How can you tell the relative age of a sedimentary rock?

Answer 1

This an area of common misconception. Most sedimentary rock, and the fossils contained therein cannot be dated directly with radiometric techniques, but can be dated indirectly. Here's the facts:

• Because most sedimentary rock is composed of particles of pre-existing rock of various types, each particle can be a different age.
• Carbon-14 or carbon dating techniques can only be used to date organic material which is of a maximum age of 70,000 years; this leaves out the vast majority of fossils found in rock, and only covering roughly .000016 percent of geologic time.
• The dating of fossils was originally done by their placement in a geologic column of rock strata. Because of the Law of Superposition, the deeper the stratum, the older it is (in an undisturbed body of rock or sediment). Fossils which were evident in a particular stratum, but missing from other strata were noted as "index" fossils. The presence of these fossils indicated the age of the stratum relative to other layers. The layers of stratum were placed in a geologic column, subdivided, and assigned names based on the presence of different types of fossil organisms. The use of fossils in this manner is known as biostratigraphy and is the basis of "relative dating" of rock layers and fossils, a dating technique that was in effect until the advent of radiometric techniques in the twentieth century.
• Radiometric dating of rock involves the measurement of the amount of decay in radioactive elements which are present in the rock. Radioactive elements decay at unique rates, depending on the isotope. This rate of decay is known as half-lives, it is the time necessary for ½ of the atoms to decay in a particular element. The decay follows a geometric scale, in that in the first half-life of an element, ½ of the atoms decay, yet in the second half-life, ½ of those remaining decay and so forth. By measuring this decay, and knowing the half life of an element, scientists can date a sample. Radiometric dating is particularly useful in dating igneous and metamorphic rock.
• Because of geologic events such as plate tectonics and volcanism, suitable material for radiometric dating, such as volcanic tuff, solidified lava, and igneous intrusions have been found as layers on, in, and cut through layers of sedimentary rock. Intrusions are always younger than the rock body they penetrate, meaning that the sedimentary rock in which intrusions are found will be older than a radiometrically dated sample of the intrusion. Lava flows and volcanic ash which form layers in rock will be younger than the rock below and older than the rock above. The solidified intrusions and lava flows can be dated with radiometric techniques.
• The radiometric dating of suitable rocks (i.e. igneous or metamorphic) in proximity to their sedimentary counterparts, therefore allows the sedimentary rock to be dated as well.

Answer 2

Sedimentary rock is dated by the use of index fossils, traces of certain organisms that were widespread, plentiful, and only existed for a certain period of time. The rock layer in which they exist has been dated by radiometric dating techniques and the fossils indicate a general age.

Answer 3

There are two general means:

• Relative dating: The rock is dated relative to neighbouring rocks by the four laws of relative dating: Superposition, Cross-cutting Relationships, Included Fragments and Unconformities. Further detail on these is provided beneath. Another means is the use of 'zone fossils'. Zone fossils are those which are known only to have occurred in a very specific time frame. Rocks including such fossils are dated in the same period as others containing the same fossils. A further means is 'way-up' evidence.
• Absolute dating: this does not apply to sedimentary rocks unless they contain traces of the original organic material. This can only be applied to Igneous rocks, or perhaps some metamorphic rocks.

The laws of relative dating are broken down more specifically here:

• Superposition: Rocks are 'piled' on top of each other, resulting in multiple strata being formed, the youngest at the top, the oldest at the bottom.
• Cross-cutting relationships: When there is an igneous intrusion especially, one can automatically assume that the intrusion entered into the country rock after the country rock was formed. Furthermore, if there was a crack in the rock, this could be in-filled with sediment, thus the sediment is younger than the cracked rock.
• Included fragments: sometimes when a volcano erupts it removes fragments of the cone with it, which are later discovered amidst the erupted igneous rock, thus this country rock is an included fragment, and is older than the igneous material it is encased in.
• Unconformities: Unconsolidated sediment which appears to cross multiple rock strata is younger than the rock it is deposited onto. Furthermore, one must consider the lava flow. After it has been identified as a lava flow (by studying baked/chilled margins), it is clear that it is younger than the rock it is on top of.
• 'Way-up' evidence: This helps you to ensure that you have not got the rock series the wrong way up, with the oldest at the top. There are a number of ways this can be identified:
• • Sedimentary features: Sole marks, dessication cracks, Flute casts and graded bedding may all contribute to finding way up evidence. There are many more sedimentary features which contribute. For example, a sole mark looks like a small dip in cross section, so it can only be a 'U' shape for it to be the right way up.
  • Baked/chilled margins: In a cross-section, a lava flow always has one side on which there is a chilled margin and a baked margin on the country rock, whereas a dyke/sill always has two which makes them useless here. Therefore, if an igneous rock appears to have just one chilled margin and matching baked margin on the country rock, it is a lava flow, and the baked/chilled margin can only be beneath the extrusion.

Answer 4

By the recognition of index fossils, which are geographically widespread rapidly evolving organisms that existed only in a previously determined specific range of geologic time.

Answer 5

Most sedimentary rocks have layers and if its does the age can be told by the amount of layers attached to the rock.

Answer 6

carbon dating