The effect of structural growth on basin floor sedimentary architecture in the Miocene Marnoso-Arenacea Formation, Italy

Abstract

Outcrops of the Miocene Marnoso-arenacea Formation in the Santerno River Valley record the evolution of basin floor deep-water fan relative to the growth of the Coniale anticline in the Apennine foreland basin. This study aimed to address if and how multiple scales of deep-water architecture change in response to this process. Collection of 1.35 km of measured sedimentological profiles integrated with 4.4 km of mapped photopanels facilitated identification of 24 sedimentary facies, 14 event bed, and 7 sedimentary body types. These attributes characterize outcrops of pre- and syn-kinematic strata on the footwall and hanging-wall of the Monte Castellaccio thrust fault. Facies record deposition from different regions of subaqueous flows sourced from large-volume Alpine (siliciclastic) and minor Apennine (calcareous) sources. Facies assemble into event bed types that occupy positions in facies tracts formed by either (1) gradual flow deceleration and transformation during longitudinal flow run-out on a flat basin floor or (2) rapid lateral deceleration and transformation onto tectonically-generated paleo-slopes where reflected flow beds and remobilized mixed-composition mudstones abound. A continuum of amalgamated to non-amalgamated lobeform and thin-bedded drape bodies reflect longitudinal and lateral proximity to the channel-lobe transition zone. Anticline growth shifts these bodies laterally generating aggradational lobeform complexes in basin axes and thin-bedded wedgeforms at basin margins. Mass-transport deposits herald major changes in basin configuration due to anticline emergence. Vertical changes in attributes define multiple scales of initiation, growth, and retreat stratigraphic cycles that reflect changes in sedimentation intensity and paleotopographic basin configuration. Mid-Serravallian pre-kinematic strata exhibit a lowdiversity of sedimentary attributes with minimal high-order cycle modulation in a monotonous basin axis succession with beds that correlate across the future site of the anticline. Late Serravallian syn-kinematic strata have greater attribute diversity and stronger attribute modulation between intermediate and high-order cycles that is overprinted on these low-order cycles. Constriction in the wedge-top basin produces a sandstone-dominated succession with a higher proportion of amalgamated lobeforms. Overlapping processes in the foredeep basin create the greatest vertical and lateral architectural variation. High-order cycles correlate to pinchout successions that indicate episodic anticline forelimb rotation and changes in basin constriction state.