Sediments

Geology 1001-section 4

Last updated: 02/13/98

SEDIMENTS AND SEDIMENTARY ROCKS (abbreviated overheads)

Importance of Sedimentary Rocks

(a) They record surface conditions at time of deposition. e.g. Grand Canyon with fossils

(b) They record sources and transport directions.

(c) They host valuable resources: oil, gas, coal, uranium, Fe, P

2. Sedimentary Rocks formed by:

(a) the accumulation and cementation (lithification) of mineral grains, rock fragments and/or remains or products of animals or plants; transported by wind, water or ice to the site of deposition produces: Clastic (or detrital) sedimentary rocks typically a mixture of rock/mineral compositions

(b) chemical or biochemical precipitation of dissolved substances at the site of deposition produces: Chemical or biochemical sedimentary rocks

(c) Mixtures of clastic and chemical sediments

3. Clastic sediments are

4. The materials that constitute the starting materials are

5. The transporting agents and the environment of deposition control the

AGENTS OF SEDIMENT TRANSPORT

1. Most transport is gravity-driven (except wind).

2. Currents - movements of fluids

· the stronger the current (faster), the

· gravity causes particles to settle

- in a fluid, particles settle as a function of

- related to velocity of

- results in sorting of particles; tendency of current to

Sorting is a measure of range of

- well-sorted - all particles nearly same size; implies

- poorly-sorted = particles are of different sizes;

-transportation is usually intermittent; velocity

3. Glaciers - ice moves downhill and incorporates large quantities of

-glacial deposits are

-glacial streams and winds modify

4. Effects of transport processes

(a) reduces grain size with more

(b) rounds the grains (clasts) and fragments the degree of rounding reflects

(c) enhances weathering

5. Dissolved Material - water carries dissolved ions that are the result of

SEDIMENTATION

Deposition of sediment of various types

1. Clastic Sedimentation - depends on transporting agent

- recall deposits are a function of velocity

(a) High energy currents carry

(b) Low energy currents carry

2. Chemical/Biochemical Sedimentation - driven by chemistry.

(a) chemical sediments are direct precipitates

(b) biochemical sediments relate to biochemical reactions

· carbonate reefs and banks

· deep sea SiO₂(chert) deposits

· preserved vegetation peat, coal, oil deposits

3. Sedimentary Environments - useful for classification of a sediment

· a geographic location characterized by a particular combination of

· the type and characteristics of a sediment (or sedimentary rock) can be used to infer

· Clastic sedimentary environments - terrigenous, originate on land.

(a) Continental:

(b) Alluvial -

(c) Shoreline:

(d) Marine:

· Chemical and Biochemical Sedimentary environments -characterized by

(a) Carbonate -

(b) Marine Evaporite -

(c) Siliceous -

Diagenesis and Lithification

Diagenesis - all physical and chemical changes (alteration) of the sediment or rock after deposition.

e.g. mineral composition, porosity

1. Cementation - addition of

chemical diagenetic change

2. Lithification - hardening of a sediment into a rock

3. Compaction - decreases in

major physical diagenetic change

CLASSIFICATION of SEDIMENTARY ROCKS

A. Clastic Sedimentary Rocks

· Account for 3/4 of all sedimentary rocks; with finer grain sizes being the most common

· classified based on grain size

(a) Conglomerates - (> 2mm, gravel) composed of large, rounded clastic grains set in a finer grained matrix

-travel distance

(b) Breccias - (> 2mm, gravel) composed of large, angular clastic grains set in a finer grained matrix; reflects

(c) Sandstones (0.062 - 2 mm, sand grains); fine, medium or coarse

i. can exhibit various degrees of sorting, size and mineralogy

- well-sorted - typical of beaches

- poorly-sorted - typical of glaciers

ii. can exhibit various degrees of rounding greater the rounding the

iii. the mineralogy of the sandstone reflects the

-Quartz arenite - dominated by quartz grains

-Arkose - dominated by feldspar grains

-Lithic sandstone - rock fragments are dominant

-Graywacke - rock fragments are with a clay matrix

iv. Mudstones, siltstones, and shales (<0.062mm grains) dominant

- mud - <0.062mm

- silt - 0.0039 - 0.062mm

- shale = rock made of these materials; finest clastic sediments

quartz and fsp detritus with abundant clays

commonly rich in organic material and can be source of

CLASSIFICATION of SEDIMENTARY ROCKS

B. Chemical/Biochemical Sedimentary Rocks

i. Limestone - calcium carbonate CaCO₃in form of mineral calcite

ii. Dolostone - calcium magnesium carbonate CaMg(CO)₃ in form mineral dolomite

- Major sites of carbonate deposition are

iii. Evaporites - calcium carbonate, CaMg(CO₃)₂,

gypsum (CaSO₄. 2H₂0), halite

form in marine environment from

iv. Chert - chemically or biochem ppt silica, SiO₂ also called Flint (=black chert) secreted by marine organisms

v. Iron Formations - Fe-oxides ppt when Earth had less atmosphere

vi. Coal - biochemically produced sedimentary rock composed of organic carbon from decaying plant material

BEDDING AND SEDIMNETARY STRUCTURES

Generally horizontal features due to gravitational setting but with some exceptions.

i. Cross-bedding - sets of bedded material inclined at ca. 35° from the horizontal

A. due to deposition on the

B. can use cross-beds to tell

- stratigraphic

- direction of

ii. Ripples - low, narrow ridge separated by troughs

- gives information on

iii. Graded beds - horizontal beds grade upward from coarse to finer grain size (reflects

- gives information on

iv. Mudcracks - desiccation plates in mud fiats

-gives information on

v. Bioturbation - Disruption of bedding structures by organisms

- information on

vi. Bedding Sequence - patterns of sedimentary rock packages indicative of a given sedimentary environment

e.g.