SEDIMENTARY SEDIMENTARY ROCKSROCKS

Dr. P. Sarathbabu M.Sc. B.Ed. Ph.D.Department of GeologyAcharya Nagarjuna University

What type of rock is formed when weathering and erosion cause sediments to press together in layers?

Characteristics

Tough rock with layers (foliation) of light and dark minerals, often curved; various colors; often glittery from mica

The Making of Sedimentary Rocks

Sedimentary Rocks• sediment: unconsolidated (“soft-rock”)• sedimentary rock: consolidated (“hard-rock”)

• Sedimentary Rocks are rocks formed when particles of sediment build up and are “cemented together” by the effect of pressure and minerals.

sea

Fragments washed to the sea

Sedimentary rocks

Rocks are brokenup by the actionof weather

Getting older

• Form at the Earth’s surface by the lithification of sediments deposited from their transporting medium (water, wind, or ice).

Types of Sediment

Sediment in a Stream

Transport: rounding, and size reduction

Sedimentary structures

*Records the sedimentary history and depositional environment; provides valuable clues as to deposition. Most basic is simple bedding (layering effect of sedimentary rocks). 

1. Ripple marksRipple marks - undulations on a sand surface produced by wind or water (asymmetrical or symmetrical)

2. Cross-beddingCross-bedding - inclined layering produced in sand by ripples or dunes at an angle to the horizontal

3. Mudcracks (desiccation cracks)Mudcracks (desiccation cracks) - polygonal pattern of cracks produced on the surface of mud as it dries.

4. Raindrop imprintsRaindrop imprints - circular pits produced by the impacts of rain on soft mud5. Graded beddingGraded bedding - progression of grain sizes from coarser at the bottom to finer

on top (or vice-versa)6. Flute marksFlute marks - scoop-shaped depressions preserved on the bottom surface of

muddy beds. Good indicator of top and bottom of bed.7. Tool marksTool marks - ridges or discontinuous marks with a preferred orientation on

bottom surfaces of beds. Indicates current direction.8. TracksTracks-footprints of organics, dinosaur footprints. TrailsTrails are caused as organism

crawls through mud or sediment. BurrowsBurrows - excavations made by organisms in soft sediment commonly filled with different sediment

9. StromatolitesStromatolites-mound-like structures formed when sediment is trapped by blue-green algae. Characteristic of limestones. Oldest life form on Earth.

10. StylolitesStylolites-pressure solution cracks formed as pressure squeezes solution through fractures in carbonates.

1. Ripple marksRipple marks - undulations on a sand surface produced by wind or water (asymmetrical or symmetrical)

Ripple marks

Asymmetric and Symmetric Ripples

river or wind currents (uni-directional)

tidal currents (bi-directional)

• 2. Cross-beddingCross-bedding - inclined layering produced in sand by ripples or dunes at an angle to the horizontal

Development of Cross-Bedding

Typical cross stratification inSand Dunes

3. MudcracksMudcracks - polygonal pattern of cracks produced on the surface of mud as it dries.

Origin of Mud Cracks

4. Raindrop imprintsRaindrop imprints - circular pits produced by the impacts of rain on soft mud

5

GRADED BEDDING

Typical cross stratification in

Sand Dunes

Formation of Coal from Swamp Deposits

6. Flute marksFlute marks - scoop-shaped depressions preserved on the bottom surface of muddy beds. Good indicator of top and bottom of bed.

7. Tool marksTool marks - ridges or discontinuous marks with a preferred orientation on bottom surfaces of beds. Indicates current direction.

8. TracksTracks-footprints of organics, dinosaur footprints. TrailsTrails are caused as organism crawls through mud or sediment. BurrowsBurrows - excavations made by organisms in soft sediment commonly filled with different sediment

9. StromatolitesStromatolites-mound-like structures formed when sediment is trapped by blue-green algae. Characteristic of limestones. Oldest life form on Earth.

10. StylolitesStylolites-pressure solution cracks formed as pressure squeezes solution through fractures in carbonates.

There are two (2) types of sedimentary rocks, based on their texturestextures:

1.1. ClasticClastic (also called “Detrital”)(also called “Detrital”)-- form from deposition of solid grains; classified based on grain size: conglomerate, sandstone, and shale

2. ChemicalChemical -- form from minerals precipitating out of water and usually involves some sort of chemical reaction; classified based on mineral content: limestone, dolomite are examples.

 

ClasticClastic Sedimentary Rocks Classification based on grain size 1. Conglomerate (rounded grains) -- more than 30% of

grains are gravel-sized (> 2mm) 2. Breccia (angular grains) -- more than 30% of grains are

gravel-sized (> 2mm)3. Sandstone -- grain bits. 1/16mm-2mm 4. Shale -- made of mud-sized grains (<1/16 mm)  ChemicalChemical Sedimentary Rocks classified by composition1.  Limestone -- made mostly of calcite 2.  Dolomite (dolostone) - made mostly of dolomite3.  Evaporites -- various mineral salts that precipitate from

evaporating water (ex: halite, gypsum)4. Bituminous Coal: organics compressed into peat then

lithified into coal 

CONGLOMERATE

ConglomerateGrain size : coarse grained >256mmUsual Colour : Brown Shape: sub angular-rounded Structure: Clastic, crude beddingComposition : Quartz, jaspecr

BrecciaGrain size : 2mm - >256mmUsual Colour : Reddish Brown Shape: Angular Structure: Clastic, crude beddingComposition : Quartz(pebbles) siliceous matrin

chert

Sedimentary rocksThere are 5 main kinds of sedimentary rocks, depending

on the mix of minerals in the rocks.

1.CONGLOMERATE Grain size : 12mm – 256 mm Usual Colour : Light (Leucocratic) Structure : clastic, crude bedding Texture : closed frame work, porphyritic texture,

very coarse grained Composition : Quartz, Jasper pebbles in calcareous

matrixOccurrence: Its occurs as rounded pebbles

indicate high energy environment and less distance transportation.

SANDSTONE

Sandstone

Grain size : coarse grained

Usual Colour : Brown

Shape: subangular-

subrounded Structure: crude

beddingComposition :

Quartz, feldspar

Sedimentary rocks2.SANDSTONE Grain size : 1 – 2mm Usual Colour : Light brown Structure : stratification, current bedding, ripples

marks, rain prints Texture : well sorted grains, >2mm-coarse

0.5 – 0.25- medium 0.25-0.1- fine

Composition : quartz as chief mineral, feldspars, mica,garnet as minor amount.

Occurrence: fine grains indicate high energy of deposition and rounded shape indicate long transportation

LIMESTONE

LimestoneGrain size : fine grainedUsual Colour : grey Shape: laminate Structure: crude beddingComposition : calcite

Sedimentary rocks3.LIMESTONE Grain size : Fine grained, compact Usual Colour : Grey Structure : crude bedding Texture : It is fine grained rock. It is commonly

compact and massive. Some Lst; may have organic structure.

Composition : Calcite, magnesite is also present is variable amounts, some contains calcareous shells

Occurrence: These occurs as non-detrital rocks. They formed either chemically, due to precipitation of calcium carbonate from surface water, or organically, due to the accumulation of hard parts of organisms.

SHALE

ShaleGrain size : Fine

grainedUsual Colour : Yellow-

black Shape: rounded Structure: Lamination,

closed fabric

Composition : clay minerals, silicates

Sedimentary rocks4.SHALE Grain size : Fine grained Usual Colour : variables Structure : Lamination, ripple marks and organic

structures Texture : very fine grained <0.01mm Composition : Its composed of kaolinite,

montmorillonite and illite,qtz,mica,chlorite are in minor amount

Occurrence: Shales formed out of mechanically transported and deposited sediments. These are made up of fine grained i.e; silty and clay, and occurrs as laminated structures.

LATERITE

Sedimentary rocks5.LATERITE Grain size : Fine grained Usual Colour : Red Structure : Lamination, ripple marks and organic

structures Texture : porus and concretionary Composition : These are essentially clay rich in

alluminium and iron hydroxides with minor amounts of silica, alluminimum hydroxides are called bauxites(pisolitic texture).

Occurrence: Its occurs as residual deposits due weathering in hot humid climate, and occurs as mantle over bed rocks.