Fault Controlled Dolomitization in the Ordovician Lexington Limestone; Using Magnetic Methods to Document Alteration and Paleoflow

Abstract:

The Ordovician Millersburg Member of the Lexington Limestone in east-central Kentucky contains a large bulb shaped exposure of dolomite along a fault. Within and immediately adjacent to the fault zone dolomite can be distinguished from the surrounding flat laying grey limestone beds due to it reddish color caused by hematite within and between dolomite crystals. The dolomite contained within the fault zone has completely obscured the skeletal grainstone fabric of the host limestone. There are several stages of dolomite, including saddle dolomite which may be of hydrothermal origin. Progressing away from the fault zone, saddle dolomite decreases, primarily occurring along fluid flow conduits leaving the majority of the grainstone texture intact. The trend in dolomite based on the petrographic results is consistent with the trends observed in f bulk susceptibility (χ) and anisotropy of magnetic susceptibility (AMS). Elevated positive χ values are observed from samples within the fault zone and those near the fault zone, while very low or negative χ values are observed in samples taken further away from the fault zone. The samples within and particularly near the fault zone are interpreted to hold their bulk susceptibility in ferroan dolomite whereas those further away indicate predominately diamagnetic minerals. AMS measurements reveal a weak inverse fabric associated with samples taken from within the fault zone, a strong inverse fabric associated with dolomitic samples near the fault zone, and a transitional to normal fabric associated with samples taken further away from the fault zone. This trend in inverse versus normal fabric can be related to the degree of dolomitization. Preliminary paleomagnetic results indicate that the dolomite in the fault zone is dominated by goethite. Sites away from the fault zone contain a southerly and shallow to moderate magnetization removed by alternating field demagnetization, indicating a magnetic carrier mineral with a low coercivity, likely magnetite. It is interpreted as a late Paleozoic -Triassic chemical remanent magnetization (CRM). A vein test is being conducted to test if this CRM is related to the alteration around the fault, and if it can date the timing of dolomitization.