Abstract
In the present paper, we examine the influence of micrite types, autochthonous or allochthonous, on the dolomitization processes. The recrystallized and dolomitized Carnian samples from Rifugio Vallandro and Alpe di Specie erratic boulders (South Tyrol, Italy) offer a unique example for studying the relationship between microbialites and dolomitization processes. The comparison between the carbonates of the well-preserved erratic boulders of Alpe di Specie and the isochronous, recrystallized, and dolomitized, samples of Rifugio Vallandro, allows for hypothesizing the role of microbialites on dolomitization processes. The Rifugio Vallandro samples represent variously dolomitized boundstone (made of corals, sponges, and peloidal crusts) with a fine texture (aphanodolomite) which contain organic matter relics, suggesting microbial-mediated mineralization. Geomicrobiological characterization of the microbialites from Alpe di Specie indicates that they formed through microbial metabolic activity of sulfate-reducing bacteria, which thrive on organic matter accumulated in the suboxic to anoxic interspaces of the skeletal framework. Similar processes can be hypothesized for the microbialite precursor of Rifugio Vallandro. Extracellular polymeric substance (EPS) and other organic compounds trapped inside the fine crystal matrix can have a role in the dolomitization processes of the microbialites. High pH and high alkalinity, derived from the degradation of organic matter, may be critical in promoting the dolomitization of microbialites because the high pH increases the concentration and activity of the dissolved CO32−, thereby increasing the dolomite supersaturation and reaction rates. This process produces very fine dolomite (aphanodolomite) that replaces the original organic-rich micrite, while the fine crystalline dolomite forming larger euhedral crystals seems to derive from the allochthonous micrite due to the presence of a large amount of siliciclastics and the absence of organic remains.
Highlights
Dolomite forms in many different surface and burial environments
The Carnian “erratic boulders” of Alpe di Specie, see Figure 5, show an excellent preservation state, including the original mineralogy of skeletal tissues and represent a very important source of knowledge about Upper Triassic reef-building organisms [36]. This is due to the embedding of patch reefs in fine basinal sediments of the San Cassiano Fm., which prevented the dolomitizing fluids from altering the primary mineralogical and biochemical characteristics [36]. The presence in this area of both dolomitized patch reefs (Rifugio Vallandro section), see Figure 4, and isochronous, preserved, carbonate bodies (Alpe di Specie “erratic boulders”), see Figure 5, allowed for a comparison to be made between the two carbonates and the advancement of a hypothesis on the control of the dolomitization processes according to the type of carbonate precursor, see Figure 8
Vallandro section allows the hypothesizing of the role of microbialites on dolomitization processes
Summary
Dolomite forms in many different surface and burial environments. It is a highly variable mineral whose molecular structure, stoichiometry and trace element composition indicate different conditions of formation. These variables do preclude seeking a common dolomitization model. Dolomite can mineralize in seawater, mixed or burial environments and the fact that dolomite is associated with normal seawater, brine, or meteoric water (all being possible) is not absolute proof of cause and effect. Based on the formation mode, dolomite is divided into two groups: primary and secondary [1]. The primary dolomite precipitates directly from an aqueous solution through microbial activities, the secondary dolomite is a diagenetic product formed from a carbonate precursor (calcite or aragonite) through a dolomitization process
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