GSA Connects 2021 in Portland, Oregon

Paper No. 161-2
Presentation Time: 9:00 AM-1:00 PM


EISENBERG, Jane, Department of Geology, Portland State University, 1721 SW Broadway Ave, Portland, OR 97213 and BERSHAW, John, Department of Geology, Portland State University, 1721 SW Broadway Ave, Portland, OR 97201

Diagenetic alteration in carbonates has long frustrated scientists who wish to use their chemistry and/or texture to understand conditions at the time of deposition. Though indicators of diagenesis are well documented, their interpretation is not always straightforward. Despite the large volume of research on the subject, the positive identification and interpretation of diagenesis is a source of ongoing debate in the scientific community. The goal of this study is to better understand the timing of, and controls on, diagenetic alteration in the Martin Bridge Formation (MBF), a Triassic-aged limestone that was altered following deposition. Thirty-nine samples of MBF with differing textures and compositions were collected from the southern Wallowa Mountains (Oregon, USA). Of these, six samples were visually and geochemically characterized using stable (d18O, d13C) and clumped (Δ47) isotope analysis. From these results, mineralizing fluid d18O values and W/R trajectories were estimated. I interpret diagenetic alteration to have occurred in a primarily closed system, at W/R ratios ranging from 0.1 to 0.5 (average value ~0.3), by an initial fluid consistent with Mesozoic meteoric water (δ18O = -3.8 to -6.5‰). My results suggest that depositional environment does not have a strong effect on alteration for MBF limestones. Rock texture does appear to have an effect on alteration, insofar as it influences original permeability, and therefore W/R ratio. My results also place new constraints on the timing of diagenetic alteration in the MBF exposed in the southern Wallowa Mountains. Relatively high temperatures (99 - 245°C) constrained by my Δ47 results suggest that peak alteration occurred during emplacement of the Wallowa batholith in early Cretaceous time.