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Structures of Sedimentary Basins

So what sort of geological structures do we encounter in sedimentary basins? In this section we will examine this question in some detail.

As already discussed, the dominant forces driving the formation of sedimentary basins are associated with extension. This naturally means that the dominant type of feature is extension related. The extension process leads to a natural thinning of the continental crust, providing the foundations for a sedimentary basin to form

Basic Structural Features I

Planar Faults

The most basic features to form in sedimentary basins under extension are normal faults. Normal faults are generally fairly steep, ie have a high dip angle. A fault will form in the crust when the extensional forces acting on it are great enough to cause failure along a fracture plane.
Fig 4.1.1. Diagram depicting a normal fault. The large arrows indicate extensional forces, smaller 'half' arrows illustrate movement of the hanging wall and foot wall along the fault plane.

Although they may occur in the field in the simple form pictured above, normal faults may also occur in groups, forming distinctive structures, or may also occur as variants of this basic type. One of the most common structures caused by normal faulting are grabens and horsts. The structures are the product of several normal faults forming in close proximity and dipping in opposite directions. This leads to the subsidence of a hanging wall (graben) between two foot walls (horst).


Fig 4.1.2 This diagram illustrates the formation of a horst and graben system. The development of opposite dipping normal faults leads to the subsidence of grabens in between horst blocks.

Fig 4.1.3 Sand box model illustrating the effects of extension on sediments. This particular diagram depicts the formation of graben and horsts.

Normal faults may not always dip in opposite directions when they form. Instead, they may actually dip in similar directions. This will lead to a 'domino' structure in which adjacent faults will slip down relative to the fault next to it. These structures are also known as half-grabens and are accompanied by a tiling of the fault blocks. Sedimentary layers may also bend towards the fault plane.
Half-grabens
Fig 4.1.4 Diagram illustrating the formation of half grabens from a series of normal faults dipping in the same direction. These structures look like fallen dominoes.

Fig 4.1.5. Sandbox model depicting the formation of half grabens or domino faults
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