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Principle of inclusion geology8/22/2023 ![]() ![]() A growth fault is a fault that continues to move as sediments are continuously delivered to the hangingwall block. An example that violates this principle can be seen with a type of fault called a growth fault. The upper sandstone is youngest of all, because it lies on top of the coal seam. So we can infer that coal seam is younger than the faults (because the coal seam cuts across them). But the faults do not appear to continue into the coal seam, and they certainly do not 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 19.8, 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. Photographs by Steven Earle (2015) CC BY 4.0 view sources left/ right 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. Figure 19.7 Applications of the principle of inclusion. This situation can make the pieces of the dyke appear to be xenoliths, but they are younger than the surrounding rock in this case. Later deformation may cause the dyke to be pulled apart into small pieces, surrounded by the host rocks. A possible situation that would violate this principle is the following: an igneous dyke may intrude through a sequence of rocks, hence is younger than these rocks (see the principle of cross-cutting relationships below). For example, a xenolith in an igneous rock, or a clast in sedimentary rock must be older than the rock that includes it (Figure 19.7). 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. But sediments can also terminate against faults or erosional features (see unconformities below), so may be cut off by local factors. ![]() The principle of lateral continuity states that sediments are deposited such that they extend laterally 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 holds true of delta foreset beds (Figure 19.6). For example, cross-bedding forms at an appreciable angle, where sand is deposited upon the lee face of a ripple. At a broad scale this is true, but at a smaller scale it may not be. The principle of original horizontality indicates that sediments are originally deposited as horizontal to nearly horizontal sheets. This situation may not be true, though, if the sequence of rocks has been flipped completely over by tectonic processes, or disrupted 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. But caution must be taken, as there may be situations in which the rules are not valid, so local factors must be understood before an interpretation can be made. There are a few simple rules for doing this. The simplest and most intuitive way of dating geological features is to look at the relationships between them. 19.2 Relative Dating Methods Relative Dating Principles ![]()
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