Clay Mineralogy

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fossil marls consist of calcite (25% to 30%), dolomite ( 1% to 3%), quartz + feldspar ( 15% to 20%), and clay minerals (45% to 55%). Among these clay minerals (in bulk) illite is largely dominant (50%), then smectite'5 (15%), chlorite (15%), kaolinite (15%), and minor amounts of mixed-layer clay. The zeolitic marls are characterized by phillipsite (up to 17%), which seems to be superimposed on the background of nannofossi! marls.

The nannofossil marls of Core 2 (early to late Pliocene, a condensed section) are very rich in calcite (up to 64%) and consequently the quartz + feldspar + clay content is modest. The base of the core consists of a lithified red dolomitic marl characterized by a very high dolomite content (80%). The high calcium content of this dolomite (Ca3B) and the total lack of calcite suggest that the dolomite is probably diagenetic, having replaced calcite (oxygen isotope data support this hypothesis [Bernoulli et al., this volume)).

The marked contrast in the carbonates/silicates ratio between Cores 1 and 2 suggests that the depositional environment during the Quaternary was significantly different from that during the Pliocene. The low terrigenous material content of the Pliocene sediments suggests that this material could hardly reach the area (Site 373), probably because of inhibited water circulation. Because of a physiography consisting of several small basins, more or less separated by barriers. In the Quaternary sediments the considerable terrigenous de-trital content (up to 70% of the sediment, quartz + feldspar up to 4.5%) implies much better conditions for transportation, which could indicate the existence of steeper continental slopes. In this hypothesis, a notable deepening of the basin occurred between Pliocene and Quaternary, probably related to the lowering of the floor of the Tyrrhenian Sea. An active volcanism (ashes, zeolites) in the area seems to support this hypothesis. We should emphasize that these conclusions are obviously highly speculative because of the very low sampling frequency at this site.

Site 374

Site 374 is located in the central part of the Messina Abyssal Plain in 4078 meters of water. The hole penetrated a 457-meter section consisting of 300 meters of nannofossil marls and muds with numerous silty layers (Cores I to 4: Quaternary), 80 meters of highly carbonaceous nannofossil marls (Cores 5 to 11: Pliocene) and 77 meters of Messinian evaporites (mainly gypsum, anhydrite, and halite; Cores 12 to 24), whose upper 20 meters (Cores 12 to 15) consist of homogeneous dark dolomitic muds barren of stratigraphie fossils. Fifty-seven samples were submitted for X-ray analysis (Figure 5),

Evaporite rocks were not sampled for routine X-ray analysis. Their study (Garrison et al., this volume;

5 In the less than 2 Jim fraction, the absence of smectite is probably a question of terminology sincc this mineral has been labeled as irregular mixed-layer illite-smectite by the clay minerals analyst, because of its poor crystallinity.

Kuehn and Hsti, this volume) established that they have been deposited in shallow brine pools (halite) progressively desiccated and subaerially exposed (nodular anhydrite). The area was covered by shallow water bodies, from time to time, when gypsum, interbed-ded with dolomitic marlstones, was cyclically deposited.

The mineralogy of the dark gray dolomitic muds (Cores 15 to 12) overlying the evaporites is rather uniform, ft consists of quartz + feldspar (15% to 20%), clay minerals (55% to 70%), and dolomite (15% to 25%). The striking lack of calcite, together with the high calcium content of the dolomite (CaK), suggest that this latter mineral was diagenetically formed by "replacement" of a calcific fraction probably of biogenic origin; this diagenesis is associated with interstitial water extremely rich in magnesium (McDuff and Gieskes, this volume). White millimetric spheres are disseminated in the sediment from place to place. They were not submitted for routine X-ray analysis, but were shown to consist of liineburgite (magnesium borophos-phate; Müller and Fabricius, this volume). The mineralogy of the clays is of special interest. In the bulk, illite is the dominant mineral (20% to 25%) followed by smectite (15% to 20%) and irregular mixed-layered illite-smectite (10%) considered to be iliite-derived, and minor amounts of chlorite (7%) and kaolinite (5%). In constrast, the less than 2 jan fraction is largely dominated by smectite (50% to 75%) followed by chlorite; illite is subordinate (15%) and kaolinite (5% to 10%) fills up the assemblage. These facts suggest that the dark gray dolomitic muds developed essentially from a terrigenous input (75% to 85% of the sediment) which settled in calm conditions (clay minerals dominant in the bulk; smectite — the finest particles — dominant in the less than 2 >im fraction). Furthermore, the depositional environment was unfavorable for the biota, as suggested by the low carbonate content (the total lack of calcite probably results from a post-depositional dolomitization as will be seen further, although a very early dolomitization is not impossible).

Overlying the dark gray dolomitic muds of the uppermost Messinian, the 80-meter-thick Pliocene section (Cores 11 to 5) displays a remarkable homogeneity in its mineralogy, except in the extreme base (Core 11) which is of diagenetic origin, as will be discussed below. The dominant features are (a) a very high carbonate content (60% to 70%), (b) a very low quartz + feldspar content {5% to 10%), and (c) a moderate clay content. The mineralogy of the clay assemblage is characterized by the presence of attapuigite which constitutes up to 50% of the less than 2 um fraction (although its contribution to the bulk never exceeds 5%). This mineral has been shown to be derived from North Africa (Chamley, 1971). With regard to the underlying dark dolomitic muds, the smectite/illite ratio decreases markedly in the less than 2 ¿<m fraction and even more in the bulk. The carbonate fraction consists mainly of calcite (nannofossils and foramini-fers) and minor amounts of calcium-rich dolomite (Ca^), except in Core 11. In this core, subjected to an

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MINERALOGY

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