50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...

(Ri > 1), there might be no ignition points, and thus
no autosuspension. Such a current would deposit
sediment, and hence decelerate.
This possibility was tested, using a theoretical
model derived from Parker et al., in which
subcritical Ri numbers were imposed, together with
different values of bottom slope, flow thickness,
and sediment grain size. The resulting values of
flow velocity, sediment content, and turbulent
energy were then entered into the turbulent-energy
equation. The energy balance was found to be
consistently negative, i.e. energy was being
consumed, and thus there would indeed be no
ignition, and no autosuspension. The important
consequence is, that if a change from super- to
subcritical flow (hydraulic jump) can be identified,
all of the channel downstream of this point must be
essentially depositional.
Natural, submarine autosuspension currents are
obviously very difficult to observe, and the reality
of autosuspension would therefore be much more
convincing if it could be demonstrated in the
laboratory. A series of experiments with this
objective has been carried out by the
Sedimentological Fluid Dynamics Group at Leeds.
Gravity currents were generated by allowing a
gravity current (saline solution or silica suspension)
to flow from a header tank down an elongated tube
(5 cm diameter), the whole rig being immersed in
plain water. Roughness elements, to improve
sediment entrainment, were positioned inside the
tube, and a test bed of silica emplaced (the same
grade as in the suspensions). The current velocity
was monitored throughout the run, together with
the shape of the descending plume as it emerged at
the lower end of the tube. Allowance was made for
any change in hydraulic resistance, due to scouring
of the test bed. The time-velocity curves, together
with measurements of plume shape, showed good
evidence of acceleration due to entrainment of testbed
sediment. Significantly, this confirmed that
autosuspension had occurred.
Lewis, K.B. & Pantin, H.M. 2002. Channel-axis,
overbank, and drift sediment waves in the southern
Hikurangi Trough, New Zealand. Mar. Geol., 192,
123-151.
Parker, G., Fukushima, Y., & Pantin, H.M. 1986. Selfaccelerating
turbidity currents. J. Fluid Mech., 171,
145-181.
ORAL
SEPTARIAN CONCRETIONS, FRACTURE
FILL ORGANICS AND THE ROLE OF
BACTERIA IN CONCRETION FRACTURING
Michael J Pearson &CampbellSNelson
Department of Earth Sciences, University of
Waikato, Private Bag 3105, Hamilton
(michaelp*waikato.ac.nz)
Stratabound concretion bodies of marine origin in
New Zealand and elsewhere commonly consist of
anhedral calcite microspar surrounding randomly
oriented clay and have high minus-cement
porosities. Calcite �13C and �18O values typically
range from -15 to -20‰ PDB and +1 to -2‰ PDB
respectively. These data indicate geologically rapid,
pervasive cementation in marine pore fluids shortly
after deposition, with bicarbonate sourced
dominantly from bacterial organic matter oxidation.
Septarian fracturing in examples from Moeraki
(Palaeocene, New Zealand) and Staffin (Jurassic,
Scotland) was probably synchronous with incipient
cementation of concretions and resulted in up to
40% volume reduction of the host material. Cracklining
brown, fibrous calcite began to precipitate in
oxic to suboxic conditions utilising the same
bicarbonate pool and marine pore fluid as the
concretion bodies but recording a relative lowering
of the redox boundary during a depositional hiatus.
The colour of the brown calcite results from an
included gel-like polar organic fraction that
probably represents bacterially degraded biomass.
Putative bacterial remains are also present in the
Staffin fracture fill.
A postulated origin for the fracturing suggests it
predates or is synchronous with carbonate
cementation. Sufficient rigidity for sediment
rupturing is provided through the binding of
flocculated clay by bacterial secretions.
Development of septarian (shrinkage) cracks in
muds is envisaged to require pervasive in situ
bacterial colonisation of sediment volumes, and to
depend on the rate of carbonate precipitation versus
breakdown of the bacterial clay-complex. Bacterial
degradation products are incorporated into early
crack-lining brown calcite crystals.
Modification of the early-formed septarian
concretions includes brittle (non-septarian)
fracturing, and precipitation of strongly ferroan
sparry white or yellow calcite cements. Fracture
morphologies and internal brecciation of earlier
cements suggests a hydraulic fracture mechanism.
Carbonate-bound lipid distributions from the
yellow calcite cement probably reflect organic
matter breakdown and aqueous solubility of
resultant fatty acids or their salts. �13C and �18O
data for such late cements are variable suggesting
no single bicarbonate source can be invoked.
Similar late fracture fills occur in non-marine
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