Quaternary limestones of San Salvador Island, Bahamas, and Jurassic sandstones and Holocene sand, United States – a first approach in comparative eolian architecture
Mario V. Caputo
Pacific Section SEPM (Society for Sedimentary Geology)
Quaternary calcarenites, forming cliffs along the northeastern and southern margins of San Salvador Island, Bahamas, display a host of eolian features that have counterparts in Jurassic quartzarenites of the southern Colorado Plateau Province USA and in modern sand at localities in California and Michigan. Such sedimentary features suggest wind at work in a nonmarine coastal setting for the San Salvador calcarenites, despite the marine nearshore origin of constituent ooid and skeletal framework grains. General clast size in most eolian deposits ranges from fine to medium sand. However in eolian calcarenites, the coarse grain-size (of some clasts) and the higher specific gravity of CaCO3 relative to that of quartz are balanced by intraparticle pores, especially in algal fragments, so that mechanical equivalence is attained. For both rock suites, eolian ripple, grainflow, and grainfall depositional mechanisms controlled geometry and distribution of corresponding wind-ripple, sandflow, and grainfall strata, in the overall architecture of an eolian dune. Size, packing, sorting, and grading of grains, and resulting porosity and permeability associated with specific eolian strata influenced:
1) retaining of pore-liquids, 2) degree of cementing, and 3) weathering of eolian strata, particularly in San Salvador limestones. Component eolian strata in exposed, rain-moistened, terrigenous sand were further distinguished by their rate of drying related to such textural traits.
Although thickness of cross-bed sets is not a definitive indicator of relative size of eolian bedforms, dune heights much greater than 3 m (10 ft) are interpreted from Jurassic eolian quartzarenites of the southern Colorado Plateau region. Depending on conditions of weather, sediment supply, and dune stabilization, the coastal dunes of San Salvador Island varied in size during Quaternary time. Thin sandflow lenses enclosed in grainfall laminae combined with thin cross-beds and cross-bed sets suggest dune heights of ≤ 3 m (10 ft). Abundant angle-of-repose sandflows in foresets that extend to lower bounding surfaces, sandflows without interbedded grainfall strata, and near absence of grainfall strata in bottomsets reflect heights of local dunes exceeding 3 m (10 ft). Thin, narrow sandflow lenses suggest low sand-volume where dune-slope avalanches were impeded by slight grain cohesion imparted by moist coastal air.
Coastal carbonate dunes around the small island of San Salvador may have developed according to the following scenario. Plant seeds that were washed in among swash debris sprouted at the shoreline and initiated the growth of coppice or “veggie” dunes. These evolved into a juvenile dune ridge and elevated wind-terrace, and later into a mature dune ridge. Wave spray and rainfall fostered dune stabilization by plants and early subaerial cementation. By comparison with dune complexes that built eolian seas in the western interior USA during Jurassic time, Quaternary eolian dunes of San Salvador Island migrated only short distances before they were stabilized and lithified. Consequently, entire dune-forms were preserved. In contrast, the build-up and long-distance migration of large, complex dunes in extensive eolian sand seas during Jurassic time in the Colorado Plateau region were favored by high sediment volume, arid weather, and sparse vegetation. The dunes were subject to erosion inherent to migrating bedforms composed of unconsolidated sand. Consequently, only lower parts of ancient siliciclastic eolian dunes were preserved by later burial lithification.