SEDIMENTOLOGY AND ICHNOLOGY OF THE LATE TRIASSIC IROHALEN MUDSTONE, ARGANA VALLEY, MOROCCO
Abstract
The Late Triassic Irohalen Mudstone is 500 m in thickness and is laterally continuous in outcrop within
Argana valley, Western High Atlas, Morocco. This mudstone-dominated unit rests conformably on the Agelgal
sandstone and represents the upper part of the Timezgadiouine Formation. It comprises two fining-upward and
coarsening-upward alluvial plain sequences that are accumulated under semi-arid climate. The two sequences
are represented by (1) Lower Irohalen Mudstone and (2) Upper Irohalen Mudstone. It is composed
predominantly of floodplain siliciclastic mudstones with interbedded proximal crevasse splay sandstones that
evolve laterally into extensive floodplain siliciclastic mudstone, ephemeral lacustrine carbonate mudstones
and distal crevasse splay siltstones. Channel sandstones and levee siltstones are also present. The lower
Irohalen Mudstone has yielded a low diversity and abundant invertebrate and vertebrate ichnofossils. The
invertebrate ichnofossil assemblages include Scoyenia, Cruziana, Rusophycus, Taenidium and Fuersichnus.
The vertebrate ichnofossils consist of the lacertoide ichnogenus Rhynchosauroides, the dinosaurian
ichnogenus Grallator, and the chirotherian ichnogenus Brachychirotherium. The purpose of this contribution
is to describe the facies and associations of facies constituting the lower Irohalen Mudstone and to review these
vertebrate and invertebrate ichnofossils and discuss their occurrence in a sedimentological and
paleoenvironmental context.