The formation of stromatactis-type fenestral structures during the sedimentation of experimental slurries - a possible clue to a 120-year-old puzzle about stromatactis


Authors: Hladil J

Published in: Bulletin of Geosciences, volume 80, issue 3; pages: 193 - 211; Received 2 February 2005; Accepted in revised form 21 April 2005;

Keywords: carbonate sedimentology, stromatactis, fenestral structures, mud mounds, polydisperse suspensions, sedimentary experiments,

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Supplementary material

Video 3 (1607 kB)

A part of the record from the late second stage (focused to vertical escape of water with small particles v. fall of bulky grains). This flow pattern develops above the middle stromatactis-rich part, and is effective during but also after the formation of

Video 1 (7608 kB)

A part of high-speed record that exemplifies the earliest stage of sedimentation (with varying and still very unstable sub-spherical and rhombic domains)

Video 2 (2142 kB)

A part of the record from the second stage (focused to formation of several small stromatactis cavities)



A critical analysis of stromatactis-type fenestrae, including the unaltered composition of the host carbonate sediments and other circumstances, led to the formulation of the hypothesis that swarms of stromatactis might originate during the process of rapid, uninterrupted sedimentation. The rationale behind this possibility concerns several points. First, the stromatactis sediment has the characteristics of particulate materials that are typically polydisperse (with polymodal frequency distributions). Also the shapes of the particles are extremely diversified and complex - the grains are often subangular, platelet-like or acicular, porous, soft, or irregularly indented. This seems to be valid not only for relatively coarse banks with stromatactis (with visible amounts of sand- and even fine gravel-sized grains), but surprisingly also for the finest available varieties, which consist rather of calcisiltites than of muds and also provide a large variety of shapes and internal fabrics (alterations), even in the finest fractions. These sedimentary materials have unusual mechanical properties, of which high coefficients of internal friction (in a dry state) and presumably high dynamic viscosities in contrast to low mass density (in dense aqueous suspensions) are the most important. The latter attributes (in suspension) can be enhanced by the occurrence of filamentous organic muds, where living bacteria significantly contribute to the production of microbubbles. The swarms of stromatactis fenestrae are often developed in the lower/middle parts of relatively homogeneous or mottled beds, being underlain by a coarser, graded base. The finest, uppermost parts of these beds are almost devoid of stromatactis. There are also other constraints, for example, that stromatactis formation can be traced from just below the fair-weather wave base to middle slope environments, and stratigraphically reach the deep Proterozoic sequences. This is a very general occurrence. Sedimentation experiments with artificially prepared slurries of comparable complexity have resulted in the production of structures that are nearly identical to stromatactis, including the details of typical stromatactis formations and the changes in the surrounding sediment. These consistently repeatable experiments show clear cause-and-effect relationships between the processes and resulting fabrics.