GSC 111 Lecture Notes - Lecture 6: Depositional Environment, Ripple Marks, Mudcrack
13 Jan 2017
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Depositional Environment
1. Sedimentary rocks
a. Clastic rocks- rocks formed through the settling of mineral or rock particles
b. Chemical/biological rocks
i. Rocks formed through chemical precipitation from a solution in the sea, a
lake or river or through the actions of living organisms
c. Organic rock- organic material that has been compacted and altered
d. Autochthonous- precipitation or formation in environment of deposition
i. Happens with chemical and organic
e. Allocthonous sediments- sediments sourced from the outside environment of
deposition
i. Common in clastic rocks
ii. Source of sediment is often a mountain or something similar that has
weathered
iii. Transport of sediment by ice, water, or wind
iv. Deposition of sediment from suspension
v. Lithification of sediment (compact and cementation)
f. Sedimentary structures
i. Number of features when sediments are deposited that can give clues as to
the environment where deposition occurred
ii. Most obvious is that they typically show some layering (bedding)
1. Can be very thick
a. Greater than 300 cm
b. Called massive bedding
c. Lava flow can cause massive bedding
d. Storms can cause massive bedding as well
2. Can also be as thin as .3 cm
a. Called thinly laminated
b. Sediments settling out in a very quiet environment
i. Bottom of a lake bed
iii. Gradded bedding
1. Normal grading: larger grains go to the bottom first and finer
grains go on top
a. Could be indication of storm
2. Reverse grading: finer grains on the bottom, heavy grains on the
top
a. Happens with sediments of different density
b. Dense, small grains can fall to the bottom
c. If you shake sediments, the larger ones will come to the top
i. Earthquakes can result in this
iv. Cross-bedding
1. Layers within a bed that are oriented at an angle to the dominant
foliation
2. Formed by
a. Sediment deposited on the leeward side of migrating
ripples or dune
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i. Leeward is side of the dune that is not being hit by
wind
ii. Sediment is pushed up windward side and cascades
down leeward side
v. Ripple marks
1. On surface of rock layer
2. Ridge of sediment resembling a ripple of water
a. Asymmetrical ripples
i. Form in response to unidirectional currents
b. Symmetrical ripples
i. Form in response to oscillatory flow (wave action)
3. Form in tidal lands
4. When we look at the ancient ripples, we can assume it was a
similar environment
vi. Mud cracks
1. Downward tapering, v-shaped fractures
2. Form as mud dries up and contracts when it desiccates
3. Provides evidence of sub-burial exposure
4. Can form in estuaries, lagoons, tidal-flats, flood plains
5. Close to a shore line
vii. Trace fossils
1. Tracks and trails of organisms that lived in this environment when
the sediments were being deposit
a. Worm or shrimp burrows
b. Feeding tracks of trilobites
c. Boring
d. Dinosaur footprints
2. Provide evidence of what types of animals were living in this
environment
g. Facies models
i. The characteristics of a sedimentary rock the distinguish one depositional
environment from another
ii. Ex: siltstone rock can occur in lots of different environments from a lake
to ocean to flood plain
1. If we add in other features, we can hone in on what type of
environment it was in
a. Lithology- texture and composition
b. Sedimentary structures
c. Chemical properties
d. Biologic properties
2. So if the siltstone had mudcracks, dinosaur tracks and bones
a. Had to be in a water environment (this is typical way for
sediment to be deposited
b. Wet before and dried out in the mud cracks
c. Could be at a watering hole or seasonal lake, shoreline of a
lake, floodplain
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