BRITTLE AND DUCTILE DEFORMATION STRUCTURES ASSOCIATED WITH CLASTIC PIPES, JURASSIC CARMEL FORMATION, SOUTHERN UTAH

Prominent brittle and ductile deformation structures in sabkha to fluvial strata are commonly associated with clastic pipes in a 30-meter section of the Paria River Member of the Jurassic Carmel Formation. The clastic pipes are syndepositional cylindrical injectites of upwardly injected material, sourced by fluvial sands during strong ground motion. This study examines numerous (>2000) pipes and their relationships to complex, multi-scale faults, fractures, folds, and brecciation that occur in the host rock adjacent to and surrounding the pipes. These structures provide important clues for understanding the formation process and the compensation for volume changes during liquefaction.

Ductile behavior and brecciation typically occur at the bottom and top 10% of the pipe-bearing section with brittle faulting dominating the remaining 80% of the section. The majority of ductile behavior manifests as folds and irregularly deformed layers indicating plastic to fluid-like properties. The ductile deformation occurs primarily within the source bed as it buckles and deforms at the time of liquefaction. Normal, reverse, and antithetical faults, with offsets of millimeters to meters represent the host rock response to the depletion and injection of material throughout the local area. Brecciation demonstrates partial cohesiveness of the host rock. Clasts from 0.01 to 2 m commonly retain the original bedding and are incorporated into the pipes.

Some larger pipes show brittle behavior surrounding the pipe itself with a damage zone of brittle structures extending outwards one to two times the diameter of the pipe. The surrounding down-dropped blocks forming normal or antithetical faults are concentric around the pipe within cross-sectional view, and show ring fault geometries in plan view. The damage zones have nested scales of deformation with a prominent ring fault bounding the damage zone and successively lesser offsets in the interior. In one case the down-dropped blocks show rotation towards the pipe indicating sediment collapse as the pipes dewatered after emplacement. These observations of deformation structures within the host rock provide the key evidence for interpreting the pipes as liquefaction features formed through instantaneous, violent injection followed by dewatering.