Diagenetic modelling to evaluate the burial history of exhumed clastic rocks: the case of Tertiary Piedmont Basin (NW Italy)

The Tertiary Piedmont Basin (TPB) is an Eocene-Pliocene episutural basin sealing the tectonic junction of the Alps and Appenines belts (NW Italy). The burial history of its easternmost part remains unknown since the whole succession has been involved in the Northern Apennines thrust and fold belt during Miocene-Pliocene time and is presently exhumed and truncated on its northern side by the Villalvernia-Varzi tectonic line, which represents a regional crustal-scale strike-slip fault zone, that have accommodated a part of the tectonic deformation in the Alps-Apennines tectonic knot during Oligo-Miocene time.

Starting from a thermal history calibrated through apatite fission tracks and microthermometric analyses of fluid inclusions in diagenetic minerals, we investigated the burial history of sediments of that part of the basin using a diagenetic software modelling approach. The rationale of this approach is to study the compaction degree of medium-size clastic rocks (sandstones), and the observed residual intergranular volume (IGV), assumed to be the best proxy for the estimation of burial suffered by clastic rocks in compaction dominated lithified clastic sequences.

Based on the regional context, three plausible but different burial histories have been tested and calibrated into the diagenetic modelling software: 1) a minimum burial history assuming no significant additional burial occurred on top of what is presently preserved in the outcrops at the top of the sequence; 2) an intermediate burial history with an hypothetical 400m burial and erosion related to the formations that are presently exposed just on the northern side of Villalvernia-Varzi line (early Miocene shallow marine sandstones); 3) a maximum burial history with 900m of Miocene sediments deposited above the youngest preserved unit (Castagnola Fm.) similarly to what is actually observed in the TPB basin westward.

What we can resume from our study is that the burial history that best explains the observed IGV data by the diagenetic modelling is the one with 900m of Miocene sediments originally accumulated on top of presently exposed rocks and then eroded. This result has important regional consequences for the reconstruction of the complex stratigraphic-tectonic evolution of the studied area, and more in general shows that diagenetic modelling has a strong potential for shedding some light on the burial history of exhumed clastic sequences.