Complex palaeosol ichnofabrics from Late Jurassic – Early Cretaceous volcaniclastic successions of Central Patagonia, Argentina

BEDATOU, E.; MELCHOR, R.N.; GENISE, J.F.

SEDIMENTARY GEOLOGY

Elsevier

Año: 2009 vol. 218 p. 74 - 102

0037-0738

Late Jurassic-Early Cretaceous volcaniclastic continental deposits of the Bajo Grande, Bajo Tigre and Punta del Barco formations from central Patagonia, Argentina were analyzed with the purpose of a detailed and integral characterization of palaeosol ichnofabrics. These units contain complex continental ichnofabrics that were also recorded in other extended volcaniclastic successions from Patagonia, ranging in age from Late Jurassic to late Miocene. It was applied herein a recently proposed methodological approach for the description and interpretation of palaeosol ichnofabrics that includes the separate measurement of ichnofabric, pedofabric and original bedding of selected intervals. This procedure allowed to determinate the degree in which the deposits are affected by soil features other than ichnofabrics and by the activity of organisms by separate. Four recurrent ichnofabrics were recognized in studied palaeosols: the Loloichnus, large Taenidium-Beaconites, diffuse boxwork, and Dagnichnus ichnofabrics. The Loloichnus ichnofabric is characterized by sub vertical Loloichnus baqueroensis and subordinate large Taenidium barretti and Beaconites coronus showing similar arrangement to L. baqueroensis. L. baqueroensis is a crayfish dwelling structure while large T. barretti and B. coronus are assigned to locomotion of the same organisms. Root traces are additional components of this ichnofabric. The Large Taenidium - Beaconites ichnofabric is formed by large T. barretti and B. coronus and by L. baqueroensis in low proportion. These burrows show a random arrangement with irregular and curved paths. This ichnofabric is also assigned to crayfish activity. The Diffuse Boxwork ichnofabric is characterized by a pervasive and intricate three-dimensional boxwork of burrows; occasionally joined to sub spherical chambers (possible Castrichnus). The diffuse boxwork is interpreted as an earthworm burrow system and the associated chambers are probably for aestivation. Rare and scattered discrete trace fossils in this ichnofabric are L. baqueroensis, T. barretti and B. coronus. The Dagnichnus ichnofabric is formed by Dagnichnus titoi, root traces and, subordinately, Loloichnus baqueroensis, Cellicalichnus meniscatus and tangled groups of meniscate burrows. D. titoi and C. meniscatus has been interpreted as crayfish breeding structures and the tangled groups of meniscate burrows are of uncertain origin and function but they are probably related to juvenile crayfishes activity. This ichnofabric is developed in patches separated laterally by massive portions of undisturbed bed. The studied ichnofabrics were formed in weakly to moderately developed palaeosols in lowland areas with frequent reworking of pyroclastic material by unconfined flows. Common hyperconcentrated and diluted flows were probably produced by heavy rainfall-induced landslides. Volcanic activity sourced in nearby eruptive centres was important and periodical as evidenced by common primary ash fall deposits. The recognized ichnofabrics show that in Patagonia, for the Late Jurassic-Early Cretaceous times, crayfishes and earthworms were the dominant soil organisms and, along with plants, rapidly colonized deposits exposed subaerially. After sediment deposition and with high soil moisture content or high water table crayfishes probably crawled in moist sediments forming the Large Taenidium - Beaconites ichnofabric. With a better drained soil profile or lower water tables, the Loloichnus ichnofabric, representing the dwelling structures of adult crayfishes, overprinted the previous ichnofabric. The Diffuse boxwork ichnofabric correspond to extensive fossil earthworm burrow systems and is usually located in the uppermost portion of palaeosols. The remaining ichnofabric, the Dagnichnus ichnofabric, is developed in very weakly developed palaeosols and is probably reflecting the ideal palaeoenvironmental conditions for breeding crayfish.