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Abstract

Magnetostratigraphic dating of sedimentary strata is often the most precise technique available for temporally constraining the evolution of and controls upon sedimentary basins over I Ma in age. Uncertainties in the absolute dates derived by this technique are often difficult to assess quantitatively, despite the desirability of specifying their precision. An explicit discrimination should be made between correlations of the local magneto-polarity stratigraphy (MPS) to the global geomagnetic polarity time scale (GPTS) based on independent biostratigraphic or radiometric time control and those based on the smoothest derived sediment-accumulation rates. Situations in which there is a single, compelling correlation and those in which the correlation is the most reasonable of several possibilities should also be explicitly distinguished. In the latter case, alternative feasible correlations should be illustrated in order to permit a qualitative assessment of the uncertainties involved. Two classes of uncertainties are associated with the temporal calibration of magnetostratigraphic sections: those related to the creation of the local MPS and those related to the GPTS. Imprecision in measured stratal thicknesses and in the position of magnetozone boundaries can produce significant (up to 50 percent) uncertainties both in magnetozone patterns and in derived rates of sediment accumulation. Uncertainties in the GPTS result from uncertainties in the radiometric calibration of magnetic anomaly patterns. Comparison of available GPTS’s indicates uncertainties of (1) as much as 100 percent for sediment accumulation rate calculations involving intervals of less than 1–2 my and (2) up to 3 my in absolute ages. An example drawn from the Late Cretaceous to Eocene Axhandle thrust-top Basin of central Utah illustrates these uncertainties.

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