Dinosaur track record on a shallow carbonate-dominated ramp (Loulle section, Late Jurassic, French Jura)

Abstract

In 2006, sauropod and theropod trackways were discovered at the locality of Loulle in the French Jura Mountains. During the Late Oxfordian (sensu gallico), Loulle was located at the southeastern margin of the carbonate-dominated French Jura platform. The upper part of the section contains a 94-cm-thick dinosaur track-bearing interval. Dinosaur tracks were recorded in a tidal-flat setting punctuated by frequent flooding and emersion phases leading to desiccation, microbial mat development, and new sediment inputs. Four types of tracks were identified relative to their morphologies and the rheology of the tracked sediment. Tracks were recorded on sediment with different degrees of water saturation, covered with a thick, cohesive, and elastic microbial mat. This mat cover prevented tracks from erosion agents but also prevented the record of trackmaker anatomical details. Early lithification, probably driven by enhanced dolomitization, allowed the rapid fossilization of the tracks, protecting them from reworking during the subsequent dinosaur tracking phases. Sequential analysis reveals that the record of dinosaur tracks was made possible by a gain of accommodation at the scale of elementary (infra-Milankovitch) and small-scale (100 kyr) sequences. The loss of accommodation observed at the scale of medium-scale (400 kyr) and very small scale (20 kyr) sequences enhanced the ramp progradation at the Bimammatum-Planula boundary, allowing dinosaurs to walk on the Loulle tidal-flat, along a narrow ENE/WSW isthmus.