Grain‐fabric alignments in a mixed carbonate–siliciclastic mass‐transport complex: The Cutoff Formation, Permian Basin, Texas

ABSTRACT Mass‐transport complexes undergo fabric modification during emplacement that results in the alignment of elongated grains. Grain‐fabric alignments promote mechanical and petrophysical anisotropy, which has broad implications for hydrocarbon extraction, carbon sequestration and geohazard assessment. These implications are likely more pronounced in mass‐transport complexes with carbonate components, where the proportion of elongated grains can be much higher as compared to siliciclastic mass‐transport complexes. However, the characteristics of grain‐fabric alignments in mass‐transport complexes are poorly understood. This study inspects the predictive properties of grain‐fabric alignments that include three‐dimensional orientation, fine‐scale heterogeneity, as well as the control of grain lengths and clay‐mineral abundance. The mixed carbonate–siliciclastic mass‐transport complex of the Cutoff Formation contains prolific spicule bioclasts (prolate‐shaped), which enabled qualitative and quantitative grain‐fabric analysis. Investigation of over 29 000 spicule bioclasts from oriented thin sections indicates that: (i) the long‐axes of elongated grains rest on the plane of lamination, which can be parallel or tilted/folded with respect to bedding; (ii) the long‐axes are transverse to palaeotransport direction; (iii) the homogeneity of alignment (termed here ‘H‐index’) of elongated grains are variable across samples, as well as locally within the same sample; (iv) H‐index increases with greater grain lengths (approximately constant aspect ratio) and with the presence of clay minerals; and (v) H‐index is likely controlled by differential stress and/or variation in microstructure types. These results improve current understanding and predictability of grain‐fabric alignments and deformation in mass‐transport complexes, as well as the associated mechanical and permeability anisotropy.