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Vol. 8, No. 1 (Feb., 1993), pp. 101-110
Published by: SEPM Society for Sedimentary Geology
Stable URL: http://www.jstor.org/stable/3515224
Page Count: 10
You can always find the topics here!Topics: Stromatolites, Algae, Cell walls, Cements, Sediments, Coral reefs, Dolomite, Geology, Carbonates, B lymphocytes
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In the Upper Permian reef complex of southern Tunisia and in Triassic (Ladinian, lower Carnian) carbonate platforms and patch reefs of the Dolomites (Italy) cushion- and pillar-shaped finely laminated structures occur which in growth form and structure strongly resemble cyanobacterial stromatolites. They consist of regular alternations of micritic (or cement) and microsparitic layers. Micritic and cement layers are 0.09 to 0.9 mm thick, with the thickness of the microsparitic layers varying from 0.05 to 5 mm. The latter show a patchily preserved structure of parallel-arranged polygonal cells which are 0.04 to 0.12 mm wide. The majority of these layers consists of one cell-layer only, but some are composed of several cell-layers. The faintly preserved calcitized cell walls are obviously recrystallized. The cell structure is similar to that of solenoporacean algae which occur in the same environment as the stromatolites. In contrast to the generally assumed Mg-calcitic composition of the Solenoporaceae, the cell walls of crustose solenoporaceans from the Carnian of the Dolomites are still aragonite. Judging from their poorly preserved wall structure, the examined Permian and Triassic stromatolitic Solenoporaceae were also probably originally aragonitic. It appears that prior to, or during an early stage of calcification, these solenoporacean algae were capable of trapping fine sediment. The muddy veneers were periodically resettled by the algae. The regular lamination and the periodical influx of muddy sediment probably reflect annual fluctuations of precipitation in nearby land areas. Growth form and lamination of the solenoporacean stromatolites resemble the Problematicum Collenella Johnson (1942) which is a common frame-building organism in the Permian reef complex of Texas/New Mexico (USA). Due to its poorly preserved internal structure, the systematic position of Collenella has hitherto remained enigmatic.
PALAIOS © 1993 SEPM Society for Sedimentary Geology