Anatomy and Evolution of the Inner Perimeter Ridge (Sites 874 and 877) of a Campanian–Maastrichtian Atoll–Like Structure (Wodejebato Guyot, Marshall Islands)

Abstract

The inner perimeter ridge of Wodejebato Guyot (Marshall Islands) is the subject of a detailed sedimentologic, seismic, and geochemical study. The deposition of the shallow-water carbonate sequence, 177.7-190.2 m thick, is related to a two-stage transgression leading to the development of fringing sand shoal facies overlain by early cemented rudist-coralgal "reef" units that constituted barrier-like features. Wodejebato Guyot exhibited, at least temporarily, an atoll-like morphology. The irregularities of the upper surface of platform carbonates, the occurrence of a trough separating the inner perimeter ridge from the outer one, and the formation of deep solution cavities in platform carbonates suggest karstification processes before the final drowning of the carbonate platform during Maastrichtian time. Drowning events were probably related to the combination of a rapid sea-level rise and climatic changes that characterize Maastrichtian time. Following the demise of the carbonate platform, subsidence possibly combined with eustatic sea-level changes carried rapidly the platform carbonates in a basinal setting below the aragonite saturation depth, resulting in dissolution of aragonite and subsequent in place precipitation of calcite cements possibly throughout the Cenozoic. Post-drowning sediments correspond to a phosphate-manganese crust that formed from the late Paleocene to the middle Eocene.

Drowning of algal mounds: records from the Upper Carboniferous Lower Pseudoschwagerina Limestone, Carnic Alps, Austria

“…Cretaceous platforms have received particular scrutiny (Grötsch and Flügel, 1992; cf. papers in Simo et al, 1993; ODP Sites 875 and 876, papers by Camoin et al, 1995 andEnos et al, 1995;Wilson et al, 1998). Further examples of drowning are documented from the Quaternary, unequivocally related to high-frequency and high-magnitude sealevel changes typical of this glacial interval (e.g.…”

“…Particularly, the gradual transition from shallow-to deeper-water facies, and subaerial exposure of some platforms prior to their final drowning (Schlager, 1989;Erlich et al, 1990;Camoin et al, 1995) often obscures or masks drowning episodes. These effects seem pronounced in greenhouse periods.…”