Clay Mineralogy

d nn¬ętHt chlorite kaolin it*

li k

W SI

WWWXi^/J

Plio-Quaternary sediments, and (b) significantly greater amounts of calcium-rich dolomite. The clay mineral assemblage is very similar to that of the Plio-Quaternary sediments. At Site 374, the upper Messin-ian sediments are characterized by (a) a high terrigenous content, and (b) notable amounts of calcium-rich dolomite. In the less than 2 /jm fraction, the clay mineralogy differs fundamentally from that of the overlying Pliocene sediments, which are dominated by attapulgite (North Africa derived); however the Pliocene attapulgite occurs in minor amounts in the bulk (less than 5%). The clay mineralogy of the upper Messinian sediments is comparable, in the bulk, with that of the Quaternary sediments, whereas in the less than 2 mm fraction noticeable amounts of chlorite occur.

Sites 371 and 374 lead to conclusions similar to those drawn from Sites 372 and 376, concerning the upper Messinian sedimentation: (a) strong influence of the terrigenous material sources, and (b) presence of magnesium-rich minerals. Thus, although the mineralogy of the upper Messinian sediments displays notable discrepancies from one site to another because of discrepancies in the mineralogy of the terrigenous material sources, the sedimentation at that time is strongly marked by the two major features of the Messinian geodynamic event: changes in the water level and the existence of specific geochemical environments.

Pliocene

The Miocene-Pliocene contact was nowhere fully recovered. Nevertheless, data from Sites 372, 374, 376, and 378 provide useful information.

The Pliocene transgression at Site 374 (the most ba-sinal location in the eastern Mediterranean) is distinct in the mineralogy of the sediments. High calcareous at-tapulgite-bearing nannofossil marls (the base of which was later dolomitized) overlie terrigenous dolomitic muds. The unrecovered transition is located somewhere within an interval at most 4.5 meters thick. The absence of a vertical trend between the facies indicates that this transition is probably sharp. At Site 376, despite a disturbance at the base of the Pliocene (a sedimentary slump), the transition is characterized by a sharp break in the mineralogy (at least on the scalc of our sampling) and the appearance of a very high cal-cite content (biogenic production), while the terrigenous input displays a marked decrease. At Site 378, the Pliocene nannofossil marlstones overlie almost immediately massive selenitic gypsum (the undocumented interval is less than 1 m thick). In the eastern Mediterranean, the Pliocene appears to have a rather sudden onset, as far as it is documented by the three drill sites. Two of these sites are located in a basinal position (374 and 378) while the third is located on the flank of a rise (376).

At Site 372, the change from Messinian to Pliocene does not appear to the authors to have been a sudden event, and seems to result from several trends initiated during the end of the evaporitic episode (decreases in terrigenous input and in dolomite, increase in calcareous biogenic input). Except for the disappearance of smectite, no sharp break is recorded in the mineralogy. Although these variations are not continuously documented, due either to poor recovery of cores or to sedimentary gaps, the Pliocene transgression appears rather different at Site 372 than at the eastern Mediterranean sites. This is because Site 372 was not located in the deepest part of the basin when the Pliocene transgression occurred; but it could also be related to the fact that this transgression was more progressive in the western Mediterranean than in the eastern Mediterranean, thus the re-establishment of connection with the Atlantic Ocean resulted first in the replenishment of the Western Basin. The Eastern Basin, probably partly filled with fresh or brackish waters during the end of the Messinian, was not refilled with marine water until the sea level, in the Western Basin, reached the top of the Tunisian-Sicilian-Calabrian barrier.

The sedimentation during the Pliocene is very poorly documented in the western Mediterranean basins, At Site 372, the mineralogy of the Pliocene sediments, as compared to that of the pre-Messinian, suggests that a rather active hydrodynamism was established within the Pliocene. Such an active circulation could have resulted from a deepening of the basin. Site 371 yields similar information.

The Pliocene sedimentation was well documented at Sites 374 and 376, and is characterized by a low terrigenous input, allowing a large contribution of the calcareous biogenic production. At Site 374, the sedimentation, partly influenced by North African sources, seems to record a compressional tectonic event that occurred within the Pliocene (MP1-2 Zone), partial emergence of the northern surrounding landmasses (Sicily, Calabria, and Peloponese).

Thus, a difference between Western and Eastern basins, in terms of hydrodynamism (and maybe of water depth), is indicated by the Pliocene sediments.

Quaternary

The mineralogy of the Quaternary sediments (poorly sampled at all sites except 376) is roughly similar to that of the Pliocene sediments, except at Site 374. At the latter site, the end of the Pliocene is characterized by (a) a notable increase in the terrigenous input, and (b) the disappearance of the North African contribution to this input. This change is related to the marked increase in amounts of detrital material supplied by the northern surrounding landmasses uplifted during the lower Quaternary, and to a deepening of'ttie Ionian Basin. At the other sites, and as far as a very poor documentation allows, the Quaternary sedimentation as compared to that of the Pliocene, appears to be more active and/or to derive from a coarser terrigenous input. These characteristics suggest a general deepening of the basins and/or upliftings in landmasses. The latter are well documented by field geology in the borders of the Mediterranean, the physiography of which seems to have been significantly modified during the Quaternary.

0 0

Post a comment