Chapter 3 Sedimentary Rocks, Sedimentary Environments and Fossils

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

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Chapter 3


Sedimentary Rocks, Sedimentary Environments and Fossils


3.1 Introduction to Sedimentary Rocks



Sedimentary rocks form at Earth’s surface

o

Earth’s surface is the interface between the solid Earth, hydrosphere,
atmosphere, and biosphere

o

Therefore,

sedimentary rocks are indicative of surface conditions and
related climatic feedbacks



There are several common types of sedimentary rocks

o

Terrigenous


derived from preexisting rocks by weathering, erosion,
transport, and deposition of sediment (such as s
and or mud)

o

Biogenic


consist predominantly of shells of dead organisms (fossils)

o

Chemical


formed by chemical precipitation from water



Besides sediment grain size and composition and fossils, sedimentary rocks may

also preserve sedimentary structures a
s clues to their origin and interpretation

3.2 Processes of Weathering



Rocks exposed at Earth’s surface break down by processes of physical and

chemical weathering

o

Physical weathering involves the formation and enlargement of cracks

o

Chemical weathering r
esults from weak acids produced by the decay of
organic matter and the dissolution of carbon dioxide in water

o

Ions dissolved from rock are carried by rivers and groundwater to the
ocean

o

In the weathering of granite, feldspar and micas form mud; highly resi
stant
minerals such as quartz are freed to produce sand and gravel

3.3 Terrigenous Sedimentary Rocks



Formation of Terrigenous Sediments


o

The three broad types of terrigenous sediment are based on the
predominant grain size: gravel, sand and mud (not their
chemical
composition)

o

Loose rock and mineral grains that result from weathering are transported
(by landslides and streams) and eventually deposited, buried and lithified

o

Lithification involves consolidation of lose sediment into rock by burial,
compaction

and cementation



What Do Terrigenous Sedimentary Rocks Tell Us About How They Formed?

o

Sedimentary rocks have histories that can be deciphered using indicators
such as the size, shape, and mineral composition of the grains

o

During transportation and depositi
on, sediment grain sizes may be
deposited together, or may be separated according to size

o

The separation according to size is the sorting; it depends upon the amount
of environmental energy available to transport the grains

o

Coarse grain sizes (pebbles and
cobbles) tend to be deposited close to the
source area because they are the least easily transported

o

Muds (silt and clay) tend to be transported the furthest because they are
easily suspended in water; they are deposited in quiet waters


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o

The grains of sands

and gravels may undergo rounding as they are
transported due to abrasion of the grains against one another

o

The extent of transport, rounding and sorting in terrigenous sediment
reflects its maturity

o

Maturity is of two types: compositional (relative conten
t of unstable
minerals) and textural (relative sorting and rounding of grains)

3.4 Biogenic Sedimentary Rocks



Biogenic sedimentary rocks are biologic in origin (generated by organisms)

o

They contain the shells of creatures that use dissolved ions to secrete

shells for skeletal support and protection

o

Limestone, a common biogenic sedimentary rock, is composed of calcium
carbonate

o

Shallow
-
water limestones form where there is little terrigenous sediment
(otherwise it would dilute the calcium carbonate)

o

Biogenic
limestone formed deeper in the ocean from the remains of
plankton is called calcareous ooze

o

Another type of ooze is siliceous ooze, which forms from the siliceous
remains of other types of plankton

3.5 Chemical Sedimentary Rocks



Chemical sedimentary rocks
are generated by precipitation from water

o

Evaporites such as gypsum, anhydrite and halite represent a type of
chemical sedimentary rock that form when water evaporates

o

Oolite is a type of limestone formed when calcium carbonate precipitates
from water as r
ounded grains (ooids)

o

Shallow water limestones may be altered to dolostones by highly saline
waters (brines) that percolate through them

o

Cave formations such as stalactites and stalagmites are made of travertine,
and are another type of chemical sedimentar
y rock

3.6 Marine Environments


environments of the ocean are referred to collectively as
marine environments



Marginal Marine Environments


environments where land and sea meet

o

Associated with coasts, bays, barrier islands, and estuaries

o

Often are heavil
y influenced by fresh waters and sediment brought by
rivers

o

Account for a small area of Earth’s surface but are the most productive of
all ecosystems

o

May be stressful to organisms because of fluctuating conditions (for
example: salinity, oxygen, and turbid
ity)



Coral Reefs

o

Among the most diverse biologic communities on Earth

o

Thrive in shallow, clear, warm tropical waters of normal marine salinity
with little terrigenous influx

o

Corals secrete massive amounts of calcium carbonate, forming the
limestone framewo
rk of the reef

o

During Earth’s history, other types of organisms besides corals have
formed reefs, but also thrived in relatively warm, shallow, clear waters


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Continental Shelves, Continental Slopes, and the Abyss

o

The neritic zone consists of the waters of t
he continental shelf (from the
coast to 200 m depth); it is a gradually sloping region

o

Beyond the shelf break is the bathyal zone, the waters of the continental
slope; its average inclination is actually only a few degrees

o

Below the lower reaches of the co
ntinental slope lie the abyssal plains,
which average 4000 m water depth

o

In certain locations, the abyssal plain may grade into the hadal
environment (deep sea trenches), greater than 5000 m depth

o

All bottom environments of the marine realm are collectivel
y referred to
as benthic (and the creatures that live there are the benthos)

o

The water column is referred to as the pelagic environment; it is occupied
by phytoplankton and zooplankton (floating plants and animals) and
nekton (swimming species)

o

Portions of

the pelagic zone may be occupied by an oxygen minimum
zone; in this zone, decay of dead organic matter results in low levels of
dissolved oxygen in the water column

o

At ocean depths greater than about 4000 m, the level of dissolved carbon
dioxide in the wa
ter results in the dissolution of calcium carbonate
sediments; this is called the CCD (carbonate compensation depth)

3.7 Terrestrial Environments



Forests

o

Tropical rain forests are among the most productive of Earth’s ecosystems

o

This was also true in the pa
st, and widespread photosynthesis on land may
have increased oxygen levels at times in Earth’s history

o

Lush vegetation can result in the deposition of peat, which in turn can be
turned into coal under burial conditions



Deserts

o

Deserts develop beneath desce
nding air masses and in the lee of large
mountains

o

Even though there is a lack of rainfall in deserts, water is nevertheless the
main agent of erosion, transportation, and deposition



Glaciers

o

Glaciers occur as alpine glaciers (in mountain valleys) or as mu
ch larger
ice caps on continents (as on Antarctica and Greenland, where the ice can
be several km thick)

o

There are several types of sediment associated with glaciers, collectively
referred to as drift



Lakes

o

Lakes constitute the lacustrine environment

o

Lakes

may contain annual layers called varves, which are related to
seasonal changes in plankton production and sedimentation

3.8 Sedimentary Structures



Various features


called sedimentary structures


can be used as indicators of

the processes and environme
ntal conditions involved in the transportation and
deposition of sediments


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o

Examples are mud cracks, ripple marks, cross bedding, and graded
bedding

3.9 Fossils



Early Processes of Fossilization

o

Fossils are the remains or traces of preexisting organisms

o

Taph
onomy is the study of the formation and preservation of fossils

o

Fossils are normally represented by hard parts such as shells, bones, or
teeth, but trace fossils (ichnofossils) such as tracks, trails and burrows also
occur



Modes of Preservation

o

Organisms’
hard parts are subject to several processes after burial; these
can grade into one another, and are referred to as petrification

o

Fossilization process include permineralization, replacement,
recrystallization if the hard parts undergo relatively minor chan
ges

o

More drastic changes to the hard parts may leave a cavity known as a
mold; if the cavity fills in, a cast may result

o

In rare cases, soft parts are preserved by being encased in amber or
mummified

o

Lagerstatten are “mother lodes” of fossils which are not

normally
preserved; they have been studied intensively to yield information on the
history of Earth’s systems