300px, thumbnail, _from_the_Devonian">Turbidite_from_the_Devonian-age_Becke-Oese_Sandstone.html" ;"title="Devonian.html" ;"title="Turbidite from the Devonian">Turbidite from the Devonian-age Becke-Oese Sandstone">Devonian.html" ;"title="Turbidite from the Devonian">Turbidite from the Devonian-age Becke-Oese Sandstone, Germany showing a complete Bouma sequence.
The Bouma Sequence (after Arnold H. Bouma, 1932–2011) describes a classic set of
sedimentary structures
Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition.
Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different partic ...
in turbidite beds deposited by
turbidity current
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. T ...
s at the bottoms of lakes, oceans and rivers.
Description
The Bouma sequence specifically describes the ideal vertical succession of structures deposited by low-density (i.e., low sand concentration, fine-grained) turbidity currents. An alternate classification scheme that is generally called the
Lowe sequence
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 t ...
exists for the ideal vertical sequence of structures deposited by
high-density flows.
The Bouma sequence is divided into 5 distinct layers labelled A through E, with A being at the bottom and E being at the top. Each layer described by Bouma has a specific set of
sedimentary structures
Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition.
Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different partic ...
and a specific
lithology
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. Lit ...
(see below), with the layers overall getting finer-grained from bottom to top. Most turbidites found in nature have incomplete sequences - Bouma describes the ideal sequence where all layers are present.
The layers are as follows.
* E: Massive, ungraded mudstone, sometimes with evidence of
trace fossils
A trace fossil, also known as an ichnofossil (; from el, ἴχνος ''ikhnos'' "trace, track"), is a fossil record of biological activity but not the preserved remains of the plant or animal itself. Trace fossils contrast with body fossils, ...
(i.e.,
bioturbation
Bioturbation is defined as the reworking of soils and sediments by animals or plants. It includes burrowing, ingestion, and defecation of sediment grains. Bioturbating activities have a profound effect on the environment and are thought to be a pr ...
). The Bouma E layer is often missing, or difficult to differentiate from the Bouma D layer below.
* D: Parallel-laminated siltstone.
* C: Ripple-laminated fine-grained sandstone. Often the
ripple laminations are deformed into
convolute laminations and
flame structures.
* B: Planar-laminated fine- to medium-grained sandstone. The base of Bouma B often has features known as
sole markings
Sole marks are sedimentary structures found on the bases of certain strata, that indicate small-scale (usually on the order of centimetres) grooves or irregularities. This usually occurs at the interface of two differing lithologies and/or gra ...
, such as flute casts, groove casts and parting lineation.
* A: Massive to normally graded, fine- to coarse-grained sandstone, often with pebbles and/or
rip-up clasts of shale near the base.
Dish structure A dish structure is a type of sedimentary structure formed by liquefaction and fluidization of water-charged soft sediment either during or immediately following deposition.
Terminology
Due to the similarity in its shape to a dish, the structur ...
s may be present. The base of the sandstone, below A, is sometimes eroded into underlying strata.
Processes
The Bouma sequence is deposited during waning flow as
turbidity current
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. T ...
s move downslope. In other words, flows steadily lose energy as they react to changes in the slope of the surface over which they travel, and/or as flows move from being confined within a channel to unconfined when they exit the channel and spread out. Surges and/or
hydraulic jump
A hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers and spillways. When liquid at high velocity discharges into a zone of lower velocity, a rather abrupt rise occurs in th ...
s caused by changes in slope can reinvigorate flows briefly to increase flow energy, but ultimately energy decreases as flows move away from their points of origin.
When energy within a flow is highest, it can carry the maximum amount of sediment and the largest grain sizes, but as energy decreases, the carrying capacity reduces, and the coarsest grains quickly settle out, sometimes almost instantaneously. High-energy flows may also erode into underlying beds, thereby incorporating new material into the flow, which will tend to decrease flow energy. Flows in channels can also undergo flow stripping, in which the upper part of the flow, where the finer grains tend to concentrate, separates and travels out over the top of the channel, leaving the lower part of the flow, where the coarser grains accumulate, within the channel. Ultimately, only clay particles remain, suspended in a stagnant water column with essentially no current movement.
As flows move downslope the following processes take place to create the layers of the Bouma sequence.
* Bouma E is the last layer deposited. It results from suspension settling where essentially no current exists. Clays generally remain suspended until the water chemistry changes and allows the clays to
flocculate
Flocculation, in the field of chemistry, is a process by which colloidal particles come out of suspension to sediment under the form of floc or flake, either spontaneously or due to the addition of a clarifying agent. The action differs from p ...
and settle out. Because the Bouma E layer, if deposited at all, is easily eroded by subsequent turbidity currents, it is often not present.
* Bouma D is deposited by suspension settling where a slight current exists. Subtle changes in current energy causes alternating laminations of coarser and finer grains of silt to settle out.
* Bouma C is deposited under lower
flow regime
A bedform is a geological feature that develops at the interface of fluid and a moveable bed, the result of bed material being moved by fluid flow. Examples include ripples and dunes on the bed of a river. Bedforms are often preserved in the r ...
conditions where there is enough energy for the flow to carry fine sand by
saltation, wherein grains hop and bounce across the surface beneath the flow. As grains settle out,
current ripples
In geology, ripple marks are sedimentary structures (i.e., bedforms of the lower flow regime) and indicate agitation by water (current or waves) or wind.
Defining ripple cross-laminae and asymmetric ripples
* ''Current ripple marks'', ''unidi ...
develop, with climbing ripples developing if sedimentation rates are high enough. If shear is imposed on the ripple beds by an earthquake and/or by an overlying turbidite/turbidity current, the ripple laminations can be deformed into
convolute laminations and
flame structure
A flame structure is a type of soft-sediment deformation that forms in unconsolidated sediments. The weight of an overlying bed forces an underlying bed to push up through the overlying bed, generally when both strata are saturated with water. T ...
s.
* Bouma B is deposited under upper
flow regime
A bedform is a geological feature that develops at the interface of fluid and a moveable bed, the result of bed material being moved by fluid flow. Examples include ripples and dunes on the bed of a river. Bedforms are often preserved in the r ...
conditions where energy is high enough to carry sand grains by
traction, wherein they slide and roll across the surface beneath the flow. The current energy is such that
sole marks such as groove casts, flute casts and parting lineation can form on top the bed beneath the flow, and be preserved as molds and casts on the underside of the Bouma B layer.
* Bouma A is the first layer deposited by a flow, provided the flow has sufficient energy. Otherwise Bouma B, C or D will be the first layer deposited. Bouma A is deposited when the flow energy is high enough that fluid
turbulence
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...
is able to keep the coarsest grains in suspension. When energy drops below a critical level, the grains tend to settle out all at once to create a massive bed. If flow energy drops more slowly, then the coarse grains may settle out first, leaving the fine grains still in suspension. This results in coarse-tail
graded bedding
In geology, a graded bed is one characterized by a systematic change in grain or clast size from one side of the bed to the other. Most commonly this takes the form of normal grading, with coarser sediments at the base, which grade upward into pro ...
, which means that there is a bimodal distribution of grain sizes with the coarse grains becoming progressively smaller towards the top of the bed, and the finer grains being randomly distributed between the coarse grains (i.e., the finer grain sizes are ungraded). As grains settle out, water displaced by grain compaction can move upward to create
dish structure A dish structure is a type of sedimentary structure formed by liquefaction and fluidization of water-charged soft sediment either during or immediately following deposition.
Terminology
Due to the similarity in its shape to a dish, the structur ...
s. Also, erosion can take place at the base of the flow and tear up shale from an underlying bed so that shale-rip clasts are incorporated into the base of the Bouma A layer. If there is some buoyancy to the rip-up clasts, then they may form a layer some distance above the base of Bouma A.
Examples
Image:DishStructureCalifornia.jpg, Bouma A interval of a turbidite showing dish structures with pillar structures between the dishes, Northern California.
File:Turbidite from Pigeon Pt Fm at Pescadero Beach, CA.jpg, Cretaceous turbidite showing Bouma A-D layers. Pigeon Point Formation, Pescadero Beach, California.
File:Convolute laminations, Cozy Dell Fm.jpg, Bouma B and convolute-laminated Bouma C layers in a sandy turbidite. Cozy Dell Fm, Topatopa Mountains, California.
File:Lake Berryessa turbidites3.jpg, Bouma C-D layers of distal turbidites deposited in a levee crevasse splay. Venado Fm, Lake Berryessa, California.
Notes
References
*
See also
*
Turbidites
A turbidite is the geologic deposit of a turbidity current, which is a type of amalgamation of fluidal and sediment gravity flow responsible for distributing vast amounts of clastic sediment into the deep ocean.
Sequencing
Turbidites were ...
*
Turbidity current
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. T ...
s
*
Sediment gravity flows
upright=1.25, This turbidite from the Devonian Becke-Oese Sandstone">Devonian.html" ;"title="turbidite from the Devonian">turbidite from the Devonian Becke-Oese Sandstone of Germany is an example of a deposit from a sediment gravity flow. Note th ...
*
High-density turbidity currents (
Lowe sequence
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 t ...
)
External links
Schlumberger Oilfield Glossary{{Webarchive, url=https://web.archive.org/web/20070927010858/http://www.glossary.oilfield.slb.com/Display.cfm?Term=Bouma%20sequence , date=2007-09-27
GeologyRocks Glossary Entry
Sedimentology