Astronomical forcing of meandering river behaviour: Milankovitch cycles in Devonian of East Greenland

Abstract

A more than 1100 m thick succession dominated by single- and multistorey sandstone bodies from the Upper Devonian Kap Graah Group in East Greenland is interpreted as a meandering river and associated overbank deposits. The sandstone bodies display well-developed lateral accretion bedding and are interpreted to be deposited as point bars. In a well-exposed section, 215 m thick, individual sandstone bodies have been analized quantitatively in order to compare the individual palaeochannel characteristics. The results indicate cyclic variations in discharge, channel slope and channel size. The cycles are of the order of c.20 m thick with superimposed c.100 m mega-cycles; both are interpreted as reflecting cyclic variations in precipitation rate. The cycles are interpreted as being governed by astronomical forcing of the climate, reflecting Milankovitch type precession cycles in the order of 20,000 years with modulation by c.110,000 years eccentricity mega-cycles. This study confirms that the precession and eccentricity cycles exhibit the main forcing on low latitude monsoonal climate.