COFFEE, PIE AND THE MIDDLE DEVONIAN

Several years ago over coffee and pie, John M. Dennison and I discussed the stratigraphic complexities of the Middle Devonian of the Appalachian basin in terms of sea-level change. Numerous researchers have studied the Marcellus and Tully intervals, but few have studied the strata in between the two using sequence stratigraphy, due to the lack of regionally traceable units. After our conversation I decided to tackle this problem. Three wells with cuttings from Preston County West Virginia where chosen to study the link between gamma-ray logs (GR), magnetic susceptibility (MS), percent CaCO₃, geochemistry (Ti, Fe, Ca, Sr, Mo, Cu, Ni, Ba, Se) and sea-level. In the subsurface GR is often used to identify shale/clay content and organic rich zones, but this method can give “false” readings. MS is another way of checking the amount of clays present in the rock. Clays have magnetically susceptible minerals incorporated into their lattice and give higher MS readings than sandstones and pure carbonates. The variation of MS can be used as a proxy for sea-level, high MS during the Lowstand/Trangressive System Tracts (LST/TST) and low MS during Highstand Systems Tract (HST). All samples where analyzed using a Delta Professional XRF on soil mode for Ti, Fe, Ca, Sr, Mo, Cu, Ni, Ba, and Se. The percent CaCO₃ in the well-cuttings were calculated using the ppm of Ca present in each sample. Increases in the level of Strontium (Sr) were also used to verify the presence of calcareous material. The percent CaCO₃ can help identify the presence of limestone and calcareous shale. A sample is considered to be limestone if there is more than 50% CaCO₃ , less than 50% would be considered a calcareous shale. The presence of limestone and calcareous shale marks the TST to maximum flooding surface (MFS). Titanium/Calcium (Ti/Ca) and Iron/Calcium (Fe/Ca) ratios where calculated for each sample. The Input from rivers during LST/TST caused increases in Titanium/Calcium (Ti/Ca) and Iron/Calcium (Fe/Ca) ratios. Increased levels of Mo, Cu, Ni, Ba, and Se are indicators of anoxia and seem to correspond with the zone most researchers believe is the Marcellus shale. The use of gamma-ray logs, magnetic susceptibility, percent CaCO₃, and geochemistry indicate the presence of ~10 cycles. These cycles appear to be regionally traceable units, but further research is needed.