Revised 8 / 06 (Monroe 6th ed.)

Sedimentary Processes and Rocks - Chapter 7




Origin of Sedimentary Materials

Environments of Deposition

Continental Deposition

Marine Deposition

Transitional Environments

Conversion into Rock

Features of Sedimentary Rocks

Which Way is Up?

Facies Changes


Classification of sedimentary rocks

Clastics - true secondary rocks

Chemical sedimentary rocks

Organic sedimentary rocks

Click here for online mineral and rock ID charts




We've studied igneous rocks & the minerals of which they are composed

Basement rocks

Most are covered by a thin veneer of debris

Consolidated into a "rock" through slow-acting processes

Usually involving pressure and fluid penetration

Relatively simple to understand

Relatively near-surface processes

As opposed to igneous & metamorphics, which usually occur at depth

Secondary (or derived) rocks

Several main categories

Clastic sedimentary rocks - The classic sedimentary rock

Accumulations of debris derived from the disintegration of pre-existing rocks

DIGRESS TO: Terrigenous sediments

Chemical sedimentary rocks - Chemical precipitates

Usually as the result of the evaporation of water

Ex. Salt (NaCl); Gypsum (CaSO4 · 2H2O)

Organic sedimentary rocks

All hydrocarbons

Coal, peat, oil, etc.

The distinction between these three categories can get pretty fuzzy at times

Ex. Limestone, chert

Hard rocks vs. Soft rocks


Origin of Sedimentary Materials

DIGRESS TO: Physical vs. Chemical weathering

Click here for additional information on water, weathering, and erosion (RCC)

Click here for additional information on surface processes (GPHS)

Clasts - derived from physical (and chemical) weathering processes

Smaller solid particles

Derived directly from the source area

Reflect lithology of the source area

Wide range of sizes, from silt to boulders

Chemical processes can result in the relative enrichment of more resistant (or inert) minerals

Ex. quartz vs. feldspar

Clay minerals

I'm not a clay kind of guy

Extremely complex mineralogy

My understanding is minimal

Easy to get confused by the term

The term "clay" refers to both a size and a mineral family

A clast can be clay size without being clay

DIGRESS TO: "clay the size" vs. "clay the mineral"

Clay formation forms small, sheet-like minerals (look like the micas)

Lots of different clay minerals

Which mineral is formed reflects primary lithology and environment

Can change to a different mineral if moved to a different environment

Downslope? Downstream?

Near-surface, low temperature environments

Hot and humid works best

Water is a universal solvent (H·OH)

Tends to work parallel to Bowen's Reaction Series

The higher temperature minerals are more susceptible to chemical weathering

Therefore, especially hard on the mafics and feldspar

To repeat what was mentioned above

Chemical processes can result in the relative enrichment of more resistant (or inert) minerals

Ex. quartz vs. feldspar

Describe "decomposed granite"


Chemical weathering also results in "ions" which are "held in solution"

The solution is usually water

Remember: Water is a universal solvent (H·OH) and will play merry hell with anything "over the course of geologic time!"

See Strickler's 3rd and 4th Laws of GeoFantasy

Some elements will dissolve and be held in solution

Ex. salt, sugar

DIGRESS TO: Solution (ions) vs. Suspension (clays)

Both make fundamentally different types of sed. rocks

Common ions include: Ca+2, Na+, CO3-2, Cl-

DIGRESS TO: What do the superscripts mean?

Atomic structure & the role of the electron

These ions are responsible for the "mineral taste" in some water

Therefore, we can tell that iron and sulfur must also be common

If the amount of ions increases relative to the amount of water, minerals can precipitate

Ex. salt (Na+ + Cl- -> NaCl)

Saturation is the key

An undersaturated solution can become oversaturated in 2 ways

Increase the dissolved ions

Decrease the solvent (water)

This is more common (probably)

Can be initiated by the evaporation process

Organisms can also extract the ions directly from the water

Use them to build shell material

Ex.: Ca+2 + CO3-2 -> CaCO3

Can result in extensive deposition of calcium or silica sediments


Environments of Deposition

(Monroe; fig. 7-3, pg. 202)

Water plays an important role in most aspects of sedimentary rocks

From weathering and erosion to transportation and deposition


Deposition occurs in a wide variety of locations

Basically, any low spot is a potential depositional environment

On both regional and local level - expand

Three major divisions - Continental deposition, marine deposition, and transitional (inter-tidal)

Infinite possible combinations of environments and materials

Results in infinite possible sedimentary rocks

Fortunately, most fall into one of several common environments

And as we already know from our study of igneous rocks, most of the rocks start with a similar chemistry

It can still be tough to recognize the depositional environment

DIGRESS TO: This is the ultimate goal of the study of sedimentary rocks

The names are important, but only insofar as they provide clues to how they got there

The interpretation of earth's history is the purpose of any geological examination

In any event, this will usually take lots of field work

And the examination of lots of different rocks

As well as copious amounts of lubricant to make sense of the data!

Multiple Working Hypotheses

Need to keep an open mind

Several working together with different ideas can be good

As can a "Devil's Advocate" to keep the group from getting cocky.

It's far too easy to only see those units and/or features which support the currently favorable model

Important factors include:

Sorting - key to interpreting the depositional environment

"The degree in similarity in particle size in a sediment"

Important in the clastic sediments

Particle size

Important in the clastic sediments

Particle composition

Important in chemical and organic sediments


Continental Deposition

Sediments trapped on land

Lots of different environments

Usually can be viewed directly, so relatively easy to understand

Rivers (click here for additional information on streams from RCC and Secondary level)

Riverbed - size directly related to energy of the stream

Can be poorly sorted (high energy) or well sorted (low energy)

Floodplain - Flat surfaces adjacent to a river

Represents sediments deposited during flooding

All different scales

Major floodplains - Nile, Amazon, Mississippi, etc.

Minor floodplains - localized along selected stretches of most streams and rivers

Sorting varies in response to local conditions

Usually well-sorted, but not always

Sorting can be a VERY local phenomena

Both laterally and longitudinally

Draw a X-section of a stream bed at a meander

Before, during, and after flood stage

Glaciers (click here for additional information on glaciers from RCC and GPHS)

Non-turbulent flow (unlike rivers)

Can and will carry all sizes of material

Commonly poorly sorted, but not always!

Alpine vs. continental glaciation

Distinctive types of deposits


By nature a temporary feature

A sure trap for sediments - Q=AV

Will certainly fill in "over the course..."

Usually well-sorted "locally"

Variations in grain size related to distance from inflow

Diagram: long section

Example: mouth of Carberry Creek during January 1997 flood

Evaporites - common to arid regions

Ex.: Bonneville Salt Flats

Also can be marine in origin

Alluvial Fans

Generally arid and semi-arid climates


Generally poorly sorted


Essentially an underwater alluvial fan

Again, Q=AV

React to the base level of a given river

Lakes are temporary base levels

The ocean is the "ultimate base level"

Eolian Deposition

Wind can also play a role in the erosion, transportation, and deposition of sediments

Can affect wide areas

Not confined to a defined channel like a river is

Can move vast quantities of material

Always well sorted (unless contaminated by other processes)

Small stuff only - no boulders!

Sand dunes

Combination of:

Lots of sand sized pieces (and smaller)

Little or no vegetation to hold material in place

Strong winds

Loess deposits - fine dust and silt

Common along margins of continental glaciers


Marine Deposition

The seafloor is the final resting place for the majority of weathered rock materials

See Strickler's 3rd Law of GeoFantasy

Factors affecting deposition include: (Monroe; fig. 7-12, pg. 211)

Distance from shore

Depth of the water

Physical & chemical properties of the water

Variety of plant and animal life

These result in 4 major zones of deposition

Relatively good sorting within each zone

DIAGRAM: X-section of seafloor (should be a review from G-101)

Shore Zone

The shore acts like a channel and restricts the "flow" of the ocean

High energy zone

Coarse sand and gravel are deposited here

Smaller material stays in suspension/solution and moves offshore

Continental Shelf

Much broader than the shore zone

Most terrigenous sediments end up here (sooner or later)

Mostly silt & clay

Some coarser material

Has to be related to times of higher energy

Storms? Seismic disturbances?

Carbonate deposits also common

Inorganic and organic deposits of CaCO3


Common to "shallow, warm water"

Topographic highs on shelf (banks, seamounts)

Minimal contamination by terrigenous sediments

Continental Slope

Debris collects in canyons traversing the slope

"Perched pre-turbidites"

Can be quite poorly sorted

Abyssal Plains

Coarse sediments common at base of slope


Very poorly sorted

Set in motion by storms and quakes

Mostly very fine grain sediments

Calcareous and siliceous oozes

Water depth and temperature generally determine which is deposited

Calcareous to siliceous to terrestrial clay ooze


Transitional Depositional Environments

Basically the inter-tidal zone

Fluctuates between marine and continental deposition


Conversion into Rock

Lithification - "the process of converting soft, unconsolidated sediments into hard rock"

(Monroe; fig. 7-4, pg. 203)

DIGRESS TO: Hard rock vs. soft rock geology

Two major factors contribute to the lithification process

Remember: we are usually starting with a loose pile of debris, which is saturated with water


Weight of overlying sediments results in compaction

Reduction in pore space

Interstitial fluids (water) may be removed

Cementation - "The most significant process"

"The deposition from solution of a soluble substance"

Fills the interstitial pore spaces

Cements the grains together

Three common types of cement


Probably the most common

Easily dissolved in groundwater

H20 + CO2 = H2CO3 (Carbonic Acid): will dissolve calcium and put it into solution

Silica - less soluble than calcite

Will form a much harder and stronger cement

Iron Oxide (Fe2O3)



Features of Sedimentary Rocks

Stratification - the most common and distinctive (Monroe; fig. 7-13, pg. 212)

Most sedimentary rocks are composed of particles which settle through water (or air)

Generally quiet water deposition results in nearly horizontal layers

Layers are called strata (stratum: singular) (like data and datum)

Greater than 1cm thick: beds

Less than 1cm thick: laminae

Differences through time result in visible layering

Variation in clast size

Direct result of fluctuating energy within the system

Variation in clast composition/mineralization

Affects color, size, amount

EXAMPLE: Limestone/Shale

Shale during storms, seismic events, turbidites, etc.

EXAMPLE: Flood into a lake

Coarser during flood (high energy)

Example: mouth of Carberry Creek during January 1997 flood

EXAMPLE: Discontinuous or intermittent deposition

Climatic or tectonic changes

Graded Bedding (Monroe; fig. 7-15, pg. 214)

Larger pieces on bottom, finer at top

Common when a poorly sorted debris is dumped into quiet water

EXAMPLES: Storms into a lake, turbidites

Cross Bedding (Monroe; fig. 7-14, pg. 213) (Monroe; fig. 7-19, pg. 218)

Non-horizontal bedding

Moderate to steeply-dipping layers

Common in arid environments - sand dunes

Wind moves grains up the windward slope, where they fall off the edge

"Slip-face" - the steep downwind side

At the Angle of Repose

"The maximum slope at which grains will remain stationary without sliding down the slope"

Fluctuating wind direction can result in thick sequences with complex, changing patterns

Also in deltaic environments

Topset, bottomset, and foreset beds (at the angle of repose)

Cross-bedding can also be the result of uplift, tilting, erosion, subsidence, and additional horizontal deposition

Roundness of the clasts (Monroe; fig. 7-5, pg. 205)

Usually reflects transport distance and/or time in transit

Long distance = rounder clasts

Obviously, composition of the material will affect this to some degree


Most igneous rocks are some variation of grey

Sedimentary rocks can be quite colorful

Different pigments can fill the void spaces between the clasts

Iron - very common

Results in shades of red, brown, pink, or yellow

Also purple, green, or black

Dark to black color commonly the result of organic material

EXAMPLE: Black shale

Also, very fine grained pyrite (with or without gold) can cause the rock to appear black

Origin of color often uncertain

Can be carried to depositional site with sediments

Or added later (exotic)

Or produced chemically in place

Mud cracks (Monroe; fig. 7-18, pg. 215)

Form in wet clays and muds which are dehydrated

Loss of water causes the silt to contract and crack

Similar to columnar jointing in thick basalt flows

Basalt - from cooling

Mud - from dehydration

Well developed mud cracks indicate repeated wet/dry cycles

Common in shallow, seasonal lake beds

NOT common in tidal flats

Drying period not long enough

Ripple marks (Monroe; fig. 7-16, pg. 214)

Develop perpendicular to direction of current

Commonly asymmetrical in cross-section

Slip-face at the Angle of Repose (like sand dunes and drumlins)

Symmetrical ripples are called "Oscillation Ripples"

Sharper crests with gently rounded troughs

Indicate alternating directions

EXAMPLE: Surface waves along a beach (marine or non-marine)

Can provide clues to ancient paleo-currents

Originally assumed to be restricted to shallow water

Recent studies on the seafloor have identified them at depths of several thousand meters

Fossils - the classic sedimentary feature (Monroe; pgs. 216-217)

Evidence of once-living organisms

Characteristic of many sedimentary rocks

Not igneous or metamorphic

Most relate to remains of "hard body parts" (bones, shells, teeth)

Any evidence is considered a fossil

Soft body molds




Some amazing parts have been preserved

Jellyfish, compound eye parts, dragonfly wings

Clues to depositional environments

EXAMPLE: Clam fossils pretty much indicate marine deposition, etc.

Used to establish the Relative Time Scale (Monroe; Fig. 1-16, pg. 24)


Nearly spherical solid bodies found in sedimentary rocks

"Composed of material that solidified around a small, hard nucleus after the sediment was deposited"

Any small, hard particle will work

CaCO3 common


Which Way is Up?

As we learned last term, sedimentary rocks are usually deposited in horizontal layers

Also learned that tectonic forces can significantly change the orientation of rocks at and near the surface of the earth

In a situation where we are trying to unravel the tectonic history of an area...

It can be pretty tough to come up with definitive clues

Igneous rocks provide few, if any

Except for flows, no original preferred orientation

Metamorphics are even worse

The very process which forms them will usually complicate (or destroy) any primary orientation features

Sedimentary beds are our only real method of figuring out the degree of tectonic deformation in a regional or local area

Most studies are based on the premise of original horizontality

How far has a sedimentary unit been rotated from the horizontal

In extreme cases, the tilting can be of such an extent that the beds are completely overturned

Even back to a horizontal position!

It can be very important to determine tops from bottoms of sedimentary beds

Can be nearly impossible to determine, and is most often inconclusive without extensive field mapping and laboratory work

Many methods can be used as appropriate to the situation

Graded bedding

Cross-bedding (concave up)

Scour & Fill

Baked zones (inter-layered flows)

Ripple marks


Facies Changes

"Lateral change in the basic properties of a sedimentary horizon"

(Monroe; fig. 7-12, pg. 211)

DIGRESS TO: Time-Stratigraphic Horizons

EXAMPLE: Conglomerate into sandstone into shale

Reflect local variations in the depositional environment

DIAGRAM: on board

Transgression / Regression (Monroe; fig. 7-12, pg. 211)



The sedimentary record is not complete

Long term gaps in the sedimentary record indicate periods of non-deposition and/or erosion

We actually can see only a small part of the earth's history in sedimentary rocks

The gaps clearly represent more time than do the beds themselves

Angular Unconformity (Monroe; fig. 9-9, pg. 272)

Easiest to recognize - describe

Non-parallel beds above and below

Represents: deposition, uplift, deformation, erosion, subsidence, and new deposition

Disconformity (Monroe; fig. 9-8, pg. 271)

Parallel beds above and below

Can be real tough to recognize

Nonconformity (Monroe; fig. 9-10, pg. 273)

Sedimentary beds overlying igneous or metamorphic rocks

Represent immense time periods (EXPLAIN)


Classification of sedimentary rocks

As we said, there are 3 general categories (Monroe; Table 7-1, pg. 207)

Clastic/fragmental; Chemical precipitates; and Organic

Distinction between different types often fuzzy in reality

Click here for online mineral and rock ID charts


Clastics - true secondary rocks

Derived from the breakdown of pre-existing rock at the surface of the crust

Most sedimentary rocks are clastics

Quick review:

Surface weathering produces small clasts (physical / chemical processes)

As soon as a clast (at whatever size) is broken from bedrock, it is involved in the erosion and transport process

Gravity is the ultimate driving force here

Clasts moved downslope to creek/river systems

Carried downstream to a suitable depositional environment

Weathering can continue during transport

Both physical and chemical

Its reasonable to assume that physical weathering dominates in the headwaters at higher elevations

Chemical weathering takes on a more active role at lower elevations

Smaller clast size = greater surface area for chemical attack

Erosion or deposition is controlled by the energy of the system

F=MA and Q=AV: review these?

Classification generally based on the size of the clasts

Also important as modifiers of the general size designation:



Degree of rounding

Large clasts

Conglomerate - cemented gravel (Monroe; fig. 7-5a, pg. 205)

Usually poorly sorted, calcium or silica cement

Well rounded

Common to upper portions of rivers and high energy shorelines

Breccia (Monroe; fig. 7-5b, pg. 205)

Same idea as conglomerate except angular fragments

Indicated deposition close to source area

Fault breccia - explain

Sand-sized clasts

Sandstone (Monroe; fig. 7-6, pg. 206)

Often inter-bedded with shale or conglomerate

Review facies changes

Indicate near shore marine - your basic beach

Often well rounded - long transport distance

These represent the final product of the weathering process

Mafics & feldspars gone, only quartz remains

Can also occur as arid eolian deposits

Commonly angular

Both types generally well sorted (especially the eolian deposits)

Calcium or silica cement

Which one is present determines hardness (induration)

Friable - breaks up easily due to weak cement

Compositional differences

Arkosic sandstone

Quartz and feldspar

Shorter distance of transport?

Graywacke - "dirty sandstone"

Generally dark in color

Quartz, feldspar, mafics, lithic fragments all present

Indicates very short distance of transport

Deposition adjacent to source area

No time for chemical weathering

Commonly very poorly sorted

Common to flanks of island arc systems

Common filling of trenches

A real mess when exposed to the surface

All the unaltered materials rapidly alter to clays

Silt & clay sized clasts (Monroe; fig. 7-7, pg. 207)

Lots of names based on size of clasts

Siltstone, claystone, mudstone

Shale works as a general descriptive name for most of them

Usually impossible to determine composition of clasts due to small clast size

Shale is composed primarily of clay minerals

With clay sized clasts of quartz, feldspar, and other minerals

Commonly exhibits fissility

The ability to split along closely spaced sub-parallel bedding planes

A result of the platy nature of the clay minerals

Mud & siltstone may not have fissility

Due to lack of clay minerals

Tend to break into chunks (like most rocks)


Chemical sedimentary rocks

Evaporites (Monroe; fig. 7-9, pg. 209)

Result from the evaporation of water

Dissolved solids (ions) precipitate as minerals as the fluids reach saturation

EXAMPLE: Halite (NaCl)

Evaporation of sea water, inland seas

Repeated flooding can produce thick deposits

Also saline lakes

EXAMPLE: Great Salt Lake, Bonneville Salt Flats

Any lake with no outlet will become saline over time

DIGRESS TO: Shattered fence posts at Bonneville

Acquisition and control of salt deposits has historically been of great importance

Empires have risen and fallen due to control of salt!

"Worth his salt" and "Salary" both indicate importance of this substance

Currently in use as storage areas for sensitive materials

WHY? (No moisture and relative tectonic stability)

Gypsum (CaSO4 · 2H2O): The Hidden Hydrosphere!!

Also common due to evaporation of seas and saline lakes

Anhydrite (CaSO4) - Gypsum without the water

Lots of other evaporites - these are only the most common

Formation of evaporites

Different minerals precipitate in a specific order

Somewhat analogous to Bowen's Reaction Series

Controlled by concentration/saturation

Gypsum & Anhydrite form first

After approx. 80% of the water has evaporated

Halite after 90%

Sodium, magnesium, and potassium salts after that

Thick deposits are really hard to explain!

1000 feet of water will produce

0.3' gypsum; 11.5' halite; 3' other salts

How do we get 1000' thick gypsum deposits?

Repeated flooding with partial evaporation?

Secondary remobilization (after deposition)?

A full understanding of evaporites will take more work!

Carbonates (Monroe; figs. 7-8, pg. 208)

Calcite (CaCO3) makes Limestone

Aragonite - an "unstable" form of CaCO3

Dolomite (CaMg(CO3)) makes dolomite (dolostone)

Seawater is generally concentrated with CaCO3

Minor changes in temperature or composition can lead to precipitation

There is a continuing argument concerning the importance of inorganic processes to the formation of the world's great limestone deposits

Most agree that organic processes are most important - see below


Hot springs deposits (Monroe; fig. 16-24, pg. 522)

Beautiful building and decorative stone

Tufa (or calcareous tufa)

Springs and lime-saturated lakes

May be related to lime-secreting algae

Oolites (Monroe; fig. 7-8, pg. 208)

Small spherical grains of CaCO3

Called ooliths (From the Greek: oo=egg; lith=stone)

Form around a nucleus - like a pearl

Grains of sand washing around in a bottom current

Tidal areas of warm, shallow seas

The origin of dolomite

A problem - we don't see it forming anywhere at this time

It is common in the geologic past

Primary precipitation vs. derived from limestone or aragonite

By the percolation of magnesium rich solutions through limestone

Many dolomites obviously Calcium replacement by Magnesium

Others well layered with limestone

Would require selective replacement of individual beds

Most favor replacement


Organic sedimentary rocks


Oil and Gas (Monroe; fig. 7-20, pg. 220)

Commonly occur in sedimentary rocks

Coal - lithified plant and animal remains (Monroe; fig. 7-11, pg. 210)

Compacted swamps, etc.

Convert to coal in an anaerobic environment

Calcium based rocks

Limestone the most common

Most limestone is organic as opposed to chemical in origin


Microscopic plants & animals extract CaCO3 from seawater and use it to build shells

These will settle to the seafloor and accumulate into Limestone deposits

Larger organisms also extract CaCO3 for shells which can accumulate on seafloor

Coquina - lithified shell debris

Can be reworked in the sea currents - broken and moved around

Are these then clastic sedimentary deposits?

Reefs (Monroe, pg. 380)

Made largely of corals and carbonate secreting algae

Like shallow, warm waters which are agitated by wave action

High in nutrients (for food)

Environment essentially free of terrigenous sediments

Can result in extremely pure limestone deposits

Commonly +/- 30 deg. of the equator

Complex structures

The reef itself - form by organisms building upward

Seaward - clastic carbonate debris

Broken off the reef by wave action

Landward - Lagoon with carbonate & clastic muds

Silica based rocks (Monroe; figs. 7-10, pg. 209)

Chert - the "general name used to cover many types of dense, hard, non-clastic, microcrystalline siliceous rocks"

Flint - dark color from included organic remains

Uniform texture - conchoidal fracture

Jasper - reddish flint

Sinter - hot springs (like travertine)

Thick beds of chert are found throughout the geologic record

Some may result from direct chemical precipitation

White smokers at spreading axes

Most are thought to be organic (like the carbonates)

Microscopic plants & animals extract silica from seawater and use it to build shells

These will settle to the seafloor and accumulate into chert deposits

Larger organisms do not use silica to build shells

WHY? (Not as much in the seawater? Less soluble so harder to extract?)

Banded Iron Formation (Monroe; figs. 7-22, pg. 221)

Much more on these in G-103 when we look at the evolution of earth's atmosphere


Click here for online mineral and rock ID charts



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