SEDIMENTARY STRUCTURES

Brief content

Introduction Body Conclusion and Economic Importance reference

INTRODUCTION:

SEDIMENTARY STRUCTURES?o These are macroscopic structures found on

sedimentary rocks and sedimentary beds. They are as a result of the interraction between the processes and conditions of flow (or transport), and the sediments (grain size and bedforms).

o Based on their process of formation, sedimentary structures are basically divided into inorganic (further classified into PHYSICAL or MECHANICAL and CHEMICAL) and organic (or BIOGENIC) structures.

o Also, they can be classified based on the time of formation, i. e. divided into primary and secondary structures.

o CLASSIFIED BELOW AS

S E D I M E N T A R Y S T R U C T U R E S C L A S S I F I E D

Physical sedimentary structures

Physical (inorganic) structures are sedimentary features formed by physical processes without the influence of organism.

Primary sedimentary structures are the most important. They are mechanical structures formed during deposition of the sediments.

PLANE BEDDING The simplest sedimentary structures is plane bedding. They form

in practically all sedimentary environments and under a variety of conditions.

Three basic mechanism can form plane bedding: sedimentation from suspension, horizontal accretion from a moving bed load, and encroachment into the lee of an obstacle.

Bed forms Generated By Unidirectional Currents

As soon as flow attains a force sufficient to erode particles from the bed, sediments are transported in a set of structures of the bed called Bedforms.

If they are latter buried and preserved, they will form sedimentary structures.

Lamination

Finer scale plane bedding (less than 1 cm thick).

It can be form by alteration of light and dark layers such as glacial varves.

Lamination in mud is usually the result of slow steady deposition.

Absence of lamination in mud is due to flocculation (clumping of clays before they settle) or to secondary bioturbation.

Laminated sands are the results of rapid deposition, often by a single hydrodynamic event.

Lack of lamination may be the result of bioturbation.

Studies have shown that there is a predictable sequence of bedforms that depend on velocity, grain size, depth of flow.

In Sand that is finer than 0.7 mm (coarse or finer) the first feature to form is ripples. Typically their spacing is 10 to 20

cm or less, and their height is less than a few centimeters.

As flow velocity increase the ripples enlarge until they form sand waves, and finally dunes, which have spacing from 0.5 to 10m or more and heights of tens of cm to a meter or more. In deeper currents, greater flow velocity is required to produce the large bedforms.

With increasing flow velocity, dunes are destroyed and the turbulent flow which was out of phase turns into sheetlike flow in phase with the bedform. It forms plane beds.

At higher velocities plane beds are replaced by antidunes of up to 5m spacing. Low dip angles of 10 degrees or less, eventually chutes and pool.

Trough cross-strat.Develops from migratingRipples & dunes

Tabular cross-strat.Is produced by migratingsand waves

Symetrical ripple marks withA distinctive lenticular x-section

Terminology for the shape of the crests of ripples and dunes formed by unidirectional currents.

Ripple types and variation with flow regime

Interference pattern formIn symmetrical ripples fromTwo coexisting wave setsIn a modern tidal flat.

Herringbone cross-stratification from alternating tidal currents.

Bedform generated by multidirectional flow

In tidal regions the most significant features are caused by the mixing of sand- and mud-sized fractions from the asymetrical currents. Lenticular bedding occur when sand is trapped in troughs in the mud as sand waves migrate across a muddy substrate. If mixing produces minor mud layer in a sandy substrate the pattern is called flaser bedding.

Lenticular bedding Flaser bedding

Lenticular bedding and flaser bedding

Small-scale erosional features on a bed surface are referred to as sole marks. They are preserved in the rock record when another layer of sediment is deposited on top leaving the feature on the bedding plane. Sole marks may be divided into those that form as a result of turbulence in the water causing erosion (scour marks) and impressions formed by objects carried in the water flow (tool marks). They may be found in a very wide range of depositional environments, but are particularly commonin successions of turbidites where the sole mark is preserved as a cast at the base of the overlying turbidite.

Graded Bed

Load Structures

These result from soft sediment deformation. Load casts are one type of smaller load structure that is closely associated with sole marks. These are irregular bulging features on the base of sandstone beds that overlay shales. Unlike flute casts, they differ in their lack of symmetry and orientation. In addition, they are not remanent casts of scours, but result from the downward protrusion of sands and deformation of upper clay laminations. They can originate in any depositional environment where sands are deposited over water saturated, unstable clays.

Flame Structures

This is a type of load structure that demonstrates there was preferred orientation during deformation. These occur due to the presence of pre-existing linear projections, usually flute marks. Very often these are shales that project into overlaying sand and resemble “flames” in the cross-sectional view.

Ball and Pillow

Sands can display “pillow” like structures, like the pillow lavas associated with submarine basalt flows. These structures form from soft sediment deformation which takes place before the emplacement of overlaying sediments. The balls can also be composed of limestone, if they were true sands at the time of Deposition. The structures are largely representative of the lower part of the affected bed.

Generally, they are imperfect spheres and ellipsoids, and occasionally form kidney and mushroom shapes. Any internal structure they possess is likely to be deformed.

Dish and Pillar

These are most common in thick turbidite sandstones. They are related to sheet dewatering structures and form as vertical to sub-vertical sheets, a few millimeters wide. These sheet dewatering structures are composed of clean sand, in comparison to their more clay-rich host sandstones.

Bio-genic Sedimentary Structures

Bio-genic Sedimentary StructuresBio-genic structures result from bioturbation, the post-

depositional disturbance of sediments by living organisms. This can occur by the organisms moving across the surface of sediment or burrowing into the first few centimeters. It is usually contemporaneous with deposition.

The organisms that cause these alterations in the sediments have a dual affect. They physically and chemically alter the original deposit, and they give some useful information about the bottom conditions at the time of deposition.

Depending on the location, these animals can affect up to 90% of the sediment surface. As a result, this biological action can completely destroy primary laminations and the original orientation of sediment particles.

There are usually significant changes in the formations porosity and permeability. This can help determine the origin and magnitude of any abnormal pore pressures. It can also enhance the tendency for slumping and sliding on steep continental margins, as the sediment shear strength is decreased.

Conclusion and Economic Importance:

Having gone through the study of sedimentary structures, it is necessary to note that they can be used to determine depositional processes, being that depositional processes occurs in several environments, and few structures are immediately diagnostic of a specific environment.

Sedimentary structures can also indicate the direction of paleocurrent flow, paleoslope, paleogeography and sand-body trend.

They record the processes that occur during deposition and so help in interpreting sedimentary and post-depositioal processes

Help in defining geological history and surface processes .Cross-beds within ancient rocks helps to tell the direction a

stream flowed millions of years ago or the direction of prevailing wind in the past.

Reference

Baker hughes INTEQ, Petroleum geology.

Principles of Sedimentation and Stratigraphy.

Sedimentary structures, Wikipedia.

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