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

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”)

Pyramid Lake, NV Grand 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

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

In sediment, near Albany, NY In 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)