Primary Sedimentary Structures

2 Primary Sedimentary Structures

Bedding Shape

level bedding ripple bedding cross-bedding

● Main types of bedding ● Determination of younging direction of beds from primary structures ● Angular unconformity ● Soft sedimentary structures

Main Bedding Types

plate

laminae

Main Types of Bedding

wedge trough

graded bedding

Graded Bedding Fine grains distribute in the whole bed, coarse grains decrease from the bottom to the top

Determination of the yanging direction of beds from graded bedding (After M.P.Billings) A, B– normal beds; C– vertical beds; D– overturned beds

Cross-bedding, Silurian System,Jiangxi,China

Determination of the younging direction from cross-bedding. the oblique laminae converge to the bottom of the bed A– normal beds, top to the top left; B– vertical beds, top to right; C– overturned beds, top to the bottom right

(Zuoxun Zeng, 1990)

1

top mold wave crest wave trough

Ripple Mark and its Mold (After R.R. Shrock,

Mud Cracks (After R.R. Shrock, 1948)

The sharp ends of the wedge point to the bottom of the bed

1948) Wave crest points to the top and wave trough points to the bottom of the bed

Stromatolite in Different Shapes The laminae protrude to the younging direction of beds

Bedding surface

scour surface cm 11 Cross section perpendicular to bedding surface 1

2

3

4

5

Scour surface and its relation to the sequence of the beds The sandstone beds are over the silicalite beds, note the gravels in the bottom bed of the sandstones 1 sandstone, 2 silicalite, 3 limestone, 4 cross-bedding 5 silicalite gravels

Determination of top surface of a bed from the roots of plants (After R.R. Shrock)

west

east

1

2

3

4

Determination of top surfaces of the beds from the scour surfaces Rain Impression and Mud Cracks

(After M.R. Billings) 1. Lava, 2. Sandstone, 3. Conglomerates, 4. Shale

2

Deposition of older rocks

Interval of nondeposition

Deposition of Younger beds

Formation of unconformities

An unconformity represents the following sequence of events • • • •

Deposition of lower strata; Tilting or other deformation of lower strata; Erosion; Deposition of upper strata.

R.J. Lisle, 1988

Angular unconformity between Cambrian and Songshan Group(Paleoproterozoic(Pt12)) in Dengfeng County, Henan Province, China (After Ma, 1981)

111111111111111

An example of unconformity Displayed on map and crosssection

3

Deformation bedding (Zuoxun Zeng,1995)

Soft Sedimentary Deformation Structures

Slump Breccia (Zuoxun Zeng, 1995)

Slump Folds (Zuoxun Zeng, 1995)

Competent folds in front of a slump sheet, Silurian, Jiangxi, China (Zuoxun Zeng, 1995)

Similar to drop stone structure in tillite Folding after Breaking

Boudins in the back of a slump sheet (Zuoxun Zeng, 1995)

Slump Drop Stone Structure, Silurian System, Xiushui, Jiangxi Province, China (Zuoxun Zeng, 1995)

4

Problems for Review • How can we determine younging direction of beds ？ • How a unconformity can be identified ？ • How do you distinguish different unconformities? • How can we identify soft sedimentary structures? • Can you list a few soft sedimentary structures?

5