EOLIAN MATERIAL IN BROAD, SANDY EPHEMERAL STREAMS: DEPOSITION, IDENTIFICATION, AND POTENTIAL FOR PRESERVATION

Ephemeral streams undergo significant periods during which no fluvial deposition takes place. During these time periods, eolian processes can dominate transport and erosional/depositional processes. Eolian processes may be of particular importance on broad, sandy streams lacking high banks or canyon walls that shield the stream from wind. Although a considerable amount of effort has been exerted in identifying and characterizing fluvial depositional features in ephemeral streams, relatively little work has been done on classifying eolian features that may be present within such stream systems.

This study is focused on the Rio Salado near Socorro, New Mexico –– a broad, sandy ephemeral stream with an abundant supply of sediment both from upstream sources and along the banks of the stream from large adjacent sand dunes and weathering Neogene deposits. The stream is characterized by considerable seasonality, where for most of the winter, spring, and early summer months, dry, eolian processes dominate sedimentation, while in late summer and early autumn the stream becomes active in the form of monsoon-driven high-energy flash floods. Such floods erode the previously deposited material and produce fining-upwards sequences as stream activity decreases. This setting provides an ideal location to examine the interaction of fluvial and eolian processes within an ephemeral stream. Several trenches were dug in the bed of the Rio Salado, both before and after fluvial activity, in the active channels and within gravel bars surrounding the channels. Field observations focused on sediment textures, cross-stratification, the nature of contacts between layers, and vertical and horizontal sequences. Samples were taken from the features observed within the trenches and analyzed for grain size and sorting. These distributions were compared to surface samples for which depositional mechanisms could be easily identified, to determine if eolian depositional features could be distinguished from fluvial features. By sampling these distributions both before and after active flood events, the potential of eolian features to be preserved could be determined. Based on data collected to date, eolian depositional features are likely to be preserved and may be useful in identification of ephemeral stream deposits in the rock record.