718 ARRHENIUS [CHAP. 25 



equator go back to only the Middle Pleistocene, about half a million years. 

 During this time the equator has remained constant within the resolution of the 

 present sampling grid, which is about four degrees of latitude. If 500-m cores 

 could be drilled in the equatorial sediment bulge. Cretaceous or older sediments 

 might be reached, corresponding to postulated positions of the North Pole in 

 the North Pacific (Runcorn, 1956, 1959). Determination of polar wandering by 

 this method would be free from the uncertainty imposed by possible continental 

 drift. 



Biostratigraphic means available for subdividing into stages and correlating 

 Upper Cenozoic formation units in their calcareous-siliceous facies are lacking 

 in the adjacent clay and zeolitic facies. The stratigraphic units so far resolved 

 in the North Equatorial Pacific clay facies consist of a Pleistocene-Recent bed 

 of buff clay, comparatively low in halmeic minerals, conformably overlying a 

 series of dark brown clay members (Pliocene), high in manganese-iron oxide 

 minerals and other halmeic phases, and in fish bone apatite (Fig. 40). Older clay 

 sediments (Middle Tertiary) occasionally sampled in outcrops on topographic 

 highs (cf. Section 5) show a petrological composition similar to the Pliocene ones. 

 Where the buff top formation is found without interruption of the sequence by 

 unconformities, the thickness amounts to a few meters, but its lower boundary 

 cannot be defined precisely. Accepting for the north equatorial Pleistocene- 

 Recent clay facies a rate of inorganic accumulation around 2 mm/ 1000 years, 

 indicated by protactinium/ionium and radiocarbon dates (see above) and 

 ionium/thorium measurements (Goldberg and Koide, 1958), it appears that the 

 buff clay formation comprises the last one or two million years. The stratigraphy 

 suggests a relatively low rate of transportation of terrigenous matter into the 

 area during Tertiary times through Middle Pliocene, periodically varying transi- 

 tion conditions in the Upper Pliocene, and a general increase in terrigenous 

 contribution during Pleistocene and Recent. 



5. Topographic and Tectonic Control of Sedimentation 



Deformation of the ocean floor into hills, mounts, and fault scarps has 

 undoubtedly occurred many different times. Much of the present small-scale 

 topographic relief in the Pacific was probably created by extensive laccolithic 

 injection of basaltic lava, which updomed the overlying sediments (Arrhenius, 

 1952, 2.57.0, 3.4.1). This process is assumed to have created the low-relief, hilly 

 topography typical of vast areas in the Pacific Ocean basins (cf. Figs. 41 and 43). 

 Lava erupting at oceanic depths — unlike conditions on land or in shallow water — 

 does not encounter marked discontinuity in the confining pressure at the 

 sediment surface. Lateral flows are consequently not particularly probable at 

 this surface, but are likely to occur at petrological discontinuities at depth in the 

 sediment. Submarine lava flows at great water dejoth are expected to spread 

 extensively owing to low viscosity resulting from the flow of interstitial water 

 or sea-water into the lava at high water pressure. The discontinuity in sound 

 velocity a few hundred meters below the sediment over wide areas in the east 



