Sediment Thickness— Kroenke 
337 
to be 90 m thick, with the top 440 m below 
sea level. No bottom samples were obtained to 
verify the profile results. However, Menard et 
al. (1962) have reported Miocene (?) fossils 
dredged southwest of Honolulu at a depth of 
500 to 520 m. If the seismically defined reefs 
are real and could all be shown to be of equiva- 
lent age, differential subsidence along the length 
of the Hawaiian Ridge, of at least 440 m north 
of Maui to 500 m south of Oahu, is indicated 
since the advent of reef formation. In their 
paper, Menard et al. suggest that dredge-hauls 
south of Oahu were from a Miocene reef in 
situ. If this is true, then approximately 0.5 km 
of subsidence has occurred since Miocene time, 
which leaves at least 1.5-2. 5 km of subsidence 
to be explained as having occurred before reef 
formation (to be in accord with the total sub- 
sidence of 2-3 km suggested by seismic-refrac- 
tion measurements). 
The fact that subsidence of more than 0.5 
km has taken place is also shown by the reflec- 
tion measurements in the flanking trench-areas 
to the north and south of the Hawaiian Ridge. 
In both areas a considerable thickness of sedi- 
ments is indicated. Two parallel profiles made 
due north of Oahu (Fig. 2) show depths to 
the lowest sub-bottom reflecting layer to be in 
excess of 600 m within the deeps, thinning to 
less than 100 m up the south flank of the Ha- 
waiian Arch. A similar situation is observed on 
two profiles northeast of Molokai (Fig. 3). 
Here the deepest sub-bottom reflecting layer 
appears to be in excess of 1 km, thinning to 
zero as the crest of the Arch is reached. 
These results are in agreement with the con- 
ditions postulated by Shor and Pollard (1964) 
from seismic refraction measurements. 
South of the Hawaiian Ridge the sediment 
thickness varies from 100-200 m in thickness. 
Probably this indicates a less active environ- 
ment of deposition, as the over-all picture is 
similar to that north of the Ridge. Both north 
and south of the Hawaiian Ridge the sediment 
thickness increases as the base of the Ridge is 
approached. Although it is more obvious in the 
northern profiles, the sub-bottom topography 
continues to dip toward the Ridge on both 
sides until the base of the slope is encountered. 
This is suggestive of faulting at the base of the 
June 1 1 HOURS 
June 1 1 HOURS 
June 9 HOURS June 10 
Fig. 4. Seismic reflection profiles south of Oahu 
(A, B) and Molokai (C). 
slope. An alternate explanation would be a 
simple migration of the axis of a depositional 
basin inward toward the Hawaiian Ridge as 
the Ridge sank. Although slumping or vol- 
canic flows appear to mask most of the base 
of the Ridge, the sub-bottom sedimentary struc- 
ture strongly suggests progressive downwarp- 
