafloor spreading ceased in the Tasman Sea in Early Eocene time but continued in the southern Pacific Ocean. A new Australia-Pacific plate boundary formed south of New Zealand, where opening of the Emerald Basin resulted in anticlockwise rotation of eastern New Zealand relative to the west. For much of the region there was only minor deformation. In Southland there was rifting and the rotation resulted in compression in the Reinga Basin. By Late Oligocene time, the land area was greatly reduced and New Zealand may have been completely submerged. The Oligocene rocks are mainly calcareous. Differential compaction across basement highs and deformation associated with the new plate boundary formed a range of structures during this time.
Neogene plate boundary – Uplift and deposition
By earliest Miocene time, a southwest-dipping subduction zone was present in northern New Zealand. Large
calc-alkaline stratovolcanoes erupted on and immediately west of what is now Northland, a part of the overriding Australian Plate. The Reinga Basin, originally a rift, became an intraplate back-arc basin. The thick Cretaceous to Oligocene passive margin sedimentary sequence which had accumulated northeast of the New Zealand landmass was obducted and emplaced part-way into the Reinga, East Coast and Raukumara basins as a series of thrust sheets (Northland and East Coast allochthons). Southwest-directed subduction in northernmost New Zealand was short-lived and calc-alkaline volcanism had all but ceased there by the end of the Early Miocene. Allochthon emplacement took place over about three million years.
Southwest-directed oblique convergence at the plate boundary east of North Island and Kermadec Ridge
continues at the present day. The Kermadec Trench and its prolongation as Hikurangi Trough extend south to meet Chatham Rise just south of Cook Strait. In South Island, deformation on the Alpine Fault is mainly strike-slip, with about 480 km of dextral offset in the last 20 million years.
The rate of plate boundary convergence accelerated from Middle Miocene time, resulting in increasingly rapid uplift, with erosion of Northland volcanoes, the axial ranges of the North Island, and the elevated mountain chain of the Southern Alps. The supply of vast amounts of sediment resulted in progressive infilling of marine depocentres and progradation of the continental shelves. Most New Zealand basins have thick Neogene successions of slope and basin floor mudstones, with intercalations of turbidite sandstones. Burial by thick Neogene sequences has raised the maturity of underlying Cretaceous and Paleogene rocks to levels
sufficient to generate and expel hydrocarbons.
Taharoa Ironsand, King Country, North Island | Doug Hood Mining Ltd