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| Titel |
Initiation of Salt Minibasins under Uneven Sedimentation |
| VerfasserIn |
R. Goteti, C. Beaumont |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250064686
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| Zusammenfassung |
| Can uneven sedimentation explain the enigmatic early initiation of salt minibasins at rifted
continental margins when the sediment fill is less dense than the underlying salt? Using
isostatic balancing arguments for compacting sediments, we first demonstrate that
minibasin-scale thickness variations in a weak sedimentary overburden can cause pressure
differences large enough to initiate salt minibasins. These contribute toward the minibasin
attaining the Rayleigh-Taylor threshold, the point at which sediments sink into an underlying
salt layer. Secondly, using two-dimensional plane-strain finite element models, we show that
uneven sediment loading on a viscous salt channel can initiate salt minibasins even when
sediments have finite strength, as is likely in natural systems. Uneven sedimentation is
idealized in the models as uniform aggradation of compacting sediment with a
superimposed smooth sinusoidal topographic perturbation, wavelength λ, maximum height
hl.
Our main goal is to estimate the minimum duration that uneven sedimentation must
persist, tP, when followed by uniform aggradation, such that the strength of the sediments is
overcome and a successful minibasin develops. We investigate models with widths, W , equal
to λ (confined salt models), and greater than λ (unconfined salt models). Our results suggest
that for models that develop successful minibasins there is an optimal value of λ, λOPT, for
which tRT, the time to attain the theoretical Rayleigh-Taylor threshold based on
isostasy arguments, and tP are the smallest. This wavelength selection exists when the
sediments have moderate (e.g., shale) to high (e.g., carbonates) density but is not
prominent for low density sediments. For confined salt models at λOPT, tP varies
from 8.0 - 2.25 Ma for low to high density sediments, respectively, when hl ranges
from 40 to 80 m. In unconfined models, where salt can be expelled over larger
distances than λ, tP is reduced and ranges from 7 - 1.5 Ma. For a given sediment
density and compaction model, there is a lower limit of λ below which uneven
sedimentation alone does not initiate minibasins. However, successful minibasins
develop over a wider range of λ in unconfined models and when W -« λ, uneven
sedimentation in one location can indirectly lead to the development of outboard
basins.
Uneven sediment may alternatively be deposited on a uniformly thick, compacting
pre-kinematic sediment layer. Under these circumstances tP is further reduced to 9.0 - 0.5 Ma
depending on the density of the sediments. Weak in-plane contraction can further reduce tP
to 3.5 - 0.375 Ma even for low density sediments. Our results indicate that under
optimal conditions a period of uneven sedimentation of this type in an otherwise
uniformly aggrading sequence need only have a topographic amplitude of 40 - 80 m
and persist from 0.05 to 2 Ma in order to seed the development of a successful
minibasin. Our idealized uneven sedimentation model can be interpreted as a proxy
for channel-levee systems, deposition of turbidite lobe complexes, and/or lateral
density variations within the sediments that cause equivalent sediment loading and
which occur both in the down-dip and along-strike directions of a rifted margin. |
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