Author Topic: LITHIFICATION  (Read 601 times)

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Re: LITHIFICATION
« on: February 04, 2017, 11:31:01 pm »
https://www.researchgate.net/figure/223634294_fig5_Fig-5-Structural-geological-map-and-related-geological-section-of-the-NW-outcropping
[There is] high contrast between the lithification rate of a shaly-dominated basal complex and the overlying calcareous succession during the D1 deformation. In fact in an underwater environment the diagenesis process is a combination of mechanical compaction and chemical reactions; in the case of the shales the last ones are not time-dependent and start with temperatures higher than 70 e 80 C, corresponding to a few kilometers depth (Hower et al., 1976; Bruce, 1984), while for the fine-grained carbonates the cementation can be a process closely following the deposition (Lasemi et al., 1990). Consequently, during the D1 phase the shales could be still unconsolidated, while the calcareous flysch succession was already lithified.

http://www.sciencedirect.com/science/article/pii/S0025322715000183
2. Beachrock: sea-level related characteristics
Beachrock is a lithified coastal deposit where lithification is a function of CO3− 2 ion concentration in seawater, microbial activity and degassing of CO2 from seaward flowing groundwater. Field experiments (e.g., Hanor, 1978) and coastal observations (e.g., Hopley, 1986) suggest that cementation occurs within a few decades where suitable coastal morphology provides sufficient accommodation space for soft sediment to settle.
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2.2. The cement

The cement by which the loose sand is locked into position is indicative of the nearshore zone between shoreface and beach, at the interface between seawater and meteoric water (Fig. 2). The interface is the mixing zone, the chemically most active zone, where beachrock forms. The zone is characterised by a pore fluid that is a mixture of different end-member solutions (Moore, 1973), originating from the adjacent environments (e.g., hypersaline waters from sabkhas; meteoric water from groundwater). The chemical characteristics of the solutions, in particular acidity and under- or supersaturation with respect to calcite, control the precipitation of the carbonate mineral when the initial pCO2 falls due to degassing ( Plummer, 1975 and Meyers, 1987). As a carbonate mineral will only precipitate from a solution that is supersaturated with respect to this mineral, the mixing of the groundwater and seawater must result in supersaturation. Plummer (1975) showed that for this to happen the mixture must contain more than 50% seawater, the end-member solutions are in equilibrium with calcite and the pCO2 drops below 10− 2 atm (Fig. 3A). The higher the temperature, the less seawater is required to achieve supersaturation (Fig. 3B) and the more CO2 escapes, the higher the pH and the faster carbonate minerals can precipitate. Thus, the sediment layer that is closest to the water table will cement first and fastest and preferred areas of the layer are those where microbes are active (Neumeier, 1998). If the end-member solution contains Mg2 +, high magnesian calcite (HMC) precipitates and the typical crystal form of this mineral is bladed or granular (Fig. 4A) or it is micritic when microbes are involved in the precipitation (Neumeier, 1998). The higher the temperature of the solution, the faster aragonite precipitates relative to calcite (Burton and Walter, 1987) and the crystal form the cement takes is mostly fibrous (Fig. 4B). Crystal arrangement and fabric is controlled by environment and gravitation. HMC and aragonite form circumgranular rim in meniscus fabric in the vadose environment (Fig. 4C) or symmetrical crusts in the meteoric environment. In most beachrocks the pore space is not completely occluded but is filled with mosaic fabric and may remain empty in the centre. Fig. 2 depicts the spatial relationship between carbonate cementation zones and Table 1 provides the details of the cement types in terms of crystal form, size and fabric.
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Diagenesis takes place in the subsurface in response to a change in water-table elevation, temperature or pressure. Diagenesis involves processes such as dissolution, reprecipitation and recrystallisation and the end-point of these processes is chemical stability. The process follows the relative thermodynamic stability of magnesian calcite and aragonite and the chemistry of the pore fluid. The thermodynamic calculations reveal the metastability of aragonite with respect to calcite, and of magnesian calcite with respect to calcite and dolomite (Morse and Mackenzie, 1990). Most effective in terms of creating the end-members calcite and dolomite is the infiltration of meteoric water depleting the cement in Mg, Sr and Na and enriching it with other elements (e.g., Fe2 +). Dissolution and subsequent creation of secondary porosity can occur through infiltration of meteoric water where the dissolution capacity of the water is largely controlled by the amount of dissolved CO2 and the permeability of the arenite frame resulting often in moulds and vugs. These can be later filled with marine cement or intraclasts.
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Table 2.
Description of beachrock samples dated using the OSL technique. The model used to determine the equivalent dose (De) is listed in Table 4.
Sample code (LV); Origin; Coordinates;
De (median)±σ (Gy); De (model, Gy); OSL age (ka,±1σ)
249   E-Mediterranean (Levant)   
32.14N
34.49E   1.20±0.33   1.1±0.1      2.3±0.1
365   Levant   
32.40N
34.56E   84±2      85±1      113±5
404   Levant   
32.49N
34.57E   0.51±0.02   0.51±0.02   1.01±0.06
426   Iberia (Torre Vieja)   
37.56N
00.42E   79±4      73±5      83±6
493   Gulf of Gabès   
33.64N
10.55E   3.37±0.09   3.3±0.1      4.3±0.2
494   Gulf of Gabès   
33.64N
10.56E   80±3      82±3      106±4
565   E-Arabia (Oman)   
22.30N
59.56E   50±3      48±3      80±3
- Table 3.
Analytical data used for OSL age estimation. For details on age modelling of carbonate-rich sediments see Nathan and Mauz (2008).
Sample code (LV); Grain size (μm);
Water content (%); U (μg g− 1); Th (μg g− 1); K (wt.%)   
View the MathML sourceD.cosm (Gy ka− 1)   Carbonate (%)
249   180–250   8±3   
0.399±0.018   0.626±0.063   0.169±0.010   0.212±0.010   71±4
365   150–200   5±3   
1.353±0.036   0.763±0.058   0.227±0.010   0.153±0.008   60±3
404   200–250   5±3   
0.247±0.012   0.288±0.069   0.194±0.010   0.21±0.01   65±4
426   200–250   5±2   
1.156±0.032   1.785±0.069   0.474±0.014   0.21±0.01   75±4
493   200–300   5±2   
1.440±0.035   0.765±0.039   0.072±0.006   0.21±0.01   87±5
494   90–150   6±2   
1.732±0.045   0.679±0.058   0.201±0.010   0.098±0.004   69±4
565   200–300   5±2   
1.708±0.045   0.796±0.085   0.122±0.009   0.172±0.009   90±3
A comparison between ages obtained from radiocarbon and OSL dating techniques shows systematically lower OSL age values regardless of the calibration curve used for radiocarbon, the differences within 14C or whether whole rock or shell was used (Bosman, 2012). Ages obtained from mollusc shells tend to agree better with quartz OSL ages (Bosman, 2012). As such, careful selection of the material used for radiocarbon dating can circumvent the effect of old carbon and diagenesis (e.g., Desruelles et al., 2009).
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The cementation rate is most rapid in the landward side of the beachrock formation zone where large carbonate crystals fill pore space and bind components within the space of years to decades. Thus, before burial the sediment is already lithified and is likely not subject to compaction that would be significant enough to impact on the vertical precision of the SLIP.
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