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

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 innerand 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 fifthorder 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. Earth’s ancient shallow marine carbonate platforms are important archives of past global change. Their in situ biological genesis makes them sensitive indicators of physical conditions in shallow marine environments, including temperature, circulation and sea level (Wilson 1975). The accommodation space afforded by water depth is thought to govern the amount and type of sediment that accumulates at a given location; water depth in turn is controlled by a combination of tectonic subsidence, near-shore deposition and global sea level, which vary significantly through time (Schlager 2005). The commonly observed stacking of metrescale shallowing upward cycles in carbonate platform deposits has led to proposals of allocyclic v. autocyclic forcing mechanisms. The most widely cited allocyclic hypothesis is astronomically controlled eustasy (Fischer 1964; Hardie et al. 1986); autocyclic mechanisms involve infilling and oceanward progradation of sedimentation without eustatic change (Ginsburg 1971). To this day, the allocycle v. autocycle debate remains unresolved in understanding the origin of 10–10 year scale lithologic cyclicity in shallow marine carbonates (Schwarzacher 2000; Burgess 2006; Schlager 2010). Full shallowing to sea level may not develop in response to every sea-level oscillation (subtidal ‘missed beats’), resulting in amalgamated cycles; conversely, prolonged episodes of subaerial exposure may occur in inner platform settings that are only briefly or never flooded with every sea-level oscillation, leading to condensed or peritidal ‘missed beats’ (Goldhammer et al. 1990). Ultimately facies analysis of shallow marine sections is limited in teasing apart the interplay of eustasy owing to climate change, and subsidence arising from local tectonics. This is in large part owing to the relatively subjective interpretation of lithofacies that is required from the stratigrapher. Therefore we need to develop facies-independent From: Jovane, L., Herrero-Bervera, E., Hinnov, L. A. & Housen, B. A. (eds) 2013. Magnetic Methods and the Timing of Geological Processes. Geological Society, London, Special Publications, 373, 325–340. First published online April 25, 2013, http://dx.doi.org/10.1144/SP373.20 # The Geological Society of London 2013. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics by guest on July 5, 2013 http://sp.lyellcollection.org/ Downloaded from