Abyssal hills in the central equatorial Pacific: Detailed structure of the sea floor and subbottom reflectors

Abstract

Abyssal hills in a 230 km 2 area of the central equatorial Pacific (8°20′N 153°W) have relief ranging from 50 to 270 m. Most hills and valleys are elongate and trend north-south. The three highest hills lie on a northwest trending ridge parallel to the crest of the former Darwin Rise. Three subbottom reflectors mapped with a reflection profiler show the same major structural features as the sea floor. The deepest of these (base of the first layer) is generally conformable with the sea floor but has about twice as much relief (540 m compared with 280 m). The first layer ranges in thickness from 100 to 440 m and is thickest in depressions and thinnest on high abyssal hills. If the base of the first layer is isochronous the rate of sediment accumulation has varied by a factor of four over a distance of less than 10 km. These variations in thickness lead to a smoothing of the surface topography. Small-scale features of the intermediate reflector are not concordant with those of the base of the first layer and the sea floor. Therefore, it seems that this reflector does not mark a time-stratigraphic horizon, but rather a change in the degree of lithification of the sediment not directly related to time surfaces. The shallowest reflector is restricted to a topographic depression covering about a quarter of the surveyed area. Its complex nature and limited areal extent point to marked (perhaps periodic) variations in the local pattern of sedimentation. The thickness of fossiliferous Quaternary sediment in 38 cores is positively correlated with the steepness of slopes, but not with the thickness of the first layer. This discrepancy can only be resolved with more extensive surveys and samples from deep drill holes.