SEDIMENTARY ENVIRONMENTS/Carbonate Shorelines and Shelves 511 absence of calcified green algae (such as those present in South Florida and the Bahamas) and geochemical evidence from the lagoon waters and the deposited muds indicate that these muds were precipitated chemically within the lagoon rather than having an organic origin, as is the case in Florida Bay The sabkhas (Arabic for area of low-lying salty ground) are broad intertidal areas and coastal plains (Figures and 10) that may be flooded by lagoon waters when storm winds blow landwards Intertidal areas have extensive microbial populations on the tidal flats growing on and within sediment deposited on the sabkhas when they are flooded by storms These processes give rise to sediments with alternating microbial-rich and sediment-rich layers, known as stromatolites (Figure 10) High aridity in the area leads to a net evaporation of floodwaters and the drawing up of saline groundwaters into the sabkha This leads to the precipitation of evaporite minerals, such as dolomite, gypsum, and anhydrite, within the microbial mats, including the stromatolites With time all these depth-related sedimentary zones will build out or prograde into the Gulf, forming a sedimentary succession that has been recognized in many ancient limestones throughout the world Ancient carbonate ramp successions are known from the Carboniferous of north-western Europe and the USA, where they build out carbonate shelves with facies similar to those described from the present-day Arabian Gulf However, in addition, the deep-water areas are characterized by carbonate mud mounds that not have an exact modern-day analogue Very extensive Jurassic and Cretaceous carbonate ramps are known from the subsurface of the eastern part of the Arabian Plate Here they host the rich oil and gas reserves of the Middle East in prograding units of inner ramp carbonate sand shoals and patch reefs The oil and gas is preserved in these porous carbonate rocks and sealed by overlying evaporites, which formed in sabkha environments similar to those that occur today in the southern Arabian Gulf grains (ooids etc.) in these cooler waters In the north Atlantic temperatures vary from an average of 7 C in winter to 15 C in the summer, and salinity levels are relatively constant at around 35% Within these waters skeletal sands and gravels are formed from a variety of different seafloor communities that contrast with those of the tropics The shelves of north-western Europe form what are termed distally steepened ramps in that they have very gentle gradients from the shoreline to a shelf margin at around 200 m water depth (Figure 11) Here, the seafloor drops away more steeply on what is known as the continental slope The shelf environment is subject to a number of different currents that have an important effect on sediment types and their distribution The North Atlantic Drift is an oceanic current that flows north-eastwards along the continental slope and is strong enough to transport sand along the upper slope in large seafloor sand dunes The shelf is also affected by westerly winds that have blown across the Atlantic Ocean and generate significant storm waves (i.e a maximum of 30 m during a 50 year period) that can rework the seafloor sediments Attached Carbonate Ramp in a Cold Temperate Environment The shelf regions of the north-eastern Atlantic form a large area of cold-water shelf carbonate sediments (Figures and 11) These extend from the narrow Spanish shelf to broad shelf regions within the Arctic Circle near Spitzbergen Similar extensive areas of cold-water carbonates are found in the southern hemisphere, particularly off the south coast of Australia (Figure 2) Cold-water carbonates differ from their warm-water counterparts in that they are formed only of skeletal material: there are no precipitated Figure 11 The north east Atlantic Ocean, showing the distribu tions of the main types of shelf and slope carbonate sediments Note that carbonate sediments are abundant on the attached shelves of north western Europe and on the isolated platforms of the Faeroes, Rockall Bank, and Porcupine Bank White areas of shelf region are those that have not been studied in detail