HIGHSTAND SHEDDING OF CARBONATE PLATFORMS

The term ''highstand shedding'' means that a depositional system sheds most sediment into the adjacent basin during highstands of sea level. In the Quaternary, highstand shedding has been observed around all ri mmed carbonate platforms studied to date. Highstand shedding in the di stant past is more difficult to prove for want of well-documented sea- level curves and limited stratigraphic resolution. Nonetheless, severa l detailed studies strongly suggest highstand shedding of ancient plat forms, too. The reasons for carbonate highstand shedding are twofold: (1) the productive area increases manyfold when the flat platform top is flooded during highstands, and (2) highstand platform sediments rap idly lithify during sea-level falls when the sea floors are winnowed d ue to lowering of wave base or when sediments become exposed to fresh water. The causes of highstand shedding also determine the limitations of this principle. Highstand shedding is most pronounced on low-latit ude, rimmed platforms. It is reduced on ramps and in cool-water carbon ate systems; furthermore, the effect is damped when the lowstand wedge can expand into a full-size platform during long-lasting lowstands of sea level. Highstand shedding ceases when the platform is drowned dur ing a sea-level rise. All in all, the principle of highstand shedding of carbonate platforms is as robust as the principle of lowstand shedd ing of siliciclastic systems. Both are caused by fundamental attribute s of the respective depositional system and both have their limitation s and exceptions.