FIG. 8 SEISMIC LINE MD40-03, BASALTIC SAMPLES
Basaltic conglomerates and breccia
cemented in a Middle Eocene (NP15-17)
calcareous matrix comprising few thin
fragments of arkosic origin (84DR14).
Photomicrograph of 84DR14. Matrix is
dated by coccolithes of Upper Eocene
(NP15-17) and contains late zeolithes
(phillipsites) that crystallize on dissolution channels.
Basaltic breccia are cemented in a bioclastic
carbonate matrix (84DR13). These breccia
form irregular beds up to 40 cm thick. Middle
Eocene matrix with coccolithes (NP15-17) and
Basalt above crystalline basement
angular fragments and suggest in situ deposition. The missing Oligocene, equally absent in several sites of the Indian
Ocean, may represent this unconformity, recognizable on
the seismic by a strong and thin reflector.
The fact that Miocene bioclastic limestones and breccia
from the crests of the ridge are locally karstified, fractured,
and directly covered by younger Pleistocene chalks, suggests
fault-controlled tilting, emersion, and erosion along the ridge
during Miocene and records an important stratigraphic hiatus which might be related with a large tectonic activity or
mersion before the sedimentation of the Pleistocene chalks.
The Upper Pleistocene oozes (NN 19/20) from the flanks of
the ridge contain significant proportions of reworked Upper
Eocene nannofauna, suggesting that the Miocene-Pliocene
time gap on the crests is a period of non-consolidation where
sediments were reworked several times due to tilting and
Submarine hydrothermalism under reducing conditions
variously affected the basalts. This period was associated
with or followed by brecciation and tuffs sedimentation. Basalt samples collected are variously affected by sea water and
have alteration features such as oxidation colors and vesicles
filled with either carbonate or green-layered lattice silicate
minerals. Pressures were probably lower than 3 kilobar and
temperatures between 150° and 200° C.
Miocene-Pliocene time gap
The Miocene bioclastic limestones rich in planktonic fauna
are commonly found on the crests of the ridge, where they
enclose basaltic fragments and lie between Mn-oxide layers.
These formations were probably exposed to subaerial alteration. The breccia from the western flank are composed of
rounded fragments and this suggests deposition at some distance from the source while those from the crest consist of