מאספי חרסיות ייחודיים במפרץ חיפה: ייצוג לחילופי ים-יבשה ולסביבות מוגבלות במהלך כמיליון השנים האחרונות

A subsurface sequence in Haifa Bay was studied for its clay mineralogy, as explored by X-ray diffraction (XRD), in order to obtain a long record for a Mediterranean Quaternary sea-land transition zone, apparently longer than ever achieved before. The study was conducted on continuous cores of three boreholes of 98.5, 119.2, and 121.2 m depths whose chronology, sedimentology and paleoenvironments were previously established. The sequence represents ~1 m.y. of alternating very shallow (<40 m) and shallow (40 – 80 m) water depths with wetland and terrestrial sediments, including soils. Another aim of the current study was to evaluate the constancy of clay sources in the central Levant shallow shelf throughout this time interval and to examine possible relations to environmental changes. Marine samples, regardless of water depth, are characterized by low amounts of kaolinite + illite (<25%) and variable mixed-layered illite-smectite (IS) compositions, in contrast to paleosols, where the sum of kaolinite + illite is generally between 30 and 60% and the IS composition is more illitic. The coastal paleosols could have developed after sea-level falls during glacial periods and relatively dry climates. Nevertheless, the sandy permeable parent material enabled efficient leaching and major pedogenic clay transformations. Clay mineralogy suggests that maximum soil evolution apparently occurred when glacial marine isotope stages were more humid (MIS 6 and MIS 26). Rapid environmental changes preserved previous clay assemblages, as indicated by wetland sediments that display pedogenic characteristics and coastal dunes that display marine characteristics. Unexpectedly, the closely-spaced boreholes yielded two unique marine clay provinces, not known before elsewhere in the Levant basin north of the Nile delta. One province includes two boreholes on both sides of the Carmel fault where marine sediments are enriched by up to 90% palygorskite. The formation of palygorskite is related to the contribution of fresh waters issuing along the fault. The second province is recorded in somewhat deeper waters where clean sand bears dark peloids, apparently fecal pellets, and the clay fraction is dominated by discrete smectite or smectitic IS. The presence of two exotic minerals is sporadically present in trace amounts: clinoptilolite, derived from the nearby northeastern hinterland; and serpentine, derived from the remote Iskenderun Bay. Both unique clay provinces prevail throughout the sequence, indicating stable environmental conditions in south Haifa Bay for the last ~1 m.y. 


תאריך 31/01/2016 10:30 12:00
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