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Sedimentary Structures

Sedimentary Structures. Clues to depositional environments. Bedding - layering!. Layering of sediments is common due to episodic deposition In most cases the layers are close to horizontal at the time of deposition Layering may survive lithification. Loose sediment - Merrimack Valley.

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Sedimentary Structures

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  1. Sedimentary Structures Clues to depositional environments

  2. Bedding - layering! • Layering of sediments is common due to episodic deposition • In most cases the layers are close to horizontal at the time of deposition • Layering may survive lithification Loose sediment - Merrimack Valley East Berlin formation - Holyoke

  3. What does a geologist see? Colorado River - Grand Canyon National Park, AZ

  4. Ripple marks - indicate currents Pyramid Lake, NV Grand Canyon Natl. Park, AZ • Ripples form on the surface of sediment (mud or sand) over which a current of wind or water is flowing • Current direction may be indicated by the asymmetry of the ripples (steep side “upstream”)

  5. Cross-bedding (inside ripples or dunes) • Sediment layers on the front slope of a ripple or dune will be inclined at the time of deposition • Current direction is indicated by the direction of tilt of those cross-beds Interior of ripples in outwash - NH De Chelley sandstone - AZ

  6. Note large scale cross-bedding in these sandstones (interpreted as old desert dunes, lithified to stone!) Canyon de Chelly, AZ

  7. Mudcracks (drying of fine sediment) In sediment, near Albany, NY In mudstone, Glacier Natl. Park, MT • Mudcracks form when wet, muddy sediment dries out and shrinks • Other sediment may fill in the cracks, preserving these features after lithification

  8. Bedding - episodic deposition, undisturbed sediment (no burrowers) Ripples marks - margins of streams, windy environments (current of wind or water) Cross-bedding - margins of streams, desert dunes (similar to ripples - the cross-beds are inside!) Mudcracks - seasonal lakes, margins of rivers or lakes (occasional drying out of environment) Implications for environments?

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