Paleosol Ichnofacies

Abstract

The ichnofacies model is the only paradigm that ichnology has by now, which allows for making predictions of which association of trace fossils we may expect in a given paleoenvironment from any part of the world. Originally the concept of ichnofacies was proposed mainly for the marine realm. Different ichnofacies were recognized for marine deposits, whereas the continental ones were grouped in a single Scoyenia Ichnofacies. Later, the ichnofacies model was improved by redefining this ichnofacies, by defining new ichnofacies for the continental realm and by improving their paleoenvironmental significance. There are six ichnofacies related to paleosols, which can be included in an evolutionary scenario. The rhizolith ichnofacies is an informal one defined to comprise associations composed only of plant trace fossils, mostly rhizoliths. It is recorded since the Paleozoic. The Scoyenia Ichnofacies, also recorded since the Paleozoic, dominated by meniscate trace fossils, trackways, trails, and vertebrate tracks indicates deposits slightly submerged and periodically exposed to air. Subaerial exposition is short precluding insect nesting or pupating. The Camborygma Ichnofacies, with an extended Mesozoic record, is dominated by earthworm (diffuse boxworks, Edaphichnium, Castrichnus) and crayfish (Camborygma, Loloichnus) trace fossils. It indicates paleosols with evidence for a high and fluctuating water table in local wetlands or swamps as a subset of forest, scrub and open herbaceous vegetation. The Celliforma Ichnofacies, with the oldest records during the Late Cretaceous, is the first including insect trace fossils, such as Celliforma, Rebuffoichnus, Fictovichnus, and rhizoliths. It is well represented in Cenozoic localities showing carbonate rich paleosols. It indicates scrubs and woodlands from arid to semiarid climates and palustrine environments, in all cases with large patches of bare soil. The Coprinisphaera Ichnofacies arose along with open habitats dominated by low vegetation, mostly grasses during the Paleogene. It is dominated by Coprinisphaera, Celliforma, Teisseirei, Feoichnus, Monesichnus, Palmiraichnus and other insect traces, rhizoliths and vertebrate burrows. The Termitichnus Ichnofacies has very low recurrency, is dominated by nests of social insects, Termitichnus, Krausichnus, Vondrichnus, Fleaglellius and rhizoliths. It indicates closed forest environments.

Dedicated to Dolf SeilacherQuizas porque mi niñez sigue jugando en tu playa…….(Joan Manuel Serrat 1972, Mediterraneo)

He could not see the green of the shore now but only the tops of the blue hills that showed white as though they were snow-capped and the clouds that looked like high snow mountains above them. The sea was very dark and the light made prisms in the water. The myriad flecks of the plankton were annulled now by the high sun and it was only the great deep prisms in the blue water that the old man saw now with his lines going straight down into the water that was a mile deep.(Ernest Hemingway 1952, The Old Man and the Sea)

De alguna manera, es como si toda la teoría (icnológica) se hubiera desarrollado desde un bote anclado a cierta distancia de la costa, en el que un observador cada vez que elaboraba una clasificación, imaginaba el continente como algo homogeneo e incluía todas las trazas terrestres en un solo item, o directamente las ignoraba(Jorge Genise, 2000, Nidos fósiles de insectos en paleosuelos)