TopicPrimary sedimentary structures

Prepared by

Azhar Mahmood

Primary sedimentary structures

Structures acquired during deposition of sedimentary rock unit are called as primary sedimentary structures.

Uses of primary sedimentary structures.

These are useful for maping purpose. These are useful to mark

paleocurrent direction. These are also use for marking of top

and bottom of strata. Environment of deposition can also

have been marked by primary sedimentary structures.

Relative age is also marked by them.

Primary Sedimentary Structures

Stratification - horizontal bedding is most common structure in sedimentary rocks

Primary Sedimentary Structures

Cross-bedding - inclined stratification recording migration of sand ripples or dunes

Primary Sedimentary Structures

Ripples - undulating bedforms produced byunidirectional or oscillating (wave) currents

Ripple marks

Uses

Asymmetrical ripple marks describe the past flow direction

Symmetrical marks top and bottum

Primary Sedimentary Structures

Graded bedding - progressive decrease in grain size upward in bed• indicator of upwards direction in deposit• common feature of turbidites

uses

It,s useful for maping purpose. These are also use for marking of top

and bottom of strata.

Primary Sedimentary Structures

Sole marks - erosional grooves and marks formed by scouring of bed by unidirectional flows

• good indicators of current flow direction

Primary Sedimentary Structures

Fossils – preserved remains of organisms, casts or moulds

• good strain indicators• determine strain from change in shape of fossil• relative change in length of lines/angle between

lines

Rip-ups

 are pieces of shale or mudstone created when an erosive current containing suspended sediment flows over a shale bed, tears up pieces of it, and carries these "rip ups" some distance

uses

Use for top and bottum Use for determine relative age

Rain-pits

 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 surfaces

Mud Cracks

are polygonal cracks that form whenmud shrinks as it dries. They indicate that themud accumulated in shallow water that periodically driedup.

uses

It,s tell us Top and bottum Environment [Arid,Semi Arid] Condition

Pebble Imbrication

Imbrication is the orderly, overlapping arrangement of flattened or sub-spheroidal grains in the direction of flow. Flow in this case is usually water flow, but it can be other types of fluid flow

Uses

To determine paleo-path of rivers and channel

Current direction {which is always opposite to dip direction}

Flute Cast

When a river flow in the center of river V type structure formed by loose material are called flute cast

Soft Sediment Deformation

In it deformation as well as sedimentation carried out side by side

S.S.D = Deformation + Sedimentation

Types 1: Slump fold 2: Flame structures 3: Clastic Dikes

a. Slump Fold

A slump is a form of mass wasting that occurs when a coherent mass of loosely consolidated materials or rock layers moves a short distance down a slope Movement is characterized by sliding along a concave-upward or planar surface. Causes of slumping include earthquake shocks, thorough wetting, freezing and thawing, undercutting, and loading of a slope

b. Flame structures A term introduced

by Walton (1956, p. 267) for a sedimentary structure consisting of sharp-crested wave- or flame-shaped plumes of mud that have risen irregularly upward into an overlying layer, generally a rapidly deposited sand

c.Clastic Dikes

A clastic dike is a seam of sedimentary material that fills an open fracture in and cuts across sedimentary rock strata or layering in other rock types. Clastic dikes form rapidly by fluidized injection (mobilization of pressurized pore fluids) or passively by water, wind, and gravity 

d. Contour Bedding

e. Growth Fault

 They extend parallel to passive margins that have high sediment supply. Their fault plane dips mostly toward the basin and has long-term continuous displacement