title: Tubular Carbonate Concretions from North Island, New Zealand: Evidence for Hydrocarbon Migration and the Subsurface Plumbing System of Cold Seeps

authors: Nyman, Stephanie Leigh

subjects: hydrocarbon migration

abstract: Among the cold seep research community, it is now appreciated that tubular carbonate concretions are important indicators of hydrocarbon migration in the subsurface. In the last few years, several publications have documented tubular concretions and interpreted a subsurface seep origin. However, the nature, timing, and relative importance of tubular concretions as records of fluid flux and chemistry over time is yet to be fully appreciated.

In North Island, New Zealand, tubular concretions are geographically widespread but occur in localised clusters in several Cenozoic (especially Miocene) sedimentary formations of the East Coast Basin (Hikurangi Margin) and Taranaki Basin. They formed by precipitation of micritic dolomite and calcite cement within host siliciclastic mudstone and contain 50 to 85% carbonate (dolomite dominated), indicating precipitation in shallow burial (0 to 300 m). Several concretion morphologies occur, especially pipe, bulbous, doughnut, corkscrew, and conical shapes, and they include some of the largest examples known worldwide, ranging up to 10 m or more in length (limited only by exposure) and 0.1 to 1 m in diameter. The concretions typically support near-central conduits, from 1 to 40 cm in diameter, which may be open or variably filled with sediment and/or late cements.

As well as diverse morphologic types, the tubular concretions also show, within limits, variable mineralogic, petrographic, and geochemical characteristics. Additionally, some examples display association with slope instability, fault control on fluid migration, and stratigraphic placement directly below ancient seafloor seep carbonates. δ13C values of the cement forming the concretions range from -52 to +13 PDB and are interpreted to reflect carbonate precipitation from the onset of methane migration to the end of a major fluid migration event. A trend from strongly negative to strongly positive δ13C values reflects either a mixing of methane and methanogenic CO2 and/or the extensive anaerobic oxidation of methane (AOM) as supported by lipid biomarkers. δ18O values range from −3 to +5 PDB suggesting an evolved fluid source influenced by cycles of methane hydrate formation and dissociation.

The tubular carbonate concretions are interpreted to represent the subsurface plumbing pathways of methane expulsion in ancient hydrocarbon seep systems in North Island. Additionally, they suggest that gas hydrates may have been forming and dissociating along the Hikurangi Margin off eastern North Island for the past 23 Ma. The diverse geologic characteristics of the tubular concretions provide a unique opportunity to construct a comprehensive 4-D model of the subsurface development of such a system. The resulting schematic model of tubular concretion formation is an analogue for the subsurface fluid migration system of hydrocarbon seeps along the modern Hikurangi Margin, and possibly for many modern and ancient hydrocarbon seep systems in general.

text: Tubular Carbonate Concretions from North Island, New Zealand: Evidence for Hydrocarbon Migration and the Subsurface Plumbing System of Cold Seeps Among the cold seep research community, it is now appreciated that tubular carbonate concretions are important indicators of hydrocarbon migration in the subsurface. In the last few years, several publications have documented tubular concretions and interpreted a subsurface seep origin. However, the nature, timing, and relative importance of tubular concretions as records of fluid flux and chemistry over time is yet to be fully appreciated.

In North Island, New Zealand, tubular concretions are geographically widespread but occur in localised clusters in several Cenozoic (especially Miocene) sedimentary formations of the East Coast Basin (Hikurangi Margin) and Taranaki Basin. They formed by precipitation of micritic dolomite and calcite cement within host siliciclastic mudstone and contain 50 to 85% carbonate (dolomite dominated), indicating precipitation in shallow burial (0 to 300 m). Several concretion morphologies occur, especially pipe, bulbous, doughnut, corkscrew, and conical shapes, and they include some of the largest examples known worldwide, ranging up to 10 m or more in length (limited only by exposure) and 0.1 to 1 m in diameter. The concretions typically support near-central conduits, from 1 to 40 cm in diameter, which may be open or variably filled with sediment and/or late cements.

As well as diverse morphologic types, the tubular concretions also show, within limits, variable mineralogic, petrographic, and geochemical characteristics. Additionally, some examples display association with slope instability, fault control on fluid migration, and stratigraphic placement directly below ancient seafloor seep carbonates. δ13C values of the cement forming the concretions range from -52 to +13 PDB and are interpreted to reflect carbonate precipitation from the onset of methane migration to the end of a major fluid migration event. A trend from strongly negative to strongly positive δ13C values reflects either a mixing of methane and methanogenic CO2 and/or the extensive anaerobic oxidation of methane (AOM) as supported by lipid biomarkers. δ18O values range from −3 to +5 PDB suggesting an evolved fluid source influenced by cycles of methane hydrate formation and dissociation.

The tubular carbonate concretions are interpreted to represent the subsurface plumbing pathways of methane expulsion in ancient hydrocarbon seep systems in North Island. Additionally, they suggest that gas hydrates may have been forming and dissociating along the Hikurangi Margin off eastern North Island for the past 23 Ma. The diverse geologic characteristics of the tubular concretions provide a unique opportunity to construct a comprehensive 4-D model of the subsurface development of such a system. The resulting schematic model of tubular concretion formation is an analogue for the subsurface fluid migration system of hydrocarbon seeps along the modern Hikurangi Margin, and possibly for many modern and ancient hydrocarbon seep systems in general.

year: 2009

title: Book review: The Revelations of St. Birgitta of Sweden, Vol. 2, Liber Caelestis, Books IV-V.

authors: Barratt, Alexandra

abstract: This article reviews the book “The Revelations of St. Birgitta of Sweden, vol 2, Liber Caelestis, Books IV-V”, translated by Denis Searby.

text: Book review: The Revelations of St. Birgitta of Sweden, Vol. 2, Liber Caelestis, Books IV-V. This article reviews the book “The Revelations of St. Birgitta of Sweden, vol 2, Liber Caelestis, Books IV-V”, translated by Denis Searby.

year: 2013

title: The Aroha block from 1880 onwards

authors: Hart, Philip

abstract: With the discovery of gold in 1880 and the pressure to open the land for mining, there was a need to determine the boundaries of the Ngati Rahiri reserves and to subdivide these amongst the owners. As well, terms for paying goldfields revenue had to be negotiated before the goldfield could be opened, and after its opening the claims of Maori with little or no basis for their claims of ownership had to be investigated. Wairakau reserve, outside the goldfield, was not subdivided until mid-1882, and arguments over the ownership of Tui Pa continued well into the twentieth century. 
At first, sales of Crown land found few takers, especially because the farm sections required draining before they could be developed. Ngati Rahiri reserves were designated as being inalienable, but they could be leased for 21 years, as some portions were, at low rates. Very quickly, officials, who feared Maori would become landless, had to fend off requests to remove restrictions on sale made by owners who in some instances lived far from Te Aroha and who all wanted to obtain money from selling land of little use to them. The indebtedness of some owners led to continued pressure to sell, though often the money they received was wasted. With the fading of the goldfield, miners sought land for farms, and were frustrated at the Crown not purchasing the Ngati Rahiri reserves. 
Land was acquired by the Crown to extend the hot springs domain, and after negotiating improved leases in the Te Aroha township the freehold of this settlement was acquired. Some Maori retained their land, which Pakeha accused them of not improving; Maori requests that the government provide training in farming methods were ignored. By the twentieth century most Ngati Rahiri had become landless, but as many examples illustrate, this was commonly because cash-strapped owners insisted on selling their interests for an immediate financial return.

text: The Aroha block from 1880 onwards With the discovery of gold in 1880 and the pressure to open the land for mining, there was a need to determine the boundaries of the Ngati Rahiri reserves and to subdivide these amongst the owners. As well, terms for paying goldfields revenue had to be negotiated before the goldfield could be opened, and after its opening the claims of Maori with little or no basis for their claims of ownership had to be investigated. Wairakau reserve, outside the goldfield, was not subdivided until mid-1882, and arguments over the ownership of Tui Pa continued well into the twentieth century. 
At first, sales of Crown land found few takers, especially because the farm sections required draining before they could be developed. Ngati Rahiri reserves were designated as being inalienable, but they could be leased for 21 years, as some portions were, at low rates. Very quickly, officials, who feared Maori would become landless, had to fend off requests to remove restrictions on sale made by owners who in some instances lived far from Te Aroha and who all wanted to obtain money from selling land of little use to them. The indebtedness of some owners led to continued pressure to sell, though often the money they received was wasted. With the fading of the goldfield, miners sought land for farms, and were frustrated at the Crown not purchasing the Ngati Rahiri reserves. 
Land was acquired by the Crown to extend the hot springs domain, and after negotiating improved leases in the Te Aroha township the freehold of this settlement was acquired. Some Maori retained their land, which Pakeha accused them of not improving; Maori requests that the government provide training in farming methods were ignored. By the twentieth century most Ngati Rahiri had become landless, but as many examples illustrate, this was commonly because cash-strapped owners insisted on selling their interests for an immediate financial return.

year: 2016