Transcript Sedimentary Rocks - Cal State LA

Sedimentary Rocks
Clues to the Past
Sedimentary Rocks
• Form from sediment that’s
been weathered or
precipitated
• Cover about 75% of Earth’s
surface.
ripples
• Provide clues to past
environments
• Only rock type that contains
fossils
fossils
Mud cracks/raindrops
Types of Sedimentary Rocks
• Detrital
– Derived from weathering
of pre-existing rocks
– Transported, deposited
and lithified
• Chemical
– Form from chemical
components dissolved in
water
– precipitation from an
aqueous solution
Detrital Rock Samples
conglomerate
sandstone
shale
Chemical Rock Examples
Halite
Chemical and biochemical
Limestone
Sedimentary Textures
• Grain Size
• Angularity
• Sorting
Grain Size
• May be classified according to grain size
pebbles
silt
sandstone
clay
• Amount of energy
• Mode of transportation
Amount of Energy
Mode of Transportation
Wind vs. Water
Which cannot transport large grains? Wind
Sorting
• Measure of the variation in the range grain sizes
- Well-sorted indicate prolonged wind or water action
• Distance traveled - reworked
• Depositional processes
Depositional Processes
Headwaters
Landslide vs. River
Which would produce a poorlysorted rock? Landslide
Plains
What Type of Sorting Would This Produce?
Poorly sorted
Angularity
• Measure of how rounded the corners are
- Rounded indicates far from source
Depositional Environments
Angularity
rounded vs. angular
Depositional Environments
Mountain vs. beach
(Near source)
(far from source)
Where would you most likely find rounded
sediments? Beach
Sedimentary Structures
• Represent past environments
– Internal Structures
• Stratification
• Cross-bedding
• Graded beds
– Surface Structures
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Ripples
Mud cracks
Rain Drops
Sole marks
– Fossils
Internal Structures – Stratification
• Most obvious feature in sedimentary rocks
• Layers visible to different color &/or texture
– Beds > 1 cm thick
– Laminations - < 1 cm thick
– Bedding planes – upper and lower surfaces of layers
Internal Structures– cross-bedding
• Formed from moving wind or water
• X-beds form below ripples and dunes
• Layering inclined at an angle – dips down in downcurrent
direction
• Tangential (curve) at base and truncated (cut) at top
• Good paleocurrent and paleo“up” indicators
Cross Bedding
Which way is paleo“up”?
Down
Which way is paleocurrent? Right to left
Internal Structures – Graded Beds
• Coarser grains at bottom and finer upwards
• May indicate paleo“up”
Where would you find graded beds?
• Turbidity currents
– Rapid sediment-laden current
– Slow as it loses energy
– Produce turbidites – graded beds that fine upwards
Surface Structures - Ripple Marks
• Undulations caused by wind or water
• Symmetrical – oscillating ripples
• Asymmetrical – rivers or streams
• Good paleocurrent indicator
Ripples
• Symmetrical vs. asymmetrical
• Left-to-right or right-to-left
Surface Structures - Mudcracks
• Polygonal cracks occur when surface mud dries and
contracts
Good paleoclimate and paleo“up” indicator
Surface Structure - Raindrops
• Produced by impact of raindrops on soft mud
Good paleoclimate and paleo“up” indicators
Sole Marks – tool marks
• Sole marks are structures preserved at the bottom of surfaces
• Tools (sticks, bones, rocks) carried by a current drags, bounce,
skip along the bottom of channel
• Parallel to direction of current
Which way is paleocurrent?
Side-to-side or top-to-bottom ?
Sole Marks – Flute Marks
• Erosion or scouring of bottom sediment
• Irregular bulbous shape
• Good paleocurrent and paleo-“up” indicators
Which Way is Paleocurrent?
Left-to-right or right-to-left?
Are we looking at top or bottom? bottom
Fossils
• Good indicator of type of
environment
– Land vs. water