Sedimentary Structures

Sedimentary Structures

Definition of primary sedimentary structures

:- it is that structure which formed at the phase of deposition and before solidification of rocks.

*Secondary structures:

1-It is that forms undergo deformation after solidification of rocks.

2- Rocks architectures that produced as a result of deformation of primary sedimentary structures/ distortion under effect of external processes.

Penecontemporaneous; means after deposition and before solidification of rock.

Internal sedimentary structure: it is that structure appears in completely horizontal planes called as bedding.

Definition of bedding: it is the basic structural unit in sedimentary succession, has an upper and lower surfaces, lateral extension. Each unit characterized by its own unique chemical and lithologic composition

Classification of bedding:

Bedding plane may be;

-Sharp/planar

-Irregular

-Wavey

Cross bedding classified according to the geometry of foreset as follows

1-Tabular/planar; foreset form acute angel with the upper and lower sets

2- Tangential; tangent to the top and bottom sets.

3- trough cross bedding; approximately parallel  to the sole of beding

Cross bedding classified according to the geometry of foreset as follows

1-Tabular/planar; foreset form acute angel with the upper and lower sets

2- Tangential; tangent to the top and bottom sets.

3- trough cross bedding; approximately parallel  to the sole of beding

Determination of depositional currents (direction)

 1- Wedge shape cross bedding

Wannwing current

2- Herring bone

Cross beds form as sand dunes or ripples migrate due to wind or water currents. Herringbone cross bedding forms when current periodically flows in opposing directions such as in a tidal flat or tidal  channel. The result is alternating layers of cross beds dipping in opposite directions. The pattern created resembles the backbone and ribs of a herring or fish.

3- Gradded bedding ;- progressive change in clast grain size from base to top

 - Normal grading:  fining from base to top (most common)

2- Reverse, or inverse, grading: coarsening from base to top

generally results from deposition from turbidity flows

5- Fissility

Fissility refers to the property of rocks to split along planes of weakness into thin sheets.^[1] This is commonly observed in shales, which are sedimentary rocks, and in slates and phyllites, which are foliated metamorphic rocks. The fissility in these rocks is caused by the preferred alignment of platy phyllosilicate grains due to compaction, deformation or new mineral growth