Sediments and Sediments and Sedimentary RocksSedimentary Rocks

Tom Bean/DRKL - 6: cd/EM - E

Sedimentary rocksSedimentary rocks

• 5 % by volume of the upper crust

• 75% by area of continental areas

• Often the only record of geologic events:

e.g. The Himalayas will someday be sandstone

Sedimentary Stages in the Rock Cycle

2 KINDS OF SEDIM2 KINDS OF SEDIMENTARY ENTARY ROCKSROCKS

A. DETRITAL & CLASTIC - Rock resulting from the consolidation of loose sediment that has been derived from previously existing rocks and accumulated in layers

B. CHEMICAL - Rock formed by the precipitation of minerals from solution by either organic or inorganic processes

Transport and deposition Transport and deposition of clastic sedimentsof clastic sediments

• Movement of sediment by wind, ice or water (GEOLOGIC AGENTS).

• Mode of transport produces distinctive deposits.

• Deposit as layers - beds/strata

Transport affects the Transport affects the sediment in several ways sediment in several ways

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SortingSorting: measure of the variation in therange of grain sizes in a clastic rock orsediment

• Well-sorted sediments indicate that they have been subjected to prolonged water or wind action.

• Poorly-sorted sediments are either not far-removed from their source or deposited by glaciers.

SortingSorting

Well-sorted Poorly-sorted

Rex Elliott

Well-sorted Sand

Poorly-sorted Sand

Rex Elliott

Transport affects the sediment in Transport affects the sediment in several ways - several ways - SHAPESHAPE

RoundnessRoundness: measure of how rounded the corners are

SphericitySphericity: measure of how much it is like a sphere

* Sorting, roundness, and sphericity all increase with LENGTH of transport.

Sorting

Roundness

Types of detrital rocks Types of detrital rocks

Largely based on the size of the particles, which may be anything.

• Conglomerate (poorly sorted/round)• Breccia (poorly sort/angular)• Sandstone (quartzite, arkose, greywacke)• Mudstone• Siltstone• Shale - most common rock on continents

Conglomerate

Breck Kent

Sandstone

Breck Kent

Shale

D. Cavagnaro/Visuals Unlimited

SEDIMSEDIMENTARY ROCKSENTARY ROCKS

CHEMICAL ROCK- Rock formed by the precipitation of minerals from solution by either organic or inorganic processes

Types of chemical Types of chemical sedimentary rockssedimentary rocks

Limestone CaCO3

Chert SiO2

Salt NaCl, KCl

Gypsum CaSO4 • 2H2O

Coal altered organic debris

Breck Kent

Limestone

Breck Kent

Halite

Breck Kent

Chert

Fossiliferous Limestone

Peter Kresan

Chemical environments: Chemical environments: EvaporitesEvaporites

Restricted environments such as Mediterranean Sea & Texas coast

Minerals precipitate according to solubility.

Gypsum Halite

CaSO4 •2H2O NaCl

One Model for theOne Model for the Formation of Evaporites Formation of Evaporites

Chemical environments: Chemical environments: CarbonatesCarbonates

Clear water — away from big rivers (or volcanoes)

Warm water — subtropical to tropicalShallow water -- two reasons:

Organic: sunlight only penetrates to

about 100 m

Inorganic: CCD so carbonates dissolve

Foraminifer in the Eye of a Needle

Chevron Corporation

Coral Reef Surrounding Volcanic Island

Jean-Marc Truchet/Tony Stone Worldwide

Common Sedimentary EnvironmentsCommon Sedimentary Environments

Clues to interpreting Clues to interpreting sedimentary depositional sedimentary depositional

environmentsenvironments

• Sedimentary structures

• Sorting, roundness, sphericity

• Sequence & kinds of beds

Sedimentary structuresSedimentary structures

Particular structural features can give information about the environment of deposition.

Structures also help determine if a bed is right-side-up — important in deformed rocks

Peter Kresan

Cross-bedded Sandstone

Turbidity currents

Varves record annual cycles in glacial lakes

S. C. Porter

Raymond Seiver

Modern Rippled Sand

Reg Morrison/Auscape

Ancient Ripple-marked Sandstone

Fig. 7.9

MODERN ANCIENT

Mudcracks

Chip Clark

Bioturbation Tracks and Tunnels

From Peat to Coal

The end