Primary Structures

Question 1

What are the main types of structures

Answer 1

Primary structures - Non-tectonic

Secondary structures - Tectonic
- Brittle structures
- Ductile structures

Question 2

Why are primary structures useful

Answer 2

-Markers
-Dating indicators (which layer/feature is younger)
Once a structure has been deformed, you can’t always go by which one is on top and which one is below
->structures not formed by tectonics are thus better used for dating

->give you direction

Question 3

What is the best marker

Answer 3

Bedding -> planar, horizontal to start
-> if bedding is no longer planar or horizontal, you know it has been deformed

primary beds are markers in structural analysis

Question 4

Younging (or facing) direction

Answer 4

direction in which beds get younger
-direction to depositional tops of the beds
-what way was originally up

Question 5

Graded bedding

Answer 5

sediments are coarser grained at the base and finer grained at the top

coarse, heavy particles settle first
->finer grained stuff is lighter, can be carried

-occurs in water

Question 6

bouma sequence

Answer 6

turbidity
a and b will be more preserved, upper layers less likely to be preserved

Question 7

Cross bedding

Answer 7

the younger one always cuts the old one (cross cutting relationships)

Question 8

ripple marks

Answer 8

angled ripple marks from currents are harder to tell what the top or bottom is bc it looks the same upsidedown. mineral deposits can give us a clue to this

Question 9

load structures

Answer 9

occur at the contact between a mud layer and a sand layer

when its muddy, sediments contain lots of water. sand is denser than mud so it will sink into the mud. mud may inject into the sand

Question 10

load structures
- ball and pillow
- flames

Answer 10

flame structure as mud injects up into the sand

sand sinks into mud and forms a ball, mud wraps around

flames turning into b&p

Question 11

dewatering structures

Answer 11

Sediments oversaturated with water
and under pressure
-Disturbed by an event (such as an
earthquake or slumping), water
wants to get out
-water goes up

sand volcanoes
-water comes up, sand creates dome shape

clastic dyke
-sandstone dyke

Question 12

scour structure or channel-and-fill structure

Question 13

describe the 3 types of contacts and how to differentiate between them

Answer 13

1. depositional contacts
   -rocks on ground, sediments deposit on top
   -conformable contact (continuous deposition, no substantial time gap)
   -unconformable contact (not continuous deposition, time gap)
   -hiatus

2.fault contact
contact caused by a fault

3.intrusive contact
-magma intrudes on rock

Question 14

what type of contact is this

Answer 14

sedimentary, depositional

Question 15

types of unconformities

Answer 15

disconformity
-major gap in time
-erosional surface
-bedding above and below are parallel

angular unconformity
-folded, often metamorphosed
-top is eroded, sediment deposited on top

nonconformity
-intrusive rock or high grade metamorphic rock

Question 16

huttons unconformity

Question 17

paleozoic sedimentary rocks overlying grenvillian metamorphic rocks

Answer 17

metamorphic rocks were metamorphosed deep underground, then got lifted close to the surface, overlying rocks got eroded, then sedimentary rocks were deposited on top

(nonconformity)

Question 17

how can you recognize unconformities

Answer 17

angular discordance

crystalline rocks in direct contact with sedimentary rocks (if the contact is not intrusive or fault)

hiatus in stratigraphic sequence (e.g. based on fossils)

weathering or erosional surface (e.g. scour features, paleosol, basal conglomerate)

Question 18

compaction and diagenesis structures

Answer 18

differential compaction, both laterally and vertically, results in pinc-and-swell structures

compression causes clay minerals to line up

Question 19

pressure solution

Answer 19

pressure solution

at certain layers there are greater concentrations of clay minerals that do not dissolve as easily
->they get left behind and form the structure
->happens in limestones

Question 20

penecontemporaneous folds

Answer 20

-folding can be primary
sediment deposits on top of a slope and slides down, forms a slump structure
-bed above and below are uniform, but between is a complex structure
-the bedding would be more regular if it was due to tectonics (moved after it was lithified)

-regular layers, but in between are chaotic structures that look like they’re sloping in one direction

Question 21

igneous intrusions

Answer 21

dykes, cross cutting relationships

intrusive foliations

pillow lava (submarine)

flow banding
cooling fractures: columnar joints

-examples of this in the rock garden