SHEAR STRUCTURES WITHIN SOFT-SEDIMENT DEFORMATION FEATURES IN THE EARLY JURASSIC NAVAJO SANDSTONE

Soft-sediment deformation features in outcrops of the early Jurassic Navajo Sandstone commonly display shear structures, at a variety of scales. These typically appear as planar zones of laminated sand that truncate primary features and are accentuated by overturned foresets, rip-ups, and other displacements of pre-existing architectural elements. In some cases, it is apparent that shear stress was accommodated by more cryptic movement along foresets.

The origin of the shear forces producing these structures is sometimes the evident consequence of larger-scale dynamics within the deformation event, driven by gravity or hydraulic pressure, after seismic destabilization (liquefaction) has occurred. Such examples constitute an essential set of case studies, greatly supplementing models derived from experimentation and observation in modern environments. In many instances, however, it is difficult to postulate coherent mechanisms to explain the preserved architectural relationships within the context of current theory and established physical constraints. Identifying these interpretive challenges is a necessary first step in developing more broadly applicable models. To this end, photodocumentation of shear structure diversity is here provided, along with a corresponding array of interpreted dynamics and emerging enigmas.