HIGH-RESOLUTION ELEMENTAL DATA IN CARBONATES, A TOOL FOR CONSTRUCTING SEQUENCE STRATIGRAPHIC FRAMEWORK, OR FANCY GAMMA-RAY WIRE-LINE LOG?

Sequence stratigraphy is a powerful tool for interpreting and understanding a wide variety of depositional systems. In shallow-water carbonate rocks, where facies shifts permit the identification of sequences and sequence boundaries, sequence stratigraphic frameworks are established to explain facies cyclicity and their relationship to sea-level changes, help predict reservoir compartmentalization, and provide the basis for stratigraphic correlations. This approach is harder to implement in deep-water carbonate deposits where the rocks are typically more homogeneous. In an attempt to address this challenge, recent studies have interpreted changes in the continental proxy elements [Al], [Si], [K], [Ti], and [Zr] to reflect variations in the influx of continentally derived siliciclastics into the carbonate system. These variations have been used to infer environmental changes, such as water depth and paleoshoreline relative location, and constrain the boundaries of stratigraphic sequences within the homogenous deep-water carbonates.

This study tests the reliability of continental proxy elements to construct a sequence stratigraphic framework in the deep-water carbonate deposits of the Llandovery (Early Silurian) succession of the Michigan Basin. The age of the Llanovery interval is well constrained by previous work, and time-equivalent carbonate depositional packages have been correlated across the basin, both of which make it an ideal case study. High-resolution (every 1 cm) x-ray fluorescence spectroscopy elemental data are presented from ten drill cores. These data suggest that the continental proxy elements exhibit systematic variations with strong positive correlations between Al-K, Ti-Zr, Al-Si, and Al-Zr within a single well. When correlated between wells, these variations crossed well-established time horizons, suggesting that these elemental variations occurred at different times at different locations across the basin. Therefore, the result of this study shows that changes in the [Al], [Si], [K], [Ti], and [Zr] are inherently diachronous in carbonate rocks and unreliable chronostratigraphic anchors similar to gamma-ray variability.