X-RAY DIFFRACTION ANALYSIS OF GLAUCONITE INTER-LAYER THICKNESS: A NEW METHOD OF ASSESSING GLAUCONITE MATURITY

The New Jersey coastal plain is home to glauconite-rich marine sections that span the K-Pg boundary. At Inversand, the K-Pg boundary is located above the contact between the Navesink and Hornerstown formations at a bonebed, the Main Fossiliferous Layer. Locating the K-Pg boundary in this locality had been complicated by the heavy bioturbation by Thalassinoides isp. burrows. Previous studies have shown that the maturity of glauconite matrix, as indicated by K₂O concentrations and morphology, increases up section, consistent with decreased sedimentation rates and deepening from inner to outer shelf environments. Changes in glauconite chemical composition should be accompanied by changes in unit cell dimensions. This study seeks to determine whether unit cell dimensions of burrow and matrix glauconite, calculated using x-ray diffraction (XRD), is another method that can be used to assess glauconitic maturity. Samples of sediment from matrix and burrows were obtained at 20 cm intervals from 100 cm to 60 cm below the Navesink/Hornerstown Formation contact, and at 10 cm intervals from 60 cm below to 50 cm above the contact. The chemical composition of purified glauconite samples was measured by x-ray fluorescence (XRF). Purified glauconite samples were filtered onto 25 mm, 0.45 µm silver membrane filters, used as an internal standard. K₂O concentrations in matrix glauconite increase from 4.8 to 8.3% up section, while Zr and Ti concentrations decrease from 106 to 52.9 ppm and from 0.534 to 0.272%, respectively, indicating slower sedimentation rates, increase in depth and more distal character of the Hornerstown Formation. K₂O concentrations in Thalassinoides burrow fill decrease slightly with depth and differ from the surrounding matrix. Matrix glauconite inter-layer thickness increases from 9.992Å to 10.015Å up section. K₂O and interlayer spacing are positively correlated (r² = 0.86). This suggests that inter-layer thickness may also be used as a glauconite maturity index.