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Clastic Dikes: The Shape of Filled Fractures

There are 4 basic shapes taken by sediment-filled fractures. I classify them as "A", "AV", "V", and "X". Shape is dependent on the depth of maximum extension, scale of the observation, and orientation of the plane in which they are observed. Fractures are 3D structures shown in cross section here. The sediment that fills each type may be sourced from above, below, or from somewhere out of the plane of observation. Infilling may be passive via gravity (i.e, sediment washed into an open crack) or involve pressurized injection. Shape plays a significant role in how filled fractures are interpreted. Taper direction, cross cutting relationships with the host strata (relative timing), and internal characteristics of the fill itself are the keys to determining how any given set of filled fractures formed. Indeed, whole categories of geologic phenomena can be ruled out if shape is correctly determined.

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