assignment 200

151PTE321 Engineering Geology

Lecture 7

Dr. Seyed Mehdi Alizadeh

Sedimentology

 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.

 The stratigraphic record of sedimentary rocks is the fundamental

database for understanding the evolution of life, plate tectonics

through time and global climate change.

Sediment Sources, Transport,

and Deposition

 Why are rounding and sorting important in sediments

and sedimentary rocks?

 Both are important in determining how liquid water, ice, and

wind move through sediments and sedimentary rocks.

 The amount of rounding and sorting depends on particle

size, distance of transportation, and depositional

processes.

 Rounding and size control, also how important subsurface

resources such as groundwater and petroleum, move

through sedimentary rocks and sediment

Sediment Sources, Transport,

and Deposition

 Eventually, the sediment comes to rest in a

depositional environment.

 Depositional environments are areas of

sediment deposition that can be defined by their

physical characteristics (topography, climate,

wave and current strength, salinity, etc.).

 Depositional environments provide clues as to

how the rock formed and what the geologic past

was like.

Sediment Sources, Transport,

and Deposition  Major depositional settings are continental (including freshwater),

transitional (shore or near shore marine), and marine.

 Each of these depositional settings includes several specific subenvironments.

How Does Sediment Become

Sedimentary Rock?

 Through the process of lithification,

sediment is converted into sedimentary rock.

How Does Sediment Become

Sedimentary Rock?

 Lithification involves two processes:

 1. Compaction - The volume of sediment

decreases as the weight of overlying sediment

causes a reduction in pore space (open space) as

particles pack more closely together.

 Compaction alone is sufficient for lithification of

mud into shale.

How Does Sediment Become

Sedimentary Rock?

 Lithification involves two processes:

 2. Cementation is a process that glues the sediments together.

 The most common cements are calcium carbonate and silica, but iron oxide and iron hydroxide are important in some rocks.

 Compaction alone will not form rocks from sand and gravel. Cementation is necessary to glue the particles together into rocks.

How Does Sediment Become

Sedimentary Rock?

Sedimentary Facies

 Geologists realize that if they laterally trace a

sedimentary layer far enough, it will undergo

changes in composition and/or texture.

 Bodies of sediment or sedimentary rocks which are

recognizably different from adjacent sediment or

sedimentary rocks and are deposited in a different

depositional (sub) environment are known as

sedimentary facies.

 Today we recognize modern facies changes when we go

from an inland area with rivers to the beach.

 The environment at any point on

the land or under the sea can be

characterized by the physical and

chemical processes that are

active there and the organisms

that live under those conditions at

that time.

 In the description of sedimentary

rocks in terms of depositional

environments, the term ‘facies’ is

often used. A rock facies is a body

of rock with specified

characteristics that reflect the

conditions under which it was

formed.

Sedimentary Environments And Facies

Rock Formation

A formation is the basic rock unit in geology. IT IS NOT A TIME UNIT. It

is defined by its properties: type (sandstone, limestone, etc. e.g. (Bell

Shale), color (Brown Niagrian), texture, geometry. The choice is fairly

obvious in A, but more difficult in B. In B and C the choice of

subdivisions is somewhat arbitrary.

Depositional Environments and

Sedimentray Facies

 Lateral variations of strata not fully appreciated

until 1838

 Facies concept relates sediments to their

depositional environment

 Block diagram

showing proximal (near

source) and distal (distant

from source) facies

relationships in a shoreline

environment.

 The source area is the

uplifted “island” which is

supplying sediment

(gravel, sand, mud in

that order) as it erodes)

Facies- Example

Facies- Example

A = Sandstone facies (beach environment)

B = Shale facies (offshore marine environment)

C = Limestone facies (far from sources of

terrigenous input)

 Each depositional environment grades laterally into other environments.

Sedimentary Facies

 Sedimentary facies are used to identify ancient changes in sea level, called marine transgressions and regressions.

Sedimentary Facies

 A marine transgression occurs when sea level rises with respect to the land, resulting in offshore facies overlying nearshore facies.

 Marine Transgressions and Regressions

Sedimentary Facies

 A marine regression,

caused when the land

rises relative to sea

level, results in

nearshore facies

overlying offshore facies.

 Note the difference in

the vertical rock

sequence that occurs

in a transgression

versus a regression.

 Marine Transgression and Regression

Onlap (Transgressive) Sequences

Shifting Facies through Time

Beach moves farther away

Water gets deeper

Sediment becomes finer

Time Rock Unit

Time Rock Unit

Time Rock Unit

Time Rock Unit

Time Rock Unit

Time Rock Unit

Beach

sandstone Near Shelf

shale Far Shelf

limestone

FUS – Fining Upward Sequence

= Transgressive Sequence

Offlap (Regressive) Sequences

Shifting Facies through Time

Beach

sandstone

Near Shelf

shale

Far Shelf

limestone

Beach moves closer

Water gets shallower

Sediment gets coarser

Prograding Regression

Time Transgressive Rock Unit

CUS – Coarsening Upward Sequence

= Regressive Sequence

Transgressive Sequence

Regressive Sequence

Beach

sandstone Near Shelf

shale Far Shelf

limestone

Beach moves closer

Water gets shallower

Sediment gets coarser

Prograding Regression

Time Transgressive Rock Unit

Beach moves farther away

Water gets deeper

Sediment becomes finer

Beach

sandstone Near Shelf

shale Far Shelf

limestone

Depositional Environments

 Areas of the Earth’s surface where distinct processes generate specific geological (sedimentary) products:

 Physical

 Biological

 Chemical

Depositional Environments

Continental environments

Transitional environments

Marine environments

 Desert environments contain an association of features found in

 sand dune deposits,

 alluvial fan deposits,

 playa lake deposits

Desert Environments

 A dune is a hill of sand built by wind or the flow of water.

 Dunes occur in different shapes and sizes, formed by interaction with the flow of air or water.

 Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind.

 Windblown dunes are typically composed of:

 well-sorted, well-rounded sand

 cross-beds meters to tens of meters high

 land-dwelling plants and animals make up any fossils

Sand Dunes

 A desert basin showing the association of:

 alluvial fan,

 sand dune,

 playa lake deposits

Associations in Desert Basin

 Alluvial fans form best along the margins of

desert basins :

 where streams and debris flows discharge

from mountains onto a valley floor

 They form a triangular (fan-shaped) deposit of

sand and gravel

Alluvial Fans

Transitional Environments

Transitional environments

Simple Deltas

1) topset beds

2) foreset beds

3) bottomset beds

 The simplest deltas are those in lakes and consist of :

– As the delta builds outward it progrades and forms a vertical sequence of rocks that becomes coarser-grained from the bottom to top.

– The bottomset beds may contain marine (or lake) fossils,

– whereas the topset beds contain land fossils.

 Wave-dominated deltas

 such as the Nile Delta of Egypt also have distributary channels

 but their seaward margin is modified by wave action

Wave-Dominated Deltas

Marine Environments

Marine environments

 Factors that control sedimentation include particle size

and the turbulence of the depositional environment.  Terrigenous sediments are those derived from the erosion of

rocks on land; that is, they are derived from terrestrial

environments.

 Terrigenous sediments strongly reflect their source and are

transported to the sea by wind, rivers and glaciers.

 Rate of erosion is important in determining nature of

sediments.

 Average grain size reflects the energy of the depositional

environment.

4-1 Sedimentation in the Sea

Hjulstrom’s Diagram

 Hjulstrom’s Diagram graphs the relationship between

particle size and energy for erosion, transportation and

deposition.

 The gently sloping area adjacent to a continent

 is a continental shelf

 It consists of a high-energy inner part that is

periodically stirred up by waves and tidal currents

 Its sediment is mostly sand, shaped into large cross-

bedded dunes

 Bedding planes are commonly marked by wave-

formed ripple marks

 Marine fossils and bioturbation are typical

Detrital Marine Environments

 Shelf, slope and rise environments

 The main avenues of sediment transport across the shelf are submarine canyons

Detrital Marine Environments

Turbidity currents carry sediment to the submarine fans

Sand with graded bedding and mud settled from seawater

 Beyond the continental rise, the seafloor is

 nearly completely covered by fine-grained deposits

 no sand and gravel

 or no sediment at all

 near mid-ocean ridges

 The main sources of sediment are:

 windblown dust from continents or oceanic islands

 volcanic ash

 shells of microorganisms dwelling

 in surface waters of the ocean

Deep Sea Environments

 Types of sediment are:

 pelagic clay, which covers most of the deeper parts

of the seafloor

 calcareous (CaCO3) and siliceous (SiO2) oozes

made up of microscopic shells

Deep Sea Environments