THE APPLICATION OF MULTI-PARAMETER MAGNETIC TECHNIQUES TO THE STRATIGRAHIC RECORD WITH IMPLICATIONS FOR RECOGNITION OF BOUNDING DISCONTINUITIES

Sequence stratigraphic subdivision of continental strata is relatively straightforward where marine marker beds are present and/or where incised valley fill complexes can be identified. However, in predominant continental stratal units, traditional indicators (e.g. tidal indicators, widespread coals, and marine maximum flooding surfaces) cannot be used in sequence stratigraphic analysis. Preliminary magnetic measurements suggest that magnetic susceptibility profiles may be useful in identifying discontinuities in continental strata where evidence for marine incursion is obscure or missing. Isothermal remanent magnetization (IRM), anhysteretic remanent magnetization (ARM), volume magnetic susceptibility (k), mass magnetic susceptibility (c), and hysteresis parameters (M_S, M_R, H_C, H_CR, and FORC) were used to determine the magnetic mineralogy, concentration, and grain size of core and outcrop samples. Data from the Hinton Formation (Upper Mississippian) and the New River Formation (Lower Pennsylvanian) indicate that sequence boundaries and discontinuities within incised-valley fills are characterized by distinct increases in magnetic susceptibility. In Upper Mississippian strata, magnetic susceptibility maxima occur immediately below lowstand surfaces of erosion and may be related to early diagenetic siderite (FeCO₃) formation. In Lower Pennsylvanian braided-alluvial incised-valley fills, magnetic susceptibility maxima define internal discontinuities related to early stage diagenetic siderite cement. Recognition of discontinuities implies that incised-valley fills are multistory. In both samples, early diagenetic siderite cement is indicative of temporal changes in drainage, surface stability, and accommodation during soil formation or prolonged sediment exposure. Overall, multi-parameter environmental magnetic techniques demonstrate that magnetic susceptibility can be used as a stand-alone method for inferring cryptic sequence boundaries and discontinuities where changes in magnetic mineralogy, concentration, and/or grain size occur as a function of changes in base level.