Abstract

A small part (300 km2) of the 2-km-deep summit of Horizon Guyot, a flat-topped submarine ridge at the eastern end of the Mid-Pacific Mountains, was surveyed in detail with a deeply towed instrument package. This followed up the work of several earlier Scripps Institution of Oceanography expeditions to the same feature. Three hundred and fifty stereo pairs of bottom photographs were taken, intensive coring and rock dredging were carried out, and near-bottom current meters were deployed.

The summit cap of Tertiary nannoplankton ooze has truncated horizons, including Eocene chert layers, which crop out at its margins. Continuing erosion has winnowed away the fine components of the sediment, while bed-load transport of the remaining foraminiferal sand creates regular trains of ripples and dunes. All three current meters recorded currents faster than 15 cm/sec within 12 m of the sea floor. Analysis of their records indicates that scouring and sediment redistribution is performed by locally accelerated tidal currents. Net movement of sand is upslope.

Hard-rock terraces underlying the pelagic sediment are exposed at the margins of the summit platform. They are probably deposits of highly mobile hyaloclastite flows, rather than relict erosional forms. Some accessory basalt clasts within the pyroclastic rocks have cracks containing an Albian (Lower Cretaceous) fauna of planktonic Foraminifera.

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