Author： Packham G. H.
Publisher： Taylor & Francis Ltd
ISSN： 0812-0099
Source： Australian Journal of Earth Sciences, Vol.50, Iss.2, 2003-04, pp. : 169-179
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Abstract
U–Pb ages for volcanic zircons from the Lower Devonian Turondale and Merrions Formations initially dated by Jagodzinski and Black (1999) have been recalculated by Compston (2000, 2001). The Turondale, Waterbeach and Merrions Formations are, in ascending stratigraphic order, three of the Lower Devonian deep‐water formations of the Hill End Trough. The recent discovery of a Lochkovian conodont fauna of the delta zone in derived limestone blocks from the uppermost beds of the Turondale Formation has established a maximum age for that horizon. The principal lithological assemblages of the formations are volcanic (megaturbidite and lavas), epiclastic turbidites and essentially hemipelagic lithologies. The mean zircon ages suggest a long duration for the Merrions Formation, 615 m thick and 91% volcanic, and a very much shorter interval for the upper two‐thirds of the Turondale Formation, 360 m thick and 34% volcanic, plus the intervening non‐volcanic Waterbeach Formation, 512 m thick. A sedimentological model based on scaled depositional rates for the facies has been used to estimate the relative accumulation rates and durations of the succession from the base of the Turondale Formation to the top of the Merrions Formation. The model indicates that the Merrions Formation accumulated in a much shorter time than the interval from the top of the lower Turondale Formation to the base of the Merrions Formation. The analytical uncertainties of the Jagodzinski and Black (1999) zircon ages are large enough to be compatible with the sedimentological model, but the smaller analytical uncertainties calculated by Compston (2000, 2001) fall far from the scaled sedimentological model. Grouping together of the lower Merrions Formation and lower Turondale Formation zircon dates in Compston (2000) for a single mid‐Lochkovian age is in conflict with the biostratigraphic and sedimentological evidence available and the derived time‐scale for the Early Devonian is highly questionable. Given the incompatibility of the recalculated zircon ages with the depositional history and the uncertainty of biostratigraphic dating of the formations, neither of the quite different sets of ages quoted in Compston (2000, 2001) are useful for the definition of a numerical time‐scale.
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