SEDIMENTOLOGY AND ENVIRONMENTAL INTERPRETATION OF THE EVAPORITE DEPOSITS IN THE QATTARA DEPRESSION, WESTERN DESERT, EGYPT MAHMOUD A. M. AREF and MOHAMED A. A. HAMDAN Geology Department. Faculty of Science. Cairo University  The sedimentary and early diagenetic features of the evaporite deposits of the floor of the Qattara Depression have been investigated by field and petrographic studies in order to develop a depositional model in the light of the present day climatic data and geographic setting. Most of the floor of the depression is occupied by recent salt flats (sabkhas) which are dominated by thick and thin halite and gypsum crusts. and displacive and inclusive gypsum and halite crystals. Quaternary evaporite sediments also form small-scattered hills at a higher altitude west of the depression. They consist of nodular gypsum/anhydrite of variable size, abundance and distribution within mudstone, siltstone and sandstone of the Moghra Formations. In the recent salt flats, thick (> 5cm) halite crusts consisting of cornet, chevron and cumulate crystals, dissolution surface and pipes, and cracked polygonal surfaces were deposited in a saline pan dominated by flooding, evaporative concentration and desiccation. Thin (< 1 cm) efflorescent halite and gypsum crusts, and chaotic halite and gypsum crystals within mudstone and siltstone represent deposition in a saline mudflat/sandflat environment. Mudstone, siltstone and sandstone with little or no displacive halite or gypsum, but with abundant cracks, phytogenic mounds, sand sheets or dunes suggest deposition in a dry mudflat or sandflat environment. The dominant nodular morphology of gypsum/anhydrite nodules in the Quaternary evaporite hills indicates deposition by displacive growth below the sediment surface in a saline mudflat/sandflat environment. The existence of halite and gypsum in the recent salt flats and gypsum/anhydrite in the Quaternary evaporite hills point to a long arid episode over the depression, with either differences in the composition of the parent brine or relative humidity and evaporation rate. The morphologic similarity of the nodular gypsum/anhydrite in the Quaternary evaporite hills and the recent salt flats with the displacive mode of growth, point to a dominant saline mudflat/sandflat setting with a groundwater derived brine.