EARTH238-2a-Primary Structures Part 1

Why are primary structures useful

markers; younging indicators

Bedding

Primary layering in sedimentary rock, formed during deposition, manifested by changes in texture, colour, and/or composition

Stratigraphic Facing

Direction to younger strata, direction to the depositional top of beds

Younging directions

same as stratigraphic facing; direction to younger strata; direction which bed gets younger, what way was originally up

Why is bedding the best marker

it is planar and horizontal

Graded bedding

Systematic variation in grain size within a bed: sediments are coarser grained at base and finer at the top

Cross Bedding

surfaces within a thicker, master bed that are oblique to the overall bounding surfaces of the master bed

Flute casts

asymmetric troughs formed by vortices (mini-tornadoes) within the fluid that dig into the unconsolidated substrate

Ripple marks

Ridges and valleys on the surface of a bed formed as a consequence of fluid flow. can be symmetric (flows back and forth) or asymmetric (uniformly flowing)

Load Structures

commonly occur at contact b/w sand layer and mud layer

Flames

Ball and pillow

Dewatering Structures

sediments oversaturated with water and under pressure; disturbed by an event, water wants to get out