sedimentary structures

Sedimentary Structures



Sedimentary Structures:

• Features that form within

sediment during accumulation

and before it is lithified.



Sedimentary Structures:


Uses of Sedimentary Structures

Sedimentary Structures & Environments

1. Paleoenvironmental indicators:

many features only form under

certain physical conditions.



1. Paleoenvironmental indicators:

many features only form under

certain physical conditions.

• on land• moving water• moving air• still water• event(s) vs. continous


From Shrock 1948


From Shrock 1948

Modern Raindrop Imprints on Clay



2. Paleo-current direction:

• some indicate direction of flow,

• may allow for regional interpretations

Late Paleozoic“Michigan River Delta”

Illinois

Indianaflow

directions

SW

NE

Late Paleozoic“Michigan River Delta”

Illinois

Indiana

flow directions



3. Stratigraphic “up” direction:

• indicates original orientation in tectonically deformed beds

?

Examples of Sedimentary Structures


1. Bedding = stratification:

• Change or break in deposition

causes a marked plane.



1a. Massive bedding:

• thick uniform bedding

• constant conditions during deposition • relatively quite water



1b. Cross bedding:

• layers within bed are inclined at time of deposition (not horizontal)

• sand moving along as ripples or dunes

• formed by water or air

• restricted to moving water



1b. Cross bedding:

Sedimentary Structures & Environmentsh

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• truncated side = up,• forset in flow direction


• truncated side = up,• forset in flow direction

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two directions = beach/tides

Sedimentary Rock Types

1c. Graded bedding:

• grain size decrease from bottom to the top of the bed.

• represents an event

Environmental significance of sandstones


Graded beds of graywacke

Environmental significance of sandstones


2. Ripples and dunes:

two kinds


2a. Asymmetrical ripples:



• long face and short face• short face points down current/wind



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Flow Direction?


2b. Symmetrical ripples:



• back and forth, wave motion

• pointed crests rounded troughs

• directional indicator, crest UP



map view

FlowDirection?

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3. Mud cracks:

• series of polygonal desiccation cracks

• sediment accumulated in alternating wet and dry environment


3. Mud cracks:

• directional indicator, curl UP


Summary:

• massive beds: <environment>

• cross bed: <up>, <flow>, <environment>

• graded beds: <up>, <environment>

• asymmetrical ripple: <flow>

• symmetrical ripple: <up>

• mud cracks: <up>, <environment>


Other structures: (not on exam)

• Raindrop imprints: <up>, <environment>



• Fossil Burrows: <up>, <environment>



• Water escape: <up>



• load casts: <up>

bottom side of sandstone over mud



• sole marks: <up>, <flow>

bottom side of sandstone over mud


Visit the

McKinney Geology Museum

sedimentary structures

Documents

graded bedding

cross bedding

massive bedding

direction of flow

kindssedimentary rock

water events

certain physical conditions

time of deposition