The Amaranth Formation of the Williston Basin: Paleomagnetic, Petrologic and Geochemical studies

Major debate continues to exist concerning the time of deposition of the Amaranth Formation in the Williston Basin of North America, with postulated ages of Pennsylvanian, Permian, Triassic, Lower and Middle Jurassic. A multidisciplinary study of the lower member of the Amaranth Formation was conducted in six wells in Manitoba. The lower Amaranth red beds are composed of red carbonate-rich and carbonate-poor interbedded sandstones/siltstones/shales containing dolomite and anhydrite and lacking diagnostic fossils. Preliminary analysis of the oxygen and carbon isotope values measured for replacive and cement dolomite show variations related to particular lithologies that can be correlated to the types of dolomite present in the rocks. The siliciclastic sections are dominated by detrital, zoned dolomite that has recrystallized rims, whereas in the more carbonate-rich and evaporitic samples with little to no clastic content, replacive matrix dolomite is the dominant phase. Dolomite samples from the siliciclastic sections are characterized by relatively depleted carbon and oxygen isotope values, the dolomite matrix samples have relatively enriched oxygen and carbon isotope values and a few samples containing replacement matrix dolomite with minor clastic input have intermediate isotope values. These variations reflect primary and diagenetic overprints. Hematite is the major magnetization carrier, with occasional softer magnetic minerals such as magnetite. Optical microscopy revealed the existence of two types of hematite: detrital specular hematite and very fine red pigment hematite. The paleomagnetic data reveals at least three episodes of magnetization. The most pervasive magnetization, B, was formed during the Permian-Carboniferous Kiaman Reverse Superchron. An isolated magnetization in a couple of wells, C, suggests a remagnetization event that happened sometime between mid-Jurassic and Neogene, possibly resulting from a localized oxidizing fluid flow event. The oldest magnetization, D, was acquired sometime between mid-Devonian and Pennsylvanian, but it is usually poorly defined and is not common within the studied samples. B and D are both carried in both specular and pigmentary hematite and are candidates for a primary magnetization preserved in the lower Amaranth samples. The paleomagnetic data presented in this study indicate that the lower Amaranth member red beds are certainly older than early- to mid-Jurassic, and probably even older than Triassic. Our results suggest that these sediments were deposited either in Pennsylvanian or during the Permian-Carboniferous Kiaman Reverse Superchron.