Stratigraphy - field skills 2 Flashcards
What are Stenos laws of stratigraphy?
Principle of superposition
Principle of initial horizontality
Principle of strata continuity
Principle of cross cutting relationships
What is the principle of superposition?
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
What is the principle of initial horizontality?
Strata are deposited horizontally and then deformed by various actions later/ layers of sediment are originally deposited horizontally under the action of gravity
What exceptions do we now accept involving the principle of horizontality?
cross-bedding
What is the principle of strata continuity?
Strata can be assumed to have continued laterally far from where they presently end
What is the principle of cross cutting relationships?
Strata of structures that cross-cut layers should postdate them
What does the principle of superposition provide relative ages for?
Sedimentary and extrusive igneous rocks
What are the 2 assumptions of the principle of superposition ?
beds have not been disturbed significantly since burial (over turning)
units formed on top of each other
How does superposition apply to graded sedimentary bedding?
In beds, coarsest grains often found at the bottom and grain size fines upwards
How can superposition be applied to roots and rootlets?
Roots grow down
Stems grow up
What is the problem with using roots and rootlets for superposition principle?
most samples not found in-situ
How does superposition apply to desiccation cracks?
occur on tops of beds, often preserved at base of overlying bed
How does superposition apply to Geopetal void infilling?
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
How does superposition apply to footprint trace fossils?
preserves imprints on tops of beds, mould at bottom of overlying bed
How can rocks be thought of as originally continuous using principle of strata continuity?
rocks that are similar but are now interrupted by a valley or other erosional feature
What are faults in rocks?
brittle fractures of rock units due to tectonic stresses and movements that
show relative displacement between the two sides of the fault
What is a fracture termed if there is no displacement?
a joint
What needs to have occurred for a rock to be faulted?
rock needs to form and lithified
What indicates a break in geological succession? (unconformities)
break in time separating two distinct lithostratigraphic units
What is the relationship between lithostratigraphic units and other geologic features described by?
Cross cutting principle
Inclusion principle
What is the cross cutting principle?
Any unit that has boundaries that cut across other strata must be younger than the rocks it cuts
What are the 3 main types of unconformity?
Angular unconformity
Disconformity
Nonconformity
How are rock layers deposited with angular unconformity?
Horizontally
What will affect the horizontal deposition with angular unconformity?
Uplift causing the rock layers to be tilted, faulter or folded
What is deposition like for disconformity?
conformable beds
What will erosion do with disconformity?
it will remove previously deposited beds (without uplift or tectonic activity)
What creates a time gap in succession with disconformity?
the fact some beds have been removed
What is nonconformity?
Places a sedimentary rock above igneous or metamorphic rocks.
metamorphic or igneous rocks typically eroded
What is the order of events with angular unconformity? (new top older descending)
Deposition of overlying unit
Tectonic uplift and erosion of previously formed strata
Break in deposition
Formation of initial rock units
What is a conglomerate?
a coarse grained sedimentary rock composed of rounded clasts embedded in a finer grained matrix
What will the unit directly overlying an unconformity usually be?
conglomerate
What do large clasts mean?
High energy deposit
What will happen to clast size as you move through succession?
Clast or grain size in a sediment gets finer (graded bedding)
