The ultimate battle of a mighty warrior: a sedimentary basin final evolution, near-dry out stages
Sea level changes and storm signatures in a sedimentary basin sea-to-lake transition toward dry out final evolution stages
The present article captures a Late Pliocene sedimentary framework of a basin transition from brackish marine to fresh-water lake environment towards his pre-Quaternary final evolution stage - the dry out.
This continuous evolution is very well illustrated in a ~100m logged sedimentary succession representing a repetition of almost identic complete or incomplete shallowing upward sequences. The main depositional facies are: massive mudstone to massive and parallel laminated siltstone alternation; massive fine sands with bivalves and gastropods shell clusters (Fig.1H); horizontal and low angle laminated (hummocky) sand with wave ripples on top (Fig.1B, C); mudstone-siltstone to fine arenites with storm induced deformational structures (lenticular bedding, load casts, water escape structures) (Fig.1G, F); wavy and flaser bedded arenites (Fig 1D); reddish to yellowish weathered bioclastic sandstone (Fig 1A). The final term of the ideal sequence is always represented by the above mentioned Fe-rich bioclastic sandstone levels; they predominantly consist of quartz and extremely few lithic fragments. In most of the samples the presence of authigenic glauconite is noticed. Quartz clasts are angular, sub-angular, rarely sub-rounded, medium to poorly sorted and cemented by iron oxides and calcite. Sediments composed from mixtures of carbonate and siliciclastic material were described as bioclastic sandstone or sandy limestone, according to the descriptive classification system proposed by Mount (1985).
Several types of storm-induced deformational structures is reported in the studied deposits. They developed before lithification in non-cohesive sediments by liquefaction and/or fluidization (Alfaro et al., 2002). These processes increase interstitial pressure within the sediment which behaves as a viscous fluid (Allen, 1982). Several trigger mechanisms can form soft-sediment-deformation (earthquakes, tsunamis, overloading and storm waves). As the present deformational structures from our deposits are always associated with tempestites, the effect of cyclic stress induced by wave storm activity (Alfaro et al., 2002) is considered to be responsible. The most favourable conditions for liquefaction under cyclic effect of storm waves are water depth between 10 and 20 m and storm wave height up to 6 m (Alfaro et al., 2002).
Fig.1 Some of the main depositional facies and sedimentary structures. (C) Fine sand with low angle, hummocky cross stratification; (A) Red bioclastic sandstone; (D) Heterolytic facies, silt and very fine sand with lenticular structures; (F) Large scale heterolytic facies; sandy lens with primary horizontal lamination embedded in clay (E) Silt and mud with wavy structures; (G) Deformational structures in silt-very fine sand; overloading structure, convolute bedding and injection dykes (H) Bioclastic sandstone with mixed brackish-marine and fresh water fauna : Prosodacna sp., Dreissena sp.,Viviparus sp. (Saulea,1952)
Chemical composition of investigated sediments suggests abundance of terrigenous compounds (Si, Al, K, Mg) and biogenic (CaO). Three main groups of elements and their trend were identified (Fig.6): A- Al2O3, K2O, MgO, TiO2 and SiO2; B-CaO, MnO2, associated with biogenic carbonates and C-P2O5 related with marine productivity. Positive correlation between SiO2 and TiO2 prove that SiO2 has a lithogenic source. CaO is reverse correlated with lithogenic elements trend. The high concentration of carbonates indicates increasing of carbonate productivity, decreasing of dissolution/increasing of terrigenous material dilution during warm periods. The high amount of Fe2O3 (mostly 10% with maximum of 33%) may suggest alteration during early diagenesis.
Fig.2 Petrographic features of red bioclastic sandstone levels. (A) Quartzitic sandstone with angular/subangular clasts of monocrystalline and glauconite; porous hematite cement (VS08b sample, N//); (B) Quartz sandstone with sedimentary lithoclasts (muddy) and hematite clusters, dominated by monocrystalline quartz and carbonate cement (VS 25 sample, N//); (C) Quartz sandstone with monocrystalline quartz and a metamorphic lithoclast fragment, hematite cemented (VS 18, N+); (D) Monocrystalline quartz, plagioclase feldspar and calcite cement (VS 20,N+); (E) Bioclastic sandstone; we can notice carbonate sparite pellicular cement and hematite cement; (F) Detail on polycrystalline quartz; more than 10 component cristals; concave-convex contacts and good roundness indicate a metamorphic source. (N+); (G) Hematite cement in bioclastic sandstone; bioclasts are gathered by sparite cement -mosaic type (VS 22, N//); (H) Sandy bioclastic limestone-clusters of bivalve fragments (VS 39, N+).
The studied sedimentary deposits show severe base water level oscillations and strong storm signatures. A petrographic framework with absence of feldspar and other lithic fragments is in contrast with presence of dominant angular quartz clasts, in terms of sediment maturity. Although, an immature transport regime and long chemical weathering in a warm-dry climate for sub-aerial exposed intervals is suggested.
Fig. 3 Studied section sedimentary logs detail
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Yang, S.Y., Li, C.X., Yang, D.Y., Li, X.S., 2004. Chemical weathering of the loess deposits in the lower Changjiang Valley, China, Paleoclimatic implications, Quaternary International, 117, pp. 27-34.
Data and photos source: GeologX
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