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USGS CMG InfoBank: Mud Cracks

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Comment: 16:36 - 17:49 (01:13)
Source: Annenberg/CPB Resources - Earth Revealed - 17. Sedimentary Rocks: The Key to Past Environments
Keywords: "Walter E. Reed", "sedimentary rock", "clastic sediment", "sedimentary structure", "mud crack", fissure, deposition, "depositional environment", "river bed", "tidal flat", desiccation, preservation, sand, river, "natural levee", floodplain, meander, "braided stream", delta, climate

Our transcription: Sometimes the surface of a fine sedimentary bed is also broken up into a pavement of fossil "mud cracks", either with the fissures still open or with the gaps filled in by later deposits of sediments.
Mud cracks are a very good indicator of environments.
We've all been in river beds or in tidal flats in which we've seen desiccation and formation of mud crack.
These are preserved in the geologic record if sand is washed over that mud crack, it infiltrates down into the crack and preserves the crack.
We can identify them in river deposits.
They show up on natural levees.
They show up on flood plains, and so forth.
They show up in meandering as well as braided streams.
They show up in deltaic deposits where a given delta distributary is stranded or abandoned.
And so they can be very useful to us because they tell us that the situation at the time of sedimentation was such that it became dry.

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