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EASC 201 Cross bedding Study Notes

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tary%20Structures.pdf

Cross bedding (planar tabular and trough xstrat)

Cross-bedding is stratification inclined to the original horizontal surface upon which the sediment

accumulated. Cross-beds are formed in beach, river and sand-dune environments. It is produced by

deposition on the downcurrent slope of a dune or sand wave. It is produced by the deposition of

fragments of sediment layers which are then eroded and truncated by later water currents or winds that

deposit new sediments at an angle on top of the erosion surface (scours). Cross-beds can be used to

determine bedding tops because of the truncation of the beds.

•

Planar tab:

Straight ripples produce planer cross lamination. Planar laminae are essentially parallel to the

bounding surfaces of a bed. Cross laminae are at an angle to these bounding surfaces.

As bedforms and troughs move downstream, broad foresets are deposited and are truncated by

broad troughs. On the parallel-to-flow face, the resulting cross stratification appears as

downstream-dipping foresets separated by slightly upstream-dipping set boundaries. On the

perpendicular-to-flow face, both the foresets and the set boundaries form what appear to be flat-

lying laminae. This structure is known as tabular cross stratification. 2D bedforms make tabular

cross stratification.

Tabular cross-bedding consists of cross-bedded units that are broad in lateral dimensions with

respect to set thickness and that have essentially planar bounding surfaces. The laminae of tabular

cross-beds are also commonly planar, but curved laminae that have a tangential relationship to

the basal surface, as explained above, also occur.

Tabular cross-bedding is formed mainly by the migration of large-scale ripples and dunes; thus, it

forms during lower-flow-regime conditions. Individual

beds range in thickness from a few tens of centimeters to a meter or more, but bed thicknesses up

to 10 m have been observed

•

From lab slides:

Planar Tabular Cross-stratification occurs in beds with

sharp, flat basal surfaces.

◆ Cross-stratification shows minimal toeset development.

◆ Foreset inclinations may reach angle of repose (32° in medium sand in water)

•

Trough:

Sinuous ripples produce trough cross lamination.

3D bedforms are characterized by narrow, bowl-shaped troughs and short, discontinuous ripple

crests. As these move downstream, the troughs cut scoop-shaped scours that are then filled in

from the sides and the rear by foresets. These foresets also connect to one another between

laterally adjacent troughs, such that an individual foreset can be visualized as a curved or wrinkled

sheet that is concave up where it fills a trough and convex up between troughs. On the parallel-to-

flow face, this cross stratification consists of downstream-dipping foresets separated by slightly

upstream-dipping set boundaries. In other words, on the parallel-to-flow face, it is

indistinguishable from tabular cross stratification. On the perpendicular-to-flow face, set

boundaries are concave-upward surfaces that truncate underlying foresets. Foresets on this face

are also concave-upward where they overlie set boundaries. The convex-upward foresets

expected between troughs are preferentially eroded away because they are on the topographic

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high points of the bed; as a result, most or all of the foresets will be concave-upward. This

structure is known as trough cross stratification. 3D bedforms make trough cross stratification.

Trough cross-bedding can originate both by migration of small current ripples, which produces

small-scale

cross-bed sets, and by migration of large-scale ripples. Trough cross-bedding formed by migration

of large-scale ripples commonly ranges in thickness up to a few

tens of centimeters and in width from less than 1 in to more than 4 m.

•

From lecture: trough cross-stratification occurs in beds with sharp, undulatory basal surfaces.

Cross-stratification shows well-developed foresets. Foreset inclinations may reach maximum angle

of repose (32 degrees in medium sand in water). Outcrop shows troughs perpendicular o flow, but

in core, stratification looks similar to that of planar tabular.

Graded bedding

Graded bedding is characterized by a gradation in grain size from bottom to top within a single bed.

“Normal” graded beds are coarse at the bottom and become finer toward the top, a product of

deposition from a slowing current (Figure 6.22). Some graded beds are reversed (coarser at the top),

and this normally results from deposition by a fast-moving debris flow (see Chapter 15). Most graded

beds form in a submarine-fan environment (see Figure 6.17), where sediment-rich flows descend

periodically from a shallow marine shelf down a slope and onto the deeper sea floor.

Graded beds are sedimentation units characterized by distinct vertical gradations in grain size. They

range in thickness from a few centimetres to a few metres or more and commonly have sharp basal

contacts. Beds that show gradation from coarser particles at the base to finer particles at the rop are

said to have normal grading. Normal graded bedding can form by several processes, eg sedimentation

from suspension clouds generated by storm activity on the shelf or deposition in the last phases of a

heavy flood, but the origin of most such graded beds has been attributed to turbidity currents. Reverse

grading can also occur and has been attributed to two types of mechanisms: dispersive pressures and

kinetic sieving. In a mixture of grains undergoing agitation, the smaller grains presumably fall down

through the larger grains as grain motion opens up spaces between the larger particles.

Bouma cycle

Ideal turbidite sequence that consists of five structural units that include the characteristics of both

types of turbidites. These structural subdivisions presumably record the decay of flow strength of a

turbidity current with time and the progressive development of different sedimentary structures and

bedforms in adjustment to different flow regimes (upper to lower) as current-flow velocity wanes. Most

turbidites do not contain all of these structural units. Thick, coarse-grained turbidites tend to show well-

developed A and B units, but C through E units are commonly poorly developed or absent. Thin, finger

grained turbidites may display well-developed C through E units and poorly developed or absent A and B

units. Bouma's D unit seems to rarely occur, and most turbidites can be divided into only two units: a

lower horizontally laminated unit (A+B), and an upper, cross laminated unit (Unit C). Unit E is a problem

because it may consist of fine material deposited slowly from the water column and thus it may not be

part of a turbidite flow unit. The deposits of a single turbidity current flow typically display horizontal

size grading in addition to vertical size grading. That is, thick, coarse grained deposits generally grade

laterally to thinner and finer grained sediments.

Bouma Cycle: idealized succession of internal units within classical turbidites

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http://strata.uga.edu/4500/index.html ftp://soest.hawaii.edu/glenn/GG309/GG309%202012/2012%20Part%201/SuplRead%202%20Sedimen tary%20Structures.pdf Cross bedding (planar tabular and trough xstrat) Cross-bedding is stratification inclined to the original horizontal surface upon which the sediment accumulated. Cross-beds are formed in beach, river and sand-dune environments. It is produced by deposition on the downcurrent slope of a dune or sand wave. It is produced by the deposition of fragments of sediment layers which are then eroded and truncated by later water currents or winds that deposit new sediments at an angle on top of the erosion surface (scours). Cross-beds can be used to determine bedding tops because of the truncation of the beds. • Planar tab: Straight ripples produce planer cross lamination. Planar laminae are essentially parallel to the bounding surfaces of a bed. Cross laminae are at an angle to these bounding surfaces. As bedforms and troughs move downstream, broad foresets are deposited and are truncated by broad troughs. On the parallel-to-flow face, the resulting cross stratification appears as downstream-dipping foresets separated by slightly upstream-dipping set boundaries. On the perpendicular-to-flow face, both the foresets and the set boundaries form what appear to be flatlying laminae. This structure is known as tabular cross stratification. 2D bedforms make tabular cross stratification. Tabular cross-bedding consists of cross-bedded units that ar ...
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Tags: Cross bedding Graded bedding Bouma cycle Oscillation ripples Aggradational Oscillation