Facies Vectors: Disentangling Environmental Variability, Diagenesis, and Secular Change in Carbonate Stratigraphies

Stratigraphic signals of environmental variables, especially from carbonate sections, are primary pieces of evidence for interpreting the coevolution of life, environments, and climate in Earth history. Modern carbonate platforms, however, are heterogenous with respect to physical and chemical environments, and this internal variability is of the same scale as the range of secular variability interpreted from the stratigraphic record. Therefore, carbonate stratigraphies are not straightforward records of regional-to-global trends, and we first must constrain the influence of shifting local depositional conditions before reading broader secular changes from signals. Traditional geologic methods that denote depositional conditions through qualitative lithologies indicate that there may be some sub-environmental component to chemical and fossil occurrence signals, but these descriptions are not reproducible nor numerical. In the interest of furnishing robust quantitative inference between local and secular influences on carbonate signals, we develop image analysis methods that capture geologic lithofacies observations of hand samples, such as bedding fabrics and grain/feature sizes, relevant to depositional setting. We analyze these geophysical data in concert with geochemical measurements and fossil occurrences to quantitatively define relationships between lithofacies, diagenesis, biodiversity, and seawater chemistry. We apply our techniques to a lower Cambrian outcrop that contains some of Earth’s earliest metazoan framework reefs. These complex environments are crucial elements of the seafloor today but are not easily described by traditional geologic techniques, and so our methods provide novel constraints on the influence reefs have on biodiversity trends in Earth history, such as those of the Cambrian radiation. In our analysis, we identify [1] a diagenetic origin for the outcrop’s trend in 𝛿 13 C values, [2] the extent of original variability in 𝛿 13 C values in this section of the fossil platform, [3] how that variability is distributed among lithofacies, and [4] that the vast majority of fossil species lived within and nearby reef environments.