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 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

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

Ripple marks - indicate currents 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”) 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”) Pyramid Lake, NVGrand Canyon Natl. Park, AZ

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 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

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

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

Implications for environments? 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)