Deciphering the Dolomitization History of the Lower Member Winnipegosis Carbonates: Basin-Scale Gradients in Magnesium and Strontium Isotopes Identify the Deep Structural Center of the Williston Basin as a Source of Reactive Magnesium

Dolomitized carbonates in the Williston Basin are common below the Middle Devonian Prairie Evaporite Formation, highlighting the conceivable role of brine reflux as the source of magnesium (Mg) for dolomitization. However, strontium isotope ratios (87Sr/86Sr), and iron (Fe) and manganese (Mn) concentrations in a dolomite body directly below the evaporite (in the Lower Member Winnipegosis Formation near Saskatoon, SK) are inconsistent with its formation using seawater as the source of reactive Mg. It has been proposed instead that the dolomitizing fluids ascended from deep in the basin where sedimentary formation waters may develop very radiogenic 87Sr/86Sr ratios through water-rock interaction with Rb-bearing siliciclastic sediments and old crystalline rocks of the Precambrian basement (Fu et al., 2006). In this thesis, I expand the geographic range of the studied dolomite samples covering most of the northern half of the Williston Basin, and I employed novel stable Mg isotopes (26Mg) as a tracer of the dolomitizing fluids. The main finding is that the Lower Member Winnipegosis records a basin scale gradient in dolomite 26Mg values, with low values (~ –2.0‰) in the deep center of the basin in North Dakota, and higher values (~ –1.3‰) near the outcrop edge of the basin in Manitoba. The 26Mg and 87Sr/86Sr data, when plotted on a geographic map of the Williston Basin and contoured using Surfer® software, reveal a radial pattern of increasing 26Mg values and decreasing 87Sr/86Sr ratios. Using water-rock interaction modeling, I show that the gradients in 26Mg and 87Sr/86Sr formed when hot Mg-bearing and 87Sr-bearing fluids flowed upwards through the Winnipegosis from a source in the center of the basin. The gradient in 87Sr occurs from dilution of the higher 87Sr/86Sr ratio of the dolomitizing fluid by the release of seawater-derived Sr to pore fluids with a lower 87Sr/86Sr ratio along the flow path. The gradient in 26Mg reflects Mg isotope exchange between: (1) an early formed replacive proto-dolomite with a uniform 26Mg value (~ –1.55‰) reflecting its formation at ambient temperature in an open diagenetic system using seawater as a source of reactive Mg (26Mg 0.15‰), and (2) a hot (~90ºC) upwardly ascending Mg-bearing fluid with a lower 26Mg value (~ –0.80‰). The gradient arises from greater exchange occurring between proto-dolomite and fluid at higher cumulative water-rock ratios in the deep basin (closer to the source of reactive Mg) and less exchange occurring near the edges at lower cumulative water-rock ratios. The resulting ~1.0‰ gradient in dolomite 26Mg signifies cryptic mixing between primary and secondary dolomite formed at two different temperatures from Mg bearing fluids with different 26Mg values at two different stages in the basin history. The burial history of the basin is punctuated by heat flow anomalies, with fluid movement indicated by the resetting of thermal remnant magnetizations in dolomite and evaporite minerals within the basin. I speculate that ultramafic rocks supplied Mg to the bottom of the Williston Basin when they were hydrothermally altered or carbonated during an anomalous heat flow event that affected the Williston Basin in the Late Devonian/Early Carboniferous. Seismic pumping drove these fluids upwards through fracture networks in the crust, and eventually into the deep center of the Williston Basin along vertically-oriented down-to-the-basement faults, pressuring deep confined aquifers like the Winnipegosis that induced up dip migrations of fluid towards the edges of the basin. This study demonstrates that Mg isotopic mapping of large dolomite bodies is a useful tool for deciphering paleofluid-flow histories in sedimentary basins, which may help exploration geologists to find petroleum that may have migrated and become trapped along the same pathways, or Mississippi Valley Type ore deposits. Lastly, contouring mapping of Fe, Mn and 13C values in the Lower Member Winnipegosis dolomite shows that these are not tracers of dolomitizing fluid flow or signals of the dolomitization process in the studied dolomites. They see through the dolomitization and record information on carbonate depositional environments