Morphologies of Gypsum on a Modern Sabkha: Clues to Depositional Conditions: ABSTRACT

Abstract

Gypsum is the dominant evaporite mineral across the Holocene marine sabkha of the northwest Gulf of California, Baja California, Mexico. This sabkha is a complex of mud flats and saline pans in which gypsum occurs as layers and isolated intrasediment crystals. Six distinct morphologies of gypsum were found: flat plates, acicular prisms, bladed prisms, blocky prisms, blocky hemipyramids, discoidal hemipyramids. Flat plates (< 0.5 mm wide) and acicular prisms (< 1 mm long), in the form of radiating clusters and interpenetrating twins, crystallize at the brine-air interface during the temporary saline lake stage of the pan. These tiny crystals settle out on the bottom of the pan as a cumulate layer. Bladed prisms (0.5-1.5 mm long) nucleate on this cumulate layer and gr w upward as vertically oriented prisms (commonly swallowtail twins) making a grass-like layer. Blocky prisms (up to 2 cm long), barrel-like in shape with swallowtail twin terminations, are found as pendant cements in shelter vugs formed by buckling of the gypsum layers of the pan. Blocky hemipyramids (up to 1 cm across) are found as a loose mush making a diapir-like mound beneath m-scale polygonal cracks in the layered gypsum of the pan. These clear, blocky hemipyramids grow diagenetically in the vadose zone during the dry stage of the pan when evaporative pumping draws subsurface brine up along the polygonal cracks. Discoidal hemipyramids (up to 2 cm long) occur as isolated crystals. clusters of crystals, and rosettes within the siliciclastic sediment of the mud flats surrounding the an. They are of vadose and phreatic origin. Gypsum morphologies clearly vary with depositional conditions across this sabkha and this finding should allow us to use morphological variation in gypsum as a sensitive interpretative tool for ancient evaporites and evaporitive sediments. End_of_Article - Last_Page 460------------