Dendritic, laminated and spherulitic growth fabrics in a Pleistocene hydrothermal travertine succession (Italy)

Dendritic, laminated and spherulitic carbonate fabric types have received attention because of their morphology and their significance in facies interpretation and sedimentology, as well as their relevance to accumulations of... [ view full abstract ]

Dendritic, laminated and spherulitic carbonate fabric types have received attention because of their morphology and their significance in facies interpretation and sedimentology, as well as their relevance to accumulations of hydrocarbons in continental carbonate reservoirs. With the aim to improve our understanding of mechanisms and conditions that lead to the precipitation of these fabrics, a multi-scale study of a Pleistocene hydrothermal travertine succession located in Italy is here presented.
This sequence (10-m-thick and 30-m-wide) is composed of a limited variety of superimposed, distinctive fabric growth, dominantly calcite in composition. They include: 1) laminated boundstone, 2) spherulite-dominated boundstone/grainstone, and 3) dendritic boundstone.
Laminated boundstone consists of sub-spherical to lens-shaped elongated cavities (ranging from a few microns up to 4 cm wide) trapped between dense laminae (ranging from wavy to irregular and often convolute, less than 1 mm up to 2 cm wide) and/or thin (less than 1 to 5 mm) brittle undulated sheets. The porous laminae vary mainly from sub-horizontal to wavy laminated.
Spherulite-dominated boundstone/grainstone consists of white spherical structures (200 Xm to 15 mm in diameter) interbedded with thin brittle undulated laminae and clay-rich matrix. The spherical structures are characterized by fan shaped dendritic- or radial internal growth pattern. Ostracods can occur.
Dendritic boundstone consists of upward expanding arborescent structures (few microns to 10 cm), surrounded by cream to light grey coloured cement. Cream to light grey coloured gently undulated laminae, up to 1 cm-thick, delimit the dendritic structures at the bottom and on top and constitute a peculiar component of the dendritic boundstone fabric types. In addition, incipient dendritic-like morphologies rise from the spherulite surfaces.
The alternation of the above described fabric types forms a sub-horizontal, plurimetric scale travertine unit.
Microscopic analyses reveal that clotted peloidal to dense micrite and turbid microspar/spar calcite crystals represent the main petrographic components. Cementation, neomorphism and dissolution have been identified as the main processes that produced modifications on the fabrics and the associated porosity. SEM-EDS analyses show that biogenic features (e.g., EPS) and nano-globules (ranging from 5 to 150 nm) are generally pervasive and well preserved.
Based on these observations, the individuated fabric types reflect the precipitation processes due to interplay between abiotic and biotic and/or a combination of both processes, which are subsequently modified by diagenesis. The studied succession constitutes a useful analogue of the South Atlantic early Cretaceous lacustrine carbonate deposits, in terms of cyclical sedimentation and carbonate fabrics.