Sedimentary Rocks
Fossils are found in Sedimentary Rocks
• composed of sediments - mineral particles
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derived from the weathering and erosion
of rock.
layered
form on the surface
preserve fossils
preserve ancient environments
• source of fossil fuel, iron, and building
materials
Types of Sedimentary Rocks
Chemical
Detrital - gravel, sand and mud
derived from the erosion of rock
Carbonates (Limestone) - sediments
crystallized by marine organisms
Evaporites - sediments crystallized
from the evaporation of water
Organic - coal (fossil plant matter)
Detrital Rocks
•Derived from weathering of rock
•Mostly quartz and clay
•Classified by texture = grain size
•Pebble
•Sand
•Silt
•Clay
Conglomerate - pebbles
Arkose
Quartz Sandstone
Sandstones
Greywacke
Siltstone
Shale
“Mudrocks”
Carbonate Rocks
Limestones - CaCO3
Dolostones - CaMgCO3
Sediments precipitated in / near environment
of deposition - usually biological
algae, phytoplankton, zooplankton, mollusks,
corals, echinoderms (sea urchins, crinoids)
Calcareous algae
Grainstone
Lime mudstone
Limestones
Packstone
Wackestone
Chalk - lime mudstone
Coccolithophorids - Haptophyta
Algal Microfossils
calcite
Chalk Cliffs, England
Evaporite Rocks
Gypsum - Ca2SO4
Halite - NaCl
Sediments precipitated in arid environments
due to evaporation of sea water or lake water
Desert basins, arid tidal flats, salt lakes,
restricted lagoons, bays, and seas
Chert / Flint
Smooth, hard, microcrystalline
quartz
Forms as nodules in limestone
Deposited as layers in the deep
ocean
Petrified Wood
- wood completely
replaced by chert
- Pretty colors come
from iron and other
mineral impurities in
the chert.
Chinle Formation, Upper Triassic, Petrified Forest,
Arizona
Organic Rocks
Coal - carbon derived from plant matter
Plants
Peat
Lignite
Tar, oil, natural gas hydrocarbon from marine algae
Coal
Coal Beds
Depositional Environments
Sedimentary rocks trap clues to the
identity of their environment of
deposition
• Sediment size and sorting - high vs. low
energy environments (stream vs. swamp)
• Sedimentary structures - subaerial,
shallow water, deep water, currents,
waves
• Rock color - organic content, oxygen levels
• Other fossils - terrestrial, fresh water,
marine
Large sediments (gravel, coarse sand) tend to
accumulate in high energy environments (beach,
stream channel).
Fine sediments accumulate in low energy places
(swamps, continental shelf, deep ocean).
Decreasing depositional energy
Increasing rounding and sorting
Alluvial fan depositional environment, Death Valley,
California
Gravel and course sand - poorly sorted
Gravel - channels
Sand - bars
River System depositional environment, New Zealand
Carbonate
sand and
gravel
Coral Reef depositional environment, Caribbean
Sedimentary Structures
Ripple marks - waves and currents
Cross beds - ripples in side view
Trace fossils - footprints, burrows
Mudcracks - subaerial exposure
Current Ripple Marks in Sandstone
Wave ripple marks and dinosaur trackway in
sandstone
Cross Beds
Cross Beds
Modern mudcracks
Ancient mudcracks
Color in Sedimentary Rocks
Red - oxidized iron (hematite) - indicates
exposure to atmosphere / subaerial
deposition.
Black - organic carbon - deposition under
oxygen-poor conditions.
Green - reduced iron, low carbon - possibly
deposition under arid, oxygen-poor conditions.
Beige - none of the above (low iron, low
organic carbon).
East Berlin Formation, Early Jurassic,
Connecticut
Mudcracks and red color indicate
subaerial deposition.
Floodplain to lake sediment cycle, New Berlin, CT
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