Reconstructing the burial history of uplifted clastic sequences using compactional indices and diagenetic modeling (a northern Italy case of study)

The reconstruction of the burial depth experienced by sedimentary successions before their uplift is crucial for various geological applications, such as thermal history analysis, subsidence reconstruction in sedimentary basins, reservoir quality prediction and, more in general, the tectonic evolution of fold-and-thrust belts. A commonly used method for addressing this problem in clastic sequences is low-temperature thermochronology (LTT), including techniques such as (U-Th)/He and fission-track analysis (AFT) on apatite grains, or organic matter maturity indices. However, these methods have two main limitations: 1) they are T-dependent, requiring knowledge or, more commonly, assumptions about the geothermal gradient for the studied sedimentary sequence over the considered time span, which can be challenging for deep-time analysis; and 2) these techniques are most effective at temperature higher than 60°-80° for (U-Th)/He and 120° for AFT. This means that for regions with a normal geothermal gradient of 30°C/km or lower (e.g., foreland basins), low-T thermochronology is less reliable for determining burial depth of less than 2-4 km experienced by rocks before exhumation.

In this contribution we aim to address these limitations by filling the “blind window” of LTT and avoiding uncertainties related to the past geothermal gradient. We do this by using the degree of compaction in sand-sized clastic rocks (COPL-CEPL indexes analysis) as a proxy to estimate the minimum burial depth experienced by exhumed clastic sequences. We apply a compaction-driven approach coupled with diagenetic modelling to estimate the burial depth of clastic units exposed in the eastern Tertiary Piedmont Basin (TPB) which occupies an episutural position on the tectonic junction between the Alps and the Northern Apennines collisional belts. Due to its complex tectonic setting, the studied sedimentary succession has undergone a largely unknown post-depositional history, making it possible to test several regional burial/exhumation scenarios for the Eocene-lower Miocene sequence. Our results suggests that the eastern part of the TPB underwent to more burial than previously expected; this implies that it continued to subside and accumulate sediment until the end of the Miocene, with uplift and erosion likely beginning at the end of Miocene due to the combined effects of Northern Apennines contractional tectonic phase and the Messinian Salinity Crisis. Overall, this case of study demonstrates that the quantitative study of the degree of compaction coupled with diagenetic modelling can be a reliable tool for maximum burial reconstruction in the depth-temperature window where current low-T thermochronological methods hardly work.