Recognition of Evaporite-Carbonate Shoreline Sedimentation: ABSTRACT

Abstract

Evaporitic-carbonate shoreline sediments are deposited in an arid or semiarid climate by tidal currents which transport the sediment from the marine environment to the shore. The sediments accumulate primarily as tidal-flat deposits which prograde seaward producing a vertical sedimentary sequence from marine to supratidal. The supratidal sediments are the most easily recognized. They are characterized by irregular laminations, desiccation features, lithoclasts, and a general lack of fossil material. The intertidal sediments are more difficult to identify. They commonly are pelleted carbonate mud with burrows and a restricted fossil assemblage. Gastropods are dominant in many places. The vertical sedimentary sequence is the most useful tool to identify intertidal sediments The sediments just below the supratidal are generally intertidal sediments. The arid or semiarid climate allows seawater to evaporate and become saturated with respect to gypsum or anhydrite when the water circulation is sufficiently restricted from the open ocean. Primary bedded gypsum is precipitated in shallow lakes in the tidal-flat environment. In order for gypsum to be deposited in lagoons connected to the open sea by a channel, the ratio of the surface area of the lagoon to the cross sectional area of the channel must be about 106. Most gypsum or anhydrite associated with shoreline sediments is present as nodular, replacement, or porefilling crystals. This type of evaporite is secondary and crystallized from hypersaline interstitial water. The hypersaline interstitial water is the result of evaporation at the sediment-air interface. Bedded g psum or anhydrite, then, indicates the existence of a hypersaline lake or lagoon environment whereas nodular, replacement, or pore-filling gypsum or anhydrite is secondary and is found in marine, intertidal, and supratidal sediments. The precipitation of gypsum or anhydrite from seawater produces a dolomitizing fluid according to the reflux dolomitization theory. This type of dolomitization starts in the supratidal sediments and spreads into the underlying sediments. Therefore, extensive dolomitization associated with tidal-flat sedimentation indicates evaporitic shoreline sedimentation. Evaporites are removed easily by shallow groundwater. They are not well preserved in outcrops, and most commonly they are absent. Comparisons of subsurface anhydrite and outcropping shoreline sediments show that, in many places, the anhydrite or gypsum has been leached from the outcrop samples leaving molds and in places producing solution collapse breccias. Calcification of anhydrite or gypsum and dolomite is common. The recognition of evaporitic shoreline sedimentation from outcrop samples commonly makes it necessary to establish whether dissolution of evaporites End_Page 729------------------------------ and calcification of dolomite and anhydrite or gypsum have occurred. End_of_Article - Last_Page 730------------