Mineralogy and geochemistry: Exshaw formation, southern Alberta
Abstract
The Exshaw Formation consists of two units - an upper silty limestone or dolomite and a lower black shale. This formation was sampled at one outcrop locality and from five subsurface cores. Fifteen elements: Si, Al, Ca, Mg, Fe, K, Ti, Sr, Ba, Mn, Rb, Zn,
U, Cu, and Ni were determined for each sample by x-ray fluorescence. The mineralogy of the samples was determined by x-ray diffraction. The chemical data were processed by a computer to give correlation coefficients and factor analyses for two groups of samples: one
group containing all the samples ( 164) and one group containing the black shale samples (32).
Many local variations in mineralogical and elemental composition show that the Exshaw Formation is not as homogenous as prev_ipusly considered. However, the black shale is characterized by a high
Si02 content, a low carbonate content, and a predominance of illite over chlorite. Much of the Si02 in the black shale has been biologically precipitated.
The removal of calcite and dolomite from the surface black shale samples is probably caused by sulphuric acid formed during the oxidation of pyrite.
Both chlorite'·and septechlorite are present in the samples. They have a basinal distribution suggesting a source area to the east.
The samples have a very high potassium content and a high K/Rb ratio. Possible causes are the formation of authigenic illite or the introduction of an illite approaching muscovite in composition.
Barium is present as trace amounts of the mineral barite. The
ba:r:ite contains a small amount of strontium in solid solution, It was probably precipitated by organic activity from an upper zone of nor-normal sea water.
Manganese is associated with carbonate minerals in most samples. The manganese and heavy mineral concentrations in samples from the Imperial Youngstown #1 well indicate that it is located in a zone transitional from reducing to normal conditions. This well may be close to a shoreline to the east.
The distribution of zinc, nickel, and uranium is controlled by organic material in the black shales. Copper and iron are present primarily in sulphide phases, except for some iron contained in sep- techlorite and siderite. The nickel content of all the samples is anomalously high compared to published values.
The environment of deposition of the black shales is inferred to have the following conditions: an Eh from O to -0. 3, a pH from 6 to 7. 8, a P COz of greater than or equal to 10 , a P 02 of less tha 10 -70 , and a Ps of greater than 10 -33 The most likely form of uranium in solution was the complex uo C03 )2 (Hz 0)2 -Z ion.
Description
Bibliography: p. 86-93.
Keywords
Citation
Havard, K. R. (1967). Mineralogy and geochemistry: Exshaw formation, southern Alberta (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/17751