Global Milankovitch cycles recorded in rock magnetism of the shallow marine lower Cretaceous Cupido Formation, northeastern Mexico

摘要: Rock magnetic cyclostratigraphy was measured in the Barremian–Aptian Cupido (‘Cupidito’) Formation, northeastern Mexico. The goal to develop an objective evaluation of palaeo-environmental variability recorded formation that is independent facies analysis and interpretation. Anhysteretic remanent magnetization (ARM) used estimate mineral concentration variations for upper 143 m formation, which characterized by metre-scale carbonate cycles representative innerand middle-shelf marine environments. Isothermal acquisition experiments scanning electron microscope (SEM) examination indicate micron-sized detrital magnetite from eolian dust carries ARM signal. At sampled sections Garcia Chico canyons, 25 km apart, records a synchronous 30–35 oscillation with maxima coinciding fourth-order sequence boundaries, superimposed prominent high-frequency variability. Calibrating 405 kyr period (long eccentricity cycle) focuses high frequencies into short eccentricity, obliquity precession index bands; precession-band signal modulates signature. correlated between sections, but decoupled interpreted fifthorder depositional cycles. amplitudes diminish up-section suggesting deepening conditions diluted concentration. This probably signals warming, increasingly humid climate, changing global circulation and/or greater dispersal grains. Earth’s ancient shallow platforms are important archives past change. Their situ biological genesis makes them sensitive indicators physical environments, including temperature, sea level (Wilson 1975). accommodation space afforded water depth thought govern amount type sediment accumulates at given location; turn controlled combination tectonic subsidence, near-shore deposition level, vary significantly through time (Schlager 2005). commonly observed stacking metrescale shallowing upward platform deposits has led proposals allocyclic v. autocyclic forcing mechanisms. most widely cited hypothesis astronomically eustasy (Fischer 1964; Hardie et al. 1986); mechanisms involve infilling oceanward progradation sedimentation without eustatic change (Ginsburg 1971). To this day, allocycle autocycle debate remains unresolved understanding origin 10–10 year scale lithologic cyclicity carbonates (Schwarzacher 2000; Burgess 2006; Schlager 2010). Full may not response every sea-level (subtidal ‘missed beats’), resulting amalgamated cycles; conversely, prolonged episodes subaerial exposure occur inner settings only briefly or never flooded oscillation, leading condensed peritidal beats’ (Goldhammer 1990). Ultimately limited teasing apart interplay owing climate change, subsidence arising local tectonics. large part relatively subjective interpretation lithofacies required stratigrapher. Therefore we need facies-independent From: Jovane, L., Herrero-Bervera, E., Hinnov, L. A. & Housen, B. (eds) 2013. Magnetic Methods Timing Geological Processes. Society, London, Special Publications, 373, 325–340. First published online April 25, 2013, http://dx.doi.org/10.1144/SP373.20 # Society London Publishing disclaimer: www.geolsoc.org.uk/pub_ethics guest on July 5, 2013 http://sp.lyellcollection.org/ Downloaded