Integrating Magnetic Susceptibility Data with Sequence Stratigraphy in the Ironstone Bearing Successions (Lower Silurian) of Eastern North America
Authors: Sullivan Nicholas B. , Brett Carlton E. 
Published: December 2023

A total of 839 samples of marine sedimentary rock were collected from Lower Silurian (Aeronian-Sheinwoodian) strata exposed at localities situated along an inferred proximal to distal transect of the Appalachian Foreland Basin. These samples were classified on the basis of lithology, and analyzed for bulk magnetic susceptibility. The application of several statistical tests suggests a significant correlation between lithology and magnetic susceptibility. The results are considered in the context of models explaining patterns of magnetic susceptibility and the origin of ferruginous strata. In shallow depositional environments rimming the Appalachian Basin, peak magnetic susceptibility values were associated with intervals containing abundant ferric iron, notably the classic “Clinton Ironstones” of east central New York. Elevated values are also associated with glauconitic shale horizons and fossiliferous, dolopackstones as well. In these same sections, beds interpreted as tempestites show lower values than those observed in the surrounding units. Magnetic susceptibility curves generated for these sections display sharp offsets in values at the base of ferric ironstones; the highest values occur near the top of these units. These observations are consistent with models that explain ironstones as sediment-starved and highly reworked transgressive units overlying unconformities. According to this framework, non-ferruginous strata overlying the ironstone would be interpreted as the product of increased sedimentation rates associated with highstand and falling stage, which would “dilute” any iron that is present. However, given continuous changes in sedimentation and sea level, one would expect a gradual tapering of magnetic susceptibility values as sedimentation increased upward through the section. This is not reflected in the data described herein, which demonstrate sharp, negative shifts in magnetic susceptibility values near the upper contact of ironstones, suggesting that additional factors may also influence the rate at which iron concentrated on the sea floor. Although sedimentation rates appear to have an important influence on magnetic susceptibility values, the exceptional concentration of authigenic iron-bearing minerals in these facies may have generated a case wherein magnetic susceptibility values and presumed rate of sedimentation are inversely correlated in certain environments. At localities interpreted as the deepest part of the Appalachian Basin, conglomeratic lag beds and black shales lack elevated magnetic susceptibility peaks found in coeval successions in the surrounding regions. Previous workers have argued for a genetic link between ironstones and the conglomeratic lag beds, and both contain abundant iron bearing minerals. However, the sulfide minerals characteristic of the conglomeratic lag beds contain iron primarily in its ferrous state, which produces magnetic susceptibility values that are not significantly different from over- and underlying strata. The high magnetic susceptibility values associated with sediment-starved horizons in shallow, oxic environments and the lack thereof in deeper, dysoxic environments, suggests that the prevailing redox conditions of a depositional regime exert an important control on magnetic susceptibility values. Although this study is narrow in scope, these findings may provide useful insight toward theories of the origin of ironstones and the controls on magnetic susceptibility values in marine sedimentary rock.