VARIATIONS IN ROCK MAGNETIC PARAMETERS IN THE FORT HAYS LIMESTONE, NIOBRARA FORMATION, BADITO, COLORADO: A RECORD OF CLIMATICALLY CONTROLLED CYCLES IN MARINE CARBONATE ROCKS?

Hemipelagic to pelagic marine deposits commonly display lithologic and/or biologic evidence of climatically controlled cyclicity. The Fort Hays member of the Upper Cretaceous Niobrara Formation consists of rhythmically interbedded massive limestones (lime mudstone/wackestone; 0.2 -1 m thick) and calcareous shale/marl (< few cm) and is well exposed in western interior seaway sections. We are evaluating variations in rock magnetic properties, which may reflect variations in atmospheric dust influx, to evaluate whether these correlate with cyclic lithologic variations in Fort Hays strata. Anhysteretic remanent magnetization (ARM) intensity, which is controlled by the concentration of fine-grained (<20 micron) ferromagnetic (e.g., magnetite/maghemite) minerals and is insensitive to carbonate concentration, and other rock magnetic data, have been obtained from a section at Badito, south-central Colorado. At this locality, the member is about 14 m thick, and exhibits at least six bundled limestone beds alternating with shale. Preliminary results from a ~1 cm sampling resolution on select parts of the section indicates that ARM varies considerably, from about 9.0x10-6 to 2.4x10-7 Am2/kg, with intervals of relatively high ARM intensity concentrated near the top of the member. Of note, shales have comparable or lower ARM values (typically 1 to 3 x10-6 Am2/kg) than those from most limestones, suggesting only minor early diagenetic modification (i.e. redistribution of calcareous material into adjacent limestones)(Laferriere et al. GSAB, 1992). Through the entire section, specific intervals of notably high ARM intensity correlate with high NRM intensity (3-5 x 10-7 Am2/kg) and excellent behavior in AF demagnetization (median destructive fields between 30 and 60 mT), suggesting additional low-frequency (member-scale) atmospheric dust influx affecting the Fort Hays section.