Unusual Ca-Rich Formation-Waters from Devonian Aquifers in Western Canada: Possible Relict Seawater?

Recently there has been a lot of controversy regarding the question of secular changes in Phanerozoic seawater and whether seawater chemistry remained similar to today (i.e., Mg-SO₄ type) or was for periods of time of fundamentally different composition (i.e., Ca-Cl₂ type). This issue has been exacerbated by many studies of formation-waters collected from geologic basins that have failed to find evidence of widespread Ca-Cl₂ type brines, and have concluded there is little or no evidence to support Ca-Cl₂ type seawater during the Phanerozoic.

In this talk we present new data obtained from two producing wells in the Alberta Basin, plus new data collected from a potash mine-shaft in the Williston Basin, combined with unpublished data obtained from a Drill-Stem-Test in the Williston Basin, and compare these data to previously published (re-sampled and further analyzed) data from potash mine-shafts in the Williston Basin. These six samples are from aquifers of roughly similar stratigraphic age (Devonian; when seawater was reportedly of the Ca-Cl₂ type) and are widespread across the Alberta-Williston Basins.

Results from across the basin are remarkably similar: calcium-chloride type brines (TDS > 425 g/L); calcium > 120 g/L; chloride >270 g/L; magnesium > 11 g/L; potassium >6 g/L; sodium < 10 g/L; bromine >6 g/L; and sulfate <100 mg/L. Using these data with stable isotopic measurements and analyses of the regional hydrogeology we conclude that each of these samples is relatively-unaltered evaporated Devonian seawater. If we assume that bromine was relatively conservative in seawater through time, we can calculate an evaporation factor, and then back-calculate the composition of the original Devonian seawater.

This talk will add new data to the discussion of secular changes in seawater chemistry as well as provide insights into a number of other paleo-hydrogeological processes in the Alberta and Williston basins.