Us Deep Sea Drilling Policy


Figure 7. A'-raj-' mineralogy data. Site 376, presence of turbidites implies the existence of continental slopes (Cyprus lies 40 km eastward), suggesting that the water depth was not negligible.

In the Tortonian (Cores 7 to 4), there is no fundamental change in the mineralogy of the sediments with regard to the Serravallian. Smectite dominates the clay mineral assemblage in the less than 2 ¿¡m fraction as well as in the bulk. Serpentine is still present. Nevertheless several differences are noticeable. The calcite decreases markedly and the range of its variations (10% to 45%) is less important than in the Serravallian (0% to 78%). This is in good agreement with the fact that the thick (400 m) Tortonian section (high sedimentation rate) consists essentially of turbidites. Consequently, the active settling of terrigenous material precluded high carbonaceous sedimentation, although the depositional environment probably remained the same. The sedimentation during the Tortonian evidences oro-genie activity on Cyprus and probably on South Turkey, inducing pronounced erosion of landmasses. This is supported by the appearance, among clay minerals, of small but noticeable amounts of corrensite (mixed-layer chlorite-smectite), which is known to exist on Cyprus in pyroclastics of the southern flank of the Troo-dos Massif (Desprairics and Lapierre, 1973). At the top of the Tortonian section (Core 4), illite and atta-pulgite increase in the less than 2 ¿im fraction and dolomite in the bulk (values ranging mostly from 10% to 15%, maximum 43%). The dolomite is calcium-rich (CaS5) and a very fine grained (clay size). This suggests some change in the nature of the detrital input. This change could be related to the very beginning of a regional environmental restriction (increase in magnesium), leading later to the deposition of the upper Miocene evaporites (Messinian). Such restricted environments would have been preferentially developed near-shore in shallow-water areas, where dolomite (and at-tapulgite ?) would have grown. This phenomenon is probably related to the beginning of lowering of the sea level. The occurrence of gypsum as corroded monocrystalline flakes (detrital) within the sediments of Core 4, indicates that gypsum was grown at (or even before) that time in areas acting as detrital material sources. This suggests a gradual increase of confinement in near-shore areas during the end of the Torto-nian. supporting the hypothesis of a very progressive onset of the upper Miocene "salinity crisis."

The Messinian evaporite rocks were not sampled for X-ray analysis. Although poorly sampled by spot coring at Site 375, they are well documented in the Upper Member, by seven cores at Site 376 (Cores 16 to 23). They are interpreted as shallow water deposits (Garrison et al., this volume; Kuebn and Hsil. this volume).

The post-evaporite sequence, cored continuously at Site 376 (Figure 7), displays numerous variations in its mineralogy. A very accurate survey of the halite content of the sediment (this halite is obviously "secondary" and results from the evaporation of the interstitial water in the laboratory when the samples are dried) was carried out, in order to attempt to characterize the paleosalinity of the depositional environment. Indeed the salinity of interstitial water does not necessarily reflect the salinity of the water trapped in the newborn sediment. Nevertheless this approach was encouraged by previous results gained from various Plio-Quater-naty sequences deposited in well known "normal marine" conditions.

On the basis of the mineralogical assemblages, the 80-meter-thick section of nannofossil marls (mainly turbidites) overlying the Messinian evaporites, can be divided into two parts. The lower part (Cores 15 to 12) is characterized by a calcite content displaying very important variations (20% to 60%), a very low dolomite content (average 1%), and a clay content strongly dominated by smectite (reaching almost 100% of the less than 2 /¿m fraction). Clinoptilolite, known in pyroclastics on western Cyprus, correlates very well with smectite. This, together with the presence of well-developed turbidites, indicates that intense transportation of terrigenous material from Cyprus occurred frequently. Because of the lack of "secondary" halite (see statement above) in the sediment (less than 0.1%), it is thought that the terrigenous input was deposited in a fresh (or nearly fresh) water body.4 The existence of

4 The presence of a fresh (or near Ircsh) water body in the hasin at that time would prohably have induced the existence of fresh water aquifers in the pre-Mcssinian sediments of the Florence Rise. Curiously, these sediments, as deep as investigated at Site 375 [Burdigahan, 821 m sub bottom), arc completely devoid of"second-ary" halite. This fact may be understood if the prc-Mcssinian sediments (obviously settled in "normal marine" conditions) had been leached by such fresh waters during the upper Messinian.

a fresh or nearly fresh water body in the upper Messinian of Site 376 is supported by the fact that the carbonate fraction consists essentially of reworked nannofossils and foraminifers from Cretaceous to middle Miocene (see Site Report, Chapter 6). The upper part (Cores 11 to 7) is characterized by significant amounts of dolomite (4% to 28%), increasing towards the top of the section, and a noticeable contribution of attapulgite (long and well-developed fibers) and chlorite in the less than 2 ¿im fraction, which, however, remains dominated by smectite. In Core 9, a major peak of smectite abundance correlates with the appearance of clinoptilolite. Calcite decreases towards the top of the section, both in content and in its variations. In this upper part, turbidites are less important than in the lower part (Cores 15 to 12), In the hypothesis that turbidites were triggered by changes in the water level, the decrease of calcite could suggest that the intensity of these changes, decreases significantly, allowing the development near-shore of some magnesium-rich pools where magnesium-rich minerals (dolomite, attapulgite, and chlorite) could have been generated. With regard to the lower part (Cores 15 to 12), "secondary" halite is detectable in the sediments in amounts which, combined with the water content, yield "salinity" values around 10%°. This would indicate that the previous fresh water environment became brackish by "marine" water influxes, possibly responsible for the magnesium paragenesis of this section. These influxes could have preceded the Pliocene transgression.

The Plio-Quaternary section (Cores 6 to 1), although disturbed at the base by slumping, displays a rather uniform mineralogical assemblage, roughly similar to that of present-day sediments. In contrast to the underlying upper Mioccnc sediments, the calcite content is high and ranges between 40% and 60%. The highest values are recorded in the Pliocene (Core 5). The clay mineral assemblage is dominated by smectite (20% in the bulk), which constitutes 80% of the less than 2 iim fraction, a value sensibly higher than in the immediately underlying Messinian sediments. The attapulgite content decreases markedly and the kaolinite content shows the highest values recorded at this site, especially in the Pliocene (Core 5), The prominence of both calcite (autochthonous biogenic production; see Site Report, Chapter 6) and kaolonite in Core 5 could suggest a warm climatic influence during the Pliocene. At the base of the Plio-Quaternary section, the "secondary" halite content (sec statement above) marks a sharp break, and, combined with the water content, yields "salinity" values ranging from 30%o to 35V00 all along the section. This suggests that "normal marine" conditions were re-established at the start of the Pliocene. The high smectite content indicates that ophiolites remain the chief source of terrigenous material, The upper Pleistocene sediments display, in the less than 2 ^m fraction, slightly higher chlorite and irregular mixed-layer contents than the lower Pleistocene sediments. This could be related to some minor change either in location of terrigeneous sources, or in the physiography of the basin due to tectonic activity.


The dominant character of the sedimentation on the Florence Rise is the prominent contribution of minerals from ophiolite terrains (mainly Cyprus and possibly southern Turkey) to the terrigenous detrital input, as demonstrated by the constantly high smectite contcnt of the sediments (smectite is by far the chief constituent in West Cyprus soils and sediments). This is not surprising since Sites 375 and 376 are located veiy close to the island.

  1. Burdigalian-Langhian: settling of high calcareous biogenic production in calm depositional conditions; discrete terrigenous input of clayey material, indicating warm and humid climatic influences; inability, for "coarse" terrigenous material input to reach the site, indicates that the physiography was already similar to that of the present (rise).
  2. Sera val lian: persistence of previous conditions, occasionally disturbed by weak turbidity currents.
  3. Tortonian: increase in turbidity current frequency, possibly related to a tectonic activity; thick sedimentary section deposited in late Tortonian probable beginning of sea level lowering preceding the onset of the upper Miocene "salinity crisis."
  4. Upper Miocene: deposition of Messinian eva-porites in shallow water; infilling of the area by fresh to brackish waters; extensive transportation of terrigenous material by turbidity currents, possibly triggered by changes in the water level. In the last part of this episode discrete influxes of "marine waters inducing magnesium-rich mineral paragensis near-shore probably occurred.
  5. Pliocene: area invaded by seawater; rapid establishment of "normal marine" conditions; possible warm climatic influence favoring the calcareous biogenic productivity.
  6. Quaternary: no drastic change in sedimentation; possible slight variation in the physiography of the basin (deepening ?) in upper Pleistocene.

Site 377

Site 377 is located in a cleft on the Mediterranean Ridge, previously drilled by Leg 13 (Site 126), in 3718 meters of water. The hole penetrated a 263-meter-thick sequence consisting of Quaternary sediments overlying disco n form ably a middle to lower Miocene section. The recovery of the disconformity at 193 meters sub-bottom is a drilling artifact; the change in lithology occurred at 161 meters as is evidenced by a change in drilling characteristics. Four cores were taken, and only six samples were submitted for X-ray analysis (Figure 8).

The Quaternary (lower Pleistocene) sediments (Core 1, Sections 1 and 2, 142 cm) consist of nanno-fossil marls, sapropels, tephras, sand with graded bedding, and slump breccias (implying reworking by bottom currents). Two samples were taken in the finest lithologies. Their mineralogy is characterized by a high calcite content (33% to 52%) and a moderate terrige nous input, dominated by illite (14% to 24%) and quartz + feldspar (12% to 17%). Illite constitutes half of the less than 2 pm fraction, followed by kaolinite, chlorite, and smectite. Because of complex reworking by bottom currents, this minerological assemblage does not necessarily reflect the initial constitution of the sediment (closely-spaced sampling would have been more suited to such variable lithologies). No sample from the base of Core 1, Section 2 (middle Miocene) was submitted for X-ray analysis.

Nannofossil marlstones of Core 2 (middle Miocene [Serravallian ?]) offer a mineralógica! assemblage very similar to that of the Pleistocene sediments, suggesting that they could have acted as sources for the latter. This would indicate an active submarine erosion of the walls of the cleft during the lower Quaternary. Dark siltstones of Core 3 (middle Miocene) are characterized by a conspicuously low carbonate content (calcite 2%). This suggests that they were deposited either by turbidity currents (masking the calcareous, biogenic production) or in an abiotic environment. Combination of these two possibilities is probable since a noticeable amount of pyrite (4%) is present, and cross-lamination and faint grading occur. The terrigenous input is strongly dominated by smectite (45%) and kaolinite (13%). These two minerals are generally mutually exclusive, from a pedogenetic point of view. Therefore the polygenic character of this material can be explained by the influence of different landmasses acting as sources of terrigenous material, or by a polygenic character of the detrital material developing ashore. Core 4 consists exclusively of a drilling breccia of material similar to that of Core 3. Consequently, the mineralógica! data have to be viewed with caution. Nevertheless, with regard to the data of Core 3, they show a considerable increase in coarse particles (quartz, feldspar, illite, and chlorite) at the expense of the fine (smectite and kaolinite). The different ratio between these minerals within the same size class, keep approximately the same values. This would confirm turbidity current sedimentation. The presence of pyrite also confirms the abiotic character of the medium: this allows the interpretation of the calcite as a terrigenous detrital component. Serpentine, present in small amounts, indicates that the terrigenous material probably originates from ophiolites of the external Aegean Arc. In this hypothesis, smectite would proceed from the same origin. Again these conclusions are of a highly speculative character because of very poor sampling frequency.

Site 378

Site 378 is located on the northern flank of a small depression in the North Cretan Basin of Aegean Sea, in 1835 meters of water. Two holes (378 and 378A) were drilled at the same location. The 343.5-meter-thick section penetrated consists of nannofossil marls and oozes of Quaternary age (Core 1, Hole 378A; Cores 1 to 5, Hole 378), nannofossil marlstones with sapropel layers of Pliocene age (Cores 6 to 11, Hole 378; Core 3, Hole 378A), and massive selenitic gyp

Was this article helpful?

0 0

Post a comment