ABSTRACT

In contrast to terrigenous margins, the stratigraphic response of carbonate platforms to relative sea level changes is influenced by carbonate productivity, dissolution, and diagenesis. Using seismic reflection and Ocean Drilling Program (ODP) drilling data from Broken Ridge, we assess the importance of these processes in controlling the stratigraphic response of a carbonate platform to relative sea level changes. Prior to middle Eocene rifting, Broken Ridge and Kerguelen-Heard Plateau constituted a large carbonate platform. ODP drilling data from the Kerguelen-Heard Plateau (Leg 120) and Broken Ridge (Leg 121) reveal that the carbonate platform had two major episodes of tectonic uplift. The early Maastrichtian uplift was confined to the southern section of the Kerguelen-Heard Plateau (Sites 747, 748) and gently tilted the platform toward the north. The middle Eocene rift-induced uplift affected both Broken Ridge and the northern section of the Kerguelen-Heard Plateau (Sites 752–755).

Watergun seismic reflection and drilling data indicate the following stratigraphic response of the platform to relative sea level changes. (1) Prograding clinoforms consisting of carbonate sediments were deposited during a long-term relative sea level rise. Submergence of the uplifted parts of the platform (southern Kerguelen-Heard Plateau) increased the area available for productivity allowing the carbonate sediment to prograde northward into the basin during the rise. (2) Onlapping sequences consisting of carbonate detritus were deposited in the basin during the middle Eocene relative sea level fall. The carbonate strata on Broken Ridge were uplifted to at least wavebase and eroded. The detritus was transported northward toward the basin and onlaps the preexisting strata. The approximate volume balance between the sediment eroded from the crest of Broken Ridge and the onlapping sediments deposited along the northern slope suggests that mechanical erosion and transportation of carbonate sediment into the deep sea were important processes controlling the stratigraphy at Broken Ridge during the relative sea level fall.

We propose that the stratagraphic response of Broken Ridge to the middle Eocene relative sea level fall is consistent with stratigraphic predictions based on terrigenous margins. However, during a relative sea level rise, biogenic carbonate production may allow carbonate sediment to prograde basinward despite the fact that water depths are increasing. Stratigraphic models based on terrigenous margins do not account for the increase in sediment supply along carbonate margins during a relative sea level rise, and thus require minor modification in order to predict the development of the prograding clinoforms.

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