Sedimentary Rocks


General Statements

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

We will concentrate on this type

Chemical sedimentary rocks - Chemical precipitates

Usually as the result of the evaporation of water - Ex. Salt (NaCl)

Organic sedimentary rocks

Limestone, all hydrocarbons - Coal, peat, oil, etc.


Origin of Sedimentary Materials

DIGRESS TO: Physical vs. Chemical weathering

Clasts - derived from physical (and chemical) weathering processes

Smaller solid particles

Wide range of sizes, from silt to boulders

Clay minerals

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

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

Near-surface, low temperature environments

Hot and humid works best - chemical weathering!!

Note: chemical weathering also results in "ions" which are "held in solution"

Can result in chemical sedimentary rocks

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 deposition of organic sediments


Environments of Deposition

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

Two major divisions - Continental and marine

Also there are inter-tidal (transitional) environmnets

Important factors include:

Sorting - 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

Rivers and streams

Riverbed - size directly related to energy of the stream

Can be poorly sorted (all different sizes) or well sorted (all the same size)

Floodplain - Flat surfaces adjacent to a river

Represents sediments deposited during flooding

Usually well sorted


Non-turbulent flow (unlike rivers)

Can and will carry all sizes of material

Commonly poorly sorted, but not always!


By nature a temporary feature

A sure trap for sediments (because Q=AV)

Evaporites - common to arid regions with seasonal lakes (playas)

Ex.: Bonneville Salt Flats

Alluvial Fans

Generally arid and semi-arid climates


Essentially an underwater alluvial fan

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

Always well sorted (unless contaminated by other processes)

Small stuff only - no boulders!

Sand dunes


Marine Deposition

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

Please refer to Strickler's 3rd Law of GeoFantasy

Remember - "The earth breaks what it makes and puts it in the ocean"

Factors affecting deposition include:

Distance from shore

Related to energy

Depth of the water

These result in 3 broad zones of deposition

Relatively good sorting within each zone

In general, the shore and shelf contain the majority of "terrigenous" sediments

Gravel ---> Sand ---> Silt ---> Clay ---> Carbonate Ooze

The 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

The Continental Shelf

Much broader then the shore zone

Most terrigenous sediments end up here (sooner or later)

Mostly silt & clay

Locally coarser material related to times of higher energy

Carbonate deposits also common

Inorganic and organic deposits of CaCO3 - Limestone

Common to "shallow, warm water"

The Abyss - much of this ends up being subducted

Mostly very fine grain sediments

Water depth important in which is deposited

Calcareous to siliceous to terrestrial clay ooze

As depth increases and/or temperature decreases


Features of Sedimentary Rocks

Stratification - the most common and distinctive

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

Generally quiet water deposition results in nearly horizontal layers

Differences through time result in visible layering

Variation in clast size

Variation in clast composition/mineralization

Special enhancements to visible layering

Graded Bedding

Cross Bedding

Size and Roundness of the clasts

Usually reflects transport distance and/or time in transit

Long distance = smaller and rounder clasts


Most igneous rocks are some shade of gray

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

Dark to black color commonly the result of organic material

EXAMPLE: Black shale

Fossils - the classic sedimentary feature

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)

But 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


Conversion into Rock

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

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

Calcium- 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)


Facies Changes

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

DIGRESS TO: Time-Stratigraphic Horizons

EXAMPLE: Conglomerate into sandstone into siltstone into shale

Reflect local variations in the depositional environment

DIAGRAM: on board

Transgression / Regression



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

Three major types of unconformities

Angular Unconformity

Easiest to recognize - describe

Non-parallel beds above and below

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


Parallel beds above and below

Can be real tough to recognize


Sedimentary beds overlying igneous or metamorphic rocks

Represent immense time periods


Classification (types of sedimentary rocks)

As we said, there are 3 general categories

Clastic/fragmental; Chemical precipitates; and Organic

Distinction between different types often fuzzy in reality

Clastic Sedimentary Rocks - 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 and 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 results in greater surface area for chemical attack

Classification generally based on the size of the clasts

Conglomerate - cemented gravel

Usually poorly sorted, calcium or silica cement

Sandstone - Sand-sized clasts

Often interbedded with shale or conglomerate (facies changes)

Indicate near shore marine - your basic beach

Calcium or silica cement

Which one is present determines hardness

Friable - breaks up easily due to weak cement

Compositional differences

Classic sandstone is generally quartz - final weathered product

Graywacke - "dirty sandstone"

Generally dark in color

Quartz, feldspar, mafics, lithic fragments all present

Indicates very short distance of transport

Silt & clay sized clasts

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

Chemical sedimentary rocks


Result from the evaporation of water

Halite (salt), Gypsum (sheetrock)


Limestone - calcite (CaCO3)


Hot springs deposits

Organic sedimentary rocks


Coal - lithified plant and animal remains

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?


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° of the equator

Silica based rocks

Chert - "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?)


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