The sedimentology of Minto Salmon Harbour Mine site and its high sulfur coals

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University of New Brunswick


The Minto coal field at the Salmon Harbour mine site is located in Central New Brunswick in the upper part of the Minto Formation of the Pictou Group, the uppermost unit within the Maritimes Basin in this area. The stratigraphic succession at the mine sitemis divided into 7 units. The first unit is the coal seam, which is a high volatile A bituminous grade that contains between 5 to 9 % sulfur. This unit is interpreted as shallow poorly drained coal swamp environment. Unit two, the shales and in places sandstones form the roof rocks and contains an abundance of organic material used in age dating. Bio-stratigraphy gives an age of Middle Carboniferous, Westphalian C. This unit is interpreted as suspension sedimentation that drowns and ends the coal swamp environment. Units three and five are mudstones, siltstones and sandstone couplets with soil horizons and are highly fractured. These units are interpreted as sheet floods and over-bank deposits. Unit four is a sandstone-conglomerate with trough cross beds, rip-up clasts and climbing ripples and is interpreted as a river channel. Unit six are small lenticular channels that laterally pinch out and have highly sinuous profile and contain lateral accretion sets. This is interpreted to be a meandering channel system. Unit 7 is the present day overburden. Sulfur/Carbon analyses (using LECO method) show, at the base of unit two shales, 3.1 wt% sulfur and 1.32 wt% carbon and lower values up-section. The other 3 units have negligible sulfur and notable carbon at the base of each unit and decreased up-section. In units 2 and 3 petrographic analysis identifies quartz as a blocky cement and kaolinite as a pore-filling vermiform clay phase. These authigenic minerals are characteristic of acid pore water environments. Up-section quartz forms "dog tooth" quartz crystals alongside calcium carbonate cement (5-10%). This may reflect the additional influence of a more late alkaline pore water environment. The Minto area is modelled as a shallow depositional sub-basin of an ancient river valley that was poorly drained. The river valley fill is dominated by normal grading fluvial facies, with trough cross bedding overlain by climbing ripples that indicates palaeoflow towards the northeast to southwest. The development of soil horizons containing peds with preserved organic material is indicative of long periods of sub-aerial exposure in better drained areas of the floodplain. The peat bog developed in the poorly drained areas. The source of the sulfur in the coal is debatable. Although sulfate from seawater would provide an abundant source of sulfur for the coals, no sedimentological evidence for marine influence is noted. Instead it is proposed that sulphate-rich freshwaters were derived from solutions from an aquifer system below the coal seam.