Global Milankovitch cycles recorded in rock magnetism of the shallow marine lower Cretaceous Cupido Formation, northeastern Mexico
Linda A. Hinnov, Kenneth P. Kodama, David J. Anastasio, Maya Elrick, Diana K. Latta, 2013. "Global Milankovitch cycles recorded in rock magnetism of the shallow marine lower Cretaceous Cupido Formation, northeastern Mexico", Magnetic Methods and the Timing of Geological Processes, L. Jovane, E. Herrero-Bervera, L.A. Hinnov, B. Housen
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Rock magnetic cyclostratigraphy was measured in the Barremian–Aptian Cupido (‘Cupidito’) Formation, northeastern Mexico. The goal was to develop an objective evaluation of palaeo-environmental variability recorded in the formation that is independent of facies analysis and interpretation. Anhysteretic remanent magnetization (ARM) was used to estimate magnetic mineral concentration variations for the upper 143 m of the formation, which is characterized by metre-scale carbonate cycles representative of inner- and middle-shelf marine environments. Isothermal remanent magnetization acquisition experiments and scanning electron microscope (SEM) examination indicate that micron-sized detrital magnetite from eolian dust carries the ARM signal. At the sampled sections from Garcia and Chico canyons, 25 km apart, ARM records a synchronous 30–35 m oscillation with maxima coinciding with fourth-order sequence boundaries, superimposed with prominent high-frequency variability. Calibrating the 30–35 m oscillation to a 405 kyr period (long eccentricity cycle) focuses the high frequencies into short eccentricity, obliquity and precession index bands; the precession-band signal modulates with an eccentricity signature. The ARM signal is correlated between sections, but decoupled from the interpreted fifth-order depositional cycles. ARM amplitudes diminish up-section with facies suggesting deepening conditions that diluted magnetite concentration. This probably signals a warming, increasingly humid climate, changing global circulation and/or greater dispersal of magnetite grains.
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Magnetic Methods and the Timing of Geological Processes
Magnetostratigraphy is best known as a technique that employs correlation among different stratigraphic sections using the magnetic directions defining geomagnetic polarity reversals as marker horizons. The ages of the polarity reversals provide common tie points among the sections, allowing accurate time correlation. Recently, studies of magnetic methods and the timing of geological processes have acquired a broader meaning, now referring to many types of magnetic measurements within a stratigraphic sequence. Many of these measurements provide correlation and age control not only for the older and younger boundaries of a polarity interval, but also within intervals. Thus, magnetostratigraphy no longer represents a dating tool based only on geomagnetic polarity reversals, but comprises a set of techniques that includes measurements of geomagnetic field parameters, environmental magnetism, rock-magnetic properties, radiometric dating and astronomically forced palaeoclimatic change recorded in sedimentary rocks, and key corrections to magnetic directions related to geodynamics, palaeocurrents, tectonics and diagenetic processes.