Day 4, Locality 7

7) Lageia, 2 km west of village; roadcut.

This video shows the exposure, panning from west to east:   MPEG-1,   WMV-1.

At the westerly end of the roadcut a few broken pillows may be seen low in the section. Much of the exposure however is occupied by rusty brown-red stratiform deposits, with varying angles of dip. Close inspection of these deposits show them to be sedimentary, with units of varying grain size. In many places coarser clastics are seen to disrupt lower, finer grained horizons.

Other parts show a jumble of stratified blocks and rafts, all at varying angles, enclosed by finer grained angular material. These are debris flows and slumps of sediment from the higher parts of the transform valley onto the floor of the transform. In some places the debrites are fine grained, and show fining upwards. These are interpreted as distal slumps and turbidites. Other areas show rafts of sediment at steep angle, completely detached and enclosed within finer, angular sediment. These are interpreted as more proximal slide-slump deposits. Much of the reddening of the sediment is due to hydrothermal precipitation of iron as part of the volcanic process. Patches of very fine grained iron-rich and ferromanganoan mudstone of hydrothermal origin may be found in places.

The easterly end of the roadcut shows small, intact pillows of boninite lava, surrounded by volcaniclastic infill. The pillows are seen to be black and glassy, and in places gradations into glassy hyaloclastites can be seen. Here, small blobs of glassy lava are surrounded by black glassy fragments. Fractured pillows show very thick chilled margins, and indicate greater viscosity of these silica-rich lavas. Again, these pillows have been erupted within the transform zone directly onto the ocean floor, with very little intermediate ponding of magmas. Shallow level hydrous melting has been aided by water ingress along fault planes within this transtensional environment. The source mantle has already been depleted, and thus a further magma extraction from an already refractory mantle under these conditions gives the highly depleted and magnesian boninite-type lava composition.