Parasequence-scale organic facies variations through an Early Carboniferous Yoredale cyclothem, Middle Limestone Group, Scremerston, Northumberland

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Abstract
The Early Carboniferous (Brigantian) Middle Limestone Group of Northumberland represents a prolonged period of rhythmic Yoredale-type deltaic sedimentation. Optical and geochemical analyses were undertaken on the minor Berwick Limestone Cyclothem, a prograding coarsening- and shallowing-upward cycle (parasequence) within the major Oxford Limestone Cyclothem. These indicate an overwhelming dominance of Type III–IV land-plant derived kerogen (woody phytoclasts). The hydrogen index shows a strong negative correlation with the palynologically determined b1ack: brown phytoclast ratio. The total organic carbon content of the fine-grained lithologies increases upwards through the cycle and is positively correlated with silt content, presumably due to the hydrodynamic equivalence and common fluvial source of phytoclasts and silt particles. The 1ath:equant ratio of the black phytoclasts increases upwards through the cycle (i.e. in a proximal direction); this pattern is ‘atypical’ and results from the lath-shaped phytoclasts being larger than the equant ones. The section is marginally mature (about 0.64% vitrinite reflectance); reflectance is correlated with hydrogen index. The distribution of pyrite and siderite is apparently related to sediment accumulation rate, with siderite most common in the interval of probable highest sedimentation in the lower half of the cyclothem above the limestone. The marine flooding surface at the base of the cycle is overlain by black shale, not limestone; the traditional (limestone) base of the cyclothem does not correspond to the true base.