The use of magnetic susceptibility and trace element geochemistry for the correlation of fine-grained siliciclastic sequences: a Late Llandovery example from northwest England

High-resolution lithological and magnetic susceptibility logs were made of two sections of the Upper Llandovery Browgill Formation in northwest Englan d: Stockdale Beck, the type section of the Browgill Formation, and Spengill . The Browgill Formation is composed of fine-grained deep marine siliciclas tics, which can be divided broadly into two facies: a homogeneous grey muds tone, deposited under oxygenated bottom-water conditions, and subordinate b eds of laminated, graptolite-bearing black mudstone deposited under low bot tom-water oxygen levels. The latter facies is often partially or fully diag enetically altered to chlorite nodules, occasionally with manganese carbona te nucleii. Magnetic susceptibility logs are shown to reflect variations in the illite-chlorite ratio of the clay mineralogy. Chlorite is a paramagnet ic mineral, so the bands of chlorite nodules produce magnetic susceptibilit y highs. Correlation demonstrates that diagenetically altered beds of lamin ated black mudstone are continuous between the two sections, now situated 3 2 km apart. This lateral continuity would favour pelagic fallout in prefere nce to gravity flow as a depositional model for both facies. A correlated s equence within the turriculatus Biozone is 3.8 times thicker at Spengill th an at Stockdale Beck, probably reflecting variations in sediment accumulati on rates. Correlation also identifies significant non-sequences in both sec tions: at least 47 % of the turriculatus Biozone (including maximus Sub-bio zone) is missing at Stockdale Beck and at least 77 % of the crispus Biozone is missing at Spengill. The identification of non-sequences may be of valu e for refining graptolite biostratigraphy.