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Neogene Deepwater Carbonate Mud-Mounds and Their Paleozoic Counterparts – Comparisons of Geometry, Sedimentology, and Petrology

Jay M. Gregg1, Ian D. Somerville2, Anneleen Foubert3, Eibhlin Doyle4, and IODP Expedition 307 Shipboard Scientific Party
1 Oklahoma State University, Stillwater, OK
2 University College Dublin, Dublin, Ireland
3 Gent University, Gent, Belgium
4 Geological Survey of Ireland, Dublin, Ireland

Deep-water carbonate mud-mounds occur in every system of the Paleozoic and are particularly common in the Devonian and Carboniferous. This depositional setting is important because of its association with hydrocarbon and metallic resources. IODP Expedition 307 cored three sites in the Neogene Challenger Mound in the Porcupine Basin of the North Atlantic, 100 km off the west coast of Ireland. These cores and related research allow a direct comparison of Neogene carbonate mounds with their Paleozoic counterparts.

Paleozoic mounds range from <0.3 km2 up to >2 km2 and several hundred meters thick. They are believed to have formed in relatively deep water on carbonate ramps, downfaulted platform margins, and the platform edge during transgressions. Neogene deep-water carbonate mounds up to 2 km2 and 200 m thick formed in water depths of 200-1000 m along the eastern Atlantic Ocean continental slope from Morocco to Norway.

Paleozoic mounds are composed of micrite which apparently was stabilized by crinoids and bryozoans. A particular characteristic of Paleozoic mud-mounds is development of stromatactis cavities and strong evidence of early marine cementation. The composition of core samples taken from the Challenger Mound is almost entirely nannoplankton (coccoliths) and varying amounts of clay to silt sized siliciclastics stabilized by cold water corals. There is no evidence of cavity development analogous to stromatactis or marine cementation in the Challenger Mound.

Although Neogene carbonate mud-mounds occupy similar depositional settings and have similar geometries to their Paleozoic counterparts they differ substantially in their composition and early diagenetic features.