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doi:10.2204/iodp.proc.330.106.2012

Structural geology

Structural features in Hole U1375A on Achernar Guyot are veins and vein networks in sedimentary clasts and geopetals in the surrounding sediment. Seven geopetal structures were observed: two are horizontal (Fig. F10A, F10B) and the other five have angles from 4° to 26° from horizontal (e.g., Fig. F10C). Importantly, these dipping geopetals show evidence for water flow during or after geopetal sediment deposition. For example, the level of sediment on either side of a small clast is not equal (blue arrow in Fig. F10C). This suggests significant water flow from right to left, which would have locally disturbed the deposition of these sediments or caused some erosion of the fine-grained sediments. In the same example, the left sides of the larger clasts also lack fine-grained yellowish sediment, suggesting scouring by water flowing from right to left. It can be concluded that these five anomalous geopetals were not originally deposited horizontally and therefore do not yield accurate information about the paleohorizontal level of deposition. In contrast, the sediments around the two horizontal geopetals in the same core show no evidence for being disturbed by water flow (Fig. F10A, F10B). From the presently horizontal nature of these two “undisturbed” geopetals, it can be concluded that this part of Achernar Guyot has not been tilted since deposition of the geopetal material.

Several of the larger clasts in Hole U1375A contain common veins and vein networks as wide as 8 mm, although typically much thinner. Because these veins were formed before erosion and redeposition of the clasts, the present orientation of the veins has no geologic significance. No orientation measurements were taken on these features. Vein widths and infilling materials are discussed in “Alteration petrology.”

In Hole U1375B, 12 veins were recorded in the 57 cm of microgabbro (dolerite) recovered. These veins are dominantly steeply dipping, with a pronounced maximum of 70°–75° (Fig. F11A, F11B). There is also a conjugate set of three veins that have a much shallower orientation of ~20° (Fig. F11A, F11B). These primary and conjugate veins may represent infilled fractures that formed by decompression during erosion of this intrusive rock. If this is indeed the case, the orientation of primary and conjugate veins is indirect evidence that this microgabbro may be in situ, rather than a large boulder in the top sediment cover of Achernar Guyot.

Summary

Only two reliable geopetals were observed in Hole U1375A. These geopetals have horizontal orientations, indicating that Achernar Guyot has not been tilted since deposition of the geopetal sediments. Several veins and vein networks as wide as 8 mm are also present in the sedimentary clasts of Hole U1375A. Within the microgabbro of Hole U1375B, 12 veins were observed. The predominant orientation of these veins is 70°–75°, with thinner conjugate veins at 20°.

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