Lowe sequence

Last updated April 20, 2019

Layers S1-S3 in a high-density turbidite exposed near Talara, Peru. DishStructureTalara.jpg — Layers S1-S3 in a high-density turbidite exposed near Talara, Peru.

The Lowe sequence describes a set of sedimentary structures in turbidite sandstone beds that are deposited by high-density turbidity currents. It is intended to complement, not replace, the better known Bouma sequence, which applies primarily to turbidites deposited by low-density (i.e., low-sand concentration) turbidity currents.

A turbidite is the geologic deposit of a turbidity current, which is a type of sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean.

Sandstone is a clastic sedimentary rock composed mainly of sand-sized mineral particles or rock fragments.

A turbidity current is most typically an underwater current of usually rapidly moving, sediment-laden water moving down a slope; although current research (2018) indicates that water-saturated sediment may be the primary actor in the process.. Turbidity currents can also occur in other fluids besides water.

Description

The Lowe sequence adds three layers labelled S1 through S3 to Bouma's terminology, with S1 being at the bottom and S3 at the top of a sandy turbidite bed. As with the Bouma sequence, each layer has a specific set of sedimentary structures and lithology. And like the Bouma sequence, the layers become finer grained from bottom to top.^[1]^[2]

Sedimentary structures include all kinds of features formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different particle sizes are deposited on top of each other. These beds range from millimeters to centimeters thick and can even go to meters or multiple meters thick.

The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lithology may refer to either a detailed description of these characteristics, or a summary of the gross physical character of a rock. Lithology is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas. In certain applications, such as site investigations, lithology is described using a standard terminology such as in the European geotechnical standard Eurocode 7.

The layers are described as follows.^[2]

S3 - Massive to graded, fine- to coarse-grained sandstones that overlie the S2 layer represent deposition from a turbulent suspension. Sometimes dish structures and dewatering pipes are present. This layer is essentially the same as the Bouma A layer.
S2 - Inverse (reverse) graded, fine- to coarse-grained sandstone layers that overlie the S1 layer represent deposition as traction carpets, where grain-to-grain collisions are an important process.
S1 - Sandstone to conglomerate that are at the base of the turbidite and display parallel-laminated to cross-laminated beds that indicate traction deposition, wherein the current moves grains, pebbles and large clasts by rolling and sliding them across the surface beneath the flow.

Traction (geology) geologic process whereby a current transports sand grains and larger clasts by rolling or sliding along the bottom. Thus, the grains and clasts interact with the substratum during transport

Traction is the geologic process whereby a current transports larger, heavier rocks by rolling or sliding them along the bottom. Thus, the grains and clasts interact with the substratum during transport. By contrast, saltation, a related sediment transport process, moves grains across the bottom by bouncing or hopping. The actual current carries the sediment load in traction and saltation flows, whereas downslope movement under the force of gravity carries the sediment in gravity flows. These processes contrast with suspension settling, in which there is no current. Traction is where large stones or boulders in the river's load are rolled along by the force of the river.

As previously mentioned, the Lowe sequence is intended to complement, not replace the Bouma sequence. Fine-grained turbidites resulting from low-density turbidity currents, in which the Bouma A through Bouma E terminology applies, are referred to in the Lowe classification as Ta through Te, in which the T acronym derives from "Traction". By contrast, because the S1-S3 terminology describes sand-rich turbidites deposited by high-density turbidity currents, the S acronym derives from "Sandstone". Lastly, R1-R3, which uses the same descriptive criteria as S1-S3, applies to conglomerates, wherein the R acronym derives from "Rubble". In practice, the S1-S3 terminology is widely used, Ta-Te, is used sometimes, and R1-R3 is seldom used.

Processes

Initially grains, pebbles and large clasts in a high-density turbidity current (i.e., a high-sand concentration flow), are moved by traction (rolling and sliding) to generate a coarse-grained to conglomeratic, parallel-laminated to cross-laminated S1 layer. However, as grains settle out and move closer together, grain-to-grain collisions begin to generate dispersive pressures that help prevent further settling. This results in smaller grains moving between larger grains and preferentially settling out beneath them. Thus, an inverse graded layer develops that is called a traction carpet, since it is thought to move as a single unit. At some point, the grains move close enough together that collisions no longer generate enough energy to keep the grains in suspension, and the entire layer freezes to create an S2 layer. This process can then repeat to create additional traction carpets.^[2]

When grains move closer together and settle out, the water between them is displaced so that it can move upward into the flow, helping to keep grains above the traction carpets in suspension. Because the flow is in motion, this upward movement of fluid quickly becomes turbulent. When the energy of the flow drops low enough that it can no longer sustain turbulence, then the entire flow freezes to create the massive to normally graded S3 layer. Subsequent reworking of the top of this new deposit by overlying remnant currents, or by new currents unrelated to the original flow can create laminations that resemble the Bouma B layer. When reworking stops, suspension settling may deposit massive mudstone (Bouma E) directly on top of the laminated layer. Alternatively, if new sediment is introduced during this reworking phase, or if sediment is sufficiently remobilized and transported, then a more complete Bouma sequence may develop on top of the S3 layer.^[2]

De-watering pipes above the Lowe S3 layer of a high-density turbidite. Cozy Dell Formation, California.
Giant shale rip-up clast in the Lowe S1 layer of a high-density turbidite. Cozy Dell Formation, California.
De-watering dike associated with a high-density turbidite. Cozy Dell Formation, California.

California is a state in the Pacific Region of the United States. With 39.6 million residents, California is the most populous U.S. state and the third-largest by area. The state capital is Sacramento. The Greater Los Angeles Area and the San Francisco Bay Area are the nation's second and fifth most populous urban regions, with 18.7 million and 9.7 million residents respectively. Los Angeles is California's most populous city, and the country's second most populous, after New York City. California also has the nation's most populous county, Los Angeles County, and its largest county by area, San Bernardino County. The City and County of San Francisco is both the country's second-most densely populated major city after New York City and the fifth-most densely populated county, behind only four of the five New York City boroughs.

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Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particles and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (minerals) or biological detritus. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies. Sedimentation may also occur as dissolved minerals precipitate from water solution.

Conglomerate is a coarse-grained clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts, e.g., granules, pebbles, cobbles, and boulders, larger than 2 mm (0.079 in) in diameter. Conglomerates form by the consolidation and lithification of gravel. Conglomerates typically contain finer grained sediment, e.g., either sand, silt, clay or combination of them, called matrix by geologists, filling their interstices and are often cemented by calcium carbonate, iron oxide, silica, or hardened clay.

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Liquified flows and fluidized flows are types of sediment-gravity flows in which grains within the flow are kept suspension by the upward movement of fluid. They form in granular substances where the concentration of suspended mud is too low to develop cohesive forces within the flow. As grains at the base of the suspension settle out, fluid that is displaced upward by the settling generates pore fluid pressures that can help suspend grains in the upper part of the flow. Application of an external pressure to the suspension will initiate flow. This external pressure can be applied by a seismic shock, which may turn transform loose sand into a highly viscous suspension as in quicksand. Generally as soon as the flow begins to move, fluid turbulence results and the flow rapidly evolves into a turbidity current. Flows and suspensions are said to be liquefied when the grains settle downward through the fluid and displace the fluid upwards. By contrast, flows and suspensions are said to fluidized when the fluid moves upward through the grains, thereby temporarily suspending them. Most flows are liquefied, and many references to fluidized sediment gravity flows are in fact incorrect and actually refer to liquified flows. Because fluid is displaced upward in these types of flows, dewatering features such as dish structures, pillars, pipes and dikes are common.

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References

↑ Bouma, Arnold H. (1962). Sedimentology of some Flysch deposits: A graphic approach to facies interpretation. Elsevier. p. 168.
1 2 3 4 Lowe, D.R. (1982). "Sediment gravity flows: II. Depositional models with special reference to the deposits of high-density turbidity currents". Journal of Sedimentology, Society of Economic Paleontologists and Mineralogists: v. 52, p. 279–297.

Lowe sequence

Contents

Description

Processes

Related Research Articles

References

See also