Ichnology of a Late Palaeozoic ice-marginal shallow marine succession: Talchir Formation, Satpura Gondwana basin, central India

Abstract

The Late Palaeozoic, glaciomarine Talchir Formation in the Satpura Gondwana basin, central India hosts sparse to profuse trace fossils of various kinds, although sporadically. Among the three principal depositional systems that gave rise to the Talchir succession, deposits of the ice-contact fan delta system and the glacial outwash braidplain delta system lack trace fossils of any kind. On the other hand, within the deposits of the storm wave-dominated, shoreface–shelf system a variety of animal traces is present including simple locomotion traces and non-descript bioturbation. Three trace fossil assemblages could be recognized within these deposits. Assemblage-I is dominated by Cylindrichnus of the Skolithos ichnofacies that occurs in storm laid, medium to fine sandstones deposited in shoreface and transitional shoreface–inner shelf environments. In contrast, assemblage-II comprising dominantly horizontal, grazing-cum-deposit-feeding traces Cochlichnus, Helminthopsis, Phycosiphon, Planolites, Archaeonassa and Psammichnites occurs on decimeter-thick sandstone beds interlayered with mudstones deposited in lower shoreface to inner shelf environments. Trace fossil assemblage-III is known from a 6.9 m thick, outer shelf to lower shoreface, progradational succession showing a vertical transition from dark gray mudstone to fine sandstone. The middle part of this particular section shows Rhizocorallium and flat-type Zoophycos, whereas the upper part hosts profuse Cylindrichnus along with Rosselia and Teichichnus in lesser proportions . This assemblage is comparable to the marine ichnofacies succession combining elements of both Skolithos and Cruziana ichnofacies. The characteristic features of the Talchir trace fossil assemblages are: (i) low ichnodiversity, (ii) presence of forms typically found the in marine environment, (iii) co-occurrences of vertical and horizontal traces of Skolithos and Cruziana ichnofacies, (iv) presence of simple structures formed by trophic generalists, (v) abundance of certain forms, and (vi) presence of monospecific suites. These characters are akin to those of brackish water ichnofaunas. In the Talchir succession, the transition from an ice-contact fan delta system to a relatively ice-free open marine system indicates an enormous flux of fresh water into the sea reducing its overall salinity. Predominance of annelid and worm traces and the rarity of crustacean burrows in the Talchir strata also suggest a stressful environment due to the prevailing cold climate and a lower than normal marine salinity. It thus appears that mixing of fresh water during deglaciation might be a key controlling factor in shaping the characters of ichnofaunas in glacially influenced shallow marine environments.