Reassessment of the Position of the Plio-pleistocene Boundary: Is There a Case for Lowering It to the Gaussimatuyama Palaeomagnetic Reversal?

Forty eight years have elapsed since formal discussions were initiated, in 1948, on the position of the PlioPleistocene boundary. At the 18th International Geological Congress held in London during that year the Commission on the Plio-Pleistocene boundary recommended that “the Lower Pleistocene should include as its basal member the marine Calabrian Formation together with its terrestrial (continental) equivalent the Villafranchian” (King and Oakley, 1950, p. 214). The suitability of the base of the Calabrian as the Plio-Pleistocene boundary was specifically linked to the first appearance, within the lowermost Calabrian as then defined, of the cryophilic mollusc species Arctica islundica and Hyalinea baltica: “according to the evidence given this usage would place the boundary at the horizon of the first indication of climatic deterioration in the Italian Neogene succession” (King and Oakley, op cit.). From the time of the adoption of this resolution until the acceptance, at the 27th International Congress in Moscow, some 36 years later, of the Global Stratotype Section and Point (GSSP) at Vrica as defining the Plio-Pleistocene boundary, considerable advances in the understanding of the marine Plio-Pleistocene succession in southern Italy, and in defining and placing within a sound chronological framework important global biostratigraphic and climatic events in both marine and terrestrial sequences, have opened to question the appropriateness of the 1948 definition. The adoption of the Vrica GSSP in 1984, at a stratigraphic position essentially in keeping with the spirit of the 1948 resolution, represents, in the view of the writer and a number of other Quatemary stratigraphers, a compromise between historical usage and recent advances in Plio-Pleistocene stratigraphy, which fully satisfies neither tradition nor the evidence of modem stratigraphy. It seems somewhat ironical that, at a time of rapid advances in marine isotope stratigraphy and both terrestrial and marine chronostratigraphy, the stamp of approval was given to a stratotype whose position had been advocated in the light of the stratigraphic knowledge prevailing a full generation earlier. There is no question that the formal requirements for the definition of the Vrica GSSP were met in 1984 and that the boundary, as placed presently at 1.8 Ma, must be accepted as dividing the Pliocene from the Pleistocene. What is, however, strongly questioned by many, including the present writer, is whether it is justified to maintain a boundary whose position is rooted in priority and historical usage rather than current evidence for the considerable earlier onset of a climatic regime which gives an essential unity to the Pleistocene and is clearly mirrored in the global stratigraphic record. Lowering of the boundary to this position would not only be in keeping with the 1948 conclusion that it be placed at the fust indication of climatic deterioration in the Neogene succession, but would facilitate correlation through the marked lithostratigraphic and palaeontological changes with which it is associated in terrestrial sequences, through the unique marine isotopic signature with which it coincides, and through its correspondence with a major palaeomagnetic reversal (the Gauss-Matuyama at 2.6 Ma). These considerations must necessarily be viewed against the background of the desirability of stability in lithostratigraphic classification and the relative recency of the acceptance of the present GSSP, intemational stratigraphic correlation should certainly not be viewed as transient and subject to the whims or fashions of the day.