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PACIFIC SCIENCE, Vol. XXII, October 1968 
beds of red ash and cinder up to 10 feet thick 
are present in the Sili Member on southeastern 
Olosega. 
The A’ofa caldera has been partly filled with 
lava flows and pyroclastic material. At the base 
of the exposed section are four dense, nearly 
horizontal lava flows of olivine basalt, three of 
which are 20-25 feet thick due to ponding 
within the caldera. These are overlain by thinner 
(1-7 feet) interbedded aa and pahoehoe flows 
of basalt and olivine basalt. An ankaramite 
boulder on the beach southeast of Tafe Stream 
probably came from one of the thick horizontal 
flows exposed near the top of the 400-foot sea 
cliff, indicating that some of the later intra- 
caldera flows were picrite. Table 3 is a strati- 
graphic section of the A’ofa intra-caldera mem- 
ber at the mouth of Sinapoto Stream. 
Just west of the mouth of Sinapoto Stream 
the lowermost thick flows have ponded against 
a cinder cone, the highest point of which is now 
about 60 feet above sea level. More than half of 
the cinder cone has been eroded away by the 
sea, however, and its summit probably was origi- 
nally about 150 feet above the present sea level. 
This cone must have been the source of thin 
(4-8 feet) aa lavas of olivine basalt that flowed 
down its northwestern flank to form Lelua 
Point. 
Nu’u formation: Nu’utele and Nu’usilaelae 
islets, off the western coast of Ofu, are the ero- 
sional remnants of a tuff cone, which was origi- 
nally about 4,000 feet in diameter at present sea 
level and approximately 300 feet high, and was 
centered off the southwestern shore of Nu’utele. 
The eruption occurred near sea level after an 
extensive period of erosion during which a sea 
cliff was cut around Ofu and Olosega. The cone 
is composed entirely of reddish-yellow palagoni- 
tized lapilli tuff with accidental blocks and 
lapilli of basalt, plus a few magmatic basalt 
lapilli. Individual beds vary from less than 1 
inch to more than 5 feet in thickness. The slopes 
of the original cone were about 30°. No coral 
fragments nor any evidences of a submarine vent 
were found, but the eruption may have been 
submarine in part. 
Along the western coast of Ofu at least two 
flows of aphanitic basalt, in places over 35 feet 
thick, have flowed down the slopes of the A’ofa 
TABLE 3 
Stratigraphic Section of the A’ofa Intra- 
caldera Member at Sinapoto, Ofu 
TOP 
(approximately 220-foot elevation) 
THICKNESS 
(feet) 
Dense gray aa flow of hawaiite contain- 
ing abundant microlites of feldspar and 
scattered microlites of olivine probably 
flowed over a fault scarp (dip =: 25° 
NW) 
8 
Clinker 
2 
Dense medium-gray aa flow identical 
with that above, but dipping only 6°N 
8 
Clinker 
2 
No outcrops; thick soil and talus cover 
15 + 
Nonporphyritic pahoehoe flows, 1-7 feet 
thick 
30 
Olivine basalt with olivine phenocrysts 
2-3 mm in diameter occurring as ap- 
proximately horizontal vesicular pa- 
hoehoe flows 1-15 feet thick; a series 
of thin-bedded pahoehoe flows cut by 
a 15-foot-thick flow that apparently 
plunged down a small fault scarp 
which had truncated the thinner pa- 
hoehoe flows 
25 
Nonporphyritic aa flow 
4 
Clinker 
4 
Olivine basalt occurring as vesicular pa- 
hoehoe flows 
3 
Clinker 
4 
Olivine basalt occurring as a dense hori- 
zontal flow 
15 
Olivine basalt occurring as a dense hori- 
zontal flow 
25 
Clinker 
1 
Olivine basalt forming a dense horizontal 
flow, a small spring issues from its 
lower contact 
25 
Olivine basalt with abundant olivine 
phenocrysts forming a dense horizontal 
flow (exposed 0.3 mile east of section) 
20 
Talus 
30 
Total thickness of section 
221 + 
shield. Nu’upule Rock, just offshore from Ofu 
Village, is an erosional remnant of one of them. 
Just south of Tufu Stream, at sea level, a 
hawaiite flow at least 25 feet thick is overlain 
by a flow of olivine basalt 20 feet thick which 
contains a few small dunite xenoliths. It appears 
that these flows poured down old, deeply eroded 
valleys and possibly out over a reef. The source 
was probably a vent near the summit at Tumu. 
Because it appears likely that these flows oc- 
curred after the formation of deep valleys and a 
