The Stratigraphy and Structure of the Arenaceous Formation of the Main Range Foothills, F.M.S

Abstract

The evidence relating to the problems raised by the Arenaceous Formation of the Main Range Foothills may be summarized thus:—(i) The Foothills are built as a rule of predominantly arenaceous sediments; belts of quartzite-conglomerate occur and argillaceous types generally are interspersed with the coarser grained rocks.(ii) Lithological variations northwards and southwards along the strike of the Formation, and eastwards and westwards through its strati-graphical succession are very great.(iii) No fossils have been found anywhere in these rocks and their age is consequently not definitely known.(iv) The Foothills have obviously been the site of orogenic pressures and metamorphism, and they appear to have formed a buttress against which the Permocarboniferous rocks have been moulded by forces from east-south-east or east.(v) There is a persistent difference of 10° to 20° between the general strike of the Foothills Formation and that of the Permocarboniferous Formation along the eastern margin of the Foothills in the 35-mile tract examined by the writer. This fact cannot be accepted as proof that an angular unconformity exists because structural complexities dependent upon (iv) above can also afford a satisfactory explanation of this divergence of strike.(vi) Scrivenor has mapped the Foothills Formation as Triassic, so that its easterly dip below the Permocarboniferous must, on this hypothesis, be due to overfolding.(vii) Between the western margin of the Foothills and the Main Range granite, occur rocks, many of them strongly metamorphosed, which may originally have been calcareous shales, pyroclasts, and impure limestones, presumed, on lithological grounds, to be of Permocarboniferous age. Their alteration has been so profound that no transitional types have been discovered between thorough-going amphibole- and epidote-schists and the presumed parental calcareous sediments. Metamorphosed argillaceous and arenaceous rocks accompany them.