--> Application of Handheld ED-XRF for High-Resolution Chemostratigraphy in Texturally Homogeneous Carbonate Mudstones: Salina A-1 Carbonate (Silurian), Michigan Basin

AAPG ACE 2018

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Application of Handheld ED-XRF for High-Resolution Chemostratigraphy in Texturally Homogeneous Carbonate Mudstones: Salina A-1 Carbonate (Silurian), Michigan Basin

Abstract

Handheld energy dispersive x-ray fluorescence spectrometry (ED-XRF) is routinely used to observe elemental variability in fine-grain siliciclastics. Although ED-XRF data have proven useful in sequence stratigraphic interpretations, there has been limited application in fine-grain carbonates. Here, we use ED-XRF chemostratigraphy in the Silurian A-1 Carbonate (A-1), Michigan Basin. In the basin-center, the A-1 is a ~55 ft thick, texturally homogenous, carbonaceous mudstone. The A-1 serves as the source rock for the underlying Niagaran “pinnacle” reefs, and is regarded as a potential unconventional reservoir. Whereas previous studies focused on the stratigraphic relationship between the A-1 and Niagaran “pinnacle” reefs, little is known about the geological variability in the basin-center. Here, high-resolution (~2.5 cm) elemental data from eight cores were integrated with petrographic and mineralogical observations to evaluate two hypotheses. (1) Texturally homogeneous carbonates in the A-1 have unique elemental signatures (i.e. chemofacies) that can be identified and correlated within the basin. (2) Elemental variations reflect temporal changes in relative sea-level, redox conditions, and sedimentation.

Redox-sensitive elements (Mo, Ni, Cu) have consistently low abundances, suggesting that either redox conditions were relatively oxic and stable during A-1 deposition, or basin restriction limited the supply of these elements from the open ocean. In contrast, strong stratigraphic variations in Si, Al, K, S, and Sr are observed that can be correlated between basin-center wells. Elevated concentrations of Si, Al, and K are associated with quartz, K-feldspar, plagioclase, and trace amounts of illite and chlorite as identified by x-ray diffraction. Increases in Si, Al, and K are interpreted to reflect intensification of siliciclastic sedimentation caused by sea-level drawdown. The associated increases in S and Sr reflect a higher abundance of anhydrite and celestine, which are interpreted as diagenetically altered gypsum deposited during periods of low sea-level. Chemofacies successions indicate two low order transgressive-regressive cycles and up to six higher order cycles. The findings presented here demonstrate the ability of ED-XRF to provide geologically relevant elemental data that can be used to refine sequence stratigraphic interpretations in fine-grain, homogeneous carbonates.