Chapter 2 Primary Sedimentary
Structures
● Main types of bedding
● Determination of younger direction of beds from
primary structures
● Types of stratigraphic break
● Angular unconformity
● Soft sedimentary structures
Main Types of
Bedding
level bedding
ripple bedding
cros
s-be
dding
graded bedding
laminaeplate
wedge
trough
Main Bedding Types Bedding Shape
Graded Bedding
Fine grains distribute in
the whole bed,coarse
grains decrease from the
bottom to the top
Determination of the top and bottom
surfaces from graded bedding
A,B– normal beds; C– vertical beds; D– overturned beds
Determination of the top and bottom surfaces
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
CROSS-BEDDING,Silurian system,Jiangxi province
Large Cross-
Bedding
Ripple Mark and its
Mold
Wave crest points to the top
and wave trough points to
the bottom of the bed
Mud Cracks
The sharp ends of the
wedge points to the
bottom of the bed
Stromatolite in Different Shapes
The laminae protrude to the top surface
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
Determination of top surface of a bed from
the roots of plants
Rain Impression and Mud Cracks
Determination of the top surfaces of the
beds from the scour surfaces
Different Stratigraphic
Break(from Roberts,1982)
diastem
paracomformity
disconformity
unconformity
An unconformity represents the
following sequence of events
? Deposition of lower strata;
? Tilting or other deformation of lower strata;
? Erosion;
? Deposition of upper strata.
Parallel unconformity(Disconformity)(K2/J3),Angular
Unconformity(J2,J3/T1,P) and Overlap(J3/J2）
Feather edge
K—Cretaceous
J —Jurassic
T—Triassic
P— Permian
? — Granite
Angular unconformity between Cambrian and Songshan
Group(Paleoproterozoic) in Dengfeng County,Henan
Province
Soft Sedimentary Deformation Structures
Deformation bedding
Slump Drop Stone Structure,Silurian System,
Xiushui,Jiangxi Province
top
bottom
Similar to drop stone structure in tillite
Sketch Showing the Curly Bedding
Slump Breccia
Slump Folds
Slump Folds,Silurian,Jiangxi
Competent Folds In The Front Of A Slump
Sheet,Silurian,Jiangxi
Competent Folds In
The Back Of A Slump
Sheet
Folding after Breaking
Slump Faults,Middle Proterozoic,Jiangxi
How to determine the top or bottom of beds？
?How many kinds of primary structures can be used
by yourself to determine the top or bottom of beds
?How to identify unconformities？
?How to identify soft sedimentary structures?
?Can you list a few soft sedimentary structures?
Problems for Thinking