STRATIGRAPHIC EVOLUTION OF THE JURASSIC NAVAJO SANDSTONE ERG, SAN RAFAEL SWELL, CENTRAL UTAH

The Jurassic Navajo Sandstone of central Utah records an intermediate region of eolian deposition between an erg margin to the east and the erg center towards the southwest. Stratigraphic analyses of the total Navajo formational thickness reveal that there are several vertical trends in grain size, bed set thickness, soft sediment deformation intervals, and facies that vary spatially across the San Rafael Swell, indicating a dynamic history of evolving depositional conditions across the erg system.

Lithofacies are broadly categorized into three facies groups based on interpretations of the depositional environment. 1) Dune deposits are characterized by different styles and thicknesses of crossbed sets and geometries of set bounding surfaces. 2) Interdune deposits contain interbedded mudstones and sandstones, with common cm-scale crossbed sets, ripple laminations, vuggy and crinkle laminations, massive sandstones, and mudstone conglomerate lags with autochthonous rip-up clasts. 3) Synsedimentary, soft-sediment deformation structures include contorted and massive beds. Local, large-scale (3-10 m) contorted beds are commonly with associated breccias, and microfaults.

The eolian Navajo Sandstone represents the largest erg recorded in geologic history. Despite the record of a vast dry desert, interdune deposits and synsedimentary structures indicate periods of high-water table conditions present as far as 130 km from the erg margin. Across the San Rafael Swell, Navajo thickness averages about 130 m on the east, to roughly 140 m in the west. Three stratigraphic “zones” show vertical changes in Navajo deposition. 1) The lowest 10 m consist of cm- to dm-scale trough crossbedded sandstones. 2) The middle 70 m interval consists of planar crossbedded eolian sandstones and lenticular interdune beds that correlate to first order bounding surfaces. 3) The upper 60 m is dominated by large scale (3-20 m) trough-cross beds. Stratigraphic data indicates that early Navajo depositional history was typified by small dune forms interspersed with interdune oases and later abruptly transitioned to a drier environment with large draas that lacked interdune oases and were subject to either periodic draas slope failures or seismicity.