ADVANCES IN THE SEDIMENTOLOGY OF DESERT BASIN MUDFLATS

Lawrence Hardie did pioneer work on the zonation of minerals in saline basins, saline pan processes, and the depositional processes and environments of desert basins. Hardie envisioned saline mudflats forming as a lateral concentration gradient of groundwater due to evaporative pumping and surface sheetflooding as the dominant processes of sedimentation in both saline and dry mudflats. Although the evaporative pumping of groundwater is evidenced by upward-fining successions of mineral crystals and the production of efflorescent crusts, additional processes affect saline mudflat minerals. In mudflat areas adjacent to saline pans, the pan brines sink into the sediment and react with pre-existing minerals making larger crystals than the underlying minerals and a vertical zonation of mineral types. Sinking brine from a saline pan may also refract groundwater flow, producing artesian springs that are a mixture of evaporated groundwater and the brine that form localized evaporite deposits different than that predicted by evaporative zonation. The sinking of saline brines also may produce saline minerals well below the surface by reactions with older evaporite crystals, sediment, or mixing with groundwater even in sediment deposited in a non-saline environment. Wind may redistribute evaporite minerals to produce intrasediment crystals and efflorescent crusts to areas without any connection to the groundwater table. Sedimentation in desert mudflats covered with stony efflorescent crusts may be due to trapping of wind-blown dust and dissolution at the base of the crust. Powdery efflorescent crusts disrupt muddy sediment and makes them susceptible to deflation and redistribution as sand and silt aggregates. Sediment from surface flooding of dry mudflats may be mostly trapped in open desiccation cracks. Infrequent flooding and complete desiccation produces complex textures of superimposed cracks, trapped air vesicles, multiple sediment infilling, and soft-sediment flowage of open cracks. The expansion and contraction of standing water bodies on desert mudflats produce shoreline features including deltas and wave-formed deposits that are interbedded with non-standing water deposits and that have features reflecting sometimes rapid fluctuations of water depth and shoreline location.