The discovery of a new sedimentary basin: offshore Raukumara, East Coast, North Island, New Zealand

2008 ◽  
Vol 48 (1) ◽  
pp. 53 ◽  
Author(s):  
Chris Uruski ◽  
Callum Kennedy ◽  
Rupert Sutherland ◽  
Vaughan Stagpoole ◽  
Stuart Henrys
Keyword(s):  
New Zealand ◽  
Oil And Gas ◽  
East Coast ◽  
Plate Boundary ◽  
Source Rocks ◽  
Fault System ◽  
Northern Coast ◽  
The South ◽  
Petroleum Potential ◽  
The North

The East Coast of North Island, New Zealand, is the site of subduction of the Pacific below the Australian plate, and, consequently, much of the basin is highly deformed. An exception is the Raukumara Sub-basin, which forms the northern end of the East Coast Basin and is relatively undeformed. It occupies a marine plain that extends to the north-northeast from the northern coast of the Raukumara Peninsula, reaching water depths of about 3,000 m, although much of the sub-basin lies within the 2,000 m isobath. The sub-basin is about 100 km across and has a roughly triangular plan, bounded by an east-west fault system in the south. It extends about 300 km to the northeast and is bounded to the east by the East Cape subduction ridge and to the west by the volcanic Kermadec Ridge. The northern seismic lines reveal a thickness of around 8 km increasing to 12–13 km in the south. Its stratigraphy consists of a fairly uniformly bedded basal section and an upper, more variable unit separated by a wedge of chaotically bedded material. In the absence of direct evidence from wells and samples, analogies are drawn with onshore geology, where older marine Cretaceous and Paleogene units are separated from a Neogene succession by an allochthonous series of thrust slices emplaced around the time of initiation of the modern plate boundary. The Raukumara Sub-basin is not easily classified. Its location is apparently that of a fore-arc basin along an ocean-to-ocean collision zone, although its sedimentary fill must have been derived chiefly from erosion of the New Zealand land mass. Its relative lack of deformation introduces questions about basin formation and petroleum potential. Although no commercial discoveries have been made in the East Coast Basin, known source rocks are of marine origin and are commonly oil prone, so there is good potential for oil as well as gas in the basin. New seismic data confirm the extent of the sub-basin and its considerable sedimentary thickness. The presence of potential trapping structures and direct hydrocarbon indicators suggest that the Raukumara Sub-basin may contain large volumes of oil and gas.

Controls on Wintertime Nonbrightband Rain Rate and Frequency in California’s Northern Coast Ranges

2021 ◽  
Author(s):  
Matthew D. Cann ◽  
Allen B. White
Keyword(s):  
Growth Efficiency ◽  
Northern Coast ◽  
Freezing Level ◽  
Warm Rain ◽  
Radar Echo ◽  
Rain Drop ◽  
Coast Ranges ◽  
Drop Size Distributions ◽  
Rain Rates

AbstractNon-brightband (NBB) rain is a shallow, orographic precipitation that does not produce a radar brightband as a result of melting ice crystals. However, NBB rain is not the same as warm rain, which excludes ice from being involved in the microphysical growth of precipitation. Despite this difference, NBB rain is often treated as warm rain in the literature, and past studies have mostly ignored the role of ice. Here, we use two wet-seasons (2015-16 and 2016-17) at four precipitation observing sites in the Northern Coast Ranges of California to show the role of echo top height and ice in determining NBB rain intensity. It was found that NBB rain was only absent of brightbands 32-46% of the time depending on location of the site. Additionally, all NBB rain rates that exceeded 10 mm hr−1 exhibited observable brightbands during the hour period. We also define Growth Efficiency (GE) as the ability of shallow rain clouds to produce raindrops larger than drizzle size (D > 0.5 mm). High-GE rain drop size distributions were composed of fewer small drops and more large drops than low-GE rain, which was mostly drizzle. High-GE rain occurred with echo top heights above the freezing level where rapid growth of precipitation was observed by radar. Echo tops that only extended 1 km or less above the freezing level suggested hydrometeor growth from mixed-phase processes, indicating that ice may be present in coastal precipitation at warmer temperatures than previously considered.

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