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Germán Otharán &Carlos Zavala
Muddy hyperpycnal flows
and organic-rich shales
To introduce arelatively new mud transport
mechanism: muddy hyperpycnal flows
To set the diagnostic criteria for the recognition of
muddy hyperpycnites in the rock record
To show examples of ancient muddy hyperpycnites
(Jurassic-Cretaceous fine-grained deposits from the
Neuquén Basin, Argentina)
To discuss the implication of mudstone depositional
processes for the origin of organic rich shales
Historically, mud deposition was related to fallout
processes in low energy depositional environments
Hemipelagic/pelagic deposits
But… Are all fine-grained sediments
deposited by suspension settling?
Mud deposits constitute the bulk of the
sedimentary record (>60%).What sedimentation
rate would be required to accumulate such
volumes of mud by suspension settling ?
Mudstone successions display basal erosional
surfaces and internal lamina truncations
Mud particles flocculate into sand-size clay
aggregates that can be transported as bedload
Mechanical compaction is a very effective
process in mud deposits…
Mechanical compaction is a very effective
process in mud deposits…
UNCOMPACTED (BEFORE DIAGENESIS)
BMW Nissan Subaru
Volskwagen
COMPACTED (AFTER DIAGENESIS)
CARS…
1. High detailed (cm-mm scale) sedimentological
analyses in cores
2. Cut and polished slabs from cemented intervals
3. Petrographic and SEM studies
4. Icnological, palinological and micro-paleontological
analyses
5. Organic/inorganic geochemestry, DRX
6. Log profiles (e.g. GR, SP)
Dirty rivers
FLOW TYPE:extrabasinal low density turbidity currents
(fluid mud flows).Erosion capacity.
FLOW COMPOSITION:fine-grained sediments and organic
debris.Ability to incorporate intrabasinal components.
TRANSPORT MECHANISM:bedload, turbulent suspension.
Lofting plume absent.
1. Basal erosional surface (scour marks and soft-
sediment deformation).
2. “Bedload(mud ripples,parallel lamination) and
suspended load facies.
3. Fining/coarsening upward trends at lamina scale
(flow fluctuations).
4. Mixed composition: Intrabasinal-extrabasinal
components (e.g. carbonate mud, microfossils,
detrital mud, plant remains).
Diagnostic criteria:
cm
cm
cm
Soft-sediment
deformation
cm
cm
Schieber et al. (2007)
cm
cm
1
2
21
cm
1
Massive
Ripples
Erosion surface
Parallel
lamination
cm
Soft-sediment
deformation
Ripples
Sharp boundary
Parallel
lamination
Erosion surface
2
Flute marks
t1
t2
t3
t4
Baudin et al. (2017)
Congo Deep-sea fan
(Angola Basin)
Seasonally, rivers carry out between 410
and 580 Tg C/y to marine basins…
Sediment-gravity
flows are the
major conveyor of
OM to terminal
turbiditic muddy
lobe complexes.
Type-III OM input
by dirty rivers
OM rapid burial and
efficient storage
Type-II OM
incorporation
The recognition of muddy hyperpycnites requires
the performance of high detailed
sedimentological analyses.
Muddy hyperpycnal flows constitute an
important mechanism for the direct transference
of mud and organic matter to inner basin areas.
Muddy hyperpycnites should be further
considered for a better understanding of OM
accumulation and preservation in organic rich
strata.
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