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Earth Geology Rock Structures Faults

The Mansfield Fault System

Click for bigger mapFaults are breaks in the rock where movement of the rocks has occurred. One side moves relative to the other. When rocks break and move, the earth shakes as the energy is released. We call this an earthquake.

Mansfield Fault MapThe faults of the Bryant Creek area are all considered to be inactive. However the proximity of the New Madrid fault zone in the southeast corner of Missouri is reason enough to make the study of even inactive faults important. The Mansfield Fault System is an inactive system of faults that runs through the Bryant watershed. 

A series of faults with a northwest trend cuts and runs parallel to Bryant Creek from northwest of Mansfield to the Arkansas border (click on maps for details). The faults are high angle and have strikes from N30W to N50W. Vertical displacement of rock layers along these faults is as much as 300 feet. At least four faults cross the rim at the north edge of the watershed between Mansfield and Cedar Gap.

The effects of one of these is visible along the north side of Highway 60 just to the east of Cedar Gap. Here greenish gray Northview shale on the west is in fault contact with tan Jefferson City dolomite on the east. The west side has been dropped around 100 feet or more relative to the east (Figures 2 and 3)

FIGURE 2 

Greenish-gray Northview shale (left side of photo) in fault contact with Jefferson City dolomite layers (right side of photo). Road cut along north side of Highway 60 just east of Cedar Gap.

FIGURE 3 

Same as Figure 2, from across Highway 60. Line in brush at right is a drainage ditch.

Another set of faults in the area lies essentially at right angles to those discussed above. These have a northeast strike and have not been mapped as extensively. The most easily observed is the Bryant Creek Fault, which crosses Bryant 2.35 miles east of Highway 5. This fault has a vertical displacement of 80 feet with the west side down (Figure 4). About seven faults have been mapped in the watershed with a northeast strike and no doubt others exist. 

The faults west of Mansfield were first mapped in association with lead mines in the area in the 1890s. A whole series of en echelon faults extends from the Mansfield area through and parallel to the main Bryant channel culminating in the fault that cuts through the junction of Bryant and North Fork just north of Tecumseh. A fault known as the Alice Mine Fault is present in an old zinc mine just south of Caulfield on the Ozark/Howell County line. About twenty faults with this northwest trend have been mapped within or adjacent to the Bryant Watershed. Due to the forest and soil cover in the area that makes mapping difficult there are no doubt many others yet undetected (see the discussion of meanders). 

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