New Zealand Journal of Geology and Geophysics abstracts
Organic geochemistry and stable isotope composition of New Zealand
carbonate concretions and calcite fracture fills
Michael J. Pearson
Organic & Petroleum Geochemistry Group
Department of Geology and Petroleum Geology
Aberdeen University
Aberdeen AB24 3UE, Scotland, UK*
Campbell S. Nelson
Department of Earth Sciences
University of Waikato
Private Bag 3105
Hamilton, New Zealand
*Present address: Department of Earth Sciences, University of
Waikato, Private Bag 3105, Hamilton, New Zealand.
Abstract Carbonate concretion bodies, representing
a number of morphological types, and associated calcite fracture fills,
mainly from New Zealand, have been studied both organically and
inorganically. Extracted organic material is dominated by a complex
polymeric dark brown highly polar fraction with a subordinate less
polar and lighter coloured lipid fraction. The relative proportion of
the two fractions is the principal control on the colour of fracture
fill calcites. Concretions are classified mainly by reference to their
carbonate stable carbon and oxygen isotope and cation composition.
Typical subspherical calcitic septarian concretions, such as those in
the Paleocene Moeraki and the Eocene Rotowaro Siltstones, contain
carbon derived mainly by bacterial sulfate reduction in marine strata
during early diagenesis. Other concretions, including a septarian
calcitic type from the Northland Allochthon, have a later diagenetic
origin. Siderite concretions, abundant in the non-marine Waikato Coal
Measures, are typically dominated by methanogenic carbon, whereas
paramoudra-like structures from the Taranaki Miocene have the most
extreme carbon isotope compositions, probably resulting from methane
formation or oxidation in fluid escape conduits. Lipids from concretion
bodies and most fracture fill calcites contain significant
concentrations of fatty acids. Concretion bodies dominated by bimodally
distributed n-fatty acids with strong even-over-odd preference,
in
which long chain n-acids are of terrestrial origin, have very
low
hydrocarbon biomarker maturities. Concretion bodies that lack long
chain n-acids often have higher apparent biomarker maturity and
prominent α–ω diacids. Such diacids are abundant in fracture fill
calcites at Rotowaro, especially where calcite infills the septaria of
a siderite concretion in the non-marine Waikato Coal Measures, and
support the view that fluid transport resulted in carbonate entrapment
of the fracture-hosted acids. Diacids also occur in Northland calcite
concretion bodies, but not in their septarian fracture fill. Release
from kerogen into migrating pore fluid during an early organic
maturation stage is suggested as a plausible origin of the diacids.
Their site of entrapment may have been serendipitous, depending on the
timing of concretion body and fracture fill carbonate precipitation.
Keywords Northland Allochthon; Waikato Coal
Measures; Moeraki boulders; concretions; calcite; siderite; dolomite;
carbon and oxygen isotopes; organic geochemistry; lipids; fatty acids;
biomarker maturity
G04037; Received 31 August 2004; accepted 20 December 2004; Online
publication date 22 August 2005
New Zealand Journal of Geology & Geophysics, 2005, Vol. 48:
395–414
0028–8306/05/4803–0395© The Royal Society of New Zealand 2005
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