Geology Lecture Outline

Sedimentary Rocks and Processes – Ch 7

I. Lecture Content

Introduction

Sediment Production and Classification

Sediment Transport and Refinement

Sediment Depositional Environments

Classic Depositional Sites of North America

The Lithification Process

Types of Sedimentary Rocks

Sedimentary Structures

Sedimentary Facies

Marine Transgressions and Regressions

Reading the Sedimentary Rock Record

Sedimentary-related Mineral Resources

Important Vocabulary Words

II. Introduction:

A. Sedimentary Rocks and Sediment on Earth

1) Constitute a small part of the Earth’s crust (~5%)

2) But they cover much of the Earth’s surface

· Nearly all of the seafloor (~ 95%)

· Much of the continents (~75%).

B. The Origin and Development of Sedimentary Rocks

1) Are a major part of the Earth’s rock cycle.

2) The Sedimentary Processes of the rock cycle include

· Weathering

· Erosion

· Deposition

· Burial

· Compaction

· Lithification

of any preexisting rock found at or near the Earth’s surface.

C. There are Two Primary Types of Sedimentary Rocks:

· Detrital

· Chemical

1) Classification of detrital sedimentary rocks is based

primarily on texture (sediment size).

2) Classification of chemical sedimentary rocks is based

mainly by composition (mineralogy).

D. The Formation of Sedimentary Rock is Controlled

Primarily by Depositional Environment

1) The Earth has a wide variety of depositional settings

· Glacial

· River/Stream

· Lake

· Desert

· Alluvial Fans

· Beach

· Delta

· Lagoon

· Tidal Flats

· Continental Shelf

· Submarine Fan

· Organic Reef

· Deep Marine

E. Sedimentary Rocks are Characteristically Layered.

1) Sedimentary rocks form characteristic layered structures

that are termed “beds” or “strata” and “formations”.

2) Places like the Grand Canyon are amazing examples of thick

stacks of sedimentary strata, which display a wide variety

of rock types, structures, depositional settings, and ages.

F. The Physical and Chemical Nature of Sedimentary

Strata Provide Clues to:

1) Source Rock

2) Sediment's Mode of Transport

3) Depositional Environment

4) Age of the rock

G. Several Groups of Important Mineral Resources Are

Found in Sedimentary Rocks and Sediments

III. Sediment Production and Classification

A. All Sedimentary rocks originate from preexisting rock

that has undergone some degree of weathering and erosion,

i.e. sediment.

1) Define weathering and erosion.

2) Contrast weathering and erosion.

3) Examples of this process occurring in nature

4) See Fig 6.3, pg 159

B. What is Sediment? Defined: Solid particles derived by

chemical and mechanical weathering of preexisting

earth material.

1) Three types of Sediment - Detrital, Chemical, and

Biochemical

C. The Major Types of Sediment - (determined by origin)

1) Detrital - derived directly from preexisting rock via

both, mechanical weathering (grains of ground-up original

composition source material); and chemical weathering

(altered rock-derived clays).

Examples: river gravel, quartz beach sand and lagoonal clays

3) Chemical - derived secondarily from the precipitation of

new minerals from ion-rich aqueous solutions, e.g. seawater,

(ions originated from weathered/dissolved rock material).

Examples: calcareous and siliceous sea-bottom oozes

4) Biochemical - derived from organisms precipitating new

minerals from aqueous solutions, e.g., seawater, (ions originated

from weathered/dissolved rock material).

Examples: shells, skeletons, and reefs

D. Classifying Detrital Sediment

1) Classification primarily based on particle size.

2) Secondary classification based on composition.

3) Sediment Size and Names --- See Table 6.1

>2mm = gravels --- large

2mm - 1/16mm = sand --- medium

1/16mm - 1/256mm = silt --- small

< 1/256mm = clay --- very small

E. General Relationship between sediment size and

particle composition:

· Boulders, cobbles, and gravels consist of rock fragments.

· Sand and silt consist mostly of mineral fragments.

· Clay-sized particles consist mainly of clay minerals.

IV. Sediment Transport and Refinement

A. Sediment transport and deposition follow/overlap with

weathering and erosion in the rock cycle (see rock cycle

diagram).

B. What is Sediment Transport? Defined: The movement of

detrital and chemical sediments by natural agents and

processes.

C. What is Sediment Refinement? Defined: The physical

changes in size, shape, and size distribution of detrital

sediment during transport by the processes of abrasion and

changing transport agent energies over time.

· Size Reduction ----- abrasion

· Rounding ------ abrasion

· Sorting ----- changes in transport agent energy.

D. Agents of Sediment Transportation

1) Running/Moving Water

· Sheet flow, streams, rivers, waves, currents

· By far the dominant transport agent on the planet.

· Best at reducing (abrading), rounding and sorting

· Dominant agent of marine and fluvial environments.

2) Wind

· A dominant agent in deserts

· Good at reducing (abrading), rounding, and sorting

· Removes clay/silt, pushes sand, trouble with gravel

· Agent of dune formation

3) Ice (glaciers)

· Active agent in high mountains and polar regions

· Excellent reducer (abrading and milling)

· Poor sorting agent

4) Gravity (mass wasting movements)

Local agent in areas of slope instability +/- flooding

Poor at reducing, rounding, and sorting

5) Volcanic eruptions (blast effect)

Move tremendous amounts of material in short time

V. Sediment Depositional Environments

A. What is a Sediment Depositional Environment?

Defined: Any geographic area in which sediment is deposited.

B. Variations in Depositional Environments

1) The Earth has a wide variety of depositional environments.

2) Each depositional environment has a unique combination

of active transport agents, surface topography, physical,

chemical, and biological processes, that together, impart

distinctive characteristics to the deposited sediment.

3) Depositional settings are classified according to prevailing

geographic conditions.

· continental vs. marine,

· high energy vs. low energy,

· detrital-rich vs. detrital-starved,

· wind vs. water vs ice vs. gravity-deposited.

C. Classification of Depositional Environments

1) Three major depositional settings (see Fig. 6.5, pg. 161)

§ Continental

§ Transitional

§ Marine

2) Continental - Interior/Non-marine

· Glacial

· Lake

· Aluvial

· Desert dunes

· Streams, rivers, floodplains (fluvial)

3) Transitional - Shorelines

§ Deltas

§ Lagoons/Estuaries

§ Beaches

4) Marine - Offshore/Interior Marine

· Barrier islands

· Continental Shelf

· Offshore reefs

· Submarine fans

· Deep-sea basins

VI. Classic Present-day Depositional Sites of North

America

· Gulf of California

· Gulf of Mexico

· Eastern Seaboard

· Basin and Range Province

VII. Classic Ancient Depositional Sites of North America

· West Central United States

· Rocky Mountains/Colorado Plateau

· Southeastern California and Nevada

VIII. The Lithification Process - Turning Sediments

Into Rock

A. What is Lithification? Defined: A series of processes,

including burial, compaction and cementation, whereby

sediment is converted to solid rock.

B. Burial ( > P and >T) + Compaction (reduced of pore space)

C. Cementation = Chemical precipitation of new mineral

crystals material between the sediment particles.

1) Cements include calcite, silica, and iron oxide.

2) Cement material comes from ions in the pore fluids

and/or circulating groundwater.

IX. Types of Sedimentary Rocks

A. What is a Sedimentary Rock?

Defined: Rock consisting of cemented detrital and/or chemical

sediment derived from one or more processes below:

1) The direct chemical and mechanical weathering of

preexisting rock (detritus);

2) From the direct precipitation of minerals from

solution (chemical);

3) Minerals secreted by organisms (bio-chemical).

B. Sedimentary rocks are classified according to the

type(s) of sediment they contain.

1) Detrital Sedimentary Rocks

§ Cemented aggregates of detrital (clastic) particles

(See Figs. 6.6 and 6.8a - pgs 162-163)

§ Exhibit "clastic" textures

§ Classification based on particle sizes

§ Examples include conglomerate, sandstone, & shale

2) Chemical/Biochemical Sedimentary Rocks

§ Cemented aggregates of new, chemically-/ bio-

chemically precipitated (crystalline) particles.

§ Exhibit "crystalline" textures

(See Fig. 6.8b - pg 163)

§ Classification based primarily on composition

§ Examples include limestone and chert

3) Sedimentary Rock Classification Chart -- Table 6.2

C. Detrital or "Clastic" Sedimentary Rocks

1) Form by a multi-step process of deposition, burial,

compaction, and cementation of detrital sediments.

2) Exhibits a "clastic" texture is defined as an aggregate of

distinct angular to rounded particles that are held

together by a matrix cement. (see Fig. 6.8a; pg 163)

3) Classification based on size of clasts. (see Table 6.2)

4) There are five general detrital sedimentary rock types:

· Breccias --- Angular gravels with a sandy matrix

· Conglomerates -- Rounded gravels with a sandy matrix

· Sandstones -- Dominantly Sand

· Siltstones -- Dominantly Silt

· Mudstones -- Mix of Silt and Clay

· Shales -- Dominantly Clay

5) Cements can be calcite, silica, and iron oxide - depends on the

pore fluid chemistry in the depositional/burial environment.

6) Quartz is the dominant mineral in the conglomerates and

sandstones. WHY?

7) Clay minerals dominate the mudstones and shales. WHY?

8) Note that 40% of all detrital rocks are shales and siltstones.

9) Rock type is dependent on depositional environment.

· Breccias --- High energy; very close to sediment source.

· Conglomerates -- High energy; further from source.

· Sandstones -- Medium energy; water/wind transport.

· Siltstones -- Low energy; water/wind transport

· Mudstone/Shales - V. low energy; water/wind transport.

10) Detrital (Clastic) Sedimentary rocks form in a wide variety of

depositional settings. (See Figs 6.5 and 6.16)

D. Chemical and Biochemical Sedimentary Rocks

1) Form by a multi-step process starting with chemical

precipitation of new minerals out of solution, which form thick

accumulations (layers) of the loose chemical sediments, that in

turn, become cemented together with cements derived from local

pore fluids or compounds found in circulating fluids.

2) Exhibits a "crystalline" texture is defined as an interlocking

mosaic of mineral crystals that are held together, in part, by a

matrix cement. (see Fig. 6.8b; pg 163)

3) Classification based on Composition. (see Table 6.2, pg 164)

4) Four general chemical/biochemical sedimentary rock types:

Limestones and Dolostones - The Carbonates

· Calcite - CaCO3

· Dolomite - (Ca, Mg)CO3

· Biochemical source-dominant

· Fossil-rich

Chert - Silica

· Microcrystalline quartz - SiO4

· Silica-producing organisms

· Varieties include flint and jasper

Evaporites - Precipitated Salts

· Halide - NaCl (table salt)

· Sylvite - KCl (table salt substitute)

· Salt residuals of dried-up lakes and tidal flats

Coal - Compressed, altered land plant remains

· Carbon-rich, organic compounds

· Forms in bogs and swamps with no free oxygen

· Increasing P+T (burial) coal changes form from:

peat to lignite to bituminous to anthracite.

X. Sedimentary Structures

A. Layered stacks of “strata” contain sedimentary

features (structures) that provide clues to its:

1) depositional environment,

2) sediment mode of transport, and

3) (rock) age.

B. Distinct Sedimentary Structures

1) graded bedding

2) sorted and unsorted bedding

3) cross-stratification

4) ripple marks

5) fossils

XI. Sedimentary Facies -- Understanding Variations In

Depositional Environments Though Time & Space

A. What is a Sedimentary Facies? Defined: Bodies of sediment

that possess distinctive physical, chemical, and biological

attributes, that are reflective of the agents and processes that

were active at the site of deposition.

· Each Sedimentary Facies Reflects its Respective

Depositional Environment

B. Depositional environments form an irregular, yet

interconnecting continuum across Earth’s surface. In many

cases one depositional environment grades laterally into

another. Therefore, the sedimentary facies that represent one

depositional environment will grade (change) laterally into

another sedimentary facies across the surface of the Earth.

(See Fig 6.16 p. 169.)

· A very important geologic observation is that, like everything else

our dynamic planet, depositional environments change over time.

· Applying the well-tested principle of uniformatarianism, regional

scale depositional environments change gradually and this change

occurs laterally through time across the Earth’s surface.

· So, two things are happening to a depositional basin through time:

1) Sediment piles up forming a column (stack) that

can be upwards of thousands of meters thick.

Each layer (bed) of sediment has a specific

facies.

2) Sediment facies shift laterally (with their

distinctly matched depositional environment).

C. Gradual changes of a depositional environment over

time in a given geographic location, are due to systematic

change in the physical, chemical, and biological processes

and transport agents that occur at that location. The major

causes for these gradual changes in depositional conditions

are:

1) Changes in sea level

2) Changes in climate

3) Changes in relief/topography

4) Changes in tectonics

D. Earth's Most Extensive Depositional Setting is the

Marine Environment

· Open Marine (Continental Shelves) and Interior

(Continental Seaways)

· This is the final resting place for most of the detrital

material being shed off the continents.

· Parallel belts of depositional marine environments

ü Near shore - Sand-dominated

ü Near offshore - Silts and mud-dominated

ü Far offshore - Fine clay and carbonate-dominated

· See Fig. 6.16

1) The greatest long-term influence on shifting marine

depositional environments is changes in sea level.

2) Causes of Sea Level Changes

· Rising Seas = "marine transgression"

ü more water in ocean basins (ice caps melt)

ü continent sinking

Falling Seas = "marine regression"

ü less water in ocean basins (ice caps grow)

ü continental uplift

E. Marine Transgressions and Regressions

1) Systematic Changes in Rising and Falling Sea Level

Reflected in the Rock Record

2) Marine Transgression = Sea Level Rise

ü Shorelines shifts (moves) inland

ü Depositional environments parallel to shoreline shift

Inland.

ü Sedimentary facies parallel to shoreline shift inland.

ü Offshore sedimentary facies will start depositing on

top of near short deposits over time.

ü The resultant sedimentary column will have the near

shore facies on bottom, the near/offshore facies

in the middle, and topped by far offshore facies.

ü This called a transgressive sequence.

ü See Figure 6.17a-d, pg 170

3) Marine Regression = Sea Level Drop/Fall

ü Exact Opposite of Transgression (see above)

ü This is called a regressive sequence

ü See Figure 617e-h, pg 170

XII. Recognizing Marine Transgressions and Regressions

in the Rock Record

1) Distinctive Sedimentary Facies Sequences in the

rock record indicative either, a marine transgression or

regression.

2) The Grand Canyon Sequence is a Beautiful example.

XIII. Sedimentary-related Mineral Resources

1) sand and gravel (construction),

2) clay (ceramics),

3) limestone chalk (cement and steel production),

4) silica (glass),

5) placer gold, diamonds, tin, and uranium (fuel and weapons),

6) petroleum and natural gas, oil shale, tar sands, and coal

7) banded iron formations.

XIV. Important Vocabulary Words

abrasion/abrading

bed (bedding)

biochemical sed' rock

breccia

burial

carbonate rock

cementation

chemical sed' rock

clastic

clasts

clastic sed' rock

clay (clay-size)

compaction

conglomerate

cross-bedding

depositional environment

detritus

detrital sed' rock

dolostone

evaporite

fossil

graded bedding

gravel

limestone

lithification

marine transgression

marine regression

mud cracks

ripple marks

rounding

sand (sand-size)

sediment

sedimentary facies

sedimentary rock

sedimentary structure

silt (silt-size)

sorting

strata (stratification)