Sedimentology Lecture 6. shelves & turbidites

The CONTINENTAL PLATFORM (or CONTINENTAL SHELF) is a submerged physiographic unit, equipped with uniform morphology and a low
degree of inclination towards the basin, which represents the ascent between the coastal area and the continental escarpment.
Its extent can vary a lot, due to the tectonic layout of the coastal area. In some areas, the continental shelf reaches several hundred kilometres
before moving to the continental escarpment. In other areas, the same is very narrow (a few kilometers) or even absent.

The sediments that characterize a CONTINENTAL SHELF system represent deposits that manage to leave the sub-coast zone and migrate to 'the wide'. Such
sediments can be of two kinds:
Fine sandy sediments, which are transported along the platform thanks to inertial currents that are generated along the coasts following storms (such
sediments are volumetrically the least important);
Muddy sediments (silts - clays), which are transported in suspension offshore and then deposited by fall out along the entire shelf.

The typical appearance of these deposits consists of a dense and rhythmic alternation of muddy intervals (thicker) and fine (thinner) sandy intervals, forming
successions even a few hundred meters thick

p i a n a
a b b i s s a l e
interna
esterna 200 m
The typical appearance of these deposits consists of a dense and rhythmic alternation of muddy intervals (thicker) and fine (thinner) sandy intervals, forming
successions even a few hundred meters thick.
Therefore, the successions that typically characterize a system of CONTINENTAL SHELF, is possibly be characterized by a progressive decrease in sandy
intercalations, within a muddy succession, proceeding from the innermost sectors towards the outer depositional system.

AUTOSUSPENDED HYPERPYCNAL PLUMES
Suspension produced by turbulence within the flow, i.e., a “normal” turbidity current (sensu latu). Gravity and
turbulence maintain the flow until frictional drag or a decreasing gradient result in deposition. These are
believed to be relatively rare on continental shelves because relatively steep gradients are required to produce
and maintain the flow.
WAVE-CURRENT ENHANCED GRAVITY FLOW
Turbulence associated with waves and/or currents, abundant sediment supply, and a gradient above 0.03
degrees. They create downslope transport and broad distribution of sediments across the shelf. Deposition
results when frictional drag, lowered gradient, and/or decreasing wave-current turbulence decelerate the flow.
SHELVES may be affected by hyperpycnal flows generated from river-mouth discharges interplaying with actively-acrreting shelf bedforms
(e.g., shelf ridges, dunes, bars, etc.).
Bentley (2003)
SHELF BEDFORMS
Modern example from the northwest Irish Shelf shows active sediment transport
occurring in isolated shelf areas under the constant action of unidirectional
currents. These bedforms occur around -30 to -60 m of depth and may
occasionally be flattened or eroded by oceanographic storms or energetic river
floods reaching the distal shelf.
Evans et al. (2004)

AUTOSUSPENDED HYPERPYCNAL PLUMES
Suspension produced by turbulence within the flow, i.e., a “normal” turbidity current (sensu
latu). Gravity and turbulence maintain the flow until frictional drag or a decreasing gradient
result in deposition. These are believed to be relatively rare on continental shelves because
relatively steep gradients are required to produce and maintain the flow.
WAVE-CURRENT ENHANCED GRAVITY FLOW
Turbulence associated with waves and/or currents, abundant sediment supply, and a
gradient above 0.03 degrees. They create downslope transport and broad distribution of
sediments across the shelf. Deposition results when frictional drag, lowered gradient, and/or
decreasing wave-current turbulence decelerate the flow.
SHELF HYPERPYCNAL FLOWS
GENERATED FROM RIVER-
MOUTH DISCHARGES IN A
POSSIBLE PRODELTA
ENVIRONMENT.
Bentley (2003)
Fraser River Delta - Ayranci & Dashtgard, 2016

Sedimentary dynamics of continental shelves and genetic processes
Swift (1972); Leeder (1999)
Nittrouer & Wright (1994)
Major physical processes influence sediment transport and deposition on clastic shelves
Bentley, 2003

Imbrie et al., 1984
Sand bodies encased into heterolithic strata may result from periods of rapid sea-
level changes, with consequent transfer and recycling of sand across the shelf.
Petter and Steel, 2006

INFERRED FACIES RELATIONSHIPS AND MESO-SCALE ARCHITECTURES
0.5 m
100 m
MplScs+Sm Sng+Srl Sig Sps

TRACTIONAL DEPOSITION OF SAND BY SHELF CURRENTS IN THE LOW-MEDIUM RANGE OF LOWER FLOW REGIME, FORMING STRAIGHT-
CRESTED OR SLIGTHLY SINUOUS-CRESTED SUBAQUEOUS 2D DUNES (HARMS ET AL.,1982). THE EVIDENCE OF FLOW REVERSALS
(OPPOSEDLY DIPPING CROSS-STRATA SETS, BACKFLOW RIPPLES, REACTIVATION SURFACES) AND BRIEF PAUSES IN SAND DEPOSITION
(INTERSTRATAL MUD DRAPES) INDICATE TIDAL CURRENTS

FACIES Scs + Sm: active migration of bedforms (i.e., subaqueous dunes) under low energetic currents
SHELF BEDFORMS
Modern example from the northwest Irish Shelf shows active sediment transport occurring in isolated shelf areas under the constant action of unidirectional currents.
These bedforms occur around -30 to -60 m of depth and may occasionally be flattened or eroded by oceanographic storms or energetic river floods reaching the distal
shelf.
Evans et al. (2004)

NE SW
OUTCROP EXAMPLE
OF CROSS-BEDDED
SAND BODY
ENCASED INTO
SHELF MUD

Garn Formation
(Bajocian-Bathonian)
Kristin Oil Field,
Halten Terrace,
offshore Norway
Messina et al., 2012
Lower Pleistocene
Messina Strait
Southern Italy
Longhitano, 2018

HYPOTHESIS ON THE SPATIAL DISTRIBUTION OF SAND
BODIES IN THE SUBSURFACE IN A SHELF
ENVIRONMENT

Sand-rich cross-sets have dimensions that can be predicted based
on their intercepted thickness. Permeability (k) changes depending
on the orientation of the fluid/gas flow.
Horizontal permeability of sand cross-set is considerably different in
the direction parallel and transverse to the strike of cross-strata,
with kx < ky.
The inherent high permeability anisotropy renders tidal dune cross-
sets (facies SCS) the main element of studied succession reservoir
heterogeneity.
Relationships compiled from Weber (1982), with the symbols denoting: kh and kv − horizontal and
vertical foreset permeability; kh(b) and kv(b) − horizontal and vertical bottomset permeability; kp and
kn − permeability parallel and normal to foreset cross-strata.
SAND BODY DIMENSIONS AND PERMEABILITY

CONTINENT
RIVER DELTAS
BASE OF SLOPE
DEEP
BASIN
Lateral confinement
exerted by localised
structural highs
SHELF
Tidally-modulated
along-shore currents
Delta-sourced, inertia-dominated density flows
reaching the inner-middle shelf
Garaguso
area

Turbidites and deep-sea fans
19

21
Deep-water marine systems
The deep-water depositional system is the one type of reservoir system that cannot be easily reached, observed, and studied in
the modern environment. The study of deepwater systems requires many different remote-observation techniques, each of
which can provide information on just one part of the entire system. As a consequence, the study and understanding of
deepwater depositional systems as reservoirs has lagged behind that of the other reservoir systems, whose modern processes
are more easily observed and documented.

TURBIDITIC depositional systems represent deep sea complexes, which originate along the continental escarpment (along subaqueous canyons) as streams
of water and sediment in rapid gravitational acceleration, and accumulate forming underwater fans at the base of the escarpment.
The triggering of turbiditic flows can be generated by earthquakes, tsunamis, abnormal storms, strong underwater currents or sediment overload accumulated
along the outer edge of the continental shelf

TURBIDITES (i.e., the sedimentary product of a turbidite current) FORM EXCEPTIONALLY-
GOOD WATER, OIL and GAS RESERVOIRS

TURBIDITIC DEPOSITIONAL SYSTEMS represent deep-marine complexes, which can be originated along the continental shelf edge, along the continental
slope (through submarine canyons) in the form of rapidly-accelerating water + sediment flows, accumulating submarine fans in the abyssal plain, at the base of
the slope.
The onset of turbiditic flows can be generated by earthquakes, tsunami or anomalous waves or, more simply, by sediment overload along the continental shelf
edge.

TURBIDITY CURRENTS can be set into motion when mud and sand on the continental shelf are loosened by earthquakes, collapsing slopes, and
other geological disturbances. The turbidity current then rushes downwards like an avalanche, picking up sediment and increasing in speed as it
flows.

TURBIDITES contain “architectural elements" which can be recognized at various scales or hierarchies in the sedimentary record. These genetically
related stratigraphic building blocks form the sedimentary architecture of the deepwater depositional system.
This hierarchical framework of the units is based solely on the physical stratigraphy of the strata and their thickness is time independent. The
elements show a progressive increase in scale from the deposit of a single sediment gravity flow (bed) to the accumulated deposits that comprise
entire slope or basin floor successions (complex system set).

27
Deep-water turbidites: main sedimentary facies
The first real recognition of deepwater (geologic definition) processes and deposits evolved from a classic paper by Kuenen
and Migliorini (1950), who described “graded beds” from laboratory flume experiments and outcrop observations. They
advanced the concept of turbidity currents as an important process by which sediment is transported from shallow water to
deep water.

28
Clastic depositional systems
Deep-water marine systems: submarine fans
Pioneering work by Bouma (1962), Mutti and Ricci Lucchi (1972), and Normark (1978) provided early
geologic models for submarine fans and their component strata. Walker (1978) attempted to combine
models into a comprehensive submarine-fanmodel composed of a feeder canyon, a proximal suprafan
lobe, and amore distal lobe fringe, all sitting on a basin-plain deposit

29
Deep-sea marine systems: submarine fans

30
Nigerian continental slope
(Pirmez et al., 2000)

Ta – Structureless
and normally-
graded coarse
sand
Tb – Laminated
medium-fine
sand
Tc – Ripple-
laminated fine
sand
Td – Plain-
parallel
laminated silt
Te – Clay
If you look at the deposit of an individual turbidtic event (layer) along a longitudinal section (parallel to the current that deposits it), the layer is
divided into INTERVALLIS. Each interval represents a different moment when the turbidity current accumulates the coarsest material (Ta)
transported by drag, then the finer material, organizing it into parallel laminae (Tb), ripples (Tc), and finally deposits the finest material (Td) until
then transported in suspension.
At the end of the turbiditic depositional event, the deep-marine clay sedimentation typical of a distal basin begins accumulating, going to 'seal' the
turbidtic layer through a hemipelagic closing interval (Te).

New turbitidic eventDeacrisingcurrentenergy
Within each individual turbiditic bed, the various intervals are vertically stacked forming the BOUMA SEQUENCE, which represents the synthesis
of the deposition that takes place by a turbiditic current that quickly loses energy, depositing first the coarsest sediment at the base and then,
gradually, the finer one upwards (fining-upward sequence).
Ta – Structureless and normally-graded coarse sand
Tb – Laminated medium-fine sand
Tc – Ripple-laminated fine sand
Td – Plain-parallel laminated silt
Te – Clay

33
The main architectural elements that comprise deepwater depositional systems are: canyons, (erosional) channels,
(aggradational) leveed channels, and sheets or lobes. It is important to note that one should include different types of data in a
reservoir characterization, because each type may provide details at a different scale.

34
Deep-water marine systems: submarine fans and associate channels
The main architectural elements that comprise deepwater depositional systems are: canyons, (erosional) channels,
(aggradational) leveed channels, and sheets or lobes. It is important to note that one should include different types of data in a
reservoir characterization, because each type may provide details at a different scale.
For example, at the reservior scale,
seismicreflection patterns for the three
elements are distinctly different. A, B, C are
three high-resolution seismic profiles from
one shallow intra-slope minibasin, northern
deep Gulf of Mexico. (A) Proximal and (B)
medial profiles cross the up-fan
channelized systems. (C) A distal profile
crosses the sheet deposits. Note that lobes
A and B have a slightly mounded
appearance among the laterally
continuous, sheetlike reflections. The
deposits are as large as 50 ms in two-way
traveltime. These have laterally continuous
reflections that lapout against the side of
basins. (D) Seismic profile of a leveed
channel complex from the western Gulf of
Mexico (Beaubouef et al., 2003).

35
Deep-water marine systems: submarine channels
Although the final internal fill of a channel normally is quite complex, channel fill often can be subdivided into an organized,
recognizable pattern or hierarchy of strata.
Confined channel hierarchy from a single channel element, through a complex of elements, through a complex set of
elements, and finally to a complex system. Multiple channel fills and intervening shale from levee deposits create significant
heterogeneity,
with many potential flow barriers and baffles.

36
Deep-water marine systems: facies models
For the past decade, most major oil and gas companies have focused on developing channel-fill or sheet-sandstone
reservoirs. Much less is known about levee-overbank deposits as potential reservoirs. Levee-overbank deposits consist
primarily of muds and thinly bedded (millimeters- to centimeters thick), laminated sands and sandstones (hereafter termed
“thin beds”) that form adjacent to sinuous channels. They sometimes exhibit excellent porosity and darcy-range permeability.

37
For the past decade, most major oil and gas companies have focused on developing channel-fill or sheet-sandstone reservoirs.
Much less is known about levee-overbank deposits as potential reservoirs. Levee-overbank deposits consist primarily of muds
and thinly bedded (millimeters- to centimeters thick), laminated sands and sandstones (hereafter termed “thin beds”) that form
adjacent to sinuous channels. They sometimes exhibit excellent porosity and darcy-range permeability.
Deep-water marine systems: facies models

Ta
base
leggermente
erosiva
Tc
Tb
Td
GRADAZIONENORMALE

Sedimentology Lecture 6. shelves & turbidites

Sedimentology Lecture 6. shelves & turbidites

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