Results And Discussion

The analytical data discussed here are listed in Appendix III (this volume). They are presented here as mineralogical logs for each site in Figures 2 to 9. Figure I shows the location of the Leg 42A drill sites. The geodynamic conclusions tentatively inferred from the minerolgical data are obviously "monochromatic" and therefore need to be compared with the other sedimentological data (petrography, geochemistry, mi-cropaleontology).

Site 371

Site 371 is located on the southern edge of the Balearic Abyssal Plain, about 90 km north of the Algerian Shelf, in 2792 meters of water, and on a slight knoll. The hole penetrated more than 500 meters of hemipelagic Plio-Quaternary sediments, in which only seven cores were taken, and was terminated at 551 meters in the upper Miocene evaporitic facies (Core 8), Thirty-five samples were submitted for X-ray analysis (Figure 2).

The recovered evaporitic sequence consists of 2 meters of homogeneous brown dolomitic marls, barren of indigenous microfossiis (its Messinian age is therefore inferred), topped by 1 meter of hard white anhydrite displaying a "chicken-wire" texture, and interpreted as a subaerial (sabkha) deposit (Garrison et al., this volume). The mineralogy of these brown dolomitic marls is characterized by a relatively high

terrigenous detrital content (quartz + feldspar = 35% to 40%) which, in the absence of turbidites, suggests the proximity of the sources of the terrigenous material. The clay mineral assemblage consists of illite, irregular mixed-layer illite-smectite, chlorite, and kaolinite. The absence of smectite indicates that the neighboring North African landmasses did not act as source for this mineral at that time. This is in good agreement with the fact that, in Algeria, the Messinian sediments of the Chelif basin (500 km west of Site 371) are characterized by a very low smectite content in the less than 2 mm fraction (Chamley et al., this volume). Therefore the absence of smectite in the Messinian sediments of the Algerian area is amazing since this mineral is believed to have been extensively produced under an arid climate in the poorly drained soils of the Messinian landscape (Chamley et al., this volume). The carbonate fraction consists of approximately equal amounts of calcite (10% to 15%) and dolomite (13% to 15%), Calcite represents tests of organisms reworked from the Cretaceous and the Paleogene. Dolomite is calcium-rich (Ca5S) and therefore is probably authi-genic or early diagenetic, which is not surprising in such an evaporitic environment. These features suggest that the late Miocene landscape consisted of very shallow basins with scant communication with the open sea.

In contrast to the upper Miocene sediments, the hemipelagic muds of the overlying lower Pliocene (Cores 7, 6, and 5) are characterized by a lower terrigenous coarse material content (quartz + feldspar = 20%), a much lower dolomite content (1% to 5%), and a higher calcite content (25% to 45%). The total amount of clay minerals in the bulk also increases significantly, although the relative composition of the clay fraction remains approximately the same. No traces of smectite can be detected in the clay mineral assemblage. These observations suggest (a) that the sources of detrital material were more distant, and (b) that the circulation of the water masses became well established, allowing a pronounced development of a marine calcareous biogenic productivity. The increase of the water column height would have been more likely due to a moderate raising of sea level rather than to sinking of the sea Door, because in the latter case a coarser terrigenous input (quartz + feldspar) would probably have been present owing to the proximity of the North African landmasses.

The entire upper Pliocene and Quaternary section {Core 4 to 1) consists of calcareous muds with sandy beds and laminae. It is characterized by a remarkably uniform mineralogy, suggesting that the sources of sediments remain constant during all this period. The coarse terrigenous content (quartz + feldspar) however is slightly higher in Cores 3 and 4 (25% mean value) than in Core 5. This, together with the presence of sandy laminae and turbidites in Section 2 of Core 3, could suggest a deepening of the basin, which would have begun in the upper Pliocene (between Core 5 and Core 4). The slight but uniform decrease in calcite content (calcareous biogenic production) could also be related to such a progressive deepening (dissolution), although this calcite decrease could be explained by some change in the ecology of the biota, or by an increase in terrigenous material input.

Resume a) At the end of the Messinian: landscape consisting of very shallow ponds where subaqueous (doio-mitic maris) to subaerial (anhydrite) deposits were deposited; proximity of landmasses suggested by high coarse terrigenous material input.

  1. Lower Pliocene: marine waters infill the area (raising sea level), supplying optimal conditions for calcareous biogenic production.
  2. Middle Pliocene: probable deepening of the area.
  3. Since the middle Pliocene: probable increase of the depth of water by continuous lowering of the sea floor, until reaching its present condition.

Site 372

Site 372 is located at the base of the Balearic Slope, 35 km east-northeast oif the Menorca Shelf, in 2699 meters of water. The hole penetrated a 885-meter section, half of which was continuously cored. Four lithologic units were distinguished. Lithologic Unit I {Cores 1 to 3) consists of Plio-Pleistocene nannofossil marls. Lithologic Unit II (Core 4 to Core 9, Section 2) is a 50-meter-thick evaporitic sequence (laminated gypsum and dolomitic marls). It represents a part of the Upper Member of the Messinian evaporites (Mauffret et al., 1973) pinching out on the pre-Messinian sediments of the Menorca Rise. Unit III (Core 9, Section 2 to Core 33, Section 3) consists of lower Tortonian to upper Burdigalian nannofossil marls to marlstones. Unit IV (Core 33, Section 4 to Core 46) consists of Burdigalian clayey mudstones, which were deposited with a high sedimentation rate (see Chapter 3, Site Report, this volume). Two-hundred fourteen samples were submitted for X-ray analysis (Figure 3).

The mineralogy of Unit IV is remarkably uniform. It consists mainly of an important terrigenous input (70% of the sediment), a modest carbonate fraction (20% to 25%), and opal-CT (5% to 10%). The terrigenous fraction is composed of approximately 20% quartz + feldspar and 50% clay minerals dominated by the association illite (25% to 30%) + chlorite (10%) (both well crystallized), and smectite (10%)3. The carbonate fraction is composed of calcite (20%) and dolomite, the amount of which varies significantly along the sedimentary column. The percentages of this latter mineral range from few percent (2% to 5%) in Cores 35, 37, 41, 42, and 43, up to 15% in Cores 34, 38, 39, 40, and even higher values (32% in Core 44, Section 4). Consequently, the calcite/dolomite ratio offers values much lower than that characterizing the recent "normal marine" sedimentation in the western Mediterranean Basin (values ranging from 15 to 20) as can be seen at Site 371 and in other Plio-Quaternary deep-sea sections. A diagenetic origin, after burial, seems to be unlikely for this because of the randomness of the calcite/ dolomite ratio variation. The nearness of Jurassic dolomites at Menorca (Bourrouilh, 1973) could suggest a detrital origin for the dolomite of Unit IV. Such an ex

^ Smectite, visible on the bulk X-ray mineralogy but not apparent on the clay mineralogy column (Figure 3), consists of poorly crystallized material giving a bump on the diffractogram (bulk natural sample) between 12A and liA. but expanding completely to 17A (well-defined peak) after ethylene-glycol treatment. These structures are considered as irregular mixed-layer illite-smectite by the clay mineral analyst. They probably arc partially illite derived.

planation could be supported by the striking correlation between dolomite and quartz + feldspar in Core 44, although it is not apparent in the other cores. In this hypothesis, the dolomite should be stoichiometric as the Jurassic one, but unfortunately this is not always the case, especially for Core 44 in which the dolomite is partly calcium rich. The problem of the dolomite of Unit IV is probably complex and needs further miner-alogical investigation, particularly on the nature of the dolomite. The relatively iow calcite content can be explained in terms of dilution of the carbonaceous biogenic production by the important terrigeneous input, which is in good agreement with the high sedimentation rate of the Unit IV. The presence of opal (low cris-tobalite, or opal-CT in the sense of Jones and Segnit, 1971) is easily understood as a product of recrystalliza-tion of previously dissolved tests of radiolarians, which are common in the sediments of the Unit IV {see Site Report, Chapter 3). The development of radiolarians in the waters of the basin during this period implies the availability of free silica, which could have been supplied by a volcanic activity. In fact, such an activity is known in this area during the Burdigalian (Mauffret et al., 1973). At some 200 km meters west-northwest of Site 372, Hole 123 or Leg 13 penetrated a volcano whose products gave ages ranging from 19 to 22 m.y. (Ferrara et al., 1973).

A major break in the mineralogy within Section 3 of Core 33 separates Unit IV from the overlying Unit III. This occurs at 18.5 m.y. (N6-N7). It is characterized by (a) a sudden decrease in the amount of the terrigeneous material content (quartz + feldspar *f clay) which drops from 70% to 40% of the sediment, although its relative constitution remains approximately the same (exccpt for a slight increase in the smectite content to ranges of 40% to 60%, Traces of kaolinite appear in the upper part of this unit. The increase in calcite content probably results from the decrease of the terrigenous input which is confirmed by the sedimentation rate in Unit III which is lower than in Unit IV. Therefore, it can be inferred that the calcareous biogenic productivity is the same in Unit III as in Unit IV. The sudden dccrcase in supply of terrigenous material, together with an increase in the content of the finest particles (smectite), suggests that the hydrodyna-mism became rather subdued, in this part of the basin. This change could have been controlled by a tectonic event such as the development of a change in threshold. This hypothesis would be supported by the interruption of the volcanic activity acting as the source of free silica (disappearance of opal-CT). In such a case, this could have marked the end of the rifting of the Balearic Margin. Another possible cause of the changc of the hydrodynamism could have been the Langhian transgression (Bourrouilh, 1973). In this hypothesis, the association clinoptdolite + smectite, occurring in the lower part of the Unit III (mainly Cores 28, 29, and 30) could be interpreted as resulting from the erosion of volcanic material previously deposited ashore during a phase of rifting. The dolomite content in Unit III is very low (1% to 3%) and the calcite/dolomite ra-

MINERALOGY

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