TECTONIC FRACTURES CONTROLLED BY PALEOSOL FABRICS: AN EXAMPLE FROM THE KOOTENAI FORMATION, LOWER CRETACEOUS, SOUTHWESTERN MONTANA, USA

Origin and development of tectonic fractures during diagenesis of sedimentary rocks depends significantly on anisotropy of sedimentary layers at multiple scales. In this regard the influence of intra-bed scale sedimentary fabrics on deformation has been comparatively neglected. This study focuses on the depositional setting of freshwater limestone units to propose that a set of pervasive, anastomosing tectonic fractures was preceded and induced by a strong subvertical pedogenic fabric that resulted from syn- /post-depositional processes of paleosol formation.

The key outcrop is that of a 2.8m thick palustrine, charophyte- and rhizolith-bearing biomicritic limestone of the Lower Calcareous member of the Aptian-age Kootenai Formation in southwestern Montana, USA, an area of large-scale upright folds and imbricate thrusts. Blocky peds at the top change to prismatic /columnar peds of downward increasing size below, controlling the morphology and downward-increasing spacing of curving, intensely anastomosing, bedding-orthogonal fractures. Fractures terminate abruptly above the base of the unit, bound by a sharp, irregular pedogenic front that cuts across bedding. Beds in the basal 10-50 cm of the unit lack bedding-orthogonal fractures, and are instead strongly buckled and /or sheared by calcite-filled fractures oblique to bedding.

Deformation style of the palustrine unit can be contrasted with that of discrete limestone bedsets above and below that do not display pedogenic features. Although similar in layer thickness and lithology, tectonic strain within these units is represented by small-scale buckle folds, minor thrusts faults oblique-to and nearly parallel to bedding, and a planar fracture cleavage of relatively homogeneous, 3-5 cm spacing. The differences demonstrate the controlling influence of a dominantly subvertical pedogenic fabric on distribution and spacing of intra-layer tectonic fractures. Such fabrics may be preserved or remain open during burial compaction of commonly occurring paleo-weathered continental deposits. Hence, recognition of paleo-pedogenic fabrics may refine interpretation of tectonic fractures (timing of origin, fill, and reactivation), which in turn is relevant to the diagenetic history of subsurface fluid flow along and across fractures.