STRATIGRAPHIC AND DEPOSITIONAL ENVIRONMENT ANALYSES OF THE AUTOCHTHONOUS TRIASSIC COVER OF THE AIGUILLES ROUGES MASSIF (VALAIS; SOUTHWESTERN SWITZERLAND)

The autochthonous Triassic sedimentary rocks of the Aiguilles Rouges Massif have not been studied in detail and thus the age and paleogeography, including depositional environments, are not well understood. The discovery of tetrapod trackways in the rocks has generated debate as to the trackmakers and age. However, recent work at the main trackway site at Vieux Emosson and the analysis of nearby newly discovered trackways reveal the presence of the ichnotaxa Isochirotherium soergeli and Chirotherium barthi. These ichnotaxa represent a “Chirothere assemblage” and indicate a Late Olenekian to Early Ladinian age.

Our work in the Lake Salanfe area has led to the discovery of 4 additional trackway locations, which support a Chirothere assemblage, and a fluvial model of deposition. The sequence overlies highly weathered crystalline basement with up to 0.5 m of local relief and likely more on a regional scale. The Triassic rocks consist of 4 lithofacies in a generally fining-upward sequence of more than 8 m: conglomerate, trough cross-bedded sandstone, thin-bedded rippled sandstone, and mudstone with thin rippled sandstones. The basal conglomerate is sublithic, matrix-supported with angular quartz clasts < 4 cm size; the sandstones are medium- to coarse-grained and quartzose, but contain some plagioclase. In some areas, a scour-based, second conglomerate bed that truncates thin-bedded sandstone, forms shallow channel fills. Mudcracks, rip-up clasts, and current and wave ripple marks are abundant in the finer grained facies. The mudstone facies contains a thin dolomite bed near the top of the sequence. Load casts at the base of rare 20-cm thick sandstones in the mudstone facies, rare burrows and no evidence of paleosols, suggest rapid deposition. Trackways are found both on top of the coarse sandstone and imprinted on rippled sandstone surfaces. Paleocurrent measurement of troughs and ripples indicate a northwest transport direction.

Deposition of conglomerate and sandstone facies were in shallow bedload-dominated streams. Fine-grained facies are interpreted as floodplain and possibly playa lake deposits. The northwest paleoslope supports drainage of the region into the Germanic Basin, and not the Tethyan realm as postulated by others. Thus, the sequence is equivalent in age and facies to the Buntsandstein.