Genesis of fossil concentrations in foreland basins: A case study of the middle Eocene Man Aike Formation, Magallanes Basin, Chilean Patagonia

Abstract

Foreland basins are tectonically active, and the accommodation for sediment deposition in shallow marine environments is continuously altered due to rapid variations in the proximal area, influencing fossil concentration. The middle Eocene Man Aike Formation in the northwest region of the retroarc foreland Magallanes/Austral Basin, Chilean Patagonia, is represented by coarse siliciclastic deposits interbedded with dense fossil concentrations. To better understand the depositional and paleoenvironmental context recorded by these strata and fossil concentrations, this study presents a multi-proxy characterization, based on facies analysis, taphonomy and geochemical analysis of bulk sediments. In the study area, the base of the sedimentary succession is interpreted as stacked transgressive facies deposits of the shoreface zone. Taphonomic features as articulation, fragmentation, orientation and bioincrustation suggests that shell concentrations were influenced by fair weather and storm waves (mass flows started by gravitational effects) in three types. The fossil concentrations that characterize episodic deposits, we interpreted as proximal and distal tempestites. The Lowest shell fossil concentrations are interpreted as fossiliferous lags, suggesting the onset of a transgression in the Magallanes Basin. The fossil concentrations composed of closed and articulated bivalves, observed in the middle portion of succession, were interpreted as event concentrations. Results of geochemical analyses (terrigenous elements/Ca, Al/Ca, Ti/Al and Sr/Ca ratios) support the occurrence of a deepening trend towards the top of the sedimentary succession. Taphonomic control related to the low accommodation of the foreland basin, combined with relatively high sea-level, was a determining factor in the genesis of the middle Eocene fossil concentration.