Authigenic carbonate chimneys as possible conduits for fluid expulsion on the outer Otago shelf

Orpin, Alan R. (Alan Russell), 1968-

Authigenically cemented chimneys and edifices are examples of submarine fluid escape structures formed by the expulsion of pore fluids at near ambient temperatures at the sediment-sea water interface. An extensive suite of cemented chimneys, slabs, and a small fault scarp were discovered on the outermost continental shelf, east of the Otago Peninsula, on a bench around 1 km2 at 200 m depth, approximately 50 m deeper than the general shelf break. The morphology of these features suggests they were formed by fluid expulsion, however they were not demonstratively active at the time of observation.
The occurrence of embedded upright chimneys and their consistently high detrital sediment content suggest dolomite cementation took place largely within the sediment. Expulsion of fluid delivered from well below the sediment-water interface is responsible for generating and maintaining conditions favourable for dolomite precipitation. The dolomite cement itself is fine grained, poorly ordered, calcic (53 mole% CaC03 ), cathodoluminescent, depleted in 13C, and slightly depleted in strontium with respect to other marine dolomites. The ol3C values (-4.9 to -12.S%o PDB) indicate some of the carbon was derived from the biogenic degradation of organic matter. The o18Q composition of the dolomite cement ( +5.9 to +6.4%o PDB) is the highest yet recorded for a New Zealand marine dolomite cement, and implies temperatures for dolomite precipitation around 3°-5°C, which is cooler than the temperature recorded near the sea bottom of present day (8°-l0°C). The cooler temperatures of precipitation and the maximum radiocarbon age of 33,000 ± 550 yrs BP of the chimneys are compatible with cementation during a period of glaciation. The geochemistry implies that the fluid was possibly derived from a shallow aquifer system. The expelled fluids were composed largely of seawater, possibly modified by a lense of "perched" ground water derived from the exposed inner-mid shelf as the shoreline regressed, culminating in the -120 m last-glacial shore line (20 ka BP).
All known carbonate chimney localities along the eastern margin of the South Island are consistently associated with submarine canyon heads and shelf breaks. This could imply that internally driven, seepage-induced slope failure has decreased the stability of the outer continental shelf and was important in the formation of submarine canyon systems.
The occurrence of actively venting pyrite-cemented chimneys on a tidal flat in Awarua Bay provides a modern example of sulphide-cemented fluid escape structures forming independently of hydrothermal systems. Furthermore, it provides a shallow level dewatering system which may be analogous to that which operated on the Otago shelf during periods of lower sea level.

1 v. (various pagings) : ill. (some col.), maps ; 30 cm. + 1 video.

1992Orpin

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