Fossil trees in ancient fluvial channel deposits: evidence of seasonal andlonger-term climatic variability

It has been established that large numbers of certain trees can survive in the beds of rivers of northeastern Australia where a strongly seasonal dist ribution of precipitation causes extreme variations in flow on both a yearl y and longer-term basis. In these rivers, minimal flow occurs throughout mu ch of any year and for periods of up to several years, allowing the trees t o become established and to adapt their form in order to facilitate their s urvival in environments that experience periodic inundation by fast-flowing , debris-laden water. Such trees (notably paperbark trees of the angiosperm genus Melaleuca) adopt a reclined to prostrate, downstream-trailing habit, have a multiple-stemmed form, modified crown with weeping foliage, develop ment of thick, spongy bark, anchoring of roots into firm to lithified subst rates beneath the channel floor, root regeneration, and develop in flow-par allel, linear groves. Individuals from within flow-parallel, linear groves are preserved in situ within the alluvial deposit of the river following bu rial and death. Four examples of in situ tree fossils within alluvial channel deposits in t he Permian of eastern Australia demonstrate that specialised riverbed plant communities also existed at times in the geological past. These examples, from the Lower Permian Carmila Beds, Upper Permian Moranbah Coal Measures a nd Baralaba Coal Measures of central Queensland and the Upper Permian Newca stle Coal Measures of central New South Wales, show several of the characte ristics of trees described from modern rivers in northeastern Australia, in cluding preservation in closely-spaced groups. These properties, together w ith independent sedimentological evidence, suggest that the Permian trees w ere adapted to an environment affected by highly variable runoff, albeit in a more temperate climatic situation than the modem Australian examples. It is proposed that occurrences of fossil trees preserved in situ within allu vial channel deposits may be diagnostic of environments controlled by seaso nal and longer-term variability in fluvial runoff, and hence may have value in interpreting aspects of palaeoclimate from ancient alluvial successions . (C) 2001 Elsevier Science B.V. All rights reserved.