ABSTRACT: SCAT Dipmeter Rules for Interpretation of Thrust Belt Kink-Fold and Detachment Structures

C. A. Bengtson, James D. Morse

Dipmeter interpretation in thrust belts can be difficult because thrust belt structure is often complicated. However, interpreters can construct reliable transverse sections by combining statistical curvature analysis techniques (SCAT) with the geometric techniques for constructing thrust belt sections. By itself, the simplistic tadpole plot furnished by logging companies is inadequate for structural interpretation even in simple settings. In contrast, the six dipmeter displays of SCAT establish the correct direction for transverse (T) sections and the correct method for projecting both the well bore and the location of stratigraphic boundaries, faults, and apparent dips onto such sections. Trend line shifts on plots of transverse dip component vs. measured depth (T-plots identify either kink planes or hanging wall or footwall cutoffs. Kink planes are entered on the T-section and extended upward to the surface, to a nonbedding parallel fault (where they bend), or to a singular point where multiple kink planes meet and terminate; and downward, to the causative thrust, or to a singular point. Charts relating axial angles of fault-propagation folds or fault-bend folds to ramp angles, together with the location of faults from well data, aid interpretation of duplex structures, antiformal stacks, back-limb thrusts, and imbricate fault-propagation folds. Curved trend lines on T-plots identify curved bedding related to listric thrusts and disharmonic or detachment folds. Transverse steps produce telltale patterns on longitudinal dip-component plots and dip vs. zimuth and polar tangent crossplots.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990