Mineralogy and geochemistry of siliciclastic Miocene Cuddalore Formation,Cauvery Basin,South India:Implications for provenance and paleoclimate

摘要

The Cuddalore Formation of the Cauvery Basin received siliciclastic detritus from inland areas of the Southern Granulite Terrain(SGT).It represented continental-fluvial sedimentation in the eastern continental margin of South India during the Miocene.Indian Summer Monsoon was thought to be initiated in the early Miocene and intensified during the middle Miocene causing major climatic shifts in the Indian subcontinent.In the present work,detailed mineralogical and geochemical studies on the siliciclastic Cuddalore Formation have been carried out to understand the provenance and paleoclimatic conditions during the Miocene.The paleocurrent direction,textural immaturity and framework detrital modes of sandstones suggest rapid uplift of basement and sediment source from nearby Madras Block of SGT.Various diagnostic immobile trace element ratios such as Th/Sc,Co/Th,La/Sc,La/Co suggest a tonalite-trondhjemite-granodiorite-charnockite provenance,and somewhat more felsic composition of source area compared to the present upper continental crust(UCC).Rare earth element mixed model suggests that sediments were dominantly(80%)sourced from felsic charnockite,with a minor contribution(20%)from mafic granulites.Higher abundance of advanced-weathering products like kaolinite,very high(>98)chemical index of alteration(CIA)values,severe depletion of mobile elements(Ca,Na,K,Mg,Ba,Rb)in comparison to UCC,significantly higher a Mg,a Ca,a Na,a K,a Sr and a Ba values(higher than the unity),all suggest the extreme chemical weathering in source terrain and/or in the sedimentary basin.Calculations based on CIA show high average surface temperature between 29.3°Cand 29.5°Cand high mean annual precipitation ranging from 2339 mm/yr to 2467 mm/yr.The geochemical data are consistent with the paleogeographic position of the depositional basin(Cauvery Basin)and suggest the deposition of Cuddalore sediments(the Cuddalore Formation)in a nearequatorial location under a warm climate condition with abundant monsoonal precipitation.