Mineralogical and Geochemical Studies of Tills in South-Western Scotland

Geology, Geochemistry, Mineralogy

Abd-Alla, Mamdouh Ahmed Ali

University:University of Glasgow

关键词: Geology, Geochemistry, Mineralogy;   

Others  :  http://theses.gla.ac.uk/77622/1/10997893.pdf

来源: University of Glasgow

【 摘 要 】

Till deposits of the NW Glasgow area and Northern Ayrshire were studied. In Part I, previous research into Quaternary problems in these areas and the nature, origin and classification of till deposits are outlined, followed by a statement of the aims of the project and the techniques used. Part II contains detailed data obtained from grain-size, clay mineralogical and major and trace element analyses of the matrices of three categories of NW Glasgow tills - Red, Weathered Grey and Grey. The Red and Weathered Grey tills have coarser-grained compositions than the Grey till. Mean size and skewness are the most diagnostic grain-size parameters for distinguishing between Red and Weathered Grey till on the one hand and Grey till on the other. All three categories contain kaolinite, illite and vermiculite. Chlorite is present only in the Grey till. The percentage of kaolinite is much lower and the percentage of vermiculite is higher in the Red till than in the Weathered Grey till. Three probable modes of origin of the clay minerals in the tills are proposed: direct inheritance, pre-glacial weathering and pedogenesis since till deposition. All three categories of till have a high SiO2 content, which is consistent with the tills having sources in the local sandstone bedrocks. With the exception of Zr, all the trace elements are preferentially concentrated in the silt and clay fractions. Zr appears to occur both in clay minerals and in the sand fraction as detrital zircon. Sr is concentrated in the calcium minerals and Ba in the K-feldspars. Study of vertical profiles shows that leaching of fine-grained material and weathering of clay minerals are common. Weathering in the Red till is difficult to detect. However, the amount of vermiculite increases upwards in the profile at the expense of illite. In the case of profiles through both Grey, and Weathered Grey till, chlorite disappears, and the amount of vermiculite increases up the profile at the expense of both illite and chlorite. The ratios Ga:Al2O3, MgO:Ni, FeO:Co and Ni:Co can be used to detect weathering trends in both Grey and Red till profiles. Mineralogical and geochemical studies of bedrock in the NW Glasgow area, showing the presence of chlorite in Carboniferous shales and sandstones and its absence in Devonian (O.R.S.) sandstones, indicate that the Grey till was derived largely from Carboniferous shales and sandstones, and the Red till largely from Devonian sandstones. Part III contains detailed data obtained by similar methods applied to the matrices of tills and associated Quaternary deposits in Northern Ayrshire. Marked similarities in the properties of samples of shelly till from five locations suggest similar sources for the shelly till at these locations. The shelly till has a high SiO2 and a low clay content, suggesting that the proportion of shell-bearing marine clay in the shelly till is not nearly as great as previously thought. The high SiO2 content in both the shelly and non-shelly tills of Northern Ayrshire reflects quartz-rich source rocks for these tills. The matrices of the non-shelly tills have higher CaO and CO2 contents than the matrix of the shelly till. This may be due to the presence of finely ground limestone in the non-shelly till matrices. Comparison of the properties of shell-bearing marine clays at Afton Lodge with those of the shelly till of N Ayrshire as a whole and with a shell-bearing deposit at Greenock Mains shows clearly that the last-mentioned is not a shelly till as formerly thought but is a marine sediment similar in composition to the deposit at Afton Lodge. The matrices of Upper and Lower grey tills at Sourlie are similar in composition, indicating similar sources, probably mainly local Carboniferous shales and sandstones. Finally, the thesis applies the results obtained to Quaternary stratigraphy. The properties of the matrices of red and grey facies of the proposed 'Wilderness Till' Formation of the Glasgow area can now be defined. The presence of Weathered Grey till overlain by Red till in the NW Glasgow area suggests at least a short period of exposure of Grey till before deposition of Red till on top of it. In Ayrshire, the discovery of shell-bearing marine deposits (at Greenock Mains) at c. 180m above present sea level and c. 30km inland from the present coast means that recent views regarding the maximum elevation and extent of Quaternary marine incursion in Ayrshire may have to be modified. The presence of these sediments also implies that the shelly till may have been derived from pockets of shell-bearing deposits picked up locally within inland Ayrshire rather than from the Firth, of Clyde. The presence of shelly till at any given location, therefore, may not be indicative of any particular direction of ice movement, as formerly thought.