Mamoso-arenacea Formation

We compared the isotopic values of calcite from the Mamoso-arenacea Formation from its upper and lower parts. Only weak differences (significance level 0.90) exist in both carbon and oxygen values. 

Samples of each formation were etched briefly in weak HCl and examined with the scanning electron microscope (SEM). Rod-shaped particles of possible nannobacteria (see Folk, 1993) or biofilms of microbially formed polymers (see Westall Rince, 1994) occur entombed in calcite cement in both Pliocene units and in the upper part of the Mamoso-arenacea Formation. The role played by microbes is uncertain, but the possibility of microbially mediated precipitation of calcite must be considered even at the depths at which these rocks were cemented (Folk, 1993). 

The lA Pliocene, Ranzano and Antognola formations were cemented by meteoric water the Bismantova Formation was cemented in part by water with a meteoric component the Loiano and the Borello formations were cemented by slightly modified marine pore water and all the foreland basin units (except the Borello) were cemented by water variably enriched in 0 (8 0 = -2 to +8) generated from silicate reactions. The most 0-enriched values for S 0, a,er are compatible with depths and temperatures of cementation of the three deepest formations, but not for the less deeply buried Loiano and upper part of the Mamoso-arenacea formations. 0-enriched fluids in these latter formations were more probably derived from underlying, more deeply buried rocks apd expelled by compaction. Possibly, the calcite in the deepest buried formations re-equilibrated with hot water after precipitation. 

Several concretions in the Pliocene, Mamoso-arenacea and Loiano formations possess calculated or whole-rock 6 C values more C depleted than -10%o. These have higher IGV values than other cemented samples (e.g. 31% for more C-depleted carbon samples vs. 14% for more C-enriched carbon samples in the Loiano), which indicates that they were cemented at shallower depths than most samples. But these earlier-phase concretions still formed after significant compaction had occurred, which would also be below the depth of sulphate survival. Mass balance calculations indicate that from 20 to 30% of the carbon in these more C-depleted cements must have been derived from the oxidation of methane, assuming that methanic carbon has a 6 C composition of at least -40%o (Curtis, 1977). 

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