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Quaternary tufa profiles, Jabal Farasan--Wadi Qedid area, southern Saudi Arabia.

1. Introduction

Tufas are fresh water carbonate deposits formed at ambient temperatures and pressure, they are commonly porous and contain calcified micromacrophytes, invertebrates, algae and bacteria (Ford& Pedley 1996, Pentecost 1995). Tufas provide excellent records of terrestrial paleoenvironments and paleoclimatic conditions (Andrews et al. 2003, Drysdale & Head 1998, Pedley et al. 2000). Paleoclimatic implications of tufa were studied petrologically, hydrologically and sedimentologically (Chafetz & Guidri 1999, Riding 2002, Pedley 2009). Environmental models define the characteristic tufa deposits in rivers, lakes and springs (Pedley 1990, Ford& Pedley 1996).

In arid/semi-arid regions phases of increased wetness accompanied with increased vegetation and organic activity promoted the formation of calcaroues tufa deposits (Candy & Black 2009, Semeniuk & Searl 1985, Andrews & al. 1998). Tufa deposits in northern Arabia (Petraglia et al. 2011, Garrard et al. 1981) and sabkhas (Engel et al., 2011) preserve abundant rhizoliths and indicate that fresh water sources were available evidencing wet climate and humid phases. Paleoclimatic and paleoenvironmental investigations in south-southeastern Arabia indicated humid phase in early--mid Holocene (Davies, 2000; Radies et al., 2005; Bray & Stokes, 2003).

Dating the deposition of wet sediments as carbonate tufa representing humid phase deposits, suggested Late Quaternary environmental changes and humid-arid transition in southeastern Arabia (Glennie & Singhvi, 2002). Bray & Stokes 2004, suggested that humid climate conditions dominated 35-25 Ka & 10-6 Ka, while arid conditions predominated from 25-10 Ka and since 6 Ka. The paleoenvironmental and paleoclimatic changes and humid-arid transitions at Wadi Atalla tufa deposits in South Eastern Desert of Egypt (Hamdan et al. 2001) at Middle- Late Pleistocene are partly correlated with the humid-arid transition in southeastern Arabia. Manifesting the prevalence of humid climate conditions at 35-25 Ka, then dominating arid conditions form 25-10 Ka.

Correlatable tufa deposits were studied and described in North Africa, at the edge of Libyan Plateau (Luo et al. 1997, Cremaschi et al. 2010), Western and Eastern Deserts of Egypt (Crombie et al. 1997, Nicoll et al. 1999, Smith et al. 2004, Smith et al. 2004, Hamdan et al. 1999, Hamdan 2000, Hamdan et al. 2001).

Jabal Farasan area represents part of a large Precambrian succession of metavolcanics, metasediments, talc-carbonates and marbles topped by Quaternary basalt flows. The area form a major syncline with minor secondary folds and dissected by major NW trending normal faults of the Red Sea system accompanied by sets of shear and open extension fractures. The Jabal Farasan marble deposits were mineralogically and geochemically investigated and classified by Qadhi 2008. The marble deposits as well as the leaching of weathered and altered mafic and ultramafic rocks express the main sources of Ca for tufa deposits in the area. Five tufa types were recorded in Farasan area. These are fluvial--barrage, cascade, perched pools, patch crust and detrital types (Fig. 1).

2. Geological and Depositional Setting of Tufa

Tufa deposits are observed in Jabal Farasan area, about 120 km northeast Jeddah (Fig.1). The carbonate deposits show patchy distribution, which seems to be controlled by structural features. The faults acted as walls of waterfalls and formed pools in depressions. Also the intersection of fractures offered sites favorable for tufa deposition. The carbonate deposits in Jabal Farasan-Wadi Qedid area exhibit mainly five morphological tufa types related to different depositional environments. The present tufa deposits exhibit small exposures localized at sites favorable for the formation of the described types. They are mainly controlled by the main wadi course. These carbonate deposits exhibit remnants of oxbow lakes at meanders as they are dissected by second order streams and structural features as faults. In general along the wadi course, three tufa types are distinguished as cascade type with associated dam, fluvial crusts and perched pools and waterhole geomorphic environments (Fig. 2a). These are mostly controlled by enechelon faults and channels developed in later stages as extensional regime faults related to Quaternary basalt flows and recent dykes.

The present fluvial and cascade types tufa as well as the fluvial crusts represent fossil tufa probably formed in the Quaternary time, but nevertheless the cascade and fluvial crusts could be still active in wet seasons during rainfalls. On the other hand pool and waterhole type is an active one forming in the present days (Fig. 2a). Tufa deposits will be studied in details concerning their internal primary features, tufa units and facies, microfabrics, diagenesis and interrupting diastem periods.

2.1. Fluvial- Barrage Tufa Type

Fluvial tufa represents barrages and mounds of mosses and stromatolites along the main course of the wadies. Barrage tufa deposits develop under adequate water supply conditions, and the flow of water must be high enough and rapid. The tufa deposits were later subjected to period of entrenchment of the river system causing destruction of parts of the tufa sections. Fossil tufas are preserved mostly at bank sections downstream the main wadi course as a set of tufa edifices distributed along the entrenched meanders formed in the water course. They mainly exhibit a banded external structure, and well preserved microphytic facies mostly as thrombolytic, stromatolitic, spongy-larval and compact facies (Fig. 2b) formed by calcification of biofilms. Lamination, rhizolithes and stromatolites are the most distinctive microstructure (Fig. 2c) indicating subaqueous situations. Local interruptions of deposition in tufa in the catchment of streams are manifested by channel development by lithoclastic facies formed of rounded and elongated clasts of tufa and non-tufa composition (Fig. 2b). These heterogeneous rudites and breccias clasts are deposited and cemented together by tufa, and finer carbonate silt and mud. Phytoclastic facies consist of calcified plant remains that leave voids of different sizes at sites of decayed organic material.

2.2. Cascade Tufa Type

This tufa deposits are exposed at the upstreams of wadies, and occur as hanging patches on the scarps of Precambrian metavolcanic bedrocks, behind faulted blocks, accumulating in open fractures, cone shape and pendant tufa on the peripheries of slopes (Fig. 2a). Cascade tufa manifest well defined stromatolitic lamination as evidence of periodic changes of physical factors, crystallization order and biological activity, due to different amount of organic material specially bacteria and cyanobacteria. The tufa exhibits sub-horizontal to sub-vertical, sometimes high angle laminations (Fig. 2d). Crumpled laminae are common, indicating down-slope creep before complete calcification. Stalactitic forms are recorded within tufa deposits in open vertical fractures formed by dripping of infiltrating carbonate water. The laminae orientation is related to the underlying substrate hill slopes and is steeply inclined at the front of the deposit (Kano& Fujii 2000, Pedley 1990). Fragments of detrital material as silty and clayey material including elongated pebbles and breccias of the surrounding basement and young basalt are incorporated in channels troughs which form small unconformity surfaces or diastems within the tufa.

2.3. Pools and Waterholes Tufa

These types are represented by small perched lakes or pools developed along the recent channels at the down thrown of the faults. Some of these pools are seasonal and other are permanent (Fig. 2a). Cyclic deposition is usually recorded in the permanent pools. These form bank tufa deposits by precipitation of carbonate on organic substrates as mosses, algae and cyanobacteria (Fig. 2e). They exhibit stromatolitic, microphytic thrombolytic facies with sedimentary fills, oncoids, oncolites, pesoids, lithoclasts and intraclasts. This recent tufa deposit expressing active precipitation of micrite on bacteria and algal filaments, mosses and roots indicate the influence and role of organisms on tufa formation. These standing water environments reflect low hydrodynamic energy zones, and CaC[O.sub.3] supersaturated water. Floated green algal crusts act as organic substrate for active precipitation of micrite and humus materials on the surface water of the pools (Fig. 2e). It can thus be manifested that the recent tufa show similar characteristics of the internal features and fabrics as the fossil tufa except for the absence of diagenetic features as the formation of sparry crusts resulting from cementation and recrystallization as well as large calcite formed by vadose and phreatic diagenesis.

2.4. Fluvial Crusts

They are represented by smooth sheet--like superficial carbonate crusts deposited mostly on inorganic substrates as the Precambrian bedrocks or the younger Quaternary basaltic flows. The carbonate crusts are usually of 1 mm up to about 3 cm in thickness. The laminated deposits are formed of micrite encrusted on filaments and small rounded colonies of algae and cyanobacteria showing laminated and nodular fabrics (Fig. 2f). Here the rocks serve as growth surfaces for the microorganisms. These features are common on bedrocks of river courses in arid regions with no permanent water supply.

2.5. Detrital Tufa

This type usually recorded downstream of the catchment area of the wadies as cement materials of the different reworked fragments of older tufa sediments and wadi fill deposits. This type has uncertain age which represent more than one cycle of rainfall and flooding regime.

3. Tufa Microfabrics

The different fabrics of tufa are studied microscopically to induce the main sedimentologic features and diagenesis. Stromatolites, microstromatolites, laminations, associated with calcified clumps, sedimentary fill and organic material (biofabrics) are the main microfabrics observed.

The laminated tufa is mainly formed of alternating layers composed of micrite (lime mud) precipitated in and on bacteria, colonized cyanobacteria and algal filaments as microphitic facies. It also contains pisoids, intraclasts, oncoids, and porous micrite.

Micrite. forms the opaque regions of alternating layers most commonly as clotted as peloidal fabrics. Peloids are subspherical structures (Riding 2000) formed by adhesion of crypto-microcrystalline carbonate on microorganisms (Fig. 3a, c). Micrite form aggregates on colonial bacteria and cyanobacteria filaments (Das & Mohanti 1997, Janssen et al. 1999) giving dendritic form or shrubs. Algae may be preserved as micritic threads with anastomosing patterns (Fig. 3a).

Unattached biotic and abiotic forms of coated grains are commonly observed.

Pisoids: are common, with a nucleus of subrounded lithoclast and a cortex sometimes asymmetric indicating variations in growth conditions (Fig. 3b, c).

Oncolites: are seen as separate unattached elliptical and ovoid grains showing concentric laminae with microbial remains around a nucleus commonly of micrite aggregates (Fig. 3b, d). Outwards the cortices consist of alternating dark micrite and lighter microspar laminae (Arenas et al. 2007).

Stromatolite: these are laminated organosedimentary structures produced by precipitation of micrite in and on microorganisms as bacteria and cyanobacteria and algae (Riding 1991). It is a prominent fabric formed as laminations that coalesce to form composite forms. The alternating laminae are manifested by dark micrite with microbial remains (microbialites) and light microspar laminae (Arenas et al. 2003). Laminated tufa deposits suggest seasonal depositional variations in cyanobacterial growth phases (Andrews and Brasier, 2005, Braiser et al. 2010).The laminations form wavy to festooned and multi-convex forms (Fig. 3c, d). Bryophytes, algae and colonial cyanobacteria dominate in the cascade type tufa, as the steep slopes of the substrate prevent colonization of larger plants (Arenas et al.2000, Kano & Fujii 2000).

Mudstone intraclasts: are observed with irregular shape consisting of silt size carbonate and mud in micrite matrix (Fig. 3b).

Larval voids are porous spongy fabrics showing mesoscopic and microscopic open spaces- fenestrae- with different size, shape and origin. These fenestrae may be due to decay of non-calcified algal filaments or organic matter (Fig. 3a, b & e). On the other hand large spaces may result from burrowing of organisms or dissolution of calcite (Fig. 3 b, e & f). They are commonly formed during or slightly after deposition. Voids are sometimes filled with secondary calcite cement (Fig. 3 e) or detrital material of lime-mud and terrigenous clayey materials (Fig. 3 f) that reduces porosity.

4. Diagenetic Features

Tufa is commonly porous when deposited, but often subsequent diagenesis reduces porosity through many processes. Tufa deposits are formed in arid to semi-arid surface environment at ambient temperature and pressure are subjected to meteoric diagenesis. They are affected by percolating rain water that results in precipitation or carbonate- cementation, recrystallization and dissolution.

Diagenetic processes starts with the crystallization of low- Mg sparry calcite (microspar) as radial aggregates normal to the depositing surface of micritic microbial filaments (Fig. 3 c, d). The bladed radiating spar grows filling voids forming meniscous and gravitational cement (Fig. 3 c, d), indicating meteoric vadose conditions and subaerial environment (Muller, 1971). The sparry laminae may form radiating fan-shaped bundles of calcite (Fig.3 c, d) oriented normal to the underlying micrite laminae (Freytet & Verrecchia 1999, Chafetz et al. 1994). Spary bundles commonly coalesce forming sparry calcite layers. Layers of radial sparry calcite may exhibit spheres or hemispheres that may be continuous around cavities forming fringe cements (Pedley 1987, Garcia del Cura et al. 2000). Radial sparry calcite increase in size forming crystals with feathery terminations. Layers of radial spar may show constant thickness forming an isopachous cement indicating phreatic conditions suggesting a period when tufa was saturated with water. This recrystallization of micrite to microspar and vug linning sparite is a process of aggradational neomorphism.

Subaerial dissolution processes may exhibit unique features observed in tufas. The presence of irregular pores and cavities represent bioturbation by small organisms as insects or warms, that dissolves calcite. The infiltrating water leaching the overlying altered basalt and basic Precambrian volcanics becomes calcium and iron- bearing and clay rich, thus it may deposite fine layers of clays and iron oxides as insoluble minerals with the vug filling calcite (Fig. 3 b, d & f). Meanwhile the dissolved CaCO3 reprecipitated as cement or veins of calcite. Sparite filling is also manifested by large crystals calcite with random orientation infilling open spaces. They may show tight mosaics indicating competitive growth or have open texture with intercrystalline spaces (Fig. 3 e).

5. Conclusions

--The incorporation of biota affected the fabric of the tufa contributing to the micro- and macromorphology

--Abiotic processes influenced the precipitation of carbonate

--Outgassingof C[O.sub.2] by turbulence and evaporation around streams banks played a great role in deposition of carbonate

--The present tufa deposits are influenced by warm monsoonal climate as they are subjected to warm temperatures, high evaporation rates and probably destructive seasonal floods. These factors and their influence was discussed by Carthew et al. 2003a, Carthew et al. 2006

--Carbonate coated grains--ooids formed in streams affected by turbulence

--Oncolites develop in slow flowing water

--Stromatlites formed in quiet conditions in shallow pools, indicating decrease in hydrodynamic energy.


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