Geology of Manu’a Islands, Samoa — Stice and McCoy 
435 
predominating in the upper part of the section. 
The flows at low elevations on the north slope 
dip 15-20°, but the uppermost layers have 
steeper dips of 25-35°. Near the summit the 
dips of these upper flows decrease to less than 
1 5 ° in the high fault scarp on the southern part 
of the island. On the eastern and western slopes 
of the Lata shield and the north side of its 
caldera, the present ground surface also con- 
forms to the lava flows, dipping about 15° away 
from the summit. 
The extra-caldera member contains numerous 
erosional unconformities, which seem to in- 
crease in number up-section, but no profound 
unconformity was noted. The change to steeper 
dips up -section apparently was a gradual one, 
accompanying a decrease in volcanism and a 
corresponding increase in the rate of erosion 
of the former slope. 
The olivine basalt lavas generally contain 
phenocrysts of olivine approximately 2-4 mm 
in diameter. Most of the basalt lavas contain 
phenocrysts or microphenocrysts 1 mm or less 
in diameter. Approximately one-third of the 
olivine basalt lavas contain plagioclase micro- 
phenocrysts 1 mm or less in diameter. A few 
flows exposed in the sea cliff on the northern 
shore between ’Ao’auli Stream and Lepula con- 
tain plagioclase crystals up to 5 cm long. The 
massive portions of these flows are only 0.5-2 
feet thick, but are separated by layers of clinker 
up to 7 feet thick. The flows are composed al- 
most entirely of plagioclase phenocrysts with 
little groundmass to bind them together and 
therefore are very friable. One 30-foot section 
of these flows is exposed at the base of ’Ao’auli 
stream valley and another 60-foot section only 
about 200 feet upstream. In several places 
higher in the section, thin massive central 
portions of aa flows only 1-2 feet thick are as- 
sociated with clinker beds up to 8 feet thick; 
these flows generally have steep dips (30-35°), 
which may account for the large accumulation 
of clinker. One tuff bed and a soil horizon 
were found at about 1,200 feet elevation in 
’Ao’auli stream valley. The section up Avatele 
Stream, given in Table 1, is characteristic of the 
extra-caldera member. 
Some of the larger Recent post-caldera cones 
of the Lata shield are Lepu’e, Olomatimu (Fig. 
4, middle photo ), Olomanu, and Olotania on 
TABLE 1 
Stratigraphic Section of Extra-caldera Member, 
Lata Formation, up Avatele Stream 
TOP 
THICKNESS 
(feet) 
Nonporphyritic, dense gray hawaiite dip- 
ping 28°N 
25 
Red vitric ash lying unconformably on an 
older erosional surface and dipping 
31°N 
1 
A series of aa flows of olivine basalt and 
oceanite 1-3 feet thick separated by 
clinker beds 2-6 feet thick, dipping 
approximately 15°N 
25 
No exposures 
100 
Aa flow of basalt dipping S5°W, ap- 
parently poured over fault scarp to 
form an angular unconformity with 
underlying aa flows 
5 
A series of thin (0.5-1. 5 feet) aa flows 
containing abundant plagioclase laths 
up to 5 cm long separated by clinker 
beds up to 7 feet thick (dip .= 28°N) 
50 
A series of thin (0.5-1. 5 feet) aa flows 
of basalt with occasional olivine pheno- 
crysts separated by clinker beds 0.5-4 
feet thick (dip = 26°N) 
20 
Total thickness of section 
226 
the northwest flank, and Sa’umane and a line 
of four or five cones near Tafetafe on the north- 
east flank. Most of the lavas issuing from the 
post-caldera cones are olivine basalt or picrite- 
basalt. Vesicular basalt and hawaiite are less 
common. Dunite inclusions approaching 2 
inches across are found in some lavas. Rarely, 
some augite occurs along with the olivine in 
these inclusions. Near the vents cinder and 
often welded spatter occur. The olivine basalts 
typically have phenocrysts of olivine approx- 
imately 2-4 mm in diameter. Plagioclase micro- 
phenocrysts approximately 1 mm in diameter 
are usually found in the basalts, olivine basalts, 
and hawaiites. The flows range from 2-3 feet up 
to 15 feet in thickness. Aa flows are predomi- 
nant, although pahoehoe flows are occasionally 
found. 
INTRA-CALDERA MEMBER OF LATA FORMA- 
TION: After formation of the caldera on the 
Lata shield, lava flows and pyroclastic deposits 
of the Lata intra-caldera member accumulated 
within the depression. They are separated from 
the volcanic rocks of the Lata extra-caldera 
