Comparing lithologic and rock magnetic cyclicity in the Upper Cretaceous Fort Hayes Limestone, southern Colorado-northern New Mexico

The Upper Cretaceous (Turonian) Ft. Hayes Limestone is composed of well-developed limestone-shale couplets (0.2-2 m) deposited in response to orbitally controlled climate changes in the Western Interior seaway. Individual precession/obliquity driven limestone-shale couplets are bundled into eccentricity cycles; these bundles have been correlated with the aid of distinct bentonite beds over distances of 800 km. Previous interpretations of the couplets suggest they are the result of alternating wet and dry climatic periods. We are examining the rock magnetic characteristics of the Ft. Hayes Limestone at several localities to determine the relationship between lithologically defined cyclicity and specific rock magnetic data. Previous rock magnetic studies of other Mesozoic and Cenozoic marine deposits have recognized orbitally controlled dust and/or fluvial-derived signals. Samples collected every 5-10 cm through three stratigraphic sections in southern Colorado and northeast New Mexico show considerable variation in intensity of anhysteretic remanent magnetization (ARM) and susceptibility (MS). Limestone and shale ARM values range from about 0.3 to about 9.0 x 10 -6 Am 2 /kg); notably the available data set shows no distinct difference between limestone and shale ARM intentities. Samples distributed laterally at constant position in the same limestone bed yield statistically indistinguishable ARM intensities, as well as multiple specimens from the same sample. Variations in ARM values do not correlate with the lithologic variations of individual limestone-shale couplets. However, we see well-developed longer term ARM variability, with low values (<2 x 10 -6 Am 2 /kg) in the lower Ft. Hayes rising to higher (2 to 6 x 10 -6 Am 2 /kg) values with variations that do not correlate with lithology or couplet bundles (describe trend) in the middle and upper part of the member. These trends can be correlated between each of the sections despite differences in limestone vs. shale abundance. ARM values in the conformably overlying Smokey Hill Shale decrease significantly (<1.5 x 10 -6 Am 2 /kg) and show little or no variability over the basal 10 meters. We interpret this longer term ARM variability as representing regional changes in fine-grained eolian and/or fluvial-derived magnetic mineral assemblages, likely dominated by a moderate coercivity cubic phase (i.e. low Ti magnetite and possibly maghemite). There appears to be little or no relationship between the orbitally controlled climatic changes that produced individual limestone-shale couplets, which sheds light on their previously interpreted wet-dry climatic origins.