Abstract

Although progradation of platform margins is a well documented process, its timing with respect to sea-level cycles is controversial. This outcrop study was designed to test the relation between prograding stratal patterns and sea-level cycles, using a sequence stratigraphic analysis and direct investigation of the sedimentological facies associations. The upper Campanian-Maastriehtian Orfento supersequence in the Maiella platform margin is an outstanding outcrop example of a large-scale prograding carbonate wedge, composed of smaller-scale, vertically and laterally stacked sequences. The evolution of the Orfento supersequence was characterized by an initial phase of aggradation, followed by progradation. At the scale of the supersequence, basinal aggradation preceded progradation of shallower-water facies, because excess relief had to be buried prior to progradation. Seven depositional facies were distinguished on the basis of depositional geometries, stratal patterns, and sedimentary features. These facies record different depositional environments on a low-angle ramp, ranging from deep-water pelagic wackestones to wave-dominated upper shoreface grainstones. Facies distributions record the overall transition from pelagic sediments, turbidites, and megabreccias into progressively shallower-water shoreface complexes. The prograding units consist of shingled offlapping sigmoidal complexes, composed of rudstones and grainstones, deposited in shallow, wave-dominated environments, and their deeper-water equivalents. Sigmoidal sequences are separated by erosive unconformities, which bring upper-shoreface facies into direct contact with underlying lower-shoreface or outer-shelf facies, indicating a sea-level fall (forced regression). Meteoric diagenesis associated with the unconformities also supports lowering of relative sea level. Depositional facies patterns within sequences record a relative rise and highstand of sea level. Repetitive downward shifts bring the shoreline successively farther basinward, indicating long-term lowering of sea level during the late stages of progradation, producing downstepping progradation. Our data suggest that changes in relative sea level, acting on a low-angle ramp, control the overall architecture of sequence distribution, whereas changes in sediment supply control changes in facies association within each sequence.

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