Texture and Structure of Sedimentary Rocks

Sedimentary rocks are the rocks resulting from the consolidation of loose sediments which have been derived from the pre-existing rocks and minerals, chemical precipitation from solution, and organic remains of plants and animals at or near the earth’s surface.

Texture of Sedimentary Rocks

The texture is the relationship among the mineral grains that include the shape, size, and arrangement of the constituent particles in rock. Following are the different textures of sedimentary rocks.

Grain Size:

Grain size is used to describe the size of the individual mineral grains, rock fragments, or organic material that are cemented together to form a clastic or chemical sedimentary rock. The principal factors that determine the size of grain are:

Mode of weathering
Composition of pre-existing rocks
The kind and amount of transport suffered by the materials

Following are the different size grades of particles:

S.N	Size (mm)	Name	Equivalent rocks	Classification
1	>256	Boulder	gravel/ conglomerate	Rudaceous rock
2	64 to 256	Cobble	Cobble gravel/ Cobble conglomerate
3	4 to 64	Pebble	Pebble gravel/ Pebble conglomerate
4	2 to 4	Granule	Granule gravel/ Granule stone	Arenaceous rock
5	1/16 to 2	Sand	Sandstone
6	1/16 to 1/256	Silt	Siltstone	Argillaceous rock
7	< 1/256	Clay	Shale/ Mudstone

Sorting

It is used to describe the grain size distribution or range of grain sizes in a rock. Following are the different types of sorting texture found in sedimentary rocks:

Poorly Sorted: They contain a variety of different sized grains. Poorly sorted rocks contain a wide range of grain sizes including fine, medium, and coarse.
Well Sorted: They contain almost all grains of the same size.
Moderately sorted: They contain particles of relatively similar grain sizes. Moderateyl sorted rocks may contain fine and medium grains, or medium and coarse grains.

Rounding

Rounding is used to describe the relative shape of the grains. Classification is described as deviations from rounded or spheroidal grain shapes. Following are the textures of sedimentary rocks:

Well rounded grains: They are smooth with rounded edge.
Moderately rounded grains: The grains are in-between the sharp, angular edges of a poorly rounded grain and the smooth roundness of a well-rounded grain.
Poorly rounded grains: The grains may be sharp or angular.

Relation Between Texture and Weathering

The texture of a sedimentary rock can provide a lot of information about the types of environments that the sediments were weathered in, transported by, and deposited in prior to their lithification into sedimentary rocks. Most sedimentary rocks consist of grains that weathered from a parent rock and were transported by water, wind, or ice before being deposited.

Grain size: It is a good indicator of the energy or force required to move a grain of a given size. Large sediments such as gravel, cobbles, and boulders require more energy to move than smaller sand, silt, and clay sized sediments. Grain size is also an indicator of the distance or length of time the sediments may have traveled. Smaller grain sizes generally indicate greater transport distances and duration than larger grains.
Sorting: It will generally improve with the constant or persistent moving of particles, and thus can indicate if particles were transported over a long distance or for a long time period. Sorting can also indicate selective transport of a particular grain size.
Rounding: is a good indicator for the amount of abrasion experienced by sediments. In general, sediments that have been transported longer distances will be more rounded than those which have traveled shorter distances.

Structure of Sedimentary Rocks

Any variety of large-scale features are produced in sedimentary rocks by sedimentary processes are called the sedimentary structure. The principal types of sedimentary structures are as follows:

Primary Sedimentary structure: Formed at time of deposition. E.g. Bedding.
Secondary sedimentary structure: Formed after the time of deposition. E.g. Nodules
Organic sedimentary structure: Developed due to action of organisms. E.g. Stromatolite, Track & Trails etc.

Primary Sedimentary Structure

Stratification: It is the arrangements of sediment in layers called the stratification which is characterized by the differences in color, composition, texture or structure, i.e. > 1 cm = Bed, < 1 cm = laminae.
Cross Bedding: Layering inclined at an angle to the main bedding planes of general stratification. Mainly in steeply dipping forest bed.
Graded Bedding: Layers are formed with sharply distinct grains. Typical in marine deposit.

Importance of Primary Sedimentary Structure

Show palaeocurrent condition through the analysis of sedimentary particles.
Rate of supply of sediments from source area.
Mode of transportation of sediment due to the analysis of particles.
Environment of deposition through sediment analysis.
Gives information about top and bottom of bed i.e. younger and older.

Secondary Sedimentary Structure

Ripple Marks: The wavy structures were developed in sedimentary strata. Such structures were formed due to the drag of waves and current of water or wind over sediments i.e. mainly in sand and silt size sediment. They may be symmetric and asymmetric in nature.
Mud Cracks: Formed due to shrinkage of mud at dry. River flood plain or floors of lakes are suitable sites of mud cracks.

Organic Sedimentary Structure:

Track & Trails: The movement of organisms on the loose & soft sediment. Organisms may develop markings, impressions, or footprints in the sediment. Such structures are called are known as tracks & trails.

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