Stratigraphy - field skills 2

Question 1

What are Stenos laws of stratigraphy?

Answer 1

Principle of superposition
Principle of initial horizontality
Principle of strata continuity
Principle of cross cutting relationships

Question 2

What is the principle of superposition?

Answer 2

In an undisturbed sequence of strata, any stratum is younger than the sequence of strata on which it rests, and is older than the strata that rest upon it

Question 3

What is the principle of initial horizontality?

Answer 3

Strata are deposited horizontally and then deformed by various actions later/ layers of sediment are originally deposited horizontally under the action of gravity

Question 4

What exceptions do we now accept involving the principle of horizontality?

Answer 4

cross-bedding

Question 5

What is the principle of strata continuity?

Answer 5

Strata can be assumed to have continued laterally far from where they presently end

Question 6

What is the principle of cross cutting relationships?

Answer 6

Strata of structures that cross-cut layers should postdate them

Question 7

What does the principle of superposition provide relative ages for?

Answer 7

Sedimentary and extrusive igneous rocks

Question 8

What are the 2 assumptions of the principle of superposition ?

Answer 8

beds have not been disturbed significantly since burial (over turning)
units formed on top of each other

Question 9

How does superposition apply to graded sedimentary bedding?

Answer 9

In beds, coarsest grains often found at the bottom and grain size fines upwards

Question 10

How can superposition be applied to roots and rootlets?

Answer 10

Roots grow down
Stems grow up

Question 11

What is the problem with using roots and rootlets for superposition principle?

Answer 11

most samples not found in-situ

Question 12

How does superposition apply to desiccation cracks?

Answer 12

occur on tops of beds, often preserved at base of overlying bed

Question 13

How does superposition apply to Geopetal void infilling?

Answer 13

Will have a void space which will be filled with mudstone but not completely the air remaining acts like a spirit level allowing the orientation to be known

Question 14

How does superposition apply to footprint trace fossils?

Answer 14

preserves imprints on tops of beds, mould at bottom of overlying bed

Question 15

How can rocks be thought of as originally continuous using principle of strata continuity?

Answer 15

rocks that are similar but are now interrupted by a valley or other erosional feature

Question 16

What are faults in rocks?

Answer 16

brittle fractures of rock units due to tectonic stresses and movements that
show relative displacement between the two sides of the fault

Question 17

What is a fracture termed if there is no displacement?

Answer 17

a joint

Question 18

What needs to have occurred for a rock to be faulted?

Answer 18

rock needs to form and lithified

Question 19

What indicates a break in geological succession? (unconformities)

Answer 19

break in time separating two distinct lithostratigraphic units

Question 20

What is the relationship between lithostratigraphic units and other geologic features described by?

Answer 20

Cross cutting principle
Inclusion principle

Question 21

What is the cross cutting principle?

Answer 21

Any unit that has boundaries that cut across other strata must be younger than the rocks it cuts

Question 22

What are the 3 main types of unconformity?

Answer 22

Angular unconformity
Disconformity
Nonconformity

Question 23

How are rock layers deposited with angular unconformity?

Answer 23

Horizontally

Question 24

What will affect the horizontal deposition with angular unconformity?

Answer 24

Uplift causing the rock layers to be tilted, faulter or folded

Question 25

What is deposition like for disconformity?

Answer 25

conformable beds

Question 26

What will erosion do with disconformity?

Answer 26

it will remove previously deposited beds (without uplift or tectonic activity)

Question 27

What creates a time gap in succession with disconformity?

Answer 27

the fact some beds have been removed

Question 28

What is nonconformity?

Answer 28

Places a sedimentary rock above igneous or metamorphic rocks.
metamorphic or igneous rocks typically eroded

Question 29

What is the order of events with angular unconformity? (new top older descending)

Answer 29

Deposition of overlying unit
Tectonic uplift and erosion of previously formed strata
Break in deposition
Formation of initial rock units

Question 30

What is a conglomerate?

Answer 30

a coarse grained sedimentary rock composed of rounded clasts embedded in a finer grained matrix

Question 31

What will the unit directly overlying an unconformity usually be?

Answer 31

conglomerate

Question 32

What do large clasts mean?

Answer 32

High energy deposit

Question 33

What will happen to clast size as you move through succession?

Answer 33

Clast or grain size in a sediment gets finer (graded bedding)