Find the Subcrop of Offset Beds

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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This fairly common problem requires that you find the intersection of a fault and some other structure (usually a bed) and locate the areas underlain by the truncated structure.

One complicating factor is the wide variety of patterns that this kind of problem can produce. In the examples below, the yellow areas (subcrop) are underlain by the bed.

The rule to follow is this. Find the relative locations of the bed and the fault at some point where the bed is known to occur. If the bed is below the fault at that point, shade in the area where the bed is below the fault. If the bed is above the fault at that point, shade in the area where the bed is above the fault.

It is also possible that the offset portion of the bed will be visible on the map as well. Depending on whether the fault is normal or reverse, and on the intersection between the bed and the fault, the subcrop patterns may be adjacent, overlap, or have a gap between them.

Examples

Separated Subcrops

 1. The sill at left intersects the fault. Find the area underlain by the sill. The outcrops of the sill and the fault are not linear because of irregular topography. 2. Construct structure contours for the sill and the fault. 3. Locate the intersections of the sill and the fault. 4. Shade in the subcrop area.

Overlapping Subcrops

 1. The sill at left intersects the fault. Find the area underlain by the sill. 2. Construct structure contours for the sill and the fault. 3. Locate the intersections of the sill and the fault. The intersection of the southwest portion of the sill with the fault is in purple and the northeast portion of the sill in green. 4. Shade in the subcrop area. Note that in this case we have a thrust fault and the two subcrop areas overlap.