Sedimentologic control of the Coprinisphaera ichnofacies

BELLOSI, E. S.

Trelew, Chubut

Congreso; First International Congress on Ichnology; 2004

Museo Paleontológico E. Feruglio

ABSTRACT The Coprinisphaera ichnofacies is a diverse assemblage of insect, plant and vertebrate trace fossils in continental deposits, considered to be mainly controlled by climate and vegetation. However, particular depositional factors may also favor its development. This ichnofacies occurs in units that range from Late Cretaceous to Quaternary in age, such as Laguna Palacios, Pinturas, Santa Cruz, Collon-Cura, Laetoli, Cangahua, Huayquerias, Monte Hermoso, Chapadmalal, Barranca de los Lobos, Vorohue, Ensenada, Buenos Aires and Lujan. It is significant that most of the known examples correspond to loess or eolian successions, which include fine volcaniclastics. This suggests that characteristic processes of loessic paleoenvironments, commonly developed in periglacial/peridesert plains, would also contribute to the origin and preservation of the Coprinisphaera ichnofacies. The explosive volcanism and high-frequency cyclic climatic changes controlled 1) the eolian sediment supply from source areas, 2) the extent of vegetated plains (sediment traps) and 3) affected directly the fauna and flora. Excellent examples of the Coprinisphaera ichnofacies to further evaluate these controls occur in tuffaceous paleosols of the Sarmiento Formation (Eocene-Miocene, Patagonia). Andisols, Alfisols, and Calcisols are intercalated with massive tuffs, bentonites and conglomerates. The tuffs resulted from distal volcanic ash fallouts (loessite) on herbaceous plains under subhumid-seasonal conditions. The relative abundance of ichnotaxa varies significantly along the unit in response to ecological and local sedimentologic factors. The ichnofauna involves: common to very abundant dung beetle brood balls (Coprinisphaera); less common coleopteran (Teisseirei, Pallichnus), bee (Celliforma) and unnamed pan-shaped trace fossils and subordinate components, such as possible termite nests, meniscate and non-meniscate burrows, rhizoliths and cocoons.