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
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.
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.
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
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
Chemical environments: Chemical environments: EvaporitesEvaporites
Restricted environments such as Mediterranean Sea & Texas coast
Minerals precipitate according to solubility.
Gypsum Halite
CaSO4 •2H2O NaCl
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
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