Primary Structures in Sedimentary Rocks

Engr. Sultan A. Khoso

Sedimentary rocks

Sedimentary rocks are those rocks which are formed by the weathered sediments of pre existing

rocks (igneous or metamorphic rocks).

The geological processes that involved in the formation of sedimentary rocks are as under:

1. Weathering,

2. Erosion,

3. Deposition,

4. Compaction

5. cementation

Sedimentary rocks

Sedimentary rocks

Primary Sedimentary Structures

Major Primary Sedimentary Structures are:

1. Bedding (Stratification)

2. Graded Bedding

3. Cross Bedding

4. Ripple Marks

5. Mud Cracks

6. Rain Drop Imprints

1. Bedding or Stratification

Bedding (or Stratification) is defined as the deposition of sediments or fragments in the

series of beds that differ from each other by mineralogy, rock type, color or other

characteristics. The upper and lower surfaces of these layers are called bedding planes.

1. Bedding or Stratification

Layer Thickness Names

> 300 cm Massive Bed

100-300 cm Very thick bed

30 - 100 cm Thick bed

10 - 30 cm Medium bed

3 - 10 cm Thin bed

1 - 3 cm Very thin bed

0.3 - 1 cm Thick lamination

<0.3 cm Thinly lamination

1. Bedding or Stratification

2. Graded Bedding

These types of structures are classified on the basis of particle size: coarser size and Finer size.

In graded bending structures, beds with particle of finer size occurred mostly at top and the

beds with particles of coarser size occurred at base. This may be due to the different in

density, weight of coarser to finer particles.

When sediments settle down in the basin or lake, heavier particles settled down first then the

lighter ones according to their specific gravity. Thus a sequence formed is called Graded

bedding

2. Graded Bedding

3. Cross Bedding or Cross-Stratification

Cross-bedding or cross-stratification is an arrangement of beds or laminations inwhich one set of layers or beds is inclined relative to the others.

The layering is inclined at an angle to the horizontal, dipping downward in thedown current direction.

Hence, cross-beds may be used as indicators of ancient current directions.

Cross-beds usually curve at the bottom edge, becoming tangent to the lower bedsurface. The upper edge of individual inclined cross-beds is usually at a steepangle to the overlying bedding plane.

4. Ripple Marks

Ripple Marks are wavelike (undulating) structures produced in the granularsediments such as sand by unidirectional of wind or water currents or byoscillating wave currents.

Ripple marks are characteristic of shallow water deposition.

Types:• Oscillatory Ripple Marks/Symmetric Ripple Marks• Current Ripple Marks/Asymmetric Ripple Marks

A. Symmetric ripple marks or wave ripples.

B. Asymmetric ripple marks or current ripples.

a. Oscillatory Ripple Marks

Oscillatory Ripple Marks (also called wave-formed or bidirectional) are produced by waves or oscillatingwater (back and forth motion of water).

These types of ripples are generally formed in the bodies of standing water. Like lake and pond.

They have almost sinusoidal profile. Means a smooth symmetric cross sections and long relatively straightcrests. That why known as symmetric ripple marks.

They have generally sharp crests peaks and rounded trough.

These ripple marks indicate an environment with weak currents where water motion is dominated by waveoscillations (sand grains are moving in both directions, forward and backward).

Sharp crest and Rounded trough.

b. Current Ripple Marks

Current Ripple Marks (also called unidirectional) form in unidirectional currents (such as in

streams or rivers). Unidirectional flow of water or wind are the main cause of these ripple marks.

Current Ripple Marks are asymmetrical in profile, means having asymmetric cross sections. That

is why called as asymmetric ripple marks.,

Asymmetric ripples have a steep slope on the downstream side, and a gentle slope on the

upstream side.

They have generally rounded crests peaks and rounded trough.

Because of this unique geometry, asymmetrical ripples in the rock record may be used to

determine ancient current direction.

Asymmetrical ripple marks with current from left to right. Rounded crest as well as Rounded trough

Differentiate

5. Mud Cracks

An irregular fracture orcrack formed by shrinkage of clay, silt, or mud under the drying effects ofatmospheric conditions at the surface, and preserved when the beds arechanged to rock.

Also known as desiccation crack; sun crack.

6. Rain Imprints

Formed due to the impact of rain drops over the soft (e.g. clay) beds.

Rain drop impressions are a geological feature characterized bysmall hole like pits with slightly raised edges that are the result of theimpact of rain on soft sediment surfaces.