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Complex Structural Features

As we have already seen, the basic structural features dominating sedimentary basins are normal faults. These may be either planar, resulting in the formation of graben and horst structures and half graben 'domino' faults. Normal faults may also be listric, often resulting in the formation of roll-over anticlines. However, in the field, geological structures are often more complicated; specific examples of structures of sedimentary basins will be discussed in the next section. A good example of this can be seen through the process of analogue modelling. This experinemtal process involves the stretching or compression of a sandbox in order to replicate geological structures found in the field. The material inside the sandbox, either clay or sand but usually the latter is known to act in similar ways to rocks in the field. Therefore analogue modelling experiments are a very effective tool for understanding geologic structures since they can be conducted in short periods of time under many and various conditions while still providing us with an accurate representation of what will occur in reality..

Pictured below are several examples of complex geologic structures modelled in a sandbox that will be encountered in the field. It is important to note that the type and sequence of structures that will form will often do so randomly and are only constrained by the specific properties of the rocks of a region. Therefore the examples below are just that: examples.

Example 1

The following figure exhibits a sandbox experiment of a listric fault pattern by Ken McClay. At the far left of the diagram is the fixed curved surface representing the listric fault. The blue and black layers represent layers present before the onset of faulting. The red regions represent bedding deposited during the faulting period. As shown in the diagram for listric faults, the collapse of the hanging wall creates a roll-over anticline that can be clearly seen. However, this diagram demonstrates that the faulting process is often more complex. Minor faulting has also occurred within and between both the deposited sediments and sediments present prior to faulting. In particular a large horst block can be seen in the middle of the blue region with grabens either side. Half grabens are also present either side of the horst block expressing some degree of tilting. The deposited sediments show shadow listric faults roughly prallel to the main listric fault. These faults are in turn cut by planar normal faults.

Fig 4.3.1. McClay's experiment

Example 2

The figures below illustrates a common process in sedimentary basins: multiple stages of extension. In this experiment conducted by Schreurs et. al. (2002) brittle and viscous layers were placed under extensional forces resulting in a series of normal faults. Two experiments were conducted. After initial extension, the forces ceased and both brittle and viscous layers of sediment were deposited on top of the grabens that formed in the brittle layer above the bottom viscous (light grey) layer. This was followed by a second stage of extension where more faults developed as well as the immediate reactivation of already existing faults.

The top two images show the experiment with only one viscous layer while the bottom two images display the two viscous layer experiment. These images were taken at the end of the experiment and show only the final result. Horst and graben systems formed in both experiments with half grabens exhibiting high degrees of rotation. Faults migrated towards the surface in the upper two images, while in the lower two, only as far as the interbedded viscous layer. This experiment shows that faulting caused by extension can be quite complex and are highly dependent on the specific geology of the region.

Click here for the complete journal article on this experiment
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