Abstract: Almost all the vertical sequences of sedimentary rocks are composed of clastic rock-carbonate rock cycles of varying sizes. An individual clastic rock-carbonate rock megacycle (first-order cycle) tends to be assembled by the lower clastic rock associations and upper carbonate rock associations. The large-scale unconformity interfaces are always found at the top and bottom of these cycles. A typical elastic rock-carbonate rock cycle covers three stages for its formation: weathering and denudation or terrestrial clastic deposition, marine clastic deposition and marine carbonate deposition. The formation of the clastic rock and carbonate rock associations is interpreted to be controlled mainly by transgression and regression which, in turn, are affected by expansion and shrinkage of the earth and cold and temperate climates which, in turn, are also related to the variations in gravitational constant when the solar system moves around the centre of the Galaxy. Thus the superposition of the elastic rock and carbonate rock associations may be responsible for the formation of the clastic rock-carbonate rock megacycles as a consequence of the periodic changes in gravitational constant with time.