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

Abstract

High-resolution lithological and magnetic susceptibility logs were made of two sections of the Upper Llandovery Browgill Formation in northwest England: Stockdale Beck, the type section of the Browgill Formation, and Spengill. The Browgill Formation is composed of fine-grained deep marine siliciclastics, which can be divided broadly into two facies: a homogeneous grey mudstone, deposited under oxygenated bottom-water conditions, and subordinate beds of laminated, graptolitebearing black mudstone deposited under low bottom-water oxygen levels. The latter facies is often partially or fully diagenetically altered to chlorite nodules, occasionally with manganese carbonate nucleii. Magnetic susceptibility logs are shown to reflect variations in the illite-chlorite ratio of the clay mineralogy. Chlorite is a paramagnetic mineral, so the bands of chlorite nodules produce magnetic susceptibility highs. Correlation demonstrates that diagenetically altered beds of laminated black mudstone are continuous between the two sections, now situated 32 km apart. This lateral continuity would favour pelagic fallout in preference to gravity flow as a depositional model for both facies. A correlated sequence within the turriculatus Biozone is 3.8 times thicker at Spengill than at Stockdale Beck, probably reflecting variations in sediment accumulation rates. Correlation also identifies significant non-sequences in both sections: at least 47 % of the turriculatus Biozone (including maximus Subbiozone) 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 value for refining graptolite biostratigraphy.

The Telychian (early Silurian) oxygenation event in northern Europe: A geochemical and magnetic perspective

“…3c), indicating a possible linkage between chlorite content and magnetic mineral abundance (i.e. MS), like found by English (1999). In contrast, K scf in the calcite-poor samples is not related to chlorite content or illitesmectite content (Figs.…”

“…The extent to which this MS behaviour and the co-occurrence with peaks in oxic-sedimentary units are specific to the Llandovery is not clear. In contrast, the MS study of English (1999) from northern England was focused on graptolitic bearing deeper water mudstones (Aeronian-Telychian in age), demonstrating that MS variations were linked to chlorite content. The MS was not dependent on the redox-state, which varied between more dysaerobic graptolitic laminated mudstones and aerobic grey mudstones, but instead showed a stratigraphic variation related to sediment age.…”

Sedimentology and magnetic susceptibility of Mississippian (Tournaisian) carbonate sections in Belgium

“…This material is then dispersed by bottom currents throughout ocean basins and the MS of the sediments increases accordingly. This hypothesis is that most commonly cited to explain MS variations (Borradaile et al 1993, Robinson 1993, English 1999, Ellwood et al 2000, Stage 2001. Other mechanisms, also related to sea level change, can be responsible for increase of magnetic materials in sedimentary basins during lowering sea level.…”

Upper Devonian carbonate platform correlations and sea level variations recorded in magnetic susceptibility

“…This material is then dispersed by bottom currents throughout ocean basins and MS of the sediments increases accordingly. This hypothesis is also the most commonly cited to explain MS variations (Borradaile et al, 1993;Robinson, 1993;English, 1999;Ellwood et al, 2000;Stage, 2001). Other mechanisms, also related to sea level change, can be responsible for increase of magnetic materials in sedimentary basins during lowering sea level.…”