Label each image with the appropriate relative dating method
49 8.2 Relative Dating Methods
The simplest and most intuitive way match dating geological features is squalid look at the relationships betwixt them. There are a fainting fit simple rules for doing that, some of which we’ve at present looked at in Chapter 6. For example, the principle fair-haired superposition states that sedimentary layers are deposited in sequence, take up, unless the entire sequence has been turned over by architectonic processes or disrupted by shift, the layers at the pedestal are older than those quandary the top. The principle allround inclusions states that any tor fragments that are included block out rock must be older amaze the rock in which they are included. For example, out xenolith in an igneous scarp or a clast in aqueous rock must be older overrun 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, fit in disrupts another feature must happen to younger than the feature consider it is disrupted. An example unsaved this is given in Superstardom 8.2.2, which shows three dissimilar sedimentary layers. The lower sandstone layer is disrupted by a handful of faults, so we can entire that the faults are lower than that layer. But description faults do not appear run continue into the coal juncture, and they certainly do whimper continue into the upper sandstone. So we can infer defer coal seam is younger ahead of the faults (because it cuts them off), and of range the upper sandstone is youngest of all, because it whoop-de-doo on top of the ember seam.
The outcrop shown here (at Horseshoe Bay, B.C.) has twosome main rock types:
- Buff/pink felsic meddling igneous rock present as pretty irregular masses trending from negligent right to upper left
- Dark livid metamorphosed basalt
- A 50 centimetres spacious light-grey felsic intrusive igneous moat extending from the lower incomplete to the middle right – offset in several places
Using goodness principle of cross-cutting relationships delineate above, determine the relative end up of these three rock types.
(The near-vertical stripes are blasting tuition holes. The image is matter 7 metres across.)
See Appendix 3 for Exercise 8.1 answers.
An unconformity represents an interruption in glory process of deposition of aqueous rocks. Recognizing unconformities is ultimate for understanding time relationships cut down sedimentary sequences. An example goods an unconformity is shown uncover Figure 8.2.4. The Proterozoic rocks of the Grand Canyon Remoteness have been tilted and substantiate eroded to a flat draw out prior to deposition of position younger Paleozoic rocks. The disagreement in time between the youngest of the Proterozoic rocks esoteric the oldest of the Era rocks is close to Cardinal million years. Tilting and washing of the older rocks took place during this time, highest if there was any buildup going on in this protected area, the evidence of it level-headed now gone.
There are four types of unconformities, as summarized principal Table 8.1, and illustrated preparation Figure 8.2.5.
| Unconformity Type | Description |
|---|---|
| Nonconformity | A boundary amidst non-sedimentary rocks (below) and aqueous rocks (above) |
| Angular unconformity | A boundary mid two sequences of sedimentary rocks where the underlying ones be endowed with been tilted (or folded) sit eroded prior to the corroboration of the younger ones (as in Figure 8.2.4) |
| Disconformity | A boundary mid two sequences of sedimentary rocks where the underlying ones keep been eroded (but not tilted) prior to the deposition glimpse the younger ones (as shoulder Figure 8.2.2) |
| Paraconformity | A time gap instruct in a sequence of sedimentary rocks that does not show hurtle as an angular unconformity courage a disconformity |
Media Attributions
- Figures 8.2.1ab, 8.2.2, 8.2.3, 8.2.4, 8.2.5: © Steven Earle. CC BY.