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Today’s Lecture:
Sedimentary structures:
• Inferring depositional processes from sedimentary rocks •Sea-level changes & the facies concept
Chapter 7: Sedimentary Rocks
Sedimentary structures: Features observed within a single bed.
Within sedimentary beds, distinctive structures can
usually be seen. These include
systematic variations in grain size and sorting, internal bedding
features, etc. that are diagnostic of
particular depositional processes.
Sedimentary Structures
Graded beds:
Show a gradual change in particle size as you move from the bottom of a bed to the top.
Bed 1
Bed 2
Sedimentary Structures
Graded beds:
Show a gradual change in particle size as you move from the bottom of a bed to the top.
Bottoms of beds: Coarser
Bed 1
Bed 2
Sedimentary Structures
Graded beds:
Show a gradual change in particle size as you move from the bottom of a bed to the top.
Tops of beds: Finer
Bed 1
Bed 2
Graded Bedding
The bed to the right shows a change fromlarge grains at the bottom, to small at the top. This is called “normal” grading.
Coarser
Finer
Graded Bedding
As transportvelocity declines, coarser particlessettle out first
(see videoon turbidity
currents).
Higher Velocity
Lower Velocity
Graded Bedding
Thus, graded beds tell us
how flow velocity changed during
deposition!
Higher Velocity
Lower Velocity
Sedimentary structures: Cross-Bedding
Cross-bedding is internal bedding
that is tilted at an angle to the
primary bedding. Cross beds are
formed by a scour and fill transport process involving
either wind or water (see
ripple movie).
Cross-bedding
Tilt-direction of cross beds indicates the direction of transport (e.g., wind direction or
direction of water flow).
Transport direction
Sedimentary Structures: Ripple Marks
Ripple marks form when moving wind or water causes sedimentary grains to “hop” along the bottom.
Fig. 7.27a
Stephen Marshak
Ripple marks can be either symmetrical (formed by waves sloshing back and forth), or symmetrical (formed by water
or wind flowing in one direction).
Sedimentary Structures: Ripple Marks
Look closely at the ripples on this surface.
Are they symmetrical, or asymmetrical?
Which way did the water
flow?
Sedimentary Structures: Ripple Marks
Look closely at the ripples on this surface.
Are they symmetrical, or asymmetrical?
Which way did the water
flow?
Paleocurrent direction
Sedimentary Structures: Ripple Marks
Study these ripple carefully. Are they symmetrical or asymmetrical? What do
they suggest aboutpaleocurrent direction?
Look at these!
Sedimentary structures: Ripple Marks
Oscillation ripples(back and forth)
Interpretation: Paleoshoreline
Sedimentary Structures: Mud Cracks
Mud cracks form when mud-
covered shorelines orlake bottoms, dry up. This produces an irregularly-
cracked surface.
Margin of a dry lake with mud cracks.
Note ripple-marked sand dunes at top
of picture.
Fig. 7.27c
Stephen Marhsak
Say you find mud cracks in an ancient sedimentary rock.
What does that suggest about the environment where the rock formed?
Describe what you see in this outcrop and interpret the
geologic history and conditions of deposition.
In class exercise:
Lower contacts cut into unitsunderneath. Erosional!
Fine layering
Wavy basal contact
Wavy basal contact
Ripple cross-bedding
Uplift of a deep-seated igneous pluton (granitic), with subsequent erosion by running water which transported igneous andmetamorphic clasts to a river which then carried them some distance from the source area,to a site of deposition (stream channel).
Transport by running is inferred by the rounding of the clasts, size grading, and sorting.
Erosional contacts at the bases of beds indicate initial turbulent transport, followed by declining flow velocity (flood event?).
What a geologist sees:
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