What are the guidelines used to determine the relative ages of rocks and events in a geologic cross-section?

Question

please help! any bullshit i can write down will do. thank you!

Answer 1

What are the Principles of Relative Geologic Age Determination?

The principle of original horizontality - sedimentary strata are initially deposited as horizontal or nearly horizontal layers.
Note: If sedimentary strata dip at an angle other than horizontal, or are folded into various angles of tilt, then the layers of rock have been tilted or folded after the layers originally formed.

The principle of lateral continuity - sedimentary strata extend sideways for some distance.
Note: If a sedimentary stratum occurs on one side of a stream valley and a seemingly identical stratum occurs at a corresponding level on the other side of the valley, then presumably they were once a single, laterally continuous layer that was later partly eroded away as the valley was eroded.

The principle of superposition - In a sequence of sedimentary strata, the stratum that is underneath is older, the stratum that is on top is younger.
Note: This is probably the simplest and yet most powerful principle of relative age determination. However, to make sure it correctly applied, you need to be sure which way was up when the sediments were initially deposited, because in some geologic structures (faults or folds) it is possible for a layer of rock to be turned completely upside-down.

The principle of inclusions - A piece of rock that is included in (completely surrounded by) sedimentary rock is older than the sedimentary rock in which it is included.
Note: If rounded pieces of granite are pebbles in a layer of conglomerate that lies on top of the granite, then the granite must have been exposed, weathered and eroded prior to the conglomerate being deposited.

The principle of cross-cutting relationships - A rock body or geologic structure that cuts off other layers or structures that would otherwise tend to continue is younger than the layers or structures that it cuts off.
Note: If sedimentary beds are cut off by a fault, then the fault must be younger than the layers of sediment.

Principle of faunal succession - Within a geologic era, period, or epoch there are certain fossil types that occur in strata of that age that are not found in strata of other ages.
Note: This principle is a powerful tool for determining the age of sedimentary rocks. Index fossils are ones that only occur within limited intervals of geologic time. Much geological research has been done to determine the extent of geologic time through which particular index fossils occurred.

Answer 2

What you are looking for is called the basic Geologic Principles. You could Google that, and it should provide you with something like what I've outlined below. Don't forget that relative age is just which layer is older than another. When you talk about how old a layer is (like in millions of years), that is called absolute age.

Sedimentary rocks are deposited in a basically horizontal manner.

In an undeformed sequence of sedimentary rocks, younger rock units were deposited on top of older rock units.

The sedimentary rocks are older than any igneous rock which intrudes into them.

A fault is younger than the rocks it cuts through

Gravels and inclusions will be older than the bed in which they are contained..

The processes of geology have the same affects and functions as they had in the past.

Where fossils are present in sedimentary rocks, the relative ages of the rocks can be determined by those fossils. This is because fossils occur in a consistent vertical order in sedimentary rocks all over the world. (This fact does not depend on evolution, and as a matter of fact was well known at least 50 years before The Origin of Species was published by Charles Darwin. However, it is nearly impossible to adequately explain this sequence without evolution)

Answer 3

thrust faulting: in additional than a number of circumstances leads to dealing with same formations two times folding: folds will be recumbent (mendacity down) so the decrease fold limb arranges the strata the different way up.