Instituted of Geology6th Semester (Morning)

Submitted by: Raja Saad Qamar Roll No:37

Submitted To: Mam ShaziaSubject: sedimentation and sedimentary basin

INSTITUTE GEOLOGY

SUBJECT SEDIMENTATION AND SEDIMENTARY BASIN OF PAKISTAN

TOPIC SEDIMENTARY STRUCTURES

Sedimentary structures

Sedimentary structures are those structures formed during

or just after sediment deposition.

Types of sedimentary structuresThere are two types of sedimentary structures

PRIMARY SEDIMENTARY STRUCTURE

SECONDARY SEDIMENTARY STRUCTURE

PRIMARY SEDIMENTARY STRUCTURE: The sedimentary structure formed during the deposition of rock are

called Primary structure Major Primary structures are: 1 Gradded bedding 2 Ripple marks 3 Rip-ups 4 Pebble imbrication 5 Mud cracks

1) GRADDED BEDDING

When sediment settle down in the basin or in lake; heavier particles settled down first then the lighter ones according to their specific gravity. Thus a sequence formed is called gradded bedding.

2) RIPPLE MARKS

The water of the ocean or river have wavy motion; because of this wavy motion wavy structures are developed on he rock bed underlying the water; is called Ripple marks

3) RIP-UPS

If the sediments of underlying rocks are found on the bottom of overlying rocks then the primary structure formed is called Rip-ups

4) PEBBLE IMBRICATION

In the moving water bodies if the disk like pebbles stacked with one another due to some obstacle in their path, formed a imbricated structure which shows paleo current direction is called Pebble imbrication

5) MUD CRACKS

Clay have swelling property in the presence of water, in arid condition when the swell clay exposed to sun water evaporates and cracks will form on the surface, these cracks have V-shaped opened structure when viewed in cross-section, which when filled with secondary material form a sedimentary structure called Mud cracks

Secondary sedimentary structure

The sedimentary structure formed after the deposition of rock are called secondary sedimentary structure

Major secondary structures are 1) Liesegang rings 2) Cone-in-cone structures 3) Flame structures 4) Raindrop impressions 5) Slump structures

1) Liesegang rings

Liesegang rings are colored bands of cement observed in sedimentary rocks that typically cut-across bedding. These secondary sedimentary structures exhibit bands of minerals that are arranged in a regular repeating pattern. Liesegang rings are distinguishable from other sedimentary structures by their concentric or ring-like appearance.

2) Cone-in-cone structures

Cone-in-cone structures are secondary sedimentary structures that form in association with deeper burial and diagenesis. Cone-in-cone structures are identifiable by their distinctive conical appearance. They are composed of concentric cones nested inside each other. The actual composition of the cones is variable and dependent on the environment in which they were formed, with the majority of the cone-in-cone structures being composed of calcite with thin layers of clay between cones.

3) Flame structures

Flame structures consist of mud and are wavy or "flame" shaped. These flames usually extend into an overlying sandstone layer. This deformation is caused from sand being deposited onto mud, which is less dense.

4) Raindrop impressions

Raindrop impressions are a geological feature characterized by small craterlike pits with slightly raised edges that are the result of the impact of rain on soft sediment surface. Raindrop impressions can appear as small craters or as small raised bumps.

5) Slump structures

Slump structures are mainly found in sandy shales and mudstones, but may also be in limestones, sandstones, and evaporites. They are a result of the displacement and movement of unconsolidated sediments, and are found in areas with steep slopes and fast sedimentation rates. These structures often are faulted.

References:

 *AILSA ALLABY and MICHAEL ALLABY. "sediment." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 8 Nov. 2010 <http://www.encyclopedia.com>.

Jump up^ Boggs, Sam jr, 2006 Principles of Sedimentology and Stratigraphy, Patrick Lynch, Principles of Sedimentology and Stratigraphy, Pearson Prentice Hall, Upper Saddle River, NJ. Ed 4 p 83-84

Jump up^ AILSA ALLABY and MICHAEL ALLABY. "antidune." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 8 Nov. 2010 <http://www.encyclopedia.com>. B

Jump up^ http://jsedres.geoscienceworld.org/cgi/content/abstract/35/4/922 C