Sedimentary Structures Flashcards
Sediment stops in an….
Environment of Deposition
What clues can we use to reconstruct ancient ones?
Sediment itself (Chemistry, sorting, rounding) Sedimentary Structures (patterns in the rock)
Layer of sediment forms in a particular environment
Beds are the large, main units of deposition.
Beds
Some environments create these internal layers
Cross Beds
Waves constantly wash back and forth
Symmetrical Ripples
Wind or stream flows in one direction all the time
Asymmetrical Ripples
High energy streams =
Boulders and gravel
Moderate energy rivers =
Sand, silt, mud
High energy beach =
Sand and gravel
Low energy deep ocean =
silt and mud
Other environments make beds that are uniform or…
Massive
Every main bed has cross-beds that slope in the same direction
Unidirectional Cross Beds
Unidirectional Cross Beds meaning:
Wind or water was always pushing sediment in the same direction (like a river)
Cross beds that change slope from one bed to another
Bidirectional Cross-Beds
Bidirectional Cross-Beds meaning:
Wind or water changed direction (beaches or deserts)
Meaning of meter-scale cross-beds:
Large desert dunes
Meaning of small-scale cross-beds:
Tidal currents or storms
Sediment grains get smaller from the bottom to the top of a single bed
Graded Beds
Meaning of Graded beds
Water was slowing down after a flood or submarine flow
Polygonal cracks in fine-grained sediment
Mud Cracks
Meaning of Mud Cracks
The environment was sometimes wet (rain, lake, ocean) and sometimes dry
Impressions of rain-drops or animal feet
Tracks
Meaning of tracks:
The environment was either dry land or shallow water
