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Sedimentary Rocks
Sedimentary Rocks: 5% of the crust, but 75%
of the surface area.
PROCESSES
Transportation: Rounding - increases with length of
transportation history.
Sorting: increases with length of transportation
history (weaker minerals broken down).
Deposition: any process that lays down material.
Environment of Deposition: location in which
deposition occurs. Use the Principle of
Uniformitarianism to study ancient environments.
Preservation: deposition and burial in a basin.
Reworking degrades preservation.
Fig. 06.01
Transportation: Rounding increases with length of
transportation history.
Boulders have
been abraided
Transportation
Sorting: increases with
length of transportation
history (weaker
minerals broken down).
Fig. 06.04
Deposition: any process that lays down material.
Environment of Deposition: location in which deposition
occurs. Use the Principle of Uniformitarianism to study
ancient environments.
Types of Sediment
Clastic or Detrital
Deposited grains of rocks and minerals derived
intact from source, transported by gravity, water,
wind, ice.
Organic
Shells, shell fragments, bones, reefs, etc. May be
transported or precipitated in situ.
Inorganic Chemical
Chemical precipitates due to precipitation of
dissolved constituents. Transport possible, but rare
(salt, limestone, gypsum).
Clastic Sediments
Grains: main or large “bits” of sediment
Matrix: fine interstitial particles.
Grain and matrix composition depends upon:
Source material – what type of rock is being
weathered and eroded?
The chemical and physical properties of the
minerals – stronger ones are at the bottom of
Bowens Reaction Series (e.g., Quartz).
Degree of weathering and erosion processes to
transport – secondary dependence upon climate.
Clastic Sediments
Texture: Depends on transport history.
Size (see Table 6.1):
Clastic Sediment Textures
Roundness/Angularity:
Transport by wind or water – rounding occurs.
Transport by ice or gravity – angular.
Degree of Sorting:
Selection/separation of grains is on the basis
of size, shape, specific gravity.
Poorly sorted: fast deposition, high energy.
Well sorted: slow deposition, less chaotic.
Poorly
Sorted
Clastic Sediment Textures
Maturity:
Chemical and physical weathering gradually
breaks down rock to quartz and clay minerals.
Maturity is a measure of this process.
Depends on: Time, Climate, Transport History,
Depositional Environment.
Immature: poorly sorted, clays + mineral fragments
(olivine, pyroxene, amphibole, etc.), rock fragments.
Grains are angular and vary in size.
Mature: well sorted, no clays, mostly made up
of rounded quartz grains.
Lithification: Compaction and Cementation.
1) Compaction: reduction of pore space due to weight of
overlying sediments.
Porosity = percentage of voids: Loose sand: 30-50%
Lithified sand: 10-20%
Siltstone:
<5%
Granite:
0%!
Permeability: ability of rock to allow water to pass.
Requires that pores are interconnected.
Reorientation of grains reduces permeability/porosity.
Lithification
2) Cementation: precipitation from pore water
(CaCO3, SiO2, Fe2O3, etc.).
Crystalline Limestone
F.O.V. = 3 mm
Fig. 06.09
Conglomerate: coarse
(> 2 mm), rounded
clasts (gravel) + finer
groundmass.
Breccia: coarse (> 2
mm), angular clasts
(gravel) + finer
groundmass.
Fig. 06.10a
Sandstone: medium (1/16-2 mm),
90% quartz + matrix.
Siltstone: fine (1/256 – 1/16 mm), fine-grained
sandstone, feels gritty.
Arkose: quartz, feldspar, matrix.
Area of deposition
close to area of
erosion - feldspar
weathers
relatively easy.
Greywacke: >15 % clay matrix, poorly sorted,
immature, formed by turbidity currents.
Earthquake triggers
slumping of sediments
down continental slopes.
Can form V-shaped
valleys in continental
slopes.
Shale: mostly clay,
fissile (splits because of
preferred orientation of
clays. Feels smooth.
Quiet deposition.
Shale
Organic Sediments
Derived due to organic activity.
Bioclastic limestone: fossils + calcite/dolomite.
Fine-grained
Coralline Mud
Coarse-grained
Bioclastic
Limestone
Fig. 06.16
Corals precipitate CaCO3 to form limestone in a reef.
Water depth about 25 feet. Warm and clear water.
A living coral-algal reef sheds bioclastic sediment
into the fore-reef and back-reef environments.
The fore reef consists of coarse, angular fragments of reef.
Coralline algae are the major contributors of carbonate sand and mud
in the back-reef environment. Beaches and dunes = bioclastic sand.
The sediments in each area can lithify to form highly varied limestones.
Coralline algae on the seafloor off the Bahamas
can produce large quantities of carbonate mud.
Organic Sediments
Coquina: rock composed of shell fragments only.
Organic Sediments
Chalk: made up of calcareous fine grained planktonic
micro-organisms.
Organic Sediments
Diatomite/Chert: Siliceous fine grained planktonic
organisms (diatoms).
Bedded Cherts
Chert Nodules
Organic Sediments
Coal: modified plant material.
Coal Bed
Table 06.02
Inorganic/Chemical Sediments
Formed by chemical precipitation from solution.
Precipitation sequence from seawater:
Calcite (CaCO3)
Anhydrite (CaSO4)
Gypsum (CaSO4.H2O)
Halite (NaCl)
Sylvite (KCl)
Salt (and mud) deposited on the floor
of a dried up desert lake, Utah.
Inorganic/Chemical Sediments
Oolitic Limestone:
Ooliths/ooids and Pisoliths
Limestone is precipitated
directly from seawater.
Sedimentary Structures
Give evidence of depositional environments.
Sedimentary rocks are deposited originally in
horizontal beds.
Later deformation causes the beds to be inclined.
Which was the original way up?
Structures can give that information.
Bedding – horizontal sheets differing in composition.
Sedimentary structures can give idea of paleocurrents.
Horizontal Bedding: Beds get younger upward.
Deposited in water.
Box 06.02.f1
Layered sedimentary rocks on Mars.
Candor Chasma, Mars
Development of Cross Bedding
Fig. 06.27
Cross-bedded Sandstone.
Sedimentary Structures
Graded Bedding
(turbidity current) – beds
grade from coarse grained
at the bottom to fine
grained at the top.
Sedimentary Structures
Mud Cracks – drying on top of bed.
Sedimentary Structures
Ripple Marks – wave action, form on top of beds.
Fig. 06.32c
Ripple Marks
Fig. 06.32d
Ripple Marks
Fossils – traces of plants
or animals preserved in
sedimentary rocks.
Formations
Extremely thick body of rock with characteristics that
distinguish it from adjacent rock units and is large
enough to map.
Formations are based on rock type.
Can be a single thick bed, several thin beds of the same
rock type, or an alternating sequence of two rock types.
Main criterion for distinguishing and naming a formation
is some distinguishable characteristic that makes it a
recognizable unit.
This could be rock-type or sedimentary structure.
Name: Geographic location followed by rock-type Chattanooga Shale.
Fig. 06.35
Formations
Source Area
Where the sediments were eroded from.
Source Area
Locating the source area.
Deposit thicker to SW
Grains coarser to SW
Cross-bedding shows
paleocurrents came
mostly from the SW
Depositional Environments
Continental: glaciers, streams (fluvial), lakes
(lacustrine), springs and groundwater, wind.
Transitional: deltas and beach deposits.
Marine: shallow (continental shelf) and deep (abyssal
plains) deposits.
Depositional Environments
Continental Deposits
Glacial: glacial sediment = till - unsorted mix of
unweathered clasts in a clay matrix.
Alluvial Fan: coarse, arkosic sandstones and
conglomerates, marked by cross-bedding and lensshaped channel deposits. Form where a river
emerges on to a valley floor from a mountain chain.
Continental Deposits
River Channel: lenses of conglomerate or sandstone
(arkosic or sand-size rock fragments).
Typically cross-bedded with ripple marks.
Continental Deposits
Lake: Thin-bedded shales, possibly with fish fossils. May
contain mud-cracks and interbedded evaporates, if it
periodically dried up.
Floodplains: thin-bedded shales with mud-cracks & fossil
footprints. Hematite may color the floodplain deposits red.
Transitional Deposits
Delta: formed when river flows into a body of standing
water (lake or sea). Mostly thick sequences of siltstone
and shale, marked by low-angle cross-bedding, cut by
coarse-grained channel deposits. Can contain peat/coal
beds as well as marine fossils.
Transitional Deposits
Beach, Barrier Island, Dune: a Barrier Island is an
elongate sand bar built by wave action. All are comprised
of well-sorted quartz sandstones with rounded grains.
Beach and Barrier Island: low angle cross-bedding
and marine fossils.
Dune: high-angle and low-angle cross-bedding and
occasional fossil footprints.
All 3 environments can also contain carbonate sand in
tropical areas producing cross-bedded clastic
limestone.
Dune Deposits
Cross-bedded Sandstone.
Barrier Island
Beach
Marine Deposits
Lagoon: semi-enclosed body of water between a barrier
island and the mainland. Fine grained dark shale cut by
tidal channels of coarse sand containing marine fossils.
Limestones may also form in lagoons adjacent to reefs.
Marine Deposits
Shallow Marine Shelves: grain size decreases offshore.
Widespread sandstones, siltstones, shales. Sandstone &
siltstone contains ripple marks, low-angle cross-beds, &
marine fossils. If tidal flats near shore are alternately
covered & exposed, mud-cracked marine shales form.
Reefs: Massive limestone in core of reef, with steep beds of
limestone breccia forming seaward, horizontal beds of
sand-sized and finer-grained limestones form landward.
All are full of fossil fragments (coral, shells, etc.).
Deep Marine Deposits: shale = quiet deposition;
greywacke sandstones (with graded bedding and
current ripple marks) deposited by turbidity currents.
General Aspects of Sedimentary Rocks
Composition of grains/clasts: tells about source.
Age of grains: tells about age of source and transportation
history (e.g., zircon).
Texture/Maturity: tells about transportation history.
Sedimentary structures: tell about depositional
environment.
Fossils: tell about depositional environment and age of
deposit.
Plate Tectonics
Convergent/Transform Plate Boundaries:
Rapid uplift = rapid erosion and sedimentation. Coarsegrained clastic sedimentary rocks.
Look at the composition of large clasts in conglomerates/
breccias to see the record of rocks already eroded.
Erosion exposes rocks from depth (igneous intrusions
and metamorphic rocks).
Marine sedimentary rocks found at the top of mountains!
Fig. 06.41
Plate Tectonics
Divergent Plate Boundary:
Rifting during formation of a new ocean basin.
Rift valley forms and fills with thick wedges of gravel
and coarse sand along faulted margins.
Lake deposits & associated evaporates in the bottom of
the rift.
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