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Fossilized microbial and algal textures in post eocene lacustrine zeolite tuff sequence, G. Abu Treifiya, Cairo-Suez road, Egypt.

Introduction

An exposed outcrop of volcaniclastic (epiclastic) deposits is observed and described at the eastern foothills of G. Abu Treifiya, in the downstream of W. Kiheiliya (Figs. 1 &2). This section was studied in details from a sedimentological point of view by Abdel-Motelib et al., 2010 (in press), revealing that the sequence was deposited in a deltaic setting reflecting fluvial to lacustrine facies deposits. Generally the area was differentiated into bouldery (Fan-Proximal facies) and conglomerate, sandstone to siltstone deposits intercalated with mudstone layers (Distal fluvial to lacustrine facies) exhibiting synsedimentary depositional structures.

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Petrographically the section was discriminated between accretionary lapilli tuffs, fine hyaloclastite tuffs and mudstones cycles. These rocks show signs of post depositional alteration processes of these water-laid deposits. These processes mainly reflect interaction of the basaltic glass clasts and glass rich matrix with the percolating water, including palagonitization, formation of authigenic clays, zeolites and calcite (Kabesh et al., in press). The proposed environments of deposition of these deposits and their lateral and vertical variations from fluvial--lacustrine to fluvial in cycles culminating by the development of anoxic paleosols, reflecting humid to subhumid paleoclimate are elaborated in Fig.3. These environments strongly suggested the presence of biological life, and thus urged seek for evidences and traces of organic matter. A lump of biological indicators as algae, framboids, disseminated pyrites, rhizolithes, rhizocretions, calcimicrobes, and filamentous bodies argue for deposition in reducing environment and extensive anoxia lead to biodegradation of organic matter.

So it raised the interest for a detailed investigation of these rocks for different biological activity, describing the traces of fossilized organic matter and clearing up their role in the intensive bioalteration and biodegradation of these tuffaceous deposits.

Recognized Microbial Alteration Textures

Detailed microscopic examination of the volcaniclastic (epiclastic) deposits was carried out, discriminating them into accretionary lapilli tuff and finer grained hyaloclastite tuffs. The first exhibit balls (tephra balls) composed of a mixture of glassy porphyritic basalt fragments, crystal fragments of olivine, pyroxenes and zeolitized plagioclase admixed with organic matter, set in a groundmass of finer calcareous muddy material with crystal fragments, clays, analcime, calcite and organic matter. The finer tuffs are formed of sand to silt- sized fragments of glassy porphyritic basalt, dispersed crystals of olivine, pyroxene and zeolitized plagioclase, surrounded by a fine matrix highly enriched with organic matter, as algae, calcimicrobes, microbial peloids and framboids (Fig. 4A). The matrix also shows analcime and calcite, mostly as later filling and cementing material. The recognized organic matter show different degrees of biodegradations and bioalterations and also some types of preservation and fossilization.

Volcanic glass is an unstable substance and is commonly readily corroded by chemical processes and attacked by microbes and bacteria itching, pitting and causing dissolution along microchannels where glass comes in contact with percolating water from the medium of deposition. The granular and tubular bioalteration textures are fossilized and preserved by precipitation and encrustation of alteration products as authigenic clay minerals and iron oxides (Thorseth et al., 2003).

The described, processes of bioalterations in basaltic glass is mainly part of the process of palagonitization. This is also accompanied by abiotic alteration and corrosion through hydration, because basaltic glass is chemically unstable and readily reacts with percolating water in the medium forming palagonite. Slight alteration form yellow--brown gel palagonite, which is in turn a metastable phase and progressively change to colored- anisotropic fibropalagonite (Stronick & Schmincke, 2002) which is a mixture of palagonite with clays (montmorillonite), zeolites and iron and manganese oxyhydroxides (Fig. 4A). The described tuffs show extensive alteration of vitric clasts leaving almost no traces of the original glass.

The groundmass surrounding the glass clasts and crystal fragments seem to be flushed with organic matter with different sizes and structures showing different degrees of biodegradation and alteration to amorphous organic matter (AOM). Well to moderately preserved honey yellow amorphous organic matter (AOM) particles consist of dull amorphous matrix with well defined margins or diffused edges with less distinct margins (Fig.4 A, E). These (AOM) particles commonly contain fine inclusions that may be unidentifiable granular structureless fragments (not phytoclasts), other inclusions are palynomorphs, pyrite and phytoclasts. These inclusions may exhibit uniform distribution or heterogeneous clotted appearance. These partially degraded particles are preserved in reducing environment of anoxic lacustrine setting.

The fine tuff matrix exhibit small rounded micritic grains formed by fine carbonate precipitated around microbes. These calcimicrobes are characterized by absence of well defined skeletal features (Chafetz et al., 1991; Scholle& Scholle, 2003). The observed peloidal- microbial shrubs grow with a branching form (Fig.3B) mainly are calcimicrobes (cyanobacterial organisms). These shrubs are encased and thus preserved by cementation with spary calcite (Fig.4B). Lumpy form of micritic peloidal accumulations of calcimicrobes are also commonly observed encrusting other rounded or elliptical framework organisms (Fig.4B, E, G). Also oncoids with tubular structure as fine micrite precipitated on filaments (calcified filaments) are seen (Fig. 4B).

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These calcimicrobes- cyanobacteria are found in hypersaline to fresh water environments as they have wide salinity and temperature tolerance.

Forms of green algae mainly calcified are recognized as skeletal calcareous algae. They commonly exhibit segments of (dasycladacead algae, according to the classification of Wray, 1977) with honeycomb structure and walls commonly of fine grained fibrous calcite (Fig. 4 B). The outer margins may show colloform bands of fine micritic zones with variation in color due to different grain size and iron content (Fig. 4B, G). Their branching habit is observed as transverse and longitudinal sections in thin sections (Scholle & Scholle 2003). The transverse sections show circular or elliptical hollow grains, or the centers may be infilled with micrite sediments (Fig. 4B). The margins of the walls of these hollow grains commonly exhibit radially arranged fine fibrous micrite, these reflect original fine holes (wall perforations) occupied by filaments and now filled by the micrite material. Longitudinal sections are seen as tubular forms, these are preserved either by filling with calcite or micritic material, or by formation of fine micrite envelope deposited on it by cyanobacterial organisms (Fig. 4B). So the cuticle is slightly preserved while the internal tissues are mostly completely degraded during diagenesis. Some algae masses may completely or partly enclose crystal or rock fragments (Fig. 4C) as they grow on them taking them as substrate. Generally the original aragonitic composition of these algae, result in their poor preservation.

Structured phytoclasts seen as small dark microstructural equant grains and lath-shaped tracheids whose external form with sharp distinct edges indicate a structural particle but with no visible internal biostructure (Fig. 4D) are found in the matrix. Biostructured phytoclasts showing internal structure are also detected and described as brown phytoclasts derived from tracheid tissues (Fig. 4F), exhibiting vertical ribbing and lack visible pitting on surface.

These green algae are most common in lacustrine setting and alkaline/ calcium rich lakes, in rocks deposited in brackish and more saline environment (Racki, 1982).

Calcispheres are problematic grains mostly attributed to algal sources (Scholle & Scholle 2003). They have varied sizes and moderately thick walls (Fig. 4D, E). They may be small spherical calcareous bodies, bricklike calcite crystals or form large volume of carbonate mud (Fig.4D,E & G). The walls of calcispheres may be uniform smooth surface of dark microgranular micrite, which is common in ancient calcispheres, or radial crystals (Fig. 4D, E & F). Other forms of problematic organisms (termed Problematica) which are unassigned to specific groups are observed in the matrix. These exhibit spherical or elliptical grains with dark central canal surrounded by a wall of radially oriented calcite (Fig. 4B & D).

Biocorrosion by microbes (cyanobacteria), fungi and algae attacking feldspar crystals produce microborings, tunnels and cavities in crystals found in the matrix (Fig. 4E). These tunnels in feldspars are due to the exuding organic acids as oxalic or citric acids that can effectively dissolve feldspar crystals, as described by Smits (2006).

Peloids are also very commonly dispersed in the rock. They are small micritic grains of no particular origin; they could be microbial, algal, fungal or even inorganic precipitation of micrite (Fig. 4B, D & G).

Small pyrite crystals are found as dispersed grains or aggregated clusters in the matrix, some may be as framboidal pyrite clusters by the action of bacteria in reducing environment probably deposited in the lacustrine facies conditions. Later during periods of fluvial, more oxidizing conditions, they were oxidized to goethite-hematite pseudomorphs (Fig. 4G).

The recognized subaerial paleosols textures are recorded along the discrete erosional surfaces which indicate general subsidence and eustatic lake level changes due to rifting mechanism associating the basaltic eruptions of the post Eocene-Oligocene. These textures include: Branched rootlet structure, rhizofilaments, oxidized iron rhizocretions, clayey curtains and encrustations, mud-crack, fresh water mosaic calcite fillings, mud balls and breccia and iron sesquoxides (Fig. 4H).

During alteration, a process of transformation or neomorphism of carbonate is recognized through recrystallization of fine micrite to microspare to coarser bladed pseudospar crystals (Fig. 4B). This recrystallization is probably due to the percolation of meteoric water.

Conclusion

A tentative model is proposed for the deposition of the volcaniclastic sequences in the area of G. Abu Treifiya by Abdel- Motelib et al., 2010 (Fig.5). The role of microorganisms was found to greatly influence the process of studied basaltic glass alteration and palagonitization. Similar alteration processes were discussed by Thorseth et al., 1995a, b; Thorseth et al., 2003; Staudigel et al., 1995, 1998; Fisk et al., 1998 and Stroncik et al., 2002. Basically bacteria and microorganisms create a local microenvironment and pH change conditions causing congruent glass microdisolution. Extensive biotic and abiotic alteration of basaltic glass resulting from interaction with percolating water which result in the formation of authigenic clays (montmorillonite), zeolites and iron and manganese oxyhydroxides.

Green algae are most common in lacustrine setting and alkaline/calcium rich lakes, in rocks deposited in brackish and more saline environment. The biodegradation of organic matter, algae, calcimicrobes, framboids, disseminated pyrites, rhizolithes, rhizocretions, and filamentous bodies argue for deposition in reducing environment and extensive anoxia. Calcite recrystallization is due to the percolation of meteoric water and subareal processes.

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ALI ABDEL-MOTELIB

aliabdelmotelib@cu.edu.eg

Cairo University, Giza

MONA ML. KABESH

monakabesh@yahoo.com

Cairo University, Giza
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Author:Abdel-Motelib, Ali; Kabesh, Mona M.L.
Publication:Geopolitics, History, and International Relations
Article Type:Report
Date:Jan 1, 2014
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