Carmen Braun

B.Sc. (Honours) Thesis

Chemical Stratigraphy of the Windsor Group Marine Sediments, Shubenacadie Basin, Nova Scotia

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This study examines a section of core from the Windsor Group in order to reconstruct
paleoclitnatic conditions at the titne of deposition. In restricted marine basins, evaporite deposits
form consistent and recognizable sequences. The typical order of precipitation and deposition in
these sequences is carbonates first, sulphates second, halites third, and other salts last. In a
partially or periodically restricted basin with evaporative conditions, this sequence will start
anew with each new marine transgression, which leads to cyclical evaporite deposits. The
Maritimes Basin (tnid-Devonian to Permian) records continuous deposition, including that of the
Windsor Group. Deposited during the Visean, it is the only marine sequence in the basin; it is
comprised primarily of cyclic evaporite deposits. These deposits will include different trace and
minor elements. Trace eletnent ratios, such as Mg/Ca, Ba/Ca, and Sr/Ca, can be important
indicators of paleo ocean conditions, including temperature. For example, the ratio ofMg to Ca
will go up, while those of Ba and Sr to Ca will go down when the temperature of the ocean
increases. Isotopic records can also serve as temperature proxies. However, diagenesis is tnore
likely to affect isotopic ratios than elemental ratios. The presence of Fe is a good indicator of
diagenesis, since it is present only in very low concentrations in normal seawater.

Analysis methods in this study include visual observations of core, hand sample, and thin
section, as well as electron microprobe and x-ray fluorescence analyses of the thin sections and
hand samples respectively. Pe-rich zones around anhydrite nodules and Fe in the dolostone show
that there has been some diagenesis. If this diagenesis occurred in a rock-dominated system, it
will have preserved the elemental ratios. The ratio of Mg/Ca is highly variable, on the foot scale,
from 1600 to 1450 ft depth, above which scale of the variations increases. The ratio decreases
overall until 1150 ft, where it begins to increase. Above 750ft, it begins to decrease again. The
ratios ofBa/Ca and Sr/Ca are also highly variable below 1450 ft, and less so above. Above that
point, the ratios trend inversely compared to Mg/Ca. The variability in the ratios on the stnaller
scales corresponds well to previously identified transgressive-regressive cycles. The longer scale
trends suggest oceanic cooling during deposition of the material from 1450 to 1150 ft (336.3-
335.3 Ma) and 725ft to the top of the study section (331.5-330 Ma), and warming between 1150
and 725 ft (335.3-331.5Ma). The overall increasing trend in Mg/Ca ratios correspond to previous
studies. Further study is required to determine the conditions under which diagenesis occurred.

Keywords: evaporites, carbonates, element ratios, Windsor Group, paleoclitnate, ocean
temperature, Visean
Pages: 40
Supervisor: Richard Cox