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 Volume 2 Chapter 13

Chapter  13

Plesiosaurs, Palaeoenvironments and the Paja Formation Lagerstätte of Central Colombia: An Overview

Leslie F. NOÈ  and Marcela GÓMEZ–PÉREZ

https://doi.org/10.32685/pub.esp.36.2019.13


Citation is suggested as: 

Noè, L.F. & Gómez–Pérez, M. 2019. Plesiosaurs, palaeoenvironments and the Paja Formation Lagerstätte of central Colombia: An overview. In: Gómez, J. & Pinilla–Pachon, A.O. (editors), The Geology of Colombia, Volume 2 Mesozoic. Servicio Geológico Colombiano, Publicaciones Geológicas Especiales 36, p. 475–516. Bogotá. https://doi.org/10.32685/pub.esp.36.2019.13


Abstract 


The Cretaceous Paja Formation of the Alto Ricaurte of the Eastern Cordillera of central Colombia was laid down under an epicontinental sea during Hauterivian – Aptian times. The Paja Formation epicontinental sea was home to a diverse, and now well–preserved, pelagic marine fauna that includes members of Plesiosauria, other marine reptiles, fish and ammonites. However, the benthic fauna is depauperate, preserving just a few thin–shelled bivalves and evidence of microbial mats. This suggests dysoxic–anoxic bottom waters, separated from oxic surface waters by a chemocline–pycnocline. The exceptional preservation of the Paja Formation fauna makes the Alto Ricaurte a unique Lower Cretaceous marine vertebrae Lagerstätte. Previous palaeoenvironmental interpretations of the Paja Formation, based on observations of the gypsiferous, dark mudrock sequence, suggested an intertidal evaporitic (sabkha) environment. However, integration of sedimentological, palaeobiological, taphonomic and diagenetic data provides evidence for deeper water conditions. The exquisite preservation and articulation of the skeletons of large marine reptiles, three–dimensionally preserved fish, beautifully ornamented ammonites, and delicate plants, do not accord with a sabkha environment. Sabkha is typical of mid–latitude, dryer climates under the descended limb of the Hadley atmospheric cell, rather than a wet tropical equatorial location of the Paja Formation. Mineralogical arguments used to infer the presence of sabkha are not primary depositional features, but due to secondary migration of mineral–rich fluids. These fluids probably had their source in the earliest Cretaceous topographic high now beneath the Sabana of Bogotá, and were driven by hydraulic pressure generated by volumetric changes due to hydration of anhydrite into gypsum due to the post–Cretaceous rise of the northern Andes mountain chain. The separation of primary and secondary diagenetic features is thereby critical for understanding the evolution of the Paja Formation sedimentary basin in the Alto Ricaurte.

Keywords:   Lagerstätte, Lower Cretaceous, Paja Formation, palaeoenvironments, Plesiosauria.