Nathan Glas

B.Sc. (Honours) Thesis

USING ANHYDRITE CRYSTALS AND WHOLE ROCK ANALYSIS IN A MISSISSIPPIAN SALT DIAPIR AS A PROXY TO DETERMINE THE PRESERVATION EXTENT OF PRIMARY SEAWATER GEOCHEMICAL FEATURES THROUGH DEFORMATION PROCESSES

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Evaporites provide direct evidence of seawater geochemistry and seawater crystallization through preserved fluid inclusions. Fluid inclusion analysis can be used to analyze the paleoenvironment that existed during evaporite formation when considering undeformed salt deposits. However, diapirism makes interpretation more difficult, owing to recrystallization processes that occur during deformation. The Wallace No. 1 core was drilled into one such salt dome located in the Cumberland Basin in northern Nova Scotia. The drill hole penetrated through the eastern flank of a large anticline cored with evaporites; mainly halite, anhydrite, gypsum and lesser occurrences of sylvite and carnallite. This deposit is heavily deformed and preserved beds dip between 40°-60° making it difficult to determine seawater chemistry from conventional methods such as fluid inclusion studies. However, preserved mudstone beds do exist, along with clear and dark grey-banded halite layers. In addition, thin sections show primary ‘snow-on-the-roof’ textures, with halite crystals being capped by anhydrite, exhibiting cyclic repetition. These preserved bedforms and textures indicate that not all of the deposit was reworked during deformation. The ability to discern preserved seawater geochemical features in salt diapirs allows for paleoclimate and paleoenvironment reconstructions to be completed. In areas that may be lacking undeformed salt deposits or other easily interpretable rocks, this can be extremely useful. This is also important considering Carboniferous evaporite basins are rare and seawater compositions changed from calcite to aragonite seas during this time period, making the ability to study salt diapirs, especially of this age, important.

Through examination of the whole rock geochemistry, including trace element compositions and anhydrite crystal chemistry, conclusions were made about the environmental conditions during primary precipitation. From these, it can be deduced that the deposit formed in a restricted basin with a large volume of brine. The basin was on the order of ~100 meters deep, with evaporite precipitation outpacing subsidence, eventually leading to a shallow sabkha-like environment persisting until the basin was closed off due to glacial or tectonic changes, with deformation beginning shortly after. The brine had a relatively consistent chemistry and only minor variation occurred throughout the existence of the basin. The basin age cannot be pinpointed exactly due to deformation, but is of Viséan age (347-331 Ma).

Keywords: Windsor Group, Cumberland Basin, Viséan age, evaporite basin, salt tectonics, salt dome preservation, snow-on-the-roof texture, paleoenvironment analysis
Pages: 94
Supervisor: Richard Cox