UPWARD TERMINATION OF COMPLEX LARGE-SCALE INTERVALS OF SOFT-SEDIMENT DEFORMATION (SSD) IN THE NAVAJO SANDSTONE: DOES EROSIONAL TRUNCATION OCCUR BEFORE OR AFTER DEFORMATION?

Cross-bedded eolian sandstones in the Navajo Sandstone contain intervals of geometrically complex, large-scale soft-sediment deformation (SSD). Some aspects of its formation and significance are poorly understood, notably the paleodepth of deformation, timing relative to sedimentation, relationship to surface paleomorphology, forces driving deformation and the initiating trigger.

Most intervals terminate upwards at an erosional discontinuity. Recent studies have shown these to be breached in places. Deformation extends into overlying sets, raising uncertainty about the origin and timing of the erosional truncations. We present several models for their development.

(A) Deformation is truncated upwards; no breaches are evident in the truncating surface. Truncation represents post-deformation erosion, limiting the subsurface depth at which SSD took place.

(B) Deformation is truncated upwards by a surface that is breached when traced laterally. Truncation represents a pre-existing set boundary that impeded water escape, acting as a ‘cap’ to deformation. Build-up of pore-water pressure led to points of rupture and burst-through. The depth beneath the surface at which SSD occurred and its timing relative to sedimentation are unclear.

(C) Similar morphology to B. The interval of SSD represents multi-phase deformation. Deformation was erosionally truncated (model A), then occurred again in the same sediment, but extended higher in the succession, leading to breached truncation surfaces.

(D) Similar morphology to B. The erosional truncation represents the paleosurface when deformation took place. Breaches represent sites of sediment extrusion and the overlying sediment represents sand-volcano-type deposits.

More than one of these models may occur, so it is important to catalogue examples that can be attributed to each model to assess their relative abundance, develop criteria for differentiating between them, and assess the paleoenvironmental, paleotectonic and/or diagenetic significance of each type.

We present examples representing different models from the Navajo Sandstone in S Utah and N Arizona. We consider the defining characteristics of different types of truncating surfaces and assess the significance of each type for understanding the processes responsible for SSD.