A UNIFYING GENETIC APPROACH TO BOUNDING SURFACES IN ERG SYSTEMS

Field recognition, classification, and interpretation of bounding surfaces has been a long-standing challenge in erg systems. Here we present a hierarchical classification that examines surfaces from lamina scale up to regional unconformities using examples from the well-exposed Lower Jurassic Navajo Sandstone erg margin of southeast Utah. Surfaces are redefined in a sequence stratigraphic context in ascending scale as: laminae deviations, bedset deviations, simple surfaces, complex surfaces, and amalgamated surfaces. These surfaces represent contiguous landscape elements and bound genetically related sedimentary deposits produced within or between the surfaces. Correspondingly, the preserved sediment accumulations between surfaces are termed laminae, lamina sets, beds, bedsets, and simple, complex, and amalgamated successions.

This descriptive approach unifies the extensive literature on previous classifications (e.g., first-, second-, and third-order bounding surfaces; deflation surfaces, supersurfaces, Stokes surfaces, and others) while capturing important hydrologic processes, pedogenesis, and hydroclimatic conditions. Although volumetrically the dune accumulations represent a large part of the preserved stratigraphic record, there is rich information preserved at the individual boundaries representing short pauses to sustained periods of erosion or bypass, or reflecting environmental autogenic to allogenic changes in the erg system. In particular, pedogenic alteration forming simple, compound, composite, or cumulative paleosols is critical to the definition and recognition of the various nested ranks of bounding surfaces. This sequence stratigraphic classification applies to the multiple hierarchical scales of surfaces in any erg system. Importantly, unlike previous classifications, the hierarchy presented here incorporates extra-dune deposits (e.g., fluvial and lacustrine) that are intermingled with the dunes at erg margins.