The Lake Tanganyika Isolated Offshore Horst Blocks: High Resolution Seismic Studies in Anticipation

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The Lake Tanganyika Isolated Offshore Horst
Blocks High Resolution Seismic Studies in
Anticipation of a Paleoclimate Deep Drilling
Program Kiram E. LezzarDepartment
of GeosciencesThe University of Arizona For
more information on this topic klezzar_at_geo.arizon
a.edu(520) 626 7797
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• Approximately 1000 km of air gun and sparker
  reflection seismic profiles collected in several
  field campaigns at Lake Tanganyika have
  demonstrated the existence of three major
• Mid-lake Structural Highs
• Also called in the literature
• Horst Blocks, Ridges
• or High Relief Accommodation Zones

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High Relief Accommodation Zone Transition area
between 2 opposite polarity basins. This forms
shallow-water horst blocks.
Opposite polarity basins with major border
faults overlap form a low relief accommodation
zone. This forms deep-water horst blocks.
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Because of their offshore settings, these highs
are isolated from major river inflows.- Several
coring campaigns have demonstrated sedimentation
rates over the last 100ka at much slower rates
(0.1-0.2 mm/year) than adjacent basins ? Deep
water open basins (0.4-0.7 mm/year) and shallow
platform flexural platforms (1-3 mm/year).
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Main ObjectivesTo conduct structural and
paleoclimatic investigations of seismic data
from the top of 3 major isolated mid-lake
structural highs and cores spanning the late
Pleistocene and Holocene, in order to propose an
ideal drilling site in Lake Tanganyika that has
the potential to provide a long and continuous
paleoclimate record.
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Talk Outline

• I- The Ubwari Horst
• -Northern Lake Tanganyika Rift Basin Case
  Study
• Kiram E. Lezzar Jean-Jacques Tiercelin
• II- The Kavala Island Ridge/Kalemie-Mahale Horst
• -Central Lake Tanganyika Rift Basin Case
  Study
• Kiram E. Lezzar Christopher A. Scholz
• III- The Kalya Horst
• -Southern Lake Tanganyika Rift Basin Case
  Study
• Kiram E. Lezzar, Andrew S. Cohen James M.
  Russell

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Case Study I The Youngest and Shallowest Lake
Tanganyika High-Relief Accommodation Zone The
Ubwari Horst Block Summary of Main
Findings (1996-2002)

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Fault-controlled sedimentary transits
High lake stand seismic facies
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The horst hemipelagic deposits alternate with
coarse detrital units from nearby river and
channel/canyon systems.
The horst shallow depth makes (50-200m) it
subject to subaerial exposure and erosion during
even moderate intensity lake level falls (350m
bpll), let along the most severe low stands
documented from seismic data (600m).
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Case Study IIThe Oldest Lake Tanganyika
High-Relief Accommodation Zone The Kavala Island
Ridge The Kalemie-Mahale Horst BlockSummary
of Main Findings(1997-2003)
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Seismic Trackline Map
P84-214
20 Km
Satellite image correlate well   with extensional
and transtensional structures mapped offshore
from seismic reflection data
-1997 SCS MCS Airgun Lines, Lake Tanganyika
(Scholz et al., 2003) -1983/1984 Probe MCS Airgun
Lines Rosendahl et al., 1988)
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The Kavala Island Ridge (KIR) Multi-channel
Seismic Reflection Data (Project Probe)
KIR
KMH
Active tectonism along and across this horst
block formed several large fault segments, both
disconnected and connected. This geometry
creates structural discontinuities atop the
ridge that influence early- and syn-rift facies
distribution.
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The Kavala Island Ridge Eastern and Western
Border Fault
Kavala Island Ridge
Kalemie-Mahale Horst Block
Fan-shaped sedimentary sequences suggest that
subsidence of the adjacent half-graben basins on
both sides of the horst was syn-depositional.
These flanking basins on the margins of the
Kavala Horst block are estimated to be 14-12Ma
in age, the oldest part of the lake.
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Major Seismic Unconformities atop the KIR
The horst ridge sediments are located far
offshore (35 Km) but their relative shallow
depth (50 to 400m) makes them subject to aerial
erosion exposure during even medium intensity
lake level drops (350m), not to mention the
severe low stands cited in the literature
(600m).
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Scholz et al., 2003
-AMS 14C dates document slow sedimentation rates
at the Kavala Horst (0.1 mm/yr), ideal for
paleoclimatic analysis on long temporal scales
(Scholz et al., 2003). -However, lithologic
data document at least one late Pleistocene lake
low-stand that truncated the piston core record.
This highlights the difficulties of obtaining a
continuous record from the shallow Kavala Island
Ridge Horst Block.
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Case Studies I II Conclusion Ubwari
Kalemie-Mahale Horst Blocks
  Major unconformities observed in both
localities are attributed to high-amplitude
fluctuations of lake level, accompanied by
tectonic tilting and relative uplift on the
ridges atop the horsts.
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Case Study IIIThe Kalya Horst BlockThe
Deepest Lake Tanganyika High Relief Accommodation
ZoneSummary of Main Findings(2000-2005)
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10 Km
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2004 Survey
Our 2004 bathymetric and seismic surveys
demonstrate both the continuity and quiescence
of the eastern border fault of the Kalya Horst,
in contrast to the border faults segmenting
the horst eastern side in Cases I II ? No
evidence for Horst displacement
Potential Drilling Sites Area
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Shallow Platform Typical Seismic Sequences
Reflectors Terminations-Erosional
Truncations-Onlap surfaces
Potential Drilling Sites Area
Not here!
E-F?
Not here!
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Continuous yes, but!

C



Kalya Horst Eastern Ridge
Not here!
Kavala Island Ridge
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High Resolution Sparker Line 06-7
10 Km
This site appears to lie in a relatively
tectonically quiescent depositional environment,
undisturbed by fault families as opposed to the
previous 2 cases.
Potential Drill Site
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High Resolution Sparker Line 08-1
10 Km
Water depths varies between 550 to 750m.
The northeastern edge of the horst lies
at sufficient water depth (gt 600 m) to have
escaped major erosional truncations as shown in
Cases I II.
Potential Drill Site (640m WD)
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Climatic Variations Recorded at the Kalya Horst
There are clear indications of climatic change
recorded in piston cores from the Kalya horst.
NP04-KH3-1K recovered 7.75 m of sediment from
640 m water depth and spans 45000 years, giving
an average sedimentation rate of 0.15 mm/yr.
Core lithology exhibits abrupt transitions that
may be linked to orbital-scale climatic change.
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Conclusion
Potential Drill Site
Kalya Horst Eastern Edge

• High resolution sparker lines with at least 400ms
  twtt penetration
• show both sequence boundary conformity and
  dominantly high
• amplitude reflectors indicative of pelagic
  sedimentation.
• There is at least 350m of continuously deposited
  sediment at this site,
• which based on Late Pleistocene/Holocene
  accumulation rates
• represents about 2Ma of accumulation. Based on
  the earlier PROBE
• lines, showing 1km of sediment cover here, there
  may be a 7Ma
• continuous paleoclimate archive in this area.

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Acknowledgements
and DOSECC of course!!!
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Scholz et al., 2003
Seismic images and core locations across the
Kavala Island ridge Note pronounced angular
unconformity near the base, or just below each
core. These unconformities are interpreted to be
produced by a combination of lake level drops and
tectonic tilting of the Kavala Island ridge
(Scholz et al., 2003).