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Modern estuarine siliceous spiculites, Tasmania, Australia: A non-polar link to Phanerozoic spiculitic cherts


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Barrett, NS, Hirst, AJ and Reid, Catherine 2008 , 'Modern estuarine siliceous spiculites, Tasmania, Australia: A non-polar link to Phanerozoic spiculitic cherts' , Geology, vol. 36, no. 2 , pp. 107-110 , doi: 10.1130/G24333A.1.

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Biosiliceous sedimentary rocks are well known from the geologic
record and many are correctly interpreted to have formed in deepwater
or cold-water environments. Shallow non-polar spiculites are
also known from the rock record, yet no modern analog has been documented
for such environments. Bathurst Harbour, an estuarine system
in southwest Tasmania, provides this much-needed modern analog. In
this system a sharp halocline separates tannin-rich low-salinity surface
waters from clear marine bottom waters. Tannins supply few nutrients
and substantially reduce light penetration to bottom environments,
resulting in a thinned photic zone and the mixing of deeper-water subphotic
biotas of soft corals, bryozoans, and sponges with other organisms
more typical of this temperate shallow-water environment. The
well-defi ned halocline allows a typically marine biota, including echinoderms,
to live in bottom waters of this estuarine setting. The bio clastic
factory, producing both carbonate and siliceous particles, exists in
marine subphotic bottom waters of incised channel and shallow rocky
environments along the shoreline. Extensive organic-rich soft sediments
in protected embayments generate few bioclasts, but contain
allochthonous sponge spicules transported from the adjacent bioclastic
factory. Trapping of organic material within the estuarine system lowers
sediment pH and promotes dissolution of carbonate biofragments,
resulting in preferential preservation of siliceous sponge spicules. This
situation implies that many biosiliceous neritic deposits in the rock
record may be the result of similar preferential preservation.

Item Type: Article
Authors/Creators:Barrett, NS and Hirst, AJ and Reid, Catherine
Keywords: Australia, spiculite, estuarine, chert, biosiliceous
Journal or Publication Title: Geology
ISSN: 0091-7613
DOI / ID Number: 10.1130/G24333A.1
Additional Information:

© 2008 The Geological Society of America.

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