FIG. 6 SEISMIC LINE MD40-32, PROJECTED SAMPLES
84DR07
Mio-Pliocene
W E
Crystalline
basement
Crystalline
basement
Arkosic sandstones in contact with
Quaternary bioclastic sediments
Bioclastic limestone highly
intruded by Mn-oxides
Burrowing, dissolution,
recrystallization of limestone
Plio-Quaternary oozes rich
in foraminifera
84DR09
Miocene limestone,
arkosic breccia
84DR08
Mio-Pliocene
with arkosic clasts
Mount Antandroy, to the north, cores recovered Late Eocene
(NP20) oozes (SU7710, 2,050 mbsl), Early Miocene (NN2)
limestone (SU7712, 2,005 mbsl) and oozes with reworked
Oligocene (SU7711, 1,920 mbsl).
The southern crests of the Sakalave Mounts between 18°
and 19°S are also covered by basaltic breccia (Fig. 8). Irregular beds of basaltic breccia are cemented in a bioclastic matrix rich in Middle Eocene nannoplancton and benthic and
planctonic foraminifera (84DR13, 600 mbsl). Basalt clasts,
up to 1 cm, are composed of diopsidic clinopyroxene, K-feldspar, and later hematite. They are oxidized and give a
reddish color to the rock.
Further south and downdip, soft blocks of altered basaltic
breccia are cemented in a chalk matrix containing few fresh
dark-green vesicular clinopyroxene-phyric basalt fragments
(84DR14, 1, 130 mbsl). Nannoplanctons of Middle Eocene
(NP15-17) dated the matrix of these breccia. Nannofauna
shows some dissolution and slight recrystallization. These
breccia, as those from the top of the Mounts (84DR13), cor-
relate with those from distinct horizons of DSDP Site 245
(Unit II) of Middle Eocene age.
Metamorphism, uplift, and erosion
The crystalline basement rocks crop out at the median parts
of the ridge. Basalts were not encountered below pelagic sediments at these latitudes, confirming the absence of magnetic signature. 10
As erosion products of crystalline provinces, these rocks
could be an intimate part of the basement, having been deposited during the early rifting which preceded the southwards drift of Madagascar along the DFZ. These rocks are
comparable with several formations from the African basement and related Karoo facies known in Tanzania and the
Malagasy craton.
