For this work, two submarine hardground crusts from the Marion Plateau were investigated (Table T1): one from the modern seafloor covering the top of the SMP (Site 1196) at 300 m water depth and one from 117 m subsurface depth (Site 1194) marking the top of a lowstand platform positioned on the upper slope of the NMP (Isern, Anselmetti, Blum, et al., 2002).
During the site survey cruise, large pieces of the surface crusts of 1–3 cm thickness were recovered by dredging (Samples 1196-DR-02 and 1196-DR-04). The dredged crusts are reddish and are extensively colonized by serpulids, bryozoans, and solitary corals (Fig. F3). Leg 194 drilling on the SMP yielded another example of such a surface crust at Site 1196, which was recovered at the top of Core 194-1196B-1R.
Thin sections of the crusts show stromatholithic reddish brown laminae (growth banding?) encrusting a biomicritic carbonate containing abundant planktonic foraminifers, less abundant bryozoans, mollusk shells, and solitary corals. Most likely, biofilms started to grow on the rigid carbonate framework of the drowned platform, which precipitated iron and manganese from seawater. As can be seen in thin section, ferromanganese laminae grew on the exposed surfaces of seafloor rocks and in small cavities and cracks (Fig. F4). In some sections laminations show abrupt changes of color and composition along fine fractures while conserving the laminated texture.
Site 1194 was drilled through the lowstand NMP and recovered a hardground consisting of ferromanganese crust at 117.4 meters below seafloor coinciding with a seismic unconformity. This crust is also characterized by a very prominent natural gamma ray peak in the downhole logging data. Section 194-1194A-14X-1 contains this ~1-mm-thick hardground crust that infilled an almost vertical crevass (Fig. F5).