Indicators of recognizing bed-parallel slip-surfaces at outcrops

ABSTRACT Faults and shear zones parallel to bedding are fundamental in sedimentary successions but are generally more difficult to recognize in the field than faults cutting across strata. Micro-scale indicators such as striations or fibrous veins may be absent or weathered, requiring larger-scale markers for reliable identification. We present a new field-based classification of kinematic markers for recognizing bedding-parallel fault at outcrops. Six structural scenarios are distinguished and grouped into two categories: (1) offset primary and preexisting structures, including disrupted primary structures, disrupted dilational fractures, and deformed cover sediments at the base of beds; and (2) characteristic fault patterns associated with bedding-parallel slip, including duplex structures, offset meso-scale faults, and synthetic shear fractures. This framework consolidates previously scattered criteria and provides a systematic approach for identifying bedding-parallel fault and slip sense. The classification has broad implications. For field geologists, it reduces errors in stratigraphic correlation and structural mapping. For exploration geoscientists, it highlights potential fluid migration pathways influencing mineralization and hydrocarbon accumulation. For engineering geologists, it distinguishes tectonic bedding-parallel fault from landslide failure planes, improving hazard assessment. By providing a practical and versatile tool for detecting bedding-parallel fault, this study advances both academic and applied structural geology in developing field-based methods for understanding folds and faults.