Mid-Pleistocene Transition at a shallowing shelf: Tectonic and eustatic forcings in the paleoenvironment of the Enza section, Northern Apennines mountain front

We present an integrated sedimentological, paleontological and oxygen isotope stratigraphic study of a Pleistocene marine to coastal section at the Enza river, allowing to highlight the role of tectonics at the mountain front of the Northern Apennines and that of climate at the onset of the Mid Pleistocene Transition. Two main evolutive steps, separated by a pronounced angular unconformity, characterize the deposition of the studied marine sedimentary succession ranging between 1.73 and 1.071 Ma. The lower part of the section shows a continuous outer shelf sedimentation, whereas the post unconformity sedimentation shows cyclothemes, documenting a progressive shallowing of the bathymetry. These are linked to enhanced eustatic oscillations leading to increasing amplitude of the transgressive-regressive cycles. The new δ18O data allowed correlating the last 14 m underneath the unconformity to the MIS 49 to 45, improving our understanding of the time duration of the sedimentary gap, estimated around 140 ka. This is primarily associated with the growth of the Quattro Castella thrust ramp anticline, when a deeper mid-crustal thrust co-occurred, causing an increased deformation and tilting rate. The consequent change in morphotectonic setting strongly impacted the environment and ecosystem of this Early Pleistocene shelf. Starting at about 1.25 Ma, the section records an increasing riverine detritus, competing with the shore-parallel shelfal sediment dynamics. In a frame of increasing slope of the Northern Apennines topography, this enhanced fluvial activity might correlate with the increasing amplitude of climate oscillations during the Mid Pleistocene Transition, as recognized in other Mediterranean settings. This is recorded by a cyclothemic deposition, linked to eustatic oscillations.