Origin, sedimentary features, and significance of low-angle eolian "sand sheet" deposits, Great Sand Dunes National Monument and vicinity, Colorado

Abstract

Low-angle eolian deposits of a sand sheet constitute an areally extensive (710 km 2 ), transitional facies between high-angle eolian dunes and non-eolian deposits near Great Sand Dunes National Monument, Colorado. The bulk of the low-angle eolian deposits of the sand sheet lie in a 15-km-wide belt between the main dune mass at Great Sand Dunes and the Rio Grande River 45 km to the southeast, or upwind. These low-angle eolian deposits have a number of distinctive sedimentary features which are useful in interpreting ancient rocks, including: (1) coarse-grained, high-index ripples (commonly horizontal); (2) gently dipping, curved or irregular surfaces of erosion (several meters in length) within the deposits; (3) abundant bioturbation traces, formed mainly by insects and plant roots; (4) patches or zones (extending up to 1 m below the ground surface) of bioturbated sand resulting from destruction of laminae by grass root growth; (5) convex upward laminations; (6) small scale cut-and-fill structures, perhaps due to scour around plant roots and subsequent infill; (7) gently dipping, poorly laminated layers resulting from adjacent grainfall deposition; (8) sets of normal- and inverse-graded laminae 1- to 4-mm-thick; (9) discontinuous thin 1- to 4-mm-thick layers of coarse sand intercalated with fine sand; (10) intercalation of non-eolian materials, including silty pond sediment and gravelly stream deposits; (11) occasional intercalation of high-angle eolian deposits. Low-angle eolian deposits of the sand sheet originate mainly by gentle deceleration of wind, in the lee of small topographic features, which produces a different style of deposition compared to the more extreme flow separation at the brink of a dune slipface. Low to moderate velocities, typical of those observed during this study, generally remove fine sand from exposed areas, and deposit the sand in nearby topographically lower or sheltered places, resulting in more texturally homogeneous sets of laminae (type a) overlying more texturally heterogeneous deposits (type b). Recognition of low-angle eolian deposits may assist in identification of margins of ancient dune fields. From an economic point of view such information may be of value in predicting either (a) stratigraphic continuity of high-angle eolian reservoir rock, or (b) proximity of high-angle eolian rock.