Highstand Shedding of Carbonate Platforms

Abstract

ABSTRACT The term 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 rimmed carbonate platforms studied to date. Highstand shedding in the distant past is more difficult to prove for want of well-documented sea-level curves and limited stratigraphic resolution. Nonetheless, several detailed studies strongly suggest highstand shedding of ancient platforms, 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 rapidly lithify during sea-level falls when the sea floors are winnowed due 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-latitude, rimmed platforms. It is reduced on ramps and in cool-water carbonate 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 during a sea-level rise. All in all, the principle of highstand shedding of carbonate platforms is as robust as the principle of lowstand shedding of siliciclastic systems. Both are caused by fundamental attributes of the respective depositional system and both have their limitations and exceptions.