Sedimentary Rocks & Environments
of Deposition
WHY STUDY SEDIMENTARY ROCKS?
- They record ENVIRONMENTS OF DEPOSITION
& are archives for FOSSILS
- Geologists can reconstruct PALEOGEOGRAPHY
(distribution of mountain belts, stable continental interiors [Cratons],
& continents themselves) & PALEOCLIMATES from this information
SEDIMENTARY ROCKS
- Form by lithification of sediments
- Lithification = Compaction and Cementation
- COMPACTION - IMPORTANT ONLY IN FORMATION OF SHALE
(MUDS COMPRESS UP TO 40%)
- Cementation - material precipitated from water into
pores
- CLASTIC SEDIMENTARY ROCKS
- Mechanical breakup of other rocks
- Transported to site of deposition by water,
ice or wind under the influence of gravity
- Sedimentary particle sizes
- GRAVEL - >2 mm
- SAND - 0.064 to 2 mm
- SILT - 0.004 to 0.064 mm [4-64 microns] also called
- CLAY - <0.004 mm [<4 microns] MUD
- Shale = clay- + silt-sized
sediments deposited in quiet water
- Sandstone = sand-sized sediments
- Conglomerate = gravel-sized
sediments
- CHEMICAL SEDIMENTARY ROCKS
- Organic or inorganic precipitation of sediments
- Limestone - CaCO3
- REEF LIMESTONE - built by corals (and other critters
in other eras)
- COQUINA - poorly cemented large shells and shell
fragments
- CHALK - poorly cemented microscopic shells
- Dolostone - altered limestone composed
of dolomite [MgCa(CO3)2]
- Chert - hard, dense rock consisting
of microcrystalline silica (SiO2)
- Evaporites - form during evaporation
of sea water (rock salt - mostly halite [NaCl]
& rock gypsum - mostly gypsum [CaSO4])
- COAL - carbon rich
FEATURES OF SEDIMENTARY ROCKS USED TO INTERPRET ENVIRONMENTS
OF DEPOSITION
- LITHOLOGY
- Composition
& Compositional Maturity
- Reflects type & degree of weathering in source
area, subsequent transport and degree of reworking for terrigenous sediments
(maturity = greater weathering intensity or longer transport path)
- quartz - increasing quartz content indicates
increasing maturity in coarser sediments
- clay minerals- increasing clay content indicates
increasing maturity in finer sediments
- feldspar & rock fragments - abundant
feldspar & rock fragments content indicates immaturity in coarser sediments
- Color - mostly due to
presence of carbon & iron sulfides & oxides, &
indicates the oxidation state of the environment of deposition
- 1. BLACK = carbon or iron sulfides, reducing conditions
(high organics, due to rapid sedimentation, stagnant currents) - restricted
basins, lagoons, tidal flats
- 2. GREEN = magnetite (Fe3O4), more oxidizing conditions, but moderate amounts of
organics - continental margins
- 3. RED = iron oxide (hematite - Fe2O3), well-oxygenated conditions, nonmarine &
arid, or deep sea
- Texture & Textural Maturity
- Aspect of rocks including SIZE, SORTING,
SHAPE, & ORIENTATION of sedimentary grains
that gives clues to CONTINUITY, TURBULENCE
& VELOCITY of flow (low energy/high energy; ice/water/wind;
continous/episodic flow) at the site of deposition
- SIZE: larger size = faster velocities
(floods [large size] vs. lagoons [small size])
- SORTING (range of particle
sizes): well sorted = small range; poorly sorted = large range
- wind & beach deposits: well sorted =
continuous flow
- glacial deposits: poorly sorted = glaciers
pick up everything
- alluvial fans: poorly sorted = rapidly deposited,
episodic
- textural maturity reflects relationship between
matrix & framework
- matrix - clay and silt sized grains
- framework - sand and gravel sized
grains
- increasing sorting & rounding of framework indicates
increasing textural maturity
- matrix present in texturally immature sediments
- GEOMETRY - 3-DIMENSIONAL SHAPE
of a sediment body
- Elongate - rivers, beaches
- Sheet-like - trangressive shoreline
- SEDIMENTARY
STRUCTURES
- Features formed during or shortly after
deposition; extremely useful for deducing environments of deposition
- 1. MUD (dessication) CRACKS = exposure to air &
drying out (subaerial) nonmarine & transitional environments
- 2. GRADED BEDDING = usually deep-marine (from
turbidity currents)
- 3. CROSS-BEDDING & RIPPLE MARKS = deposition
from a current
- i. symmetrical ripples = waves, tides; asymmetrical
ripples = rivers
- ii. can get paleocurrent directions
- FOSSILS & TRACE FOSSILS
- Organisms are adapted to specific environments,
for example ECHINODERMS = normal, open marine, or BLUE-GREEN ALGAE (mats)
= abnormal salinities
ENVIRONMENTS
OF DEPOSITION
- NONMARINE (CONTINENTAL) ENVIRONMENTS
- Fluvial - meandering rivers &.
braided streams
- BRAIDED STREAMS - coarse with horizontally-bedded
gravel & cross-bedded sand with little mud
- MEANDERING RIVERS - channel deposits (point
bar) = cross-bedded sand to gravel; levees = silt & fine sand,
floodplains = clays; crevasse = fine sand
- produce elongate sand bodies surrounded by mud
- Desert
- SAND DUNES (eolian) = cross-bedded sand
- PLAYA LAKES = clays & evaporites
- ALLUVIAL FANS = coarse sediments from braided streams
- Glacial
- TILL - unsorted, unstratified drift deposited &
in front of the ice (moraines) & under the ice
- GLACIAL LAKE SEDIMENTS - with seasonal varves
(alternating light & dark layers: light = warm w/silt & clay, dark
= winter w/clay & organics)
- OUTWASH - deposited by braided streams from melting
glaciers
- Lacustrine - lakes: fine-grained laminated
sediments with freshwater fossils
- Swamp - fine-grained sediments with
lots of organic matter
- TRANSITIONAL ENVIRONMENTS
- Deltas - river delivers sediment faster
than marine processes can redistribute it - prograding; lobe switching
- DELTA-PLAIN DEPOSITS - uppermost part of delta,
many subenvironments, essentially flat lying
- DELTA-FRONT DEPOSITS - inclined layers of sand &
silt
- PRODELTA DEPOSITS - essentially flat lying silts
& clays
- Exact configuration depends on interplay between
river supply, tides & waves
- Tidal flats - form where tidal
ranges are large (>2 m)
- ESTUARIES = mud, under water
- Barrier islands - form where tidal
ranges are small (<1 m)
- BEACHES, DUNES, & WASHOVERS = sand, mostly exposed
- TIDAL "DELTAS" = sand on either side of
tidal inlet, mostly submerged
- LAGOONS = mud, under water
- MARINE ENVIRONMENTS
- Shallow-marine continental shelves
(<200 m)
- TERRIGENOUS (DETRITAL) SHELVES - sandy inner shelf
with large sand waves, muddy outer shelf
- CARBONATE PLATFORMS
- barrier reefs & oolite shoals form marginal
topographic high
- lagoons (muddy) & patch reefs
lie between barrier & shore
- tidal flats (muddy) form the shore [sabkhas
in arid regions]
- Deep-marine ocean basins (>200
m)
- CONTINENTAL MARGINS (slopes & rises) = terrigenous
mud & turbidite sand in submarine fans
- OCEAN BASINS (abyssal plains) = eolian clay, turbidite
sand & SiO2 ooze
- TOPOGRAPHIC HIGHS (Mid-Ocean Ridge & seamounts)
= CaCO3 ooze
Study Questions
1.
How can quartz, clay and feldspar/rock fragment content be used to establish
environments of deposition?
2. How can black & red coloration
be used to establish environments of deposition?
3. How can large versus small size
be used to establish environments of deposition?
4. How can well sorted versus poorly
sorted sediments be used to establish environments of deposition?
5. What is cause
& significance of mud cracks, graded bedding & cross-bedding/ripple
marks?
6. What are examples
of environments of deposition associated with each?
7. What are
the major environments of deposition?
8. What are
the important subenvironments of each major environment of deposition &
the lithologic characteristics of the sediments deposited in each?