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Cross Beds - Structural Geology - Lecture Slides, Slides of Geology

Alagappa UniversityGeology

In these lecture slides, following are the main points that have been discussed : Cross Beds, Erosion Truncates, Juxtaposing, Topset, Bottomsets, Foreset Angles, Tangential Angles, Stratification, Direction Indicates, Transport Direction

Typology: Slides

2012/2013

Uploaded on 07/22/2013

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Cross Beds
Erosion truncates the topset and upper part of the
foreset, juxtaposing younger bottomsets on the older
foreset; this forms higher foreset angles at the upper
bedding compared to the tangential angles below
(used for facing).
The foreset beds are inclined at an angle to the main
planes of stratification.
- Truncated at top
- Tangential at bottom .
- Dip direction indicates transport direction
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Cross Beds

Erosion truncates the topset and upper part of the

foreset, juxtaposing younger bottomsets on the older

foreset; this forms higher foreset angles at the upper

bedding compared to the tangential angles below

(used for facing).

The foreset beds are inclined at an angle to the main

planes of stratification.

  • Truncated at top
  • Tangential at bottom.
  • Dip direction indicates transport direction

Ripple Marks

Ridges and valleys on the surface of a bed, formed due to current flow. Cross stratification with wave amplitude < 6 “

(1) Oscillation or Symmetric Ripple Marks

Oscillation wave produced ripples (current moving in two opposite directions)

  • Crests are pointed and troughs are curved Symmetrical concave up small scale (amplitude < 6") cross stratification.
  • Good facing indicator

(2) Current or Asymmetric Ripple Marks

Asymmetric cross stratification produced by current moving in one direction; i.e., uniformly flowing current

  • Good current direction indicator

Mud Cracks

Polygon shape in map view.

Result from desiccation into an array of polygons

separated by mud cracks.

Thin (typically sand filled) fractures that taper

down in cross section because each polygon curls

upwards along its margin.

Good facing indicator (individual cracks taper

downward.

Mud Cracks

Sole Marks - Load Casts

Bulbous protrusions of denser sand into less dense mud

layers

Forms due to density instability when sediment is still

soft (i.e., still unlithified)

The sinking is triggered by the disturbance during

earthquake, storm, or slump

At greater depths, partially consolidated mud breaks

into pieces and sink into underlying sand, forming

disrupted bedding

Contacts

Contact: Boundary between two geologic units of

any kind.

1. Depositional contact: a sedimentary unit is

deposited on top of another.

2. Fault contact: two units are juxtaposed by a fault.

3. Intrusive contact: an igneous cuts across another

rock body.

Unconformity