Clay Mineralogy








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Figure 8. X-ray mineralogy data. Site 377.

Figure 8. X-ray mineralogy data. Site 377.

sum topped by a brecciated and vuggy dolomite, of late Miocene age (core catcher 11, Hole 378; Cores 4 to 9, Hole 378A). Thirty-three samples were submitted for X-ray analysis {Figure 9),

The mineralogy of the whole section (except the late Miocene evaporites not investigated by XRD) is remarkably uniform in the bulk as well as in the less than 2 ^m fraction. Calcite, averaging 40%, is the dominant mineral, with minor amounts (5%) of Mg-calcite {10% MgC03) occurring in Core 1, Hole 378A (Quaternary). Dolomite content is low (average 5%); this mineral is always calcium-rich (Ca^fj), except in Core 1, Hole 378A, where it is stoichiometric. Quartz + feldspar content ranges from 10% to 15%, in the less than 2 ^m fraction, illite dominates the clay mineral assemblage with values ranging from 40% to 50%; smectite, chlorite, and kaolinite, in approximately equal amounts, complete this assemblage. Serpentine is present everywhere in very small amounts (average 1%); its content is slightly higher in the early Pliocene sediments (2% to 3%).

This very monotonous mineralogical assemblage suggests that the sources of terrigenous and biogenic materials have not changed since the early Pliocene. The only slight variation occurs in Core 1 of Hole 378A (upper Pleistocene), where the appearance of Mg-calcite (thought to be a shelf product) indicates a more pronounced detrital input. This is supported by the presence of stoichiometric dolomite (thought to be terrigenous) instead of calcium-rich dolomite (early diagenetic) occurring in the underlying sediments. A marked increase in plagioclase content towards the top of Core 1 (378A) seems to support this hypothesis. This change in detrital input may be explained by an active downslope transportation of the terrigenous material. This could result from a deepening of the basin. Foundering in the Aegean area during the Pleistocene is known to be a consequence of north-northwest-south-southeast cxtensional tectonics (Mercier et al., 1976; Keraudren, 1975). Indications of these events appear to be recorded within Core 1, Hole 378A.


The Pre-Mcssinian sediments are documented at two sites: 372 on the Balearic Rise (western Mediterranean) and 375 on the Florence Rise (eastern Mediterranean). Despite similarities in the thicknesses and ages of the sedimentary sections (885 and 821 m respectively, both holes reaching the Burdigalian), the sedimentation differs completely between the two.

On the Balearic Rise, the Burdigalian sedimentation is dominated by a large terrigenous input consisting mainly of fine particles (clays and silts). Despite the nearness of Menorca, the terrigenous material appears to derive from far away continental sources, probably Alpine areas. This material was deposited at high sedimentation rates but also continuously, since there are no turbidites, in a marine environment. Free silica, probably supplied by a volcanic activity in the vicinity, allows radio] aria ns to develop. These conditions change abruptly within the uppermost Burdigalian: the hydrodynamism of the basin becomes rather subdued. Consequently the terrigenous input decreases markedly (although still deriving from the same sources) and this induces an increase in the carbonaceous biogenic contribution to the sediments. These conditions prevail during the Langhian. the SerravalKan, and probably the Tortonian (see Bizon, Site Report, Chapter 3), The lowering of sea level (Messinian regression) causes an important erosion of the upper part of this sequence, while thick evaporites are deposited in the deepest part of the Balearic Basin.

Contrasting with this sedimentary history, the pre-Messinian sedimentation on the Florence Rise is strongly influenced by turbidity current activity. The terrigenous material originates from neighboring landmasses (Cyprus and possibly South Turkey), as is demonstrated by its high smectite content, this mineral being the chief constituent of soils (ophiolite derived) on Cyprus. After calm sedimentation during the Burdigalian and the Langhian, the activity of turbidity currents begins in the Serravallian and increases markedly in the Tortonian during which a thick sequence (400 m) is deposited. These turbidity currents are probably a consequence of tectonic activity on Cyprus and South Turkey. During the end of the Tortonian, a progressive trend towards restricted conditions leads to the upper Miocene evaporitic episode.

These two sections (372 and 375), although encompassing a similar time span, show fundamental differences appearing mainly in (a) the nature of the sources of terrigenous material and (b) the depositional environment. These differences obviously reflect their different physiographic locations (local as well as regional) and different structural histories of the areas in which they were drilled.


Evaporite rocks were not submitted for routine X-ray analysis. Therefore the comparisons involve only the upper Messinian sediments, which were recovered at Sites 371, 372, 374, and 376.

Sites 372 and 376 allow comparison of the upper Miocene sedimentation during the end of the Messinian Event, with that during the Pre-Messinian. At Site 372 the Messinian sediments are characterized by (a) an increase in quartz 4- feldspar + detrital dolomite content, while the clay mineral assemblage remains approximately the same, and (b) noticeable amounts of calcium-rich dolomite. At Site 376, the upper Messinian sedimentation is strongly influenced by terrigenous input. After the deposition of the evaporite rocks, the terrigenous input consists essentially of detrital calcite (reworked faunas from Cretaceous to middle Miocene) and detrital products derived from ophiolites (mainly smectite). These constituents are derived from neighboring land masses (Cyprus). Calcium-rich dolomite and attapulgite, occurring in the upper part of the section, are tentatively interpreted as detrital constituents reworked from neighboring magnesium-rich environments, where these minerals were generated at approximately the same time. At both Sites 372 and 376, the upper Messinian sedimentation is characterized, on the whole by (a) an influence of the terrigenous sources more pronounced than that during the pre-Messinian, and (b) the presence of magnesium-rich minerals.

The mineralogy of upper Messinian sediments from Sites 371 and 374 can only be compared with that of the Plio-Quaternary section, assuming the latter yields information on "open marine" sedimentation. At Site 371, the upper Messinian sediments are characterized by (a) a higher coarse terrigenous content than the o—


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