GEOL 342 Sedimentation and StratigraphyLecture 12: clastic marine and pelagic environments29 March 2005Assoc. Prof. A. Jay Kaufman

Clastic marine and pelagic environments

Compared to every other kind of depositional environment, we know the least about deep-water marine and pelagic environments, even though the oceans cover over 75% of the Earth’s surface. Even if it was possible to view these environments, most of these systems are not presently active. Only during relative sea level low stands are these systems really active, so we must look into the ancient record to elucidate these environments, which accumulate below the base level of erosion and hence have a high preservation potential.

Clastic shelf deposits

Sedimentation on continental shelves (where ocean depths are less than 200 m) are continuous with coastal plain sequences. In tropical environments these accumulate abundant carbonates, but in cold water or areas with high siliciclastic inputs, the shelves are covered by fine sands, silts, and muds. During sea level highstands continents are flooded creating large epeiric seas that may accumulate abundant sediments. The continental shelves exposed during the LGM were submerged at a rate of almost 1 cm/yr due to the melting of glaciers.

Due to reworking of fluvial or deltaic sediments deposited on present-day shelves by marine processes, modern continental shelf deposits may not be good analogs for ancient sedimentation. Depositional processes on shelves are dictated by whether the sediments accumulate above or below wave base, or whether tidal currents are strong enough to redistribute particles.

Where tidal ranges are large (>2 m) and currents are fast (50 to 100 cm/s) sand asymmetrical sand ribbons or tidal ridges are formed on the continental shelf. At tidal currents

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of less than 50 cm/s, sheets or waves of sand develop. A tidal sand wave has a crest of 3 to 15 meters and wavelengths of 150 to 500 meters. They are composed of low angle surfaces (dipping at 5 to 6 degrees, which along with cross sets that are no more than a few meters in thickness, differentiates them from eolian sand dunes).

Storm dominated coasts have linear sand ridges with variable cross bedding and hummocky cross-stratification, with low-angle curved intersections and upward domed laminae. This bedform is formed at water depths of 5-15m between fair weather and storm wave base.

Wave-ripple cross-bedding displays storm and wave influence, but no tidal effects. Wave ripples have an irregular undulatory lower bounding surface, a less trough-like shape, bundled forests, swollen lenticular sets, and draping foresets. These are unusual in that they display the effects of rapidly reversing wave flow, and often show lenticular and flaser bedding.

The typical sequence of a shallow siliciclastic shelf is dominated by storm and tidal processes, but changes in relative sea level are the primary source of sediments and structures on which these forces act. Characteristic stratigraphic profiles are recognized based on whether sea level is regressive, transgressive, or balanced.

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Continental slope and rise deposits

The continental slope between the shelf and deep ocean floor is relatively narrow (10 to 100 km) and slopes downward at an average angle of 4 to 6o. Sediments are moved downslope by gravity, disloged from the shelf/slope break by storms or earthquakes.

The characteristic sedimentary features include olistoliths, slumped and defomed shales, debris flows, and turbidites.

The vast majority of sediment in the deep ocean is brought there by turbidity currents. A turbidity current is a gravity current that suspends particles through fluid turbulence. Coarse particles fall rapidly and require fast flows – fine particles can remain in suspension for a long time. Current velocity is a function of concentration, column height, and gradient. Sediment concentration is highest and grain size coarsest at the base of the flow, decreasing upwards.

A turbidity current has three parts – a head, body, and tail. The head is tall, and thus has the most energy and does most of the erosive work. The body carries most of the sediment and can

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be very long-lived and large. It can both erode and deposit. The tail is the low concentration part of the flow, and is always decelerating. It deposits most places, but not very much.

Turbidity currents mix with the ambient water, which decrease their concentration thereby slowing the flow and depositing sediment. This feedback process promotes deposition of a turbidite. However, a flow may additionally erode its substrate thus adding mass to its body. This would increase sediment concentration, accelerating the flow and increase erosion. This condition is called ignition and promotes erosion. Due to this effect, continental slopes are generally places of sediment bypass, while basin floors are sites of sediment aggradation.

When turbidity currents decelerate they deposit turbidites. Material falling from suspension forms a Bouma sequence -- Ta (massive), Tb (planar bedded), Tc (current rippled), Td (planar laminated), and Te (suspension fallout only). These can vary in proportion, and not all parts of the Bouma sequence are likely to be present. Material may also move below the current as bed load. Bed load material will form dunes, ripples, and imbricated clasts, just like bed load material elsewhere.

Recognition

In the modern, deep-water systems are below the continental shelf edge. When sea level was at the shelf edge, deep water meant deposition below storm wave base. Basins may be 100’s or 1000’s of meters deep. Commonly, they are characterized by margins steeper than 0.5º, and commonly 1º-3º gradients. Deep-water systems are one of the few types of EOD’s where significant volumes of massive sand accumulate (most other environments have cross-bedded sands). The types of trace fossils found in deep-water assemblages is not well understood.

Classification

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Deep-water deposits can be broken into two main categories and associated environments. All environments are channelized due to the action of turbidity currents.

Name Position Geometry Depth of erosion

Channel-fill deposits

Overbank deposits

Fans (Distributary channel complexes)

Basinal (local or abyssal)

Paddle-shaped

Small (5-20 m)

Ta dominated, m–vf sand &

mud

Pretty sandy, Ta-Te

Fans (Sheet complexes)

Basinal (local or abyssal)

Paddle-shaped

V. small (> 5 m)

Ta – Te, m–vf sand &

mud

Pretty sandy, Ta-Te

Confined channel complexes

Slope Straight-sinuous

V. large (20-300 m)

The full range, including v.

coarse bedload

V. muddy, thin-bedded

Levee channel complexes

Slope Straight - sinuous

Large(20-120 m)

Bed-load, lots of mud

Thin-bedded sand & mud, thinning & fining away from

channel

Fans – Distributary Channel ComplexesFan-shaped bodies of coarse sediment accumulate where turbidity current flow is

unconfined. This usually occurs at prominent decreases in local gradient, usually to less than 0.5º. As confinement and gradient decrease, flows decelerate and drop their load. This means that proximal fans are dominated by Ta beds whereas distal fans can have more Tb and Tc beds. Deceleration also decreases erosional capacity of flows. This means that fans are commonly more channelized in their proximal positions and more sheet-like in their distal positions. Channel networks on fan avulse regularly, shifting the short term site of deposition.

Slope channels -- Confined channel complexes Long channels of varying sinuosity are cut into the continental slope by ignitive turbidity currents. These channels increase confinement, promoting flow acceleration and incision. Sedimentary successions in slope environments are thus dominated by erosion and only locally aggrade large sediment volumes. The full range of turbidite deposits can occur, including large volumes of bedload-dominated material.

Leveed channels:

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When turbidity currents are tall enough to overflow their channels, they become unconfined and decelerate. However, only the low concen-tration portion of the flow does so. As flows move away from the channel, they get progressively finer grained & lower in concentration. This effect builds levees with the following characteristics: (1) they are generally fine grained, (2) beds thin away from the channel, and (3) beds fine away from the channel.

Other deposits

Many other kinds of sediments can be found in deep-water environments.

Name Depositional Process

Distinguishing structures

Other

Slope mudstones Dilute turbidites,

pelagic fall-out

Lots of Td, Tde beds Very large volumes of sediment; can be deposited by

weak bottom currentsSlumps & Slides(Mass-transport complexes)

Mass-wasting Contorted bedding, inherited bedding

Can be extremely large

Debris flows Mass-wasting Contorted – massive bedding, outsized clasts

Can be rather large

Abandonment/Drape

Dilute turbidites,

pelagic fall-out

Lots of Td, Tde beds; organic enrichment

Can be very thin and hard to map

Pelagic sediment Strictly water-column fall-out

Micro-laminated texture; lack of normal grading

Commonly basinal in occurrence

Slope mudstones:Silts and clays are brought into the deep water in two ways. These are pelagic

sedimentation, or settling out of the water column, and fine-grained turbidity currents. Many of these turbidites can be associated with small or muddy rivers as well as very large ones. Much of this material can be overbank to channels of various kind, but can also be re-distributed by weak flows on the sea floor. Grain-size and composition will vary with distance from the source rivers.

Mass-transport complexes:The continental slope accumulates more or less at the angle of repose for water-saturated

muds. As such, it is highly susceptible to failure and mass-wasting. This failure will begin as slumps or slides, which preserve much of the original bedding of the parent. If the failed material shears enough and moves enough, it can homogenize into a debris flow.

Debris flows, slumps, and slides can accumulate into very thick packages of sediment. These are called mass-transport complexes (MTC’s) and are usually made of slope mudstones that fail and accumulate locally. As such, MTC’s are usually very fine grained. The individual failures can be VERY large, covering 1000’s of km2 and be 10’s to 100’s of meters thick.

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Abandonment/Drape:During transgression, sediments are sequestered high on the continental shelf. As such,

almost no sediment reaches the deep-ocean. This leads to abandonment of active portions of the continental slope and abyss. Abandonment successions are enriched in pelagic sediments and are often assoiciated with organic enrichment.

Pelagic sedimentation:These fine-grained sediments rain out of the ocean column slowly to accumulate on the

sea floor. The sediments may enter as muddy plumes at delta fronts or low conenctration turbidity currents that flow into the water column itself. Also, very fine sediments can be carried into the ocean by wind energy. Biological sediments (e.g., diatoms, forams) that are born in the water column also contribute to pelagic “rain”.

Depending on the state of ocean chemistry and circulation, carbonate will dissolve at very great depths. This level is called the carbonate compensation depth or CCD. Think of this like a snow line in the ocean, which due to lower pH below (rather than higher temperatures) carbonate dissolves.  The ocean floor deeper than about four kilometers iscorrosive to carbonate, so these sediments do not accumulate there.

Why is the deep ocean so corrosive to carbonate?

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