Laboratory Manual for Introductory Geology 4e

EXERCISE 6.9
Gaining Insight into Depositional Environments of Sedimentary Rocks
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Sedimentary structures, fossils, cements, and other features of sedimentary rocks provide insight into the environment in
which the sediment was deposited. Match the letter of each of the sedimentary rocks and features in the left-hand column
with an aspect of the depositional environment listed in the right-hand column
Rock and/or feature of the rock
(a) poorly sorted arkose
(b) contains fossils of intact coral
(c) red mudstone containing dinosaur footprints
(d) black shale containing some pyrite crystals
(e) sandstone containing symmetric ripples
(f) cross-bedded sandstone
(g) contains >50% carbon and fossil leaves
(h) contains large, rounded pebbles and cobbles
(i) contains fossils of feathers
(j) very angular grains
Aspect of the depositional environment
deposited in a desert dune or in a current
formed from sediments accumulated in a swamp
deposited in very quiet (stagnant) water
deposited in an anoxic (oxygen-free) marine setting
deposited by a swiftly moving stream
formed from warm-water, shallow marine reef
has not undergone a lot of transport
immature, deposited close to the source
formed from terrestrial mud (riverbank deposits)
deposited on a beach
6.7 Fossils: Remnants of Past Life
Fossils are the remains or traces of animals and plants that are preserved in rocks,
and are the subject of a major branch of geology called paleontology. Fossils record
the history of life on the Earth and, as discussed elsewhere in this book, provide
clues to the age of strata. We mention them briefly here because (1) fossils are
essentially a special kind of sedimentary structure, and (2) most fossils are found
in sedimentary rocks because the organisms lived (or died) in the environments in
which sedimentary rocks were deposited.
Because different kinds of organisms live in different environments, fossils are
clues to the depositional environment of the sedimentary rock in which they are
found. Applying the principle of uniformitarianism, we can deduce that sediments
containing fossil corals probably formed in warm, clear seawater; sediments with
fossils of tropical plants were probably deposited in equatorial latitudes; sediments
with fossils of dinosaurs generally accumulated on land rather than in the deep
ocean; and sediments containing fossil starfish accumulated in the oceans, not on
land. It is important to recognize that fossils do not provide a complete record of
ancient life because most fossils preserve only the shape of the hard parts (shells,
bones, or teeth) of an organism. Soft-bodied organisms are preserved only in very
special cases where deposition happens before the organism decays (FIG. 6.15).
6.8 Applying Your Knowledge to Stratigraphy
Ultimately, geologists use observational data about sedimentary rocks to interpret
the conditions of deposition in which the sediment forming the rock accumulated.
6.8 APPLYING YOUR KNOWLEDGE TO STRATIGRAPHY
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