Subsurface sediment remobilization as an indicator of regional‐scale defluidization within the upper Tortonian Marnoso‐arenacea formation (Apenninic foredeep, northern Italy)

Abstract

ABSTRACTThe upper Tortonian Marnoso‐arenacea Formation displays 10–30 m‐thick sand bodies encased within up to 100 m‐thick packages of thin bedded turbidites and hemipelagites. In situ soft‐sediment deformation resulting in load casts, flames, ball‐and‐pillow and water escape structures often affected the sand body sedimentation units. It is interpreted as being due to a combination of overloading through the rapid deposition of thick massive sandstones and of uneven loading due to the topography created by erosional processes within the sand bodies. Post‐depositional deformation structures are also present. A variety of small‐scale sediment remobilization structures, such as pillars and plastically deformed consolidation laminae are associated with mound‐shaped deformed sandstones. Frequently, the deformed and intruding sand caused the erosion and break up of the overlying fine‐grained beds resulting in mudclast layers and networks of dykes and sills. Thus, the deformation assemblage resembles that associated with larger scale sandstone intrusions. Moreover, an intrusion complex occurs just below the base of one of the sand bodies. It comprises dykes that intrude along synsedimentary extensional faults formed adjacent to a slide scar. The sand intrusion occurred at a shallow burial depth suggesting a close relationship between seafloor instability, sliding and sediment injection along the faults. The suite of post‐depositionally deformed and injected sands is interpreted to result from overpressure build‐up within the sand bodies. In the study area, fluid seepage at the seafloor is marked by ‘calcari a Lucine’ chemiosynthetic carbonates. Thus, it is hypothesized here that the post‐depositional deformation of the sand bodies and injection structures was favoured by an upward flow of deep, hydrocarbon‐rich fluids caused by the ensuing tectonic compressive regime.