Photographs by Steven Earle (2015), CC BY 4.0. The pieces of shale were eroded as the sand was deposited, so the shale is older than the sandstone. Right- Rip-up clasts of shale embedded in Gabriola Formation sandstone, Gabriola Island, BC. The lava flow took place some time after the diorite crystallized (hammer head for scale). Left- A xenolith of diorite incorporated into a basalt lava flow, Mauna Kea volcano, Hawai’i. For example, a xenolith in an igneous rock, or a clast in sedimentary rock must be older than the rock that includes it (Figure 19.8).įigure 19.8 Applications of the principle of inclusion. The principle of inclusions states that any rock fragments that are included in a rock must be older than the rock in which they are included. The next three principles could be considered specific varieties of the principle of cross-cutting relationships, because they involve one geological feature disrupting another in some way. In this case, the lower portion of the fault that cuts the lower sediments may have originally formed before the uppermost sediments were deposited, despite the fault cutting through all of the sediments, and appearing to be entirely younger than all of the sediments. View source.Īn example that violates this principle is a type of fault called a growth fault, where the fault continues to move as sediments are added on top of the hangingwall block. The upper sandstone is youngest of all, because it lies on top of the coal seam.įigure 19.7 Superposition and cross-cutting relationships in Cretaceous Nanaimo Group rocks in Nanaimo BC. So we can infer that coal seam is younger than the faults (because the coal seam cuts across them). But the faults don’t appear to continue into the coal seam, and they certainly don’t continue into the upper sandstone. The lower sandstone layer is disrupted by two faults, so we can infer that the faults are younger than this layer. An example of this is given in Figure 19.7, which shows three different sedimentary layers. The principle of cross-cutting relationships states that any geological feature that cuts across or disrupts another feature must be younger than the feature that is disrupted. The main idea here is that sedimentary layers don’t just stop abruptly for no reason. But sediments can also terminate against faults or erosional features (see unconformities below), and so may be cut off by local factors. The principle of lateral continuity states that sediments are deposited such that they extend sideways for some distance before thinning and pinching out at the edge of the depositional basin. Source: AntanO (2017), CC BY 4.0 View source. The delta foresets are labeled “Delta deposits” in this figure, and you can quickly see that the front face of the foresets are definitely not deposited horizontally. Figure 19.6 A cross-section through a river delta forming in a lake. The same is true of delta foreset beds (Figure 19.6). The exception happens on small scales, like cross-beds which form at an angle to the main bedding surfaces. The principle of original horizontality indicates that sediments are originally deposited as horizontal to nearly horizontal sheets. The exception is if the sequence of rocks has been flipped completely over by tectonic processes, or disrupted and re-stacked by faulting. The principle of superposition states that sedimentary layers are deposited in sequence, and the layers at the bottom are older than those at the top. These situations are generally rare, but they should not be forgotten when unraveling the geological history of an area. Local factors must be understood before an interpretation can be made with confidence. There are a few simple rules for doing this, but caution must be taken because there may be situations in which the rules are not valid. The most basic way of dating geological features is to look at the relationships between them. 110 19.2 Relative Dating Methods Relative Dating Principles
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