Paper No. 8
Presentation Time: 10:45 AM
INFLUENCE OF ORBITALLY INDUCED CLIMATE CHANGE ON DEPOSTITIONAL PROCESSES IN THE GREENHORN LIMESTONE FORMATION, HUERFANO COUNTY, COLORADO
We are testing the hypothesis that depositional processes of the mid-Cretaceous Greenhorn Formation deposited in the Western Interior Basin of North America during the Late Cenomanian to early Turonian, were influenced by orbitally-driven climate change using rock magnetic data. Correlation of rock magnetic data, including anhysteretic remanent magnetization (ARM), magnetic susceptibility, isothermal remanent magnetization in different DC fields to saturation, and hysteresis properties, from two overlapping sections of the complete Greenhorn Formation will provide detailed spatial distribution for the depositional processes and magnetic mineral climate encoding. The Greenhorn Formation includes the Lincoln Limestone, Hartland Shale, and the Bridge Creek Limestone members and consists of calcareous shales and limestones deposited in the central part of the basin as sea level reached peak highstand. Both sampled sections, each about 30 m thick, extend from the upper Graneros Shale, through the Greenhorn Formation, to the lower Carlisle Shale, with samples collected every 2 to 5 cm. Sections collected are at Badito, CO, and north of Redwing, CO.The 576 samples were hand crushed to granule size pieces and packed into 7cc IODP boxes. Bulk magnetic susceptibility, anhysteretic remanent magnetization (ARM) intensity at different peak AF levels, and isothermal remanent magnetization (IRM) intensity record variations in magnetic mineral concentration and are proxies to determine orbital scale cycles and precise stratigraphic correlation between sections. ARM intensities in a peak field of 100 mT at both sites show periodic variation within the Greenhorn Formation displaying differences in ferromagnetic mineral content of detrital origin. Intensities in the Greenhorn from both sites vary between 1.2 x 10-3 and 1.3 x 10-4 A/m which is a lower range compared to the bounding upper Graneros and lower Carlisle Shales. Magnetic susceptibility also shows periodic variation with a strong correlation between sites throughout the entire measured section. MS within the Greenhorn Formation ranges from 3.5 x 10-2 and 2.86 x 10-3.