Taphonomical Analysis of Storm-Influenced Shallow-Marine Deposits in the Tubul Formation (Pliocene-Pleistocene), Chile

The Late Pliocene – Early Pleistocene Tubul Formation of south-central Chile holds some of the richest pre-Holocene marine faunas in the southeastern Pacific margin. Although the fossil content, and particularly the mollusk fauna, has been thoroughly examined from a taxonomic point of view, the mostly fine-grained shallow-marine unit has received little attention in terms of sedimentologic studies, with still prevalent uncertainties regarding its depositional environment and associated sedimentary processes. Given its mostly homogenous lithology with little preservation of sedimentary structures, interpretations are difficult. However, the fossil content provides several inconspicuous clues. The mollusk-dominated fossil assemblages occur mostly in the form of fossil concentrations, allowing for the use of a taphonomical approach for their study. The taphonomic analysis and the information provided by the also highly abundant ichnologic record provide crucial information for both paleoenvironmental and paleoecologic interpretations, revealing how unit was deposited in the lower shoreface zone of a wave-dominated coast in a protected environment, with sporadic influence of storm events and low sedimentation rates. This setting provided optimal conditions for the colonization by benthic organisms which pervasively bioturbated these deposits. The changes in faunal composition along with a slight reduction in grain size towards the upper levels of the sequence also suggest a progressive marine transgression during this time. The Tubul Formation fossil assemblages and their taphonomic analysis prove to be a useful tool in the study of storm-influenced coastal deposits, particularly as a case in which the lack of well-preserved sedimentary structures and a relatively homogenous sequence difficult its study through a solely sedimentologic or ichnologic approach.