Geology Reference
In-Depth Information
4
Inclination Shallowing in Sedimentary
Rocks: Evidence, Mechanism and Cause
EARLY LABORATORY COMPACTION
EXPERIMENTS
accuracy of sediment. Their work with natural marine
sediments did show that inclination could be fl attened
by dewatering of sediment. The initial inclination of
70° for their compaction experiment was decreased to
49° and the amount of shallowing could be explained
by the volume loss of the sediment. However, Blow and
Hamilton used evaporation of the sediment's pore fl uid
as a model of burial compaction. Evaporative drying is
clearly not the mechanism by which deep-sea sedi-
ments are naturally dewatered. In fact, work by Noel
(1980) soon after Blow and Hamilton's study sug-
gested that drying alone could cause inclination shal-
lowing due to surface tension forces in the pore spaces
of the sediment. What was needed was a better model
of natural dewatering to see how sediment inclinations
would be affected.
In 1987, Anson & Kodama (1987) detailed an
experiment designed to observe the effects of burial
compaction on the inclination of marine sediment.
They carefully dewatered their synthetic sediment in a
laboratory compaction device initially designed by
Hamano (1980) to study the lock-in of sedimentary
DRM during the porosity decrease that occurs at the
very top of the sediment column. Since Hamano only
examined horizontal sediment magnetizations, he
The classic paper by Opdyke & Henry (1969) showed
clearly that recent marine sediments could accurately
record the inclination of the geomagnetic fi eld. This
seemed to settle the question raised by the early labo-
ratory re-deposition experiments suggesting that
sedimentary rock and sediment inclinations were inac-
curate and, as mentioned in the earlier chapter on
DRM, the shallow inclinations in the re-deposition
experiments were attributed to an artifact of the exper-
iments. However, the Opdyke and Henry experiment
did not consider the effects of burial compaction
because the short gravity cores they measured were
only a meter or so long; this is not deep enough to see
the effects of the approximately 50% volume loss asso-
ciated with burial compaction at depths of hundreds
of meters.
Not all paleomagnetists were convinced that the
inclination of sediments and sedimentary rocks was
always accurate, particularly after compaction had
affected the sediments. Blow & Hamilton (1978) con-
ducted one of the earliest experiments designed to
determine the effects of compaction on the directional
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