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| Titel |
The interaction between deepwater channel systems and growing thrusts and folds, toe-thrust region of the deepwater Niger Delta |
| VerfasserIn |
Byami Jolly, Alex Whittaker, Lidia Lonergan |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250108659
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| Publikation (Nr.) |
 EGU/EGU2015-8423.pdf |
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| Zusammenfassung |
| Gravity-driven seaward-verging thrusts, landward-verging back-thrusts and associated folds
often characterize the slope and deepwater settings of passive margins. These structures,
found in the “toe-thrust” region of the system, exert a significant control on sediment gravity
flows because they create and determine the location and configuration of sediment
depocentres and transport systems. Consequently, a quantitative understanding of the
interaction between sediment gravity flows and seabed topography is required to
understand these systems effectively. Here we make quantitative measurements of the
geomorphic response of submarine channels to growing tectonic structures with the aim of
providing new constraints on the long-term erosional dynamics of submarine channel
systems.
This study exploits 3D seismic data in the outer toe-thrust region of the deepwater Niger
Delta to analyze the interaction between Plio-Pleistocene channel systems and actively
growing folds and thrusts. We mapped folds and thrusts from the seismic data and we used
this data to reconstruct the history of fold growth.ÂÂWe then used the sea-bed seismic horizon
to build a 50 m resolution Digital Elevation Model (DEM) of the sea floor in Arc-GIS. We
extracted channel long- profiles across growing structures from the DEM, and made
measurements of channel geometries at regular intervals along the channel length. This
information was used to infer morphodyanamic processes that sculpted the channel systems
through time, and to estimate the bed shear stresses and fluid velocities of typical flow
events.
The bathymetric long profiles of these channels are relatively linear with concavity that
range from -0.08 to -0.34, and an average gradient of ~1o. Actively growing thrusts are
typically associated with a local steepening in channel gradient by a factor of up to 3, and
this effect extends 0.5 – 2 km upstream of the thrust. Within these knickzones,
channel incision increases by approximately by a factor of > 2, with a corresponding
width decrease of approximately 25%. Channel incision across growing structures is
achieved through enhanced bed-shear stress driven incision (up to 200 Pa) and
flow velocity (up to 5 ms-1), assuming typical bulk sediment concentrations of
0.6%.
Comparison of structural uplift since 1.7 Ma, and channel incision over an equivalent
period, shows that some of these channels are able to keep pace with the time-integrated
uplift since 1.7 Ma, and may have reached a topographic (bathymetric) steady-state with
respect to on-going thrusting. However, some of the sea-bed channels are yet to reach
topographic steady-state because of factors which include recent change in gradient caused
by structural uplift, and the impact of active channel diversion by growing structures.
Generally, bed-shear stresses of ~150 Pa are sufficient to keep pace with structural strain
rates of 10-15 s-1. More widely, our data demonstrates that submarine channel systems
dynamically adjust their geometry and basal gradient in order to keep pace with growth of
tectonic structures and our results suggest that these factors must be incorporated into models
to fully predict the downslope pathways of sea-bed channels in structurally complex areas. |
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