8/7/2019 Forereef to Slope Environment [Compatibility Mode]
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Forereef to Slope Environment
Lecture Outline
• Forereef ‐ Slope Deposition‐Margin Types
• Deposit Types‐talus, grain flow, debris flow,
slumps, turbidites, suspension deposits
• Progradational Vertical Sequence
Forereef & Slope deposition
• May merge gradational with bank margin and
basin floor.
• Two types of margin profiles
1. Steep = ang es o >30 or near vertica
towards top and slope gradient decreases
downslope.
2. Gentle or Ramp = < 30°
Steep Margin
• Angles of 30° or near vertical towards top and
slope gradient decreases downslope.
• Located seaward of reef ‐rimmed shelves, platforms or atolls and sandy platform margins.
Steep Margin• Spur and groove erosional
features in reef allow
sediment by‐pass across the reef and cause
sediments to be funneled
from grooves or channels.
• These ullies are closel
spaced producing a line‐
source of sediment down
the slope.
• Unlike siliciclastic slope
deposits which are brought out as a point source fed by
channels/rivers.
Gentle slopes or ramps ‐
• Located seaward of sand shoal bank margin or low biohermal buildups.
• Produces relatively continuous line source of sediment to the shelf.
• Tidal channels through banks may supply focused
se men s.
8/7/2019 Forereef to Slope Environment [Compatibility Mode]
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Sediment sources to
upper slope or fore‐reef
• 1. Shelf: reef, sandy bank margin
• 2. Pelagic: coccoliths, planktonic forams,
gastropods
– Gravity driven sediment transport down slope and
as particles settle out of water column due to
gravity.
Deposit types:
• Talus
• Grain flow
• Slumps
• Debris Flows
• Turbidites
Talus blocks and Talus Fall
• Fore‐reef deposits derived from reef rimmed
margin, platforms, or atolls.
• Form cones of sediment directly adjacent to the
reef wall.
• Arrangement of blocks is chaotic, unsorted.
• Matrix is mud or sand that filters into cavities forming perched cones or geopetal fillings.
Slumps• Can form anywhere on the slope, but are favored by steep
slopes or rapid sedimentation rates.
• More common in the Upper slope
• Form coherent, but discordant blocks to highly contorted
soft‐sediment folds or faults.
• No loss of internal cohesion that results in a fluidized
matrix.
• Tend to have discordent attitudes and lithologies often
contrast with the surrounding lithologies.
• Internal deformation may be slight or extreme
• Lithologies within the slump are generally not highly
variable.
• Adjacent beds may be distorted by drag or bulldozing of the
slump mass
Slumped reef block emplaced into
slope deposits
Intraformational Truncation Surfaces –
Slump Scars
8/7/2019 Forereef to Slope Environment [Compatibility Mode]
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Debris flows• Also called megabreccia, debris sheets, olistostromes, submarine
mass flows, and debris avalanches)
• Form breccia beds (1‐10's meters thick).
• Require slopes of less than 1°
•
Can
form
widespread
sheets
or
confined
to
channels• Sharp planar bases with load casts/generally no sole marks
• Sharp or abruptly gradational upper planar surfaces, but blocks may
project out.
• Lateral margins are abrupt and steep.
Debris Flows• Poorly sorted large blocks in matrix of
mud, tends to appear matrix‐
supported, but may also be clast‐supported.
• Size grading is not very common.
• Clasts may be rounded or angular depending on transport history prior to incorporation into the debris flow. Once in the debris flow they are not rounded due to cushioning of surrounding mud matrix.
Debris Flows
• Delicate fossils may be well‐preserved,
because interior of flow is rafted downslope
as rigid plug.
Grain Flows‐
• Grains move downslope and
are kept from settling by
knocking into each other.
• Inversely graded, contain
floating clasts, and have
base.
• Require slopes near the
angle of repose and high
supply of grainy material (generated close to bank
margin).
Turbidites‐• Also called allodapic limestones
• Deposits formed by transport of sediment in a turbid, density current.
• Can be carried far out into the basin and deposited over wide areas.
• Characterized by graded beds, Bouma sequences, sharp erosive bases, interbedded grainy horizons and pelagic muddy horizons, and lack of shallow‐water indicators.
Suspension Deposit Types
• hemipelagic ‐ shallow water shelf derived;
composition varies from fine‐grained
carbonate, siliceous ooze, pyroclastic material,
.
• pelagic ‐ generated in the upper water column
of the open ocean by organisms (globigerinid
forams, coccoliths, radiolarians (SiO2).
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Suspension Deposits
• Both types are general mud sized
material, laminated, with bedding
plane bioturbation (Zoophycus), Generally not highly bioturbated.
• Contains nodular or interbeddedchert, and is spiculitic,
• om nate y pe ag c auna
(planktonic forams, gastropods, radiolarians, coccoliths, etc.)
Steep Margin Profile and Vertical
Sequence
Gentle Margin Profile and Vertical
Sequence
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