SEDIMENTS and SEDIMENTARY ROCKS

plumbergamMechanics

Feb 22, 2014 (3 years and 5 months ago)

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SEDIMENTS

and

SEDIMENTARY ROCKS

Part II

Inferring Environments



The environments



Fluvial

associated with streams, rivers, such as channels, floodplains,
deltas



Lacustrine

associated with lakes



Low energy



Deep versus shallow



Eolian



Wind deposited



Sand dunes
or loess



Glacial



Ice
-
deposited such as till, drumlins



Glaciofluvial such as eskers, outwash

Inferring Environments



Various sedimentary features give us clues to the environment of
deposition



Bedding



Most obvious characteristic of sedimentary rocks



Beds are

made visible by changes in grain size or composition



Beds are time markers



Similarity of beds suggests stable conditions



Rapidly changing beds imply unstable conditions



Sedimentation rates are difficult to infer

Bedding

Bedding on the Coastal Plain

Inferr
ing Environments



Cross bedding



Layers inclined at an angle to obvious strata



Can tell us about depositional environment



Festoon or scour and fill



Planar cross beds

Scour & Fill Cross Beds in the Fountain formation,
Colorado

Dune Cross Beds

Lyons Quarry

Mod
ern Dune Cross Beds

Cross Beds

Lyons Sandstone

Ancient Dunes

Checkerboard Mesa

Inferring Environments



Grain shape



Tells us about the distance of transport



Rounded, subrounded, subangular, angular from long distance
of transport to short distance of transpo
rt

Inferring Environments



Sorting



Degree of similarity of grain sizes



Well sorted implies slower rates of sedimentation



Poor sorting implies rapid deposition

Inferring Environments



Graded bedding



Tells us something about the depositional process



Tells us w
hich way is up



Coarse material in sharp contact with underlying layers at bottom



Gradual change to finer material toward the top

Inferring Environments



Ripple Marks



Oscillation



Symmetrical



Water sloshed back and forth



Which way is up



Current ripples



Asymme
trical



Gives current direction



Size tells us something about water depth and velocity


Modern Current Ripples

Modern Ripples

Ancient Ripples

Inferring Environments



Mudcracks



Tell us low energy environment

only happens in fine
-
grained
materials



Tell us shal
low water

has to dry up



Can sometimes tell us which way is up

Mudcracks

Flagstaff Mt.

Inferring Environments



Sole Marks



Marks which appear on a bedding surface



Dragging twigs and the like



Can tell us current direction


Inferring Environments



Bioturbation



B
iologically disturbed indicating the presence of animal or plant life



Fossils can be used to determine age and correlation



Fossils can tell us something about the environment



Color



Problematic because we don’t really understand rock color



Red is usually ir
on oxide & implies oxidizing environment at time of
deposition or later



Black usually implies carbon and a reducing environment

Modern Raindrops

Raindrop Impressions

Lithification



Rock making or turning detrital sediments into rock



Two components



Compactio
n



Drives out excess water



Packs grains closer together



Important in fine
-
grained materials



Shales compact as much as 60%



Squeezes clay flakes together to make them adhere

Lithification



Cementation



Gluing sediments together



Crucial in holding siltstones, sa
ndstones, and conglomerates
together



Common cements



Iron oxide



Carbonate

calcite or dolomite



Silica



Clay matrix



Cements carried and deposited by groundwater

Sedimentary Facies



Once we understand how to determine environments, we can look
at those environme
nts to decipher paleogeography



Imagine a lake



Coarse sediments near the shore



Finer
-
grained materials in center

Sedimentary Facies



Sediments of the same age deposited in different environments



They differ because they were deposited under different conditi
ons



Subsidence of lake or rise or fall of sea level can cause position of types of
sediment to shift



Cartoon of Newark Basin from south on the left to north on the right

Sedimentary Facies

Turbidity Currents



Mixture of sediment and water



Higher density tha
n surrounding water



Gravity driven



Move down slope at depths at least as great as 5 km



May achieve velocities as high as 90 km/hour



Can carry up to 3 kg/m
3

of sediment



Can carry sediment 1000 km from source



Produce sequences of graded beds known as turbidi
tes

Turbidite Deposit Along the Coast of
Washington

Turbidity Currents

Deep Sea Fans

Economic Deposits

Coal



Decomposition of land plants (swamp environment)



Requires a reducing environment



Progression: Peat


Lignite


Bituminous coal


Anthracite coal


Metanthracite



Increasing carbon content



Decreasing volatiles



Decreasing moisture



Increasing heat content to bituminous, then slight decrease to anthracite

Economic Deposits

Coal



Important fossil fuel



U.S. has extensive reserves



Most anthracite is gone (eas
t coast)



Much bituminous and lower grade coals in west



Problems



Environmental



Air pollution



Carbon dioxide (greenhouse gas)



Acid rain



Particles


Economic Deposits

Coal



Mining



Strip mining scars



Underground mine collapses



Acid mine drainage



Workers’ health



Mine reclamation



Inconvenient to transport and store



Dirty to handle



Skilled miners are a declining breed



Long lead time to get new mines up and operating with heavy
equipment

Economic Deposits

Oil



Accumulation of plant and animal remains in marine sedimen
ts



Accumulation must occur where biological productivity is high but
oxygen content is low



Complex and poorly understood process to transform biological
products in oil



Oil is produced in source rocks



It migrates to reservoir rocks where it must be trapped


Economic Deposits

Oil



Traps



Anticline



Faults



Salt domes

Oil Traps

Banded Iron Ore



Layers of iron
-
rich sedimentary rocks



Unusual



Chemical precipitates interbedded with clastic sediments



Must have formed when there was a lot less oxygen available



Since mos
t of these deposits are Precambrian, it implies that the
Precambrian atmosphere contained much less oxygen

Banded Iron Ore