CH. 3 – SEDIMENTARY RX
Sediment
1) weathering products (gravel, sand, clay minerals)
2) chemical precipitates3) organic remains
Sedimentary Rocks
“sedimentum” = “settling”Sediment settles out of wind or
water- forms layers at the surface
Importance of Sedimentary Rx
1) 75% of exposed rx at surface (outcrops)
2) Clues to past environments3) Fossil record4) Economic value
Sedimentary Rx Classification
1) Detrital = weathering products (mechanical or chemical)
2) Chemical
Detrital Sedimentary Rocks
Named by particle sizeParticle Size NameGravel, pebbles Conglomerate
Detrital Sedimentary Rocks
Particle Size NameSand Sandstone
(quartz sand)
Clay-size Shale (red, green, black)
Detrital Rock Process
1) Weathering2) Erosion (transportation)3) Deposition (sediment settles)4) Lithification – turning sediment
into rock (“lithos” = rock) (after sediment is buried)
Types of Lithification Processes
a) Compaction- most effective on fine-grained rx
(ex: shale)Exception: St. Peters SandstoneStarved Rock State Park, IL
b) CementationMinerals dissolved in groundwater
precipitate around sedimentEx: silica, calcite, hematite
Lithification Processes
Sedimentary Rx Classification
2) ChemicalNamed based on compositionEx: calcite = limestone
halite = rock saltplant remains = coal
Chemical Sedimentary Rocks
a) Inorganic – formed by chemical reactions in environment
Ex: Evaporites - rock gypsum, (Fig. 3.23) rock salt
Ex: Travertine (limestone)
Chemical Sedimentary Rx
b) Organic (“biochemical”)Plant remains = coalMicroscopic sea shells = chalkBroken sea shells = ___________
Detective Analogy
Geologists are solving a mystery
Scene of Clues Mysterythe Crime Solved
Mystery Solved (Sed. Rx)
Depositional environment- any area on surface where
sediment accumulatesEx:
Principle of Uniformitarianism
“Present is the key to the past”Study modern depositional
environments for cluesEx: Coquina limestone
Clues
1) Sediment sizeDistance deposited from source
areaLarge = (closer to, further from)Small = (closer to, further from)
Clues (sediment size)
High vs. low energy environmentLarge particles = (high, low)
Ex: ___________Small particles = (high, low) Ex: ___________
Clues
2) Sediment shapeDistance deposited from source
area: close to or far from?angular = ______rounded = ______
Breccia
Clues
3) Sedimentary Structures- features produced in sediment
BEFORE lithificationEx: mudcracks, ripple marks,
trace fossils
Mudcracks
Environmental conditions:a)b)
Ex: river floodplain
Ripple Marks
Indicates current direction in shallow water
Trace Fossils
Evidence an organism was in the environment but no physical remains of the organism exist
Ex: footprints, coprolites
Clues
4) Fossils – evidence of prehistoric life
Fossil record is incomplete- shows remarkable pattern of
change from simple to complex life forms
Steps to becoming a fossil
Step 1: DeathStep 2: Hard parts – bones, teeth,
shells (Who you are!)Step 3: Rapid burial (Where you
die!)Step 4: Time (prehistoric)
Fossils
Fossil record biased towards marine environment
- abundant life in oceans- many critters have hard parts- lots of sediment for quick burial
Fossils
Good evidence for reconstructing past environments
Ex: Deep marine shale vs. muddy river shale