GEOL 1110 Lecture Notes - Lecture 15: Lithosphere, Great Oxygenation Event, Lithification

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1 Jul 2018
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Chapter 6
Sedimentary Structures
sedimentary structure refers to the layering of sedimentary rocks, for surface
features on layers formed during deposition, and for the arrangement of
grains within layers
The language used for layers:
bed: a single layer of sediment or sedimentary rock with a recognizable top and
bottom
the boundary between two beds is a bedding plane
strata: several beds together
bedding: overall arrangement of sediment into a sequence of beds
stratigraphic formation: a sequence of strata that is distinctive enough to be traced
as a unit across a fairly large region
bedforms: sedimentary structures that develop at the interface between the sediment
and the fluid. bedforms that develop at a certain location reflect such factors as the
velocity of the flow and the size of the clasts
Why does bedding form?
Because of sediment accumulated due to changes in the climate, water depth and
current velocity
Where do many clastic sediments accumulate?
In moving fluids (wind, rivers or waves)
Define and explain the two types of bedforms
Ripple Marks – relatively small, elongated ridges that form on a bed surface at right
angles to the direction of current flow. you can find ripples on modern beaches and
preserved on bedding planes of ancient rocks
Dunes – looks like a ripple, but much larger
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there are distinct internal laminations that are inclined at an angle to the main
sedimentary layer called cross beds. They are developed from eroded sediment
loads that build up until they slip down the leeward face. With time, the dune or ripple
builds in the downstream direction. The surface of the slip face establishes the
shape of the cross beds.
The boundary between two successive layers is called main bedding.
What is a turbidity current?
A moving submarine sediment suspension caused when sediment deposited on a
submarine slope tends to be unstable, so an earthquake or storm might disturb this
sediment and cause it to slip downslope and mix with water to produce a murky,
turbulent cloud. This cloud, which is denser than clear water, flows downslope like an
underwater avalanche.
Bed-Surface Markings
a number of features develop on the surface of a bed as a consequence of events
that happen during deposition or soon after, while the sediment layer remains soft.
Such bed-surface markings include the following:
mud cracks: if a mud layer dries up after deposition, it cracks into roughly hexagonal
plates that typically curl up at their edges. The openings between the plates are
referred to as mud cracks
scour marks: as currents flow over a sediment surface, they may erode small
troughs, called scour marks, parallel to the current flow
fossils: fossils are relicts of past life. some fossils are shell imprints or footprints on a
bedding surface
Burial and lithification of bed-surface markings can preserve the markings
Why study Sedimentary Structures?
They are important clues that help geologists understand the depositional
environment
in which sediments accumulated. For example, the presence of ripple
marks and cross bedding indicates that layers were deposited in a current, mud
cracks indicate that the sediment layer was exposed to the air and dried out, and
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Document Summary

Sedimentary structure refers to the layering of sedimentary rocks, for surface features on layers formed during deposition, and for the arrangement of grains within layers. The language used for layers: bed : a single layer of sediment or sedimentary rock with a recognizable top and bottom. Because of sediment accumulated due to changes in the climate, water depth and current velocity. Define and explain the two types of bedforms. Ripple marks relatively small, elongated ridges that form on a bed surface at right angles to the direction of current flow. you can find ripples on modern beaches and preserved on bedding planes of ancient rocks. Dunes looks like a ripple, but much larger there are distinct internal laminations that are inclined at an angle to the main sedimentary layer called cross beds . They are developed from eroded sediment loads that build up until they slip down the leeward face.

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