INCOMPLETE AND NONSYSTEMATIC DIAGENETIC DESTRUCTION OF PROVENANCE INDICATORS IN THE LOWER AND UPPER PETROFACIES OF THE MACIGNO FORMATION (NORTHERN APENNINES, ITALY)

Incomplete and nonsystematic diagenetic destruction of provenance indicators in the lower and upper petrofacies of the Macigno Formation (Northern Apennines, Italy)

 

Renzo Valloni, Giovanni Mezzadri, and Abhijit Basu

Department of Earth Sciences, University of Parma, Parma 43100, Italy

Department of Geology, Indiana University, USA

 

The Oligo-Miocene Macigno Formation, consisting of about 1300 alternating arenaceous and muddy turbidite beds organized in 50 depositional units, is exposed in full (2159m) near Zeri (~75km east of Genoa). The rocks are immature, arkosic (QFL=56±8 : 37±8 : 7±3), and with a high (10.7±3.5%) content of detrital mica and heavy minerals. Feldspars are extensively albitized and argillized. The lithic fraction (4.3±1.9%) is dominated by L_M (85±5%). From the lower ~700m to the upper ~1450m of the section, L_V decreases from 14±3% to 8±3%. Commensurate changes occur in opaque minerals (18.2±10.9% to 8.2±1.4%), weight% heavy minerals (0.3±0.2% to 1.2±0.7%), Niggli fm (40.7±3.8 to 36.7±3.8) and c (1.5±0.7 to 3.5±1.1), ppm Ni (95±15 to 58±12), Cr (179±38 to 121±21), Co (10±1 to 8±1), and V (75±5 to 67±6). A change in provenance resulting in a decreased supply of volcanic and ultramafic detritus across the 700m boundary best explains the data. In contrast, no change is seen in modal composition (QFL; P/F ratio; 24 variables), distribution of transparent heavy minerals (Zr, Tou, Rt, Gt, Brk, Sph, Epi, All, Zoi, Pmp, Px, Amp), major, minor and trace element concentrations (47 elements), compositions of feldspar (Si, Al, Ca, Na, K, Ba, Fe), garnet (Si, Al, Ca, Mg, Fe, Mn, Cr, Ti), zircon (Si, Zr, Hf), tourmaline (Si, Al, Ti, Fe, Mg, Ca, Na, B), and Cr-spinel (Al, Ti, Fe, Mg, Mn, Cr). The lack of change and loss of provenance information are results of diagenetic alteration of Macigno sediments. Regardless of any or no change, all variables show high dispersion (relative standard deviation generally varies between 20% to 80%). Diagenesis, therefore, did not homogenize the rocks. Rather, diagenetic fluids selectively destroyed the distinction between the lower and the upper petrofacies by discriminatory grain alteration and dissolution. This resulted in bringing the average values of petrologic variables in the two units closer together but with large standard deviations.