Geology of Manu’a Islands, Samoa — Stice and McCoy 
449 
sea cliff around Ofu and Olosega, they are 
tentatively included among the Nu’u Formation. 
intrusive rocks: The intrusive rocks in- 
clude numerous dikes exposed in the cliffs of 
Ofu and Olosega, and one or possibly two plugs 
on eastern Ofu at Fatuaga Point. Only one dike 
was found intruding the A’ofa intra-caldera 
member, and nearly all of the intrusive rocks 
are probably older than the A’ofa intra-caldera 
deposits. The plug at Fatuaga Point appears to 
be the intruded core of the Fatuaga explosion 
breccia cone. There may also be a smaller, re- 
lated plug about 1,500 feet to the east, where 
a hill has an ovoid shape suggestive of an 
intrusive plug. The outcrop on this small hill 
is aphanitic basalt and is similar to the dikes 
which form the razorback ridge on eastern 
Ofu. Perhaps the fine-grained borders of a 
plug have not yet been eroded to reveal its 
coarser grained center. 
The coarser grained plug (Fig. 6, middle ) at 
Fatuaga Point is a hypabyssal intrusion of an- 
karamite. It was recognized and described by 
Daly (1924:134) as an elliptically shaped plug 
with a maximum diameter of 120 feet and a 
minimum diameter of 80 feet. Actually it is 
considerably larger than this, probably at least 
500 by 300 feet. The highest Bouguer gravity 
anomaly in the Manu’a Islands (more than 
-(-310 milligals) was recorded near this plug 
(Machesky, 1965). There is a gradation in 
grain size from olivine-titanaugite gabbro in 
the central portion of the plug to ankaramite 
near the peripheries. There is also a gradation 
in shape from a roughly ovoid plug near sea 
level to a much more elongate ankaramite dike 
at higher elevations. The general trend of the 
plug’s longest diameter is approximately N 15° 
W and vertical. 
Near sea level at Sunu’itao, the western edge 
of the plug cuts beds of explosion breccia 
that trend approximately N 85° W and dip 
13° N. Near the top of the shark’s-tooth peak 
at Vainu’ulua, this breccia appears to be trend- 
ing N 30° E and dipping much more steeply, 
about 30° W. Some of the blocks included in 
the explosion breccia are identical with the in- 
trusive olivine gabbro, except that they are 
slightly finer grained. Thus, the intrusion prob- 
ably occurred nearly contemporaneously with 
the deposition of the breccia. Included within 
the plug is a pod of brecciated pahoehoe flows 
of vesicular basalt, which apparently was broken 
off from the chamber wall and carried up with 
the magma during the intrusion. 
Near sea level the plug is cut by numerous 
thin dikes that strike approximately E-W and 
dip 55-85° N. Most of these dikes are dense 
and aphanitic, but some of the thicker ones 
are vesicular. This vesicularity, along with the 
open miarolitic texture of the olivine gabbro, 
suggests proximity to the surface at the time 
of intrusion. These dikes parallel those of the 
dike complex extending from Le’olo Ridge 
eastward to Olosega, as described below. 
The razorback ridge of eastern Ofu is the 
topographic expression of a dike complex about 
400 feet wide. The dikes are nearly vertical, 
but some may dip steeply northward. Most are 
dense basalt, although olivine basalt, ankara- 
mite, and feldspar-phyric basalt also are present. 
Large talus blocks of diabase, which came from 
thick dikes at the top of the ridge, lie along 
the shoreline north of Vainu’ulua. 
The dike complex continues across Asaga 
Strait to the 2,000-foot cliff behind Sili Village 
(Fig. 6, middle). Near Tamatupu Point, the 
westernmost tip of Olosega, thick dikes with 
dips as low as 50° N may be slightly curved 
concentric to the Sili caldera, but the individual 
dikes could not be traced far enough to confirm 
this. Northeast of Piumafua the number of 
dikes paralleling the face of the cliff behind 
Sili Village decreases sharply. In the cliff behind 
Olosega Village north-dipping and vertical 
dikes related to this complex decrease markedly 
in number both up-section and away from the 
caldera. A few apparently related dikes trend- 
ing about E-W cut steeply across flows dipping 
10-20° away from the caldera as far as 3,000 
feet south of the cliff behind Sili. 
A sill about 400 feet long and 30 feet thick 
can be seen in the cliff at Faiava near Sili 
Village. A low-angle dike near the extreme 
eastern side of the sill discordantly intrudes a 
series of pahoehoe flows, striking about N 5° E 
and dipping 15° E, and may be the feeder dike 
for the sill, but this relationship cannot be seen 
clearly because of the vegetative cover. In this 
portion of the cliff most of the other dikes 
strike about N 30° E and are vertical, whereas 
