Clay Mineralogy

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tio is therefore very different from that of Unit IV, being similar to that of sediments settled in "normal marine" conditions. In the uppermost part of Unit HI (Core 9), one notes a very slight increase in both quartz and dolomite content towards the top of the core, which could be interpreted as a trend toward more restricted conditions. The dramatic change in the faunal assemblage occurring in Core 9, Section 2 at 45 cm, indicates a sudden transition from normal marine lower middle bathyal conditions (below) to shallow water lagoonal environment (above), is not noticeably recorded in the mineralogy. A detailed X-ray mineralogy study of Core 9 (Sections 1 and 2) was carried out on a set of narrowly spaced samples (M6Iieres, this volume), All of Core 9 (except four small pieces of laminated gypsum, topping the sediment of Section I, which are a drilling artifact) belongs mineralogically to Unit nr.

In Unit II (Messinian evaporites), the recovery was extremely poor and only four samples were submitted for X-ray analysis. The nannofossil marls interbedded with laminated gypsum are characterized by a quartz + feldspar content somewhat higher than in the underlying pre-Messinian sediments, suggesting that the sources of at least part of this detrital material were now probably closer to the depositional area (Site 372), This terrigenous fraction shows a marked decrease towards the top of the unit. The clay minerals do not change significantly either in the percentages recorded or in their relative compositions. The calcite content is low (10% in Core 6), but increases towards the top of the unit (35% in Core 4). In contrast, the dolomite content is relatively high (17% in Core 6) and displays an opposite trend (5% in Core 4). This dolomite actually consists of a mixture, in approximately equal amounts, of a stoichiometric dolomite, thought to be detrital from Jurassic dolomites of Menorca, and a calcium-rich dolomite (Ca^), which is probably autogenic in a restricted environment. These features confirm that the Messinian evaporites were deposited in a restricted environment unfavorable to the biota (low calcite content in the sediment). The reason for this restriction is probably due to a drop in sea level which caused an increase in the terrigenous detrital input on the margin of the basin; and Site 372 is actually in such a physiographic position. However, this drop was probably not sufficient to isolate the area completely from the open sea as it is shown by the stability of the clay mineral input. In the less than 5 ^m fraction, the day mineral assemblage is not significantly different from that of the pre-Messiniam. However, it cannot be excluded that the clay minerals of the Messinian marls could have been reworked from the pre-Messinian sediments, which were exposed to air by the lowering of the sea level.

The mineralogy of the nannofossil marls of Unit 1 (Pleistocene and a condensed section of Pliocene) also offers many similarities to that of the pre-Messinian sediments, confirming that sedimentation, after the Messinian event, returned to normal marine conditions.

This return is associated with changes in the mineralogy towards the top of the Unit II, confirming that the Messinian section penetrated at Site 372 belongs to the Upper Member of the Messinian evaporites. However three significant differences in Unit III are noted in Unit I: (a) the quartz + feldspar content is slightly higher, (b) there is a marked increase in illite (the coarsest among the clay minerals) in the fine fraction, and (c) the smectite (the finest among the clay minerals) disappears completely from the clay assemblage. This could be interpreted as resulting from the establishment of a new hydrodynamic regime, characterized by stronger currents than those prevailing during the deposition of the sediments of Unit III. This hypothesis is supported by the presence of numerous sandy layers within the nannofossd marls of Unit I, and by the condensed nature of the Pliocene section, suggesting that some periods of non-deposition or of submarine erosion took place at that time. This new hydrodynamic regime is probably related to a deepening of the basin. Unfortunately, no information is available from Hole 372 on the Quaternary history of the area.

Resume a) Burdigalian: fast infilling of a subsiding basin limited by fault blocks of the Balearic rifted margin (Biju-Duval et al., this volume). Infilling by fine terrigenous detrital material (clay minerals dominant) probably of Alpine origin (association dlite-chlorite). Marine environment with free silica available (radio-larians diagentically yield opal-CT) probably provided by volcanic activity.

  1. Upper Burdigalian (limit N6-N7: 18.5 m,y. B.P.): sudden decrease of the infilling rate of the basin, thought to be controlled by some tectonic activity possibly related to the end of the Burdigalian volcan-ism. Establishment of rather calm conditions of marine sedimentation persisting from the uppermost Burdigalian until at least the lower Tortonian.
  2. After an erosional gap (about 5 m.y.), probably related to a marked drop in sea level, the Upper Member of the Messinian evaporites is deposited in restricted shallow subaqueous environment.
  3. Re-establishment of "normal marine" conditions within the Pliocene and setting up of a new and active hydrodynamic regime as a probable consequence of the deepening of the basin.

Hole 373A

Hole 373A is located on the flank of a seamount in the central Tyrrhenian Abyssal Plain, in 3517 meters of water. A Plio-Quaternary sequence of nannofossil marls, zeolite marls, and volcanic ashes overlies a basaltic complex of calcareousle-cemented basalt breccias and flow basalts. The hole was terminated at 457,5 meters. Only 10 samples from Cores 1 and 2 were submitted for X-ray analysis (Figure 4).

The mineralogy of the sediments of Core 1 (Quaternary) displays numerous variations depending on the different types of sediments encountered. The nanno-

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MINERALOGY

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