Christian Meister, Martin Aberhan, Joachim Blau, Jean-Louis Dommergues, Susanne Feist-Burkhardt, Ernie A. Hailwood, Malcom Hart, Stephen P. Hesselbo, Mark W. Hounslow, Mark Hylton, Nicol Morton, Kevin Page, Greg D. Price. Episodes 2006;29:93-106. https://doi.org/10.18814/epiiugs/2006/v29i2/003
The establishment of chronostratigraphic units such as geological Systems and Series depends upon an ability to equate succession in rock strata with the passage of time, and upon a pervasive Law of Superposition. These assumptions hold true at a gross scale. But, at fine scales of stratigraphic resolution, they commonly break down. Thus, bioturbation in Phanerozoic marine deposits typically homogenizes sedimentary packages spanning millennia, affecting biostratigraphic, isotopic and paleomagnetic signals, and post-burial mass transport phenomena such as large-scale sedimentary slumps and intra-stratal diapirs locally disrupt superpositional relationships on a larger scale. Furthermore: the multi-stage transport of microfossils prior to final burial complicates the relationship between depositional and biostratigraphic ages; paleomagnetic signals, imposed at shallow burial depths, may be distinct from depositional ages; and high precision zircon U-Pb dates from tuff layers determine time of crystallization in the magma, rather than depositional age. In such circumstances, depositional units cannot be unambiguously equated with time units: because they include multiple temporal components, they cannot be subdivided precisely into time-rock units. By contrast, the different phenomena which have contributed to constructing sedimentary deposits, pre-, syn- and post-depositional, may be effectively accommodated within a unitary geological time framework.
Abstract The age of the beginning of magnetic polarity Chron M0r, a proposed marker for the base of the Aptian Stage, is disputed due to a divergence of published radioisotopic dates and ambiguities in stratigraphic correlation of sections. Our magnetostratigraphy of core DH1 from Svalbard, Norway, calibrates a bentonite bed, dated by U-Pb methods to 123.1 ± 0.3 Ma, to the uppermost part of magnetozone M1r, which is ∼1.9 m.y. before the beginning of Chron M0r. This is the first direct calibration of any high-precision radioisotopic date to a polarity chron of the M sequence. The interpolated age of 121.2 ± 0.4 Ma for the beginning of Chron M0r is younger by ∼5 m.y. than its estimated age used in the Geologic Time Scale 2012, which had been extrapolated from radioisotopic dates on oceanic basalts and from Aptian cyclostratigraphy. The adjusted age model implies a commensurate faster average global oceanic spreading rate of ∼12% during the Aptian–Santonian interval. Future radioisotopic dating and high-resolution cyclostratigraphy are needed to investigate where to expand the mid-Jurassic to earliest Cretaceous interval by the required ∼4 m.y.
The Helmholtz coil system of the MOLSPIN anisotropy delineator is shown to be partly sensitive to the specimen shape, particularly if the sample departs from the optimum shape. The effect is found to be most marked when measuring weakly anisotropic rocks which have an anisotropy below about 13 × 10−12 m3 and susceptibility less than 50 × 10−10 m3. However, comparison of MOLSPIN data to measurements undertaken on low and high field torque magnetometers, suggest the results are not unduly affected by sample shape if the rock anisotropy signal is sufficient to dominate this spurious effect. Some suggestions are also made for minimising noise factors and thus improving instrument sensitivity.
Abstract The magnetostratigraphy of two cores from the late Triassic Lunde Formation of the northern North Sea are compatible with the calibrated magnetostratigraphy for the Norian stage determined by Gallet et al. (1993) from Tethyan carbonates. The polarities of the cores indicate a lower to upper Norian age for core 9/13A–36 and lower to middle Norian age for core 9/13A–A45. The isolated characteristic magnetizations are due to a Triassic detrital remanent magnetization, which is partially overprinted with a drilling-induced remanence of two origins. The drilling-induced remanences are associated with (1) the main coring procedure, producing an overprint which is oriented downcore and (2) the subsample coring procedure, which produces an overprint parallel to the subsample core axis and directed outwards from the main core axis. The elimination of those specimens with a significant drilling-induced remanence is important for the determination of the magnetostratigraphy in these cores.
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