Arenites hybrid

Diagenetic evolution of synorogenic hybrid and lithic arenites (Miocene), northern Apennines, Italy... 

This study aims to decipher and compare the dia-genetic evolution of hybrid arenites and arenites rich in carbonate rock fragments belonging to the Bismantova-Termina succession, a synorogenic sequence of the northern Apennines (Fig. 1). Special emphasis is given to the roles of the complex detrital composition, provenance and facies organization of the arenites in their carbonate cementation. 

Fig. 2. Schematic stratigraphy of the northern Apennines Epi-Ligurian sequences. (A) Upper Eocene-Lower Miocene succession. (B) Bismantova-Termina succession. S1-S4, depositional sequences B1-B3, arenite petrofacies. Bl, hybrid arenites B2, arkosic arenites B3, feldspathic lithic arenites 1, marls 2, resedimented arkosic and lithic arenites 3, hybrid arenites 4, silty marls. Modified from Amorosi Spadafora (1995).

The base of sequence 1 (SI) is bounded by an angular unconformity marked by a glaucony-rich horizon, and consists of hybrid arenites of marginal marine facies with shelf silty-marly deposits which locally interfinger with resedimented hybrid arenites (Fig. 3). An inversion of the sedimentation trend from coarsening upwards to fining upwards is associated with an increase in glaucony content, and has been chosen to define the boundary between SI and sequence 2. 

Generally the calcite cements are characterized by relatively low average mol% of Fe (1.0%), Mg (0.6%) and Mn (0.2%) (Table 2). Cements in the turbiditic arenites have somewhat more Fe than those in the shelf hybrid arenites (av. 1.4% and 0.6%, respectively) (Table 2). Bioclasts such as echinoderms, originally composed of high-Mg calcite, show low Mg contents, which confirms their pseudomorphic replacement. Calcite which replaced feldspar grains is occasionally Sr rich (up to 2.1% SrC03) (Fig. 6F Table 2). 

Fig. 10. Generalized paragenetic sequence in the shelf hybrid arenites and in turbidite feldspathic litharenites.

The absence of kaolinite cement and kaolinized grains, as well as the high 5 Opj,B values of calcite cement in the hybrid and arkosic arenites (-3.6 to 0%o) (Table 3 Fig. 8), indicates that meteoric fluids did not play an important role in carbonate cementation. Considering that the succession remained buried at shallow depths in a region of rugged relief, the lack of meteoric influence is surprising and probably related to the early and pervasive destruction of the porosity and permeability of the arenites caused by intense cementation and compaction. 

Calcite cementation in bioclastic hybrid and lithic arenites of the Bismantova-Termina succession is pervasive along layers and concretionary horizons. Cementation in the hybrid shelf arenites was mostly precompactional and began with marine calcite rims, syntaxial overgrowths on echinoderms, K-feldspar and dolomite overgrowths, chloritic clay rims, framboidal pyrite and heulandite, followed by... 

ZuFFA, G.G. (1980) Hybrid arenites their composition and classification. J. sediment. Petrol, 50, 21-29. 

---