Title: Sedimentary Structures
1Sedimentary Structures
- Clues to depositional environments
2Bedding - 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
3What does a geologist see?
Colorado River - Grand Canyon National Park, AZ
4Ripple 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)
5Cross-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
7Mudcracks (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
8Implications 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)