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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 sand 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 as 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 results 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 resistant
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 deposition, 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 content 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 rounded 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 sedimentary 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 heavily 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 turbidity)
 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 framework 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 the 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 continental 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 collectively 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 water 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 past, 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 descending 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 much 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 environmental 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 Taphonomy 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 changes
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
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