GEOLOGICAL FIELD TRIP

Chandipur

Objectives-

To study the sedimentary facies and sedimentary structures in

and around Chandipur Beach

To understand the direction of current and wave

CONCEPTS LEARNED -

Sedimentology is the study of the processes of formation, transport and

deposition of

material that accumulates as sediment in continental and marine

environments and

eventually forms sedimentary rocks.

Stratigraphy is the study of rocks to determine the

order and timing of events in Earth history: it provides the time frame that

allows us to

interpret sedimentary rocks in terms of dynamic evolving environments.

Silt is defined as the grain size of material between 4 and 62 microns in

diameter. This size range is subdivided into coarse, medium, fine and very

fine. The coarser grains of silt are just visible to the naked eye or with a hand

lens. Finer silt is most

readily distinguished from clay by touch, as it will feel ‘gritty’ if a small

amount is ground between teeth,

whereas clay feels smooth.

Clay is a textural term to define the finest grade of clastic sedimentary

particles, those less than 4 microns in diameter.Clay minerals are a group of

phyllosilicate minerals that are the main constituents of clay-sized particles.

When clay- and silt-sized particles are mixed in unknown proportions as the

main constituents in unconsolidated sediment

Sedimentary Facies-A sedimentary facies is a body of sediments with

specified characteristics that reflect the conditions under which it was

formed (Reading & Levell 1996).Describing the facies of a body of sediment

involves documenting all the characteristics of its lithology,texture,

sedimentary structures and fossil content that can aid in determining the

processes of formation

Four different types of sedimentary facies -

Sand Berm – The sedimentary facies having steeper slope with

vegetation on it and having a dry land. Pile of sand formed due to

Aeolian activities. It consists of heavy particles.

Backshore – The sedimentary facies which extends from the limit of

high water foam lines to dunes or sand berms .It is affected by waves

during exceptional high tides or severe storms.

Front Shore- The part of a shore between high and low water marks

or between the water and cultivated or developed land

Tidal flat- the coastal wetlands that form when mud is deposited by

tides or rivers. They are found

The below figure explains the transportations of sediments which

results in the formation of sedimentary ripples in berm region .

The below figure shows the ripples in tidal flat .

DUNES - Beds of sand in rivers, estuaries, beaches and marine environments

also have bedforms that are distinctly larger than ripples. These large

bedforms are called dunes. the term ‘megaripples’ is also sometimes used,

although this term fails to emphasise the fundamental hydrodynamic

distinctions between ripple and dune bedforms.

SHARP CHANGE IN TOPGRAPHY –

As we move from sand dunes / sand berms to tidal flat , the dip angle

decreases from 17 degrees in sand berms to 2-3 degrees in backshore

and frontshore to less than 1 degrees in Tidal Flat.As we move

perpendicular to shore , the morphology of the rocks changes from

sand particles to silts to clay .Also , as we move from sand berms to

tidal flat ,the hardness of the region increases and the stickiness

decreases.

Changes in patterns of ripples are observed. These patterns changes

everyday. As we move away from the coast line, the ripples patterns

changes –

It becomes angular to the coast line

It becomes edge shaped (Rounded)

They become elongated (Rectangular)

Becomes wavy structures

WHY ALGAE ONLY IN TIDAL FLAT?

Algae get nutrients in clay, that’s why they stay there. Nutrients

in tidal flat region are carried by ground water because the slope of ground

water beneath the surface is much lower than any other sedimentary facies.

The figure below shows the algae in tidal flat region.

PRIMARY AND SECONDARY SEDIMENTARY STRUCTURES –

Primary structures are those which we see in modern sediments and

are preserved in rocks Secondary structures are those which are formed

after the rocks have been formed

DEPOSITION HISTORY –

Firstly , Sediment are carried from balasore to chandipur through Aeolian

and water activities .During this carry , minerals like plagioclase feldspar and

alkali feldspars get destroyed.

Secondly, the deposition is from sea, which leaves patterns on ocean floor.

The above Hjulstrom diagram shows the relationship between the velocity

flow and the transport of loose grains on the basis of their grain size

The heavier particles are settled near the berms and the finer grained

particles are carried way deep in the ocean resulting in the different

sedimentary facies.

As we move away from the shore line the grain size decreases -

Four basic divisions are recognised:

clay (<4 mm)

silt (4 mm to 63 mm)

sand (63 mm or 0.063 mm to 2.0 mm)

gravel/aggregates (>2.0 mm)

MARINE TRANSGRESSION AND REGRESSION -

Transgression – time when sea goes to land in geologic time scale is called

as transgression

Regression – time when land moves away from sea in geologic time scale is

called as regression

Dynamic equilibrium between coastline and sea line.Ocean is the primary

basin. Capacity depends on tectonic forces and topography

EFFECT OF GLOBAL WARMING –

Due to global warming , the ice caps will melt and the sea level will rise

(approx. 1 m) ,the marine transgression occurs because the sea level rises

and submerges the coast in it . Thus the coast line shifts backward (i.e .

Coast Regression ) occurs .The proof of this hypothesis is that , while moving

perpendicular to the shore line , it is observed that some charcoal tracers

were found parallel to the shore line which is the proof that the coast line

millions of years ago was here.

SYMMETRIC AND ASYMMETRIC RIPPLES -

Symmetry ripples are formed from Bi-directional forces.When these forces

are equal in magnitude and opposite in direction , they forms stoss and lee

sides which are equal in lengths .

Asymmetry ripples are formed from Uni-Directional forces. Thus , due to a

uni directional force , sediments are eroded from stoss side of the ripple and

gets deposited on lee side of the ripple thus , the lengths of stoss and lee

sides differ from each other .

CURRENT RIPPLES –

Current ripples are small bedforms formed by the effects of boundary layer

separation on a bed of sand (Baas 1999). The small cluster of grains grows to

form the crest of a ripple and separation occurs near this point. Sand grains

roll or saltate

up to the crest on the upstream stoss side of the ripple. Avalanching of

grains occurs down the downstream or lee side of the ripple as accumulated

grains become unstable at the crest. Grains that avalanche on the lee slope

tend to come to rest at an angle close to the maximum critical slope angle

for sand at around 308. At the flow attachment point there are

increased stresses on the bed, which result in erosion and the formation of a

small scour, the trough of the ripples.

WAVE AND CURRENTS –

Distinguishing between wave and current ripples can be critical to the

interpretation of palaeoenvironments. Wave ripples are formed only in

relatively shallow water in the absence of strong currents, whereas current

ripples may form as a result of water flow in any depth in any subaqueous

environment. These distinctions allow deposits from a shallow lake are to be

distinguished from offshore or deep marine environments .for example. The

two different ripple types can be distinguished in the field on the

basis of their shapes and geometries. In plan view wave ripples have long,

straight to sinuous crests which may bifurcate (divide) whereas current

ripples are commonly very sinuous and broken up into short,curved crests.

When viewed from the side wave ripples are symmetrical with cross-

laminae dipping in both directions either side of the crests. In

contrast,current ripples are asymmetrical with cross-laminae dipping only in

one direction, the only exception

HEAVY MINERALS DEPOSITED IN BERMS –

Heavy minerals get deposited in sand berms but not in front shores and tidal

flats region because as the sediments are carried from balasore to chandipur

region , the heavier material falls downwards with sand particles and only

finer grain particles are carried forwards , hence , giving the ripples a

blackish colour.

The figure below shows the black minerals clearly visible over the ripples

formed by aeolian deposits.

RIPPLE INDEX AND SYMMETRY INDEX –

Ripple index – The ratio of wavelength of the ripple is to the height of the

ripple formed by wave or current action.

Symmetry Index – The ratio of lengths of stoss side to lee side of ripple is

called symmetry Index.

RI < 4 – Ripples are formed by Wave Action

RI >15 – Ripples are formed by Current Action.

SI = 1 ( Symmetric Ripples are formed )

SI <1 ( Asymmetric Ripples are formed )

OBSTACLE MARKS – The marks which are left away when water recedes

away from front shore during its interaction with shales .

SWASH RIPPLES – The unique patterns of ups and downs obtained in the

Front shore is called swash ripples .

The figure below shows the horizontal swassh ripples clearly visible .

PARTING LINEATIONS – The groove where shales are deposited orthogonal

to the shore line and occur in alternate patterns is called parting lineations.

The figure below shows the parting lineations clearly visible perpendicular to

the shore line.

LADDER RIPPLES – The ripples obtained in the area of depression between

two ripples

ROSS DIAGRAM – Frequency Distribution of the plunge of the ripples in

different sedimentary facies

For example – obstacle marks in front shore , sand ripples in sand berms /

Dunes.

MUD BALLS –

Found in the area between front shore and tidal flat.Mud balls are formed

as a result of collection of water. If the water has sufficient energy such that

it exceeds the cohesive force between clay particles. It scoops out the clay

and roll it along the minimum diameter to form mud balls.

Once the mud ball is formed . Coming in Water will provide sufficient

torque ot rotate it aloing the longest direction thus , helps in determining

the water direction .

Now the mud ball is aligned in the direction of water flow , thus when

further water comes it acts as a obstacle to water , thus water will

continuously erode the sediment and mud ball will topple in the depth

formed . Thus these mud balls are preserved in the sedimentary sequence.

The below figure shows the mud ball which is nearly spherical ( Figure 1 )

The Second Figure shows the Mud ball naturally placed within the eroded

depth .

FIG – 1 FIG-2

BARRIER BAR –

A sand body having huge amount of sand deposited very quickly in front of

shore because of events like storm.

HOW MUDDY AREA IS FORMED IN OPEN ( I.E BETWEEN FRONTSHORE AND

TIDAL FLAT ) ??

It is because of coast regression . Previously , a sand barrier may have been

lying opposite to the muddy area (i.e in the area of tidal flat), now as the

water level increased , the barrier bar got destroyed/eroded and the coast

shifted backward (i.e coast regression took place) , Thus this transgression of

sea shore is called Local Transgression Sequence.

Thus the water collected in between the two barriers resulted in the

formation of marshy land .

WALTHERT’S LAW –

It states that the sedimentary facies which are occurring in vertical sequence

today , they have been lying adjacent to each other many years ago.

The figure below shows the alternate bands of sediments and clay

depositions which proves the walthert’s law and marine transgression and

regression .

REEL MARKS – Due to enough pressure in the surroundings nearby, the

ground water discharge takes place occasionally along a valley which occurs

in front shore region . This valley is called Reel Marks

The figure below shows the reel marks which are ground water coming out

of the land fgrom the pressure of the rocks above the land.

FIELD WORK (OBSERVATIONS ) -

Sand berms-

o Steep Slope around 17-20 degrees.

o Consists of mostly silica with traces of heavy black

minerals on the crests of ripples. They might be

hornblende ,tourmaline.(Figure)

o Particles are relatively coarser grained because this region

has a high energy environment created due to the

interaction of land and sea breezes.

o Ripples formed due to Aeolian activities

o Heavy Minerals are clearly observed over the ripples.

The below figure shows the ripples formed in the sand berms due to the

Aeolian deposition .

Back Shore-

o Few obstacle marks

o Swash Ripples- low amplitude ripples with long

wavelengths.

o Slope varies from 2-5 degrees

o Mostly sand particles

o Heavy minerals (Hornblende + Pyroxene) are found in

valley,Biotites are observed on the surface ( or crest) of

the ripples.

Fore Shore-

o Slope varies from 1-2 degrees

o Consists mostly of fine grained sand particles

o Large number of obstacle marks were found

o Obstacle marks- parabolic pathways along which the water

flowed around the shells

o Parting Lineation-Due to the formation of eddy currents

one side gets eroded and other gets accumulated by shells

(figure)

o Direction of obstacle marks mostly N-W( appendix)

o Parting Lineations are observed.

Tidal Flat

o Slope is less than 1 degree

o Consists of silt and clay minerals

o Pattern keep on changing across the tidal flat.

o Cross stratigraphy is observed.

o Ripples are observed to be formed from cross currents

which originate due to Wind , Air and Gyration of Earth.

o Different structures of ripple marks-Elongated, Linear,

angular, eye shaped, net shape

o Green coloring of tidal flat indicating presence of Algae

due to ground water which leads to presence of nutrients

helping in algae formation

o Presence of charcoal indicating the transgration

movement(definition )(theory of vegetation).

o Biotite presence at crests and fine particles of hornblende,

tourmaline at trough.

o Mostly asymmetric and some symmetric ripples

o Ripples vanishing across the shore .formation of ripples

depends on energy and height

o Due to pressure , Iron on the surface gets oxidized to ferric

oxide and we get brown colour liquid. As shown in the

figure below

Barrier Bars

o Sand particles

o Topography in the region entirely different

o High vegetation between barrier bars and dunes due to

accumulation of water(marshy lands).Clay particles found

o Area after barrier Bar is called Paleo Mud which might

have formed due to the presence of another barrier bar

further ahead indicating trangration

o Alternate Clay and Sand layers found on digging proving

local transgration and regration

o Mud balls found which might have due to the scooping of

sediments

o Direction of mud balls indicating current direction

o Imbricated mud ball

o

TIDAL FLAT RIPPLES DATA:

FUNDAMENTALS of HISTORICAL GEOLOGY and STRATIGRAPHY

- Ravindra Kumar (20006)