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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 4. (April)

First Page: 502

Last Page: 502

Title: Carbon Isotope Composition of an Upper Triassic Evaporite Section in Israel: Evidence for Meteoric Water Influx: ABSTRACT

Author(s): Mordeckai Magaritz, Yehezkeel Druckman


Upper Triassic (Upper Carnian) extensive evaporites occur in shallow-water basins that are surrounded by supratidal dolomites. The evaporites consist of laminated and nodular anhydrites (gypsum in outcrop) interlayered with dolomitic mudstones and, rarely, with algal or oolitic grainstones. The evaporitic section reaches a thickness of about 180 m (590 ft), whereas the surrounding dolomitic facies amount to 80-100 m (260-330 ft).

Systematic ^dgr13C profiles of the carbonates of the entire Triassic section in an outcrop and two boreholes revealed an extreme 13C depletion in the evaporitic section (the Mohilla Formation) in both the basins and highs, relative to the lower parts of the Triassic section in all three investigated sections. The ^dgr13C values range from -2^pmil to -14^pmil in the Mohilla sections, whereas in the lower parts of the section the ^dgr13C values range from +1^pmil to -5^pmil.

The systematic repetition of ^dgr13C-depleted rocks in different basinal sections as well as from dolomitic highs, rules out the possibility of post-depositional diagenetic changes in the ^dgr13C composition.

Alternatively, one may assume changes in the ^dgr13C composition of the Upper Carnian Tethys ocean. However, such low values are not plausible to reflect the oceanic ^dgr13C composition. It is therefore proposed to relate the ^dgr13C values to an influx of fresh continental water floating upon the dense evaporated brine. Such a periodic influx of continental water is also compatible with the repetitive alternation of evaporites and carbonates within the Mohilla Formation.

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