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UMD GEOL 342 - Clastic marine and pelagic environments

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GEOL 342 Sedimentation and StratigraphyLecture 12: clastic marine and pelagic environments29 March 2005Assoc. Prof. A. Jay KaufmanClastic marine and pelagic environmentsCompared 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’ssurface. Even if it was possible to view these environments, most of these systems are notpresently active. Only during relative sea level low stands are these systems really active, so wemust look into the ancient record to elucidate these environments, which accumulate below the baselevel of erosion and hence have a high preservation potential.Clastic shelf depositsSedimentation on continental shelves (where ocean depths are less than 200 m) arecontinuous with coastal plain sequences. In tropical environments these accumulate abundantcarbonates, but in cold water or areas with high siliciclastic inputs, the shelves are covered by finesands, silts, and muds. During sea level highstands continents are flooded creating large epeiricseas that may accumulate abundant sediments. The continental shelves exposed during the LGMwere 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 bymarine processes, modern continental shelf deposits may not be good analogs for ancientsedimentation. Depositional processes on shelves are dictated by whether the sedimentsaccumulate above or below wave base, or whether tidal currents are strong enough to redistributeparticles.Where tidal ranges are large (>2 m) and currents are fast (50 to 100 cm/s) sandasymmetrical sand ribbons or tidal ridges are formed on the continental shelf. At tidal currents ofless than 50 cm/s, sheets or waves of sand develop. A tidal sand wave has a crest of 3 to 15 meters1and wavelengths of 150 to 500 meters. They are composed of low angle surfaces (dipping at 5 to 6degrees, which along with cross sets that are no more than a few meters in thickness, differentiatesthem from eolian sand dunes).Storm dominated coasts have linear sand ridges with variable cross bedding andhummocky 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. Waveripples have an irregular undulatory lower bounding surface, a less trough-like shape, bundledforests, swollen lenticular sets, and draping foresets. These are unusual in that they display theeffects 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 tidalprocesses, but changes in relative sea level are the primary source of sediments and structures onwhich these forces act. Characteristic stratigraphic profiles are recognized based on whether sealevel is regressive, transgressive, or balanced.2Continental slope and rise depositsThe continental slope between the shelf and deep ocean floor is relatively narrow (10 to100 km) and slopes downward at an average angle of 4 to 6o. Sediments are moved downslope bygravity, 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. Aturbidity current is a gravity current that suspends particles through fluid turbulence. Coarseparticles fall rapidly and require fast flows – fine particles can remain in suspension for a longtime. Current velocity is a function of concentration, column height, and gradient. Sedimentconcentration 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 themost energy and does most of the erosive work. The body carries most of the sediment and can bevery long-lived and large. It can both erode and deposit. The tail is the low concentration part ofthe flow, and is always decelerating. It deposits most places, but not very much.3Turbidity currents mix with the ambient water, which decrease their concentration therebyslowing the flow and depositing sediment. This feedback process promotes deposition of aturbidite. However, a flow may additionally erode its substrate thus adding mass to its body. Thiswould increase sediment concentration, accelerating the flow and increase erosion. This conditionis called ignition and promotes erosion. Due to this effect, continental slopes are generally places ofsediment bypass, while basin floors are sites of sediment aggradation.When turbidity currents decelerate they deposit turbidites. Material falling fromsuspension 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 allparts of the Bouma sequence are likely to be present. Material may also move below the currentas bed load. Bed load material will form dunes, ripples, and imbricated clasts, just like bed loadmaterial elsewhere. RecognitionIn the modern, deep-water systems are below the continental shelfedge. When sea level was at the shelf edge, deep water meant depositionbelow storm wave base. Basins may be 100’s or 1000’s of meters deep.Commonly, they are characterized by margins steeper than 0.5º, andcommonly 1º-3º gradients. Deep-water systems are one of the few types ofEOD’s where significant volumes of massive sand accumulate (most otherenvironments have cross-bedded sands). The types of trace fossils found indeep-water assemblages is not well understood.ClassificationDeep-water deposits can be broken into two main categories and associated environments.All environments are channelized due to the action of turbidity currents.4Name Position Geometry Depth oferosionChannel-filldepositsOverbank


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