The Tesnus Formation in the Marathon region, Texas, consists of interbedded sandstone turbidite and shale of a late geosynclinal filling phase. Deposition was on an unstable slope as indicated by the occurrence of deformational features of preconsolidation origin including rolled sandstone bodies (tens of feet long), sandstone dikes (up to 1 in. wide and 5 ft long), diapiric structures, and numerous normal faults of small displacement.

Throw on these microfaults is less than 1 in. and averages 0.25 in. The displacement is seen only on the undersides of sandstone beds and cannot be traced through the beds in which they occur. The faults intersect the base of beds at an average angle of 55° from the base, whereas most fractures of postconsolidation origin are oriented normal to the base. The measured intersections with the base of beds of 100 faults show that their mean orientation is 97° to the direction of paleoslope as indicated by flute casts. The standard deviation is 15°. Hence, the faults generally parallel paleoslope contours. Of hundreds of faults examined, more than 95% were downthrown down the paleoslope.

Each individual fault has a fault zone less than 1 mm wide. Petrographic evidence for a soft-sediment origin includes (1) boundaries between fault zones and country rock are not sharp, (2) fractured grains are not present in the fault zones, and (3) all healed fractures of postconsolidation origin cut the fault zones.

Soft-sediment faults have been found useful for providing current directions for sole markings, such as groove casts, which normally give only orientation. If used carefully, the faults also can provide paleoslope information where sole markings are absent.

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