Abstract

Time series based on determinations (wt%) of calcium carbonate and total organic carbon have been generated for the entire Lower Jurassic hemipelagic Belemnite Marls, Dorset, southern Britain. This formation was deposited during early Pliensbachian time, at a paleolatitude of about 35°N, in an epicontinental sea that was largely enclosed by the supercontinent of Pangea. The sequence contains compositionally diverse light and dark marl bedding couplets, the thicknesses of which are notably reduced in the upper third of the section. The regularity of the couplets in segments of the sequence, combined with a regular amplitude modulation, indicates an origin that is related to the orbital-precession cycle (i.e., one of the Milankovitch parameters). A timescale is developed by assigning a duration of 20 k.y. per couplet, and this suggests that the entire formation represents at least 1.78 m.y.

Using the new timescale, bundles of bedding couplets are shown to have periods consistent with the 123 k.y. component of the orbital-eccentricity variations. The amplitude of the couplets varies at the same frequencies as the bundle cycles, in accordance with the interpretation that the couplets record precession and the bundles record eccentricity. However, despite having the same frequency of variation, there is no consistent relationship (coherence) between the variations in amplitude of the couplet cycle and the bundle cycles as would be predicted by the eccentricity-precession relationship. This mismatch can be explained in terms of nonlinear behavior of a climatic system characterized by a varying response time or nonlinear response of the sedimentary system itself. The data contain no evidence for orbital-obliquity cycles. Because the obliquity cycle affects insolation principally at high latitudes, the climatic factors that indirectly controlled the sedimentary cyclicity must have arisen at relatively low latitudes.

The Belemnite Marls timescale indicates highly variable minimum durations for ammonite zones and subzones. True durations cannot be determined because it is possible that the succession is incomplete as a result of undetected erosion and/or nondeposition. A combination of the new results with cyclostratigraphic data from Yorkshire, northeast England, and the Southern Alps, Switzerland, indicates, based on cycle counts, that the Pliensbachian Stage lasted at least 4.82 m.y. Marine Sr-isotope ratios appear to have decreased linearly from the start of Jurassic time until the end of Pliensbachian time. The rate of decrease in 87Sr/86Sr established using the Belemnite Marls timescale was 0.000042/m.y. (or less if the main part of the formation is incomplete). Using this rate, with the observed changes in Sr-isotope ratios, gives minimum durations of 2.86 m.y. for Hettangian time, 7.62 m.y. for Sinemurian time, and 6.67 m.y. for Pliensbachian time.

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