Sedimentary rocks
-composed of materials derived from weathering of existing rocks (could be
igneous, metamorphic, or sedimentary)
-weathered materials eroded (transported), deposited at various places on earth’s
crust…sedimentary rocks composed of these materials from existing rocks termed clastic
-weathering processes dissolve materials that may be transported in solution to
places where chemical/biological conditions proper for their precipitation and
accumulation; sedimentary rocks composed of these materials are termed organic or
chemical
-example: Great Bahama bank, across Florida straits from Florida—
isolated from source of clastic sediment; warm, shallow water; marine organisms take
CaCO3 from solution to form their shells; after death, these shells accumulate on seafloor
to form limestones, organic sedimentary rocks
Also, lots of evaporation in Bahamas; resulting high salinities cause
precipitation of calcium carbonate as crystals; accumulate on seafloor to produce
chemical sedimentary rocks
Mineral composition of sedimentary rocks
Only a few common minerals dominate sedimentary rocks because chemical
weathering destroys unstable mineral components (ex—feldspars weather to clay).
Common sedimentary minerals:
Quartz
Clay minerals (feldspars weather straight to clay)
Calcite (precipitated by chemical/biological processes)
Rock Fragments (not disintegrated during weathering/erosion)
Feldspar—not chemically stable; require rapid erosion or erosion in a cold
or dry climate for preservation
Clastic Sedimentary Rocks
Texture—refers to size, shape, arrangements of grains (just like in igneous rocks)
Figure 4-1. Grain Size scale used in classifying clastic sed. rocks
Grain Size
Definition
Gravel
>2.0mm; visible
Sand
>0.062mm, <2.0mm; visible
Silt
>0.004mm, <0.062mm; granular feel but
not visible. Smaller than fine sugar/salt
Clay
<0.004mm. Rock surface feels smooth.
Not visible
Grain angularity
Grains angular when eroded from source area; during transport become
Rounded; degree of angularity indicates distance of transport
Ex.—coarse grained clastic rocks—conglomerate (rounded) vs.
Breccia (angular).
Grain sorting
Measure of how equal in size the grains are:
Poorly sorted—varied grain sizes throughout
Well sorted—same grain size throughout
Ex—beaches w/ wave action—well sorted (due to fines being removed,
coarse left behind) vs. glaciers—poorly sorted (glaciers transport all sizes)
Not important in identification of sed rocks but provides clues to physical
conditions of deposition
Figure 4.2—Identification of clastic sedimentary rocks
Grain Size
Composition and Shape
Coarse: >2.0mm
Rounded/angular grains of
qtz, feldspar, chert
Medium: 0.062-2.0 mm
Quartz grains/feldspar
grains
Fine: 0.004-0.062mm
Too fine for identification;
probably qtz
Very fine: 0.004mm
Too fine for identification;
Probably clay minerals
Rock Name
Conglomerate/breccia
Qtz sandstone/arkosic
sstone
Siltstone
Shale
Organic and Chemical Rocks
-sedimentary rocks formed by biologic activity or by direct precipitation from sea
water
Types:
Limestone
Fossiliferous limestone (organic rock)—composed of calcite; may contain
fossils, skeletal fragments of marine organisms.
Chalk (organic rock)—composed of microscopic skeletons of tiny
organisms; have white, smooth surfaces due to size of fossils
Oolitic limestone (chemical rock)—smooth, spherical grains of calcite—
sand size. Oolites form due to high energy environment, being rolled back and
forth by wave action.
Micrite (chemical rock)— composed of microscopic calcite crystals.
Smooth surface as a result
Dolomite
Some of calcium replaced with magnesium—probably from groundwater
Chert
Composed of microscopic qtz crystals. Some biologic from organisms
that use silica for shells; also can be chemical (groundwater replacement), with the silica
source being volcanic ash (ex—Boone limestone cherts)
Coal
Composed of carbon from plant remains that have been buried and
compressed. Individual plants have lost their form from the compression. From low to
high grade, depending on pressure and temperatures of formation:
LigniteBituminousAnthracite
Bauxite
Formed from clay minerals. In humid, tropical conditions, silica removed
from clays forming an aluminum-rich soil (oxisol or laterite); with loss of water, this may
harden and form bauxite. This is refined to produce aluminum. Syenite in Central
Arkansas sufficiently weathered 35-50 million years ago to produce bauxite. How?
Syenite high in feldspars (aluminum-rich); feldspars weather straight to clay; aluminumrich clays dry to become bauxite.
Identification and classification of organic and chemical rocks—Figure 4.3
Composition
Characteristics
Rock Name
Calcite
Visible fossil fragments
Fossiliferous Limestone
Calcite
Visible ooliths
Oolitic Limestone
Calcite
Microscopic fossils; white; Chalk
chalky surface
Calcite
Microcrystalline; gray,
Micrite
brown, black in color
Dolomite
Microcrystalline; feeble
Dolostone
acid reaction
Quartz
Microcrystalline; very hard Chert
Carbon
Black; blocky; will mark
Bituminous Coal
paper
Clay minerals
Red; soft, with pisoliths
Bauxite
Halite
Light to medium gray; salty Rock Salt
taste
Depositional Environments
Before they were rocks, sediments accumulated at specific places under specific
conditions in the geologic past. Characteristics of sed rocks, plus knowledge of how
modern sediments accumulate provide clues about conditions that existed where the
sediment was deposited.
Beaches, eolian dunes, organic reefs all characterized by specific physical,
chemical, biological conditions; each is a depositional environment. Each environment
imparts characteristics that can be recognized in a rock long after the environment is
gone.
Geologists interested in reconstruction of past depositional environments.
Importance? Example—Ancient reefs and river channels have permeabilities that allow
circulation of petroleum, uranium-bearing fluids
Coarser grained rocks—higher energy environments; finer grained rocks—lower
energy environments; organic rocks—protected environments (ex-limestones only in
clear, shallow, warm water (none in gulf of mexico!))
Clastic depositional environments
Alluvial fans—at foot of mtn ranges in arid climates. Streams carry sediments
from mts but lower elevation deserts unable to maintain flow to carry sediment, so it gets
deposited at the mountain front. Sediment poorly sorted; may contain unstable Feldspars
due to short transport time. Ex.—Sangre de Cristo mts, southern CO, Death Valley, CA
Eolian dunes—composed of wind-blown sand. Well-sorted, rounded grains;
example—great sand dunes nat’l park, Sangre de Cristo mts, southern CO
Alluvial sediments—deposited in channel or on floodplain during floods.
Moderately well sorted in channels (gravel and sand); Flood plain sediment poorly
sorted, composed of silt and clay. Ex—white river flood plain/Mississippi river delta,
eastern Arkansas
Marshes—low, swampy areas on coastal plains nr. Sea; occur on alluvial flood
plains and on emergent deltas. Sediment accumulates very slowly; vegetation grows
well; perfect place for accumulation of plant remains which eventually become coal.
Ex—E Arkansas, in Mississippi embayment
Delta—masses of sediment deposited by rivers at their mouth. Form seaward,
characterized by several depositional environments. Sand accumulates on seaward edge
and in distributary channels; clay, silt deposited atop as delta builds seaward—ex.,
Mississippi delta. Builds birds-foot seaward because of dredging by corps of engineers
Lagoons—bodies of water between barrier islands and mainland—protected by
islands from waves/turbulence; clay and silt deposition; no plant remains
Barrier Islands—deposited down current from deltas—sand well sorted by wave
activity
Shelf—moderately shallow sea floor, extends seaward from mainland to edge of
continental slope. Fine grained sediments deposited here due to quiet water not affected
by current activity.
Non-clastic depositional environments
Oolith shoals—shallow, turbulent water; occur in narrow shoals around margin of
shallow shelves
Skeletal sand shoals—slightly deeper water than oolith shoals.
Organic reefs—form at margin of shallow shelves where there are nutrients from
open sea for organisms. Reef builders have strong shells to withstand waves that attack
shelf margins
Lagoons—between barrier reefs and shallower parts of shelf, or part of shelf
interior. Protected from turbulence. Often produce micrites; because of lack of energy,
fine particles accumulate
Open Sea—Deep ocean not favorable for carbonates—carbonate not stable at
depth; chert can accumulate, though...because silica stable below carbonate compensation
depth
Uses of sedimentary rocks
Limestone—crushed stone for “gravel,” concrete aggregate
Sandstone—building stone
Bituminous coal—source of energy
Bauxite—ore of aluminum