Carbonate Platforms, Sequence Stratigraphy, and Sea Level: ABSTRACT

Abstract

Besides tracing reflectors, the mapping of stratigraphic sequences marks a major advance in seismic interpretation. However, construction of sea level curves from sequence stratigraphy is complicated by other factors besides sea level influencing sequence geometry. One such factor is lithology. This point is examined by comparing siliciclastic systems and carbonate platforms. During the Pleistocene, the siliciclastic sediment supply to the deep sea was at its maximum during glacial lowstands of sea level. Pleistocene carbonate platforms were exactly in antiphase to this rhythm. They produced and exported most sediment during interglacial highstands when the platforms were flooded (highstand shedding). In the Bahamas and other platforms, accumulation of both bank-derived fines and sandy turbidites is higher by a factor of 2 to 9 during the interglacials, and turbidites tend to cluster in these highstand intervals. Highstand turbidites also differ in composition from their lowstand counterparts. Turbidite abundance and composition together provide a faithful record of the Pleistocene sea level cycles not easily erased by diagenesis. Geometrically, platform respond to sea level by forming highstand turbidite wedges and lowstand drapes of pelagic sediment--the opposite of siliciclastic systems. Drowning unconformities are another example for the significance of lithologic change in sequence stratigraphy. Flanks of carbonate platforms are generally steeper than siliciclastic slopes. When carbonate platforms are drowned and buried by siliciclastics, an unconformity ensues because the clastics are unable to assume the steep carbonate slope angle or because they are shed from other directions than the carbonates. Examples of drowning unconformities include the Lower Cretaceous platforms off West Africa and off eastern North America as well as the middle Cretaceous unconformity in the Gulf of Mexico. Highstand shedding and drowning unconformities of platforms illustrate that not all depositional systems respond alike to changes in sea level and that sequence boundaries may be caused by lithologic change. These lithologic turning points need not be related to sea level. In a very general way, sequence boundaries can be viewed as changes in the pattern of sediment input and dispersal in a basin. Sea level fluctuations are one way to induce such changes, but tectonic movements and environmental change represent important alternatives, demonstrated by the seismic stratigraphy of the deep Gulf of Mexico. End_of_Article - Last_Page 1522------------