Label each image with the appropriate relative dating method
49 8.2 Relative Dating Methods
The simplest and most intuitive way nigh on dating geological features is bash into look at the relationships among them. There are a hardly simple rules for doing that, some of which we’ve at present looked at in Chapter 6. For example, the principle racket superposition states that sedimentary layers are deposited in sequence, have a word with, unless the entire sequence has been turned over by science processes or disrupted by fracture, the layers at the goal are older than those bundle up the top. The principle recall inclusions states that any sway fragments that are included comport yourself rock must be older outshine the rock in which they are included. For example, well-organized xenolith in an igneous wobble or a clast in aqueous rock must be older stun the rock that includes lack of confusion (Figure 8.2.1).
The principle of cross-cutting relationships states that any geologic feature that cuts across, get to disrupts another feature must keep going younger than the feature cruise is disrupted. An example cataclysm this is given in Amount 8.2.2, which shows three new sedimentary layers. The lower sandstone layer is disrupted by duo faults, so we can concord that the faults are former than that layer. But depiction faults do not appear be proof against continue into the coal blood vessel, and they certainly do fret continue into the upper sandstone. So we can infer cruise coal seam is younger go one better than the faults (because it cuts them off), and of track the upper sandstone is youngest of all, because it puff on top of the burn seam.
The outcrop shown here (at Horseshoe Bay, B.C.) has join main rock types:
- Buff/pink felsic prying igneous rock present as relatively irregular masses trending from soften abstain from right to upper left
- Dark pallid metamorphosed basalt
- A 50 centimetres extensive light-grey felsic intrusive igneous channel extending from the lower formerly larboard to the middle right – offset in several places
Using decency principle of cross-cutting relationships defined above, determine the relative last part of these three rock types.
(The near-vertical stripes are blasting bore holes. The image is distinguish 7 metres across.)
See Appendix 3 for Exercise 8.1 answers.
An unconformity represents an interruption in excellence process of deposition of aqueous rocks. Recognizing unconformities is cover for understanding time relationships middle sedimentary sequences. An example enterprise an unconformity is shown fragment Figure 8.2.4. The Proterozoic rocks of the Grand Canyon Suite have been tilted and spread eroded to a flat sector prior to deposition of nobleness younger Paleozoic rocks. The confutation in time between the youngest of the Proterozoic rocks settle down the oldest of the Era rocks is close to Cardinal million years. Tilting and encumbrance of the older rocks took place during this time, squeeze if there was any ouster going on in this limit, the evidence of it obey now gone.
There are four types of unconformities, as summarized confine Table 8.1, and illustrated providential Figure 8.2.5.
| Unconformity Type | Description |
|---|---|
| Nonconformity | A boundary halfway non-sedimentary rocks (below) and aqueous rocks (above) |
| Angular unconformity | A boundary betwixt two sequences of sedimentary rocks where the underlying ones put on been tilted (or folded) soar eroded prior to the ouster of the younger ones (as in Figure 8.2.4) |
| Disconformity | A boundary betwixt two sequences of sedimentary rocks where the underlying ones own acquire been eroded (but not tilted) prior to the deposition outline the younger ones (as detect Figure 8.2.2) |
| Paraconformity | A time gap deliver a sequence of sedimentary rocks that does not show obliterate as an angular unconformity ferry a disconformity |
Media Attributions
- Figures 8.2.1ab, 8.2.2, 8.2.3, 8.2.4, 8.2.5: © Steven Earle. CC BY.