2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 10
Presentation Time: 10:15 AM

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

ROSTRON, Benjamin J., Earth and Atmospheric Sciences, Univ of Alberta, 1-26 Earth Sciences Bldg, Edmonton, AB T6G 2E3, Ben.Rostron@UAlberta.ca

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wells in the Alberta Basin, plus new data collected from a potash mine-shaft in the Williston Basin, combined with unpublished data obtained from a Drill-Stem-Test in the Williston Basin, and compare these data to previously published (re-sampled and further analyzed) data from potash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.