Abstract

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

Throw on these micro-faults is less than one inch and averages inch. The displacement is seen only on the undersides of sandstones and cannot be traced through the beds in which they occur. The faults intersect the base of beds at an average angle of 55 degrees from the base, whereas fractures of post-consolidation origin are nearly all oriented normal to the base. The measured intersections of 100 faults with the base of beds shows that their mean orientation is 97 degrees to the direction of paleoslope as indicated by flute casts. The standard deviation is 15 degrees. Hence, the faults tend to parallel paleoslope contours. Of hundreds of faults examined, more than 95 percent were downthrown down the paleoslope.

Each fault has a fault zone less than one millimeter wide. Petrographic evidence for a soft-sediment origin includes the following: (1) boundaries between fault zones and country rock are not sharp; (2) fractured grains are absent in the fault zones; and (3) healed fractures of post-consolidation origin always 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, soft-sediment faults can also provide paleoslope information where sole markings are absent.