Abstract

Microfaults are common deformational features in the Tesnus Formation and, to a lesser extent, other units in the Marathon Basin. Microfaults previously have been interpreted as the product of downslope extension of unconsolidated sediments but are here interpreted to be the product of tectonic deformation. Several lines of evidence demonstrate that the formation of these features is tectonically controlled: 1) microfaults are limited to those beds within or immediately adjacent to the décollement zones of major thrust faults, 2) clear evidence for grain cataclasis in microfault zones, coupled with slickensides on microfault slip surfaces, suggests that deformation occurred when the sediments were at least partially lithified, 3) poles to microfault planes define three major fault sets whose orientations and spatial distribution are inconsistent with formation by downslope extension, 4) coeval clastic injection features are absent in major thrust fault zones, indicating that the sediments were at least semi-lithified during thrusting, 5) sandstones and shales in thrust zones clearly exhibit deformational styles indicative of high ductility contrast. Observed geometries and spatial association of microfaults with major zones of thrusting in the study area and good agreement between principal stress orientations inferred from microfaults and features clearly of tectonic origin (particularly joint sets) suggest that microfaults in the Tesnus Formation serve as important kinematic indicators. Because of their small scale, microfaults should be identifiable in core and may aid in the determination of the position of intraformational thrusts and in tying well logs to seismic data.