Interdune areas of the back-island dune field, North Padre Island, Texas

Abstract

The small, young (about 100 yrs) back-island dune field on north Padre Island, south Texas, consists of fairly persistant oblique dunes (up to 6 m high) with well-developed interdune areas that grade northwestward to small, ephemeral transverse and barchan dunes with interconnected “interdune” areas, thence sheet sand areas. The subhumid climate is marked by rain associated with frontal systems and tropical storms. Winds are seasonally bimodal—prevailing southeasterly are punctuated by northerly and northwesterly winds with the passage of frontal systems in winter. The entire dune field and individual oblique dunes show a net migration of about 15 m yr −1 to the northwest. The dunes however are on a seasonally reversing track, changing their slipface direction and migration direction with frontal systems. One year of monitoring shows sand transport in the dune-interdune system to be complex and cyclic. During the wind reversals of winter, dunes are very ineffecfive sand traps owing to loss of flow separation, and much sand is lost to the interdune areas. Interdune areas store sand during these wet winter months as a result of the wind reversals and higher moisture content. During the summer, the interdune areas deflate and the dunes build in size. The overall dune field deposit appears to consist of three laterally contiguous zones from southeast to northwest: (1) continuous, climbing oblique dune and interdune deposits; (2) discontinuous lenses of dune sand in overall “interdune layers”; and (3) a chaotic mixture of dune and horizontal deposits of the sheet sand areas. One year's mapping and trenching documents that interdune sedimentary structures are extremely variable laterally and vertically reflecting specific microenvironments within the interdune flat. Wet-surface features consist of current and wave ripples, channel fill, miniature deltas, wrinkle marks, mini-ripples, rills, algae and sand volcanoes. Abundant adhesion structures, rain-impacted ripples, brecciated surfaces and microtopography reflect damp-surface deposits. Dry-surface features are predominately wind ripples; others include small isolated barchan and shadow dunes, organic debris lag surfaces, deflation scours, beetle bioturbation, plant-root structures associated with shadow-dunes, and grainfall from the adjacent dunes. Interdune deposits account for about 40% of the total dune field deposits, which seems reasonable compared to some ancient examples. By virtue of occupying a relative “basin”, interdune deposits are selectively preserved compared to dune deposits. In general, interdune sedimentation is enhanced by non-eolian depositional mechanisms, a high water table, early evaporatic cements, and a variable wind regime. The actual thicknesses of individual dune and interdune deposits are less on Padre than ancient examples, reflecting the relative scale of the bedforms. In many respects, sequences of sedimentary structure in Padre Island interdune deposits are typical of ancient, coastal interdune strata, but some marked departures occur. Adhesion structures, relatively rare in some ancient examples but abundant within Padre interdune deposits, seem favored by the small size of dune and interdune area, the climate and a variable wind regime. Penecontemporaneous deformation, absent in Padre interdune deposits but pronounced in some ancient examples, probably reflects dune size and the nature of the deposits. Wavy laminae in ancient interdune deposits probably result from many causes, but seem best represented by modern examples of evaporitic algal/bacterial-formed structures.