Chemical remanent magnetization due to deep-burial diagenesis in oolitic hematite-bearing ironstones of Alabama

Oolitic hematite-bearing ironstones of the Silurian Red Mountain Formation of Alabama are shown to carry a single-component remanence stable enough to have survived major folding (of probable Permian age). Nevertheless, the remanence direction (ten sites yielding a paleopole at 38.0°N, 132.4°E with dm = 3.6°, dp = 1.9°), its reverse polarity and a negative intraformational conglomerate test show that the remanence was very likely acquired during the Pennsylvanian—some 130 Ma after deposition. This remanence is likely a chemical remanent magnetization (CRM) acquired during diagenesis induced by heating due to deep burial under a Pennsylvanian clastic wedge. Two possible mechanisms for acquisition of CRM during deep-burial diagenesis are considered. In hypothesis I, the oolitic hematite transformed from original geothite when heated to about 80°C, acquiring CRM. In hypothesis II, the oolitic hematite originated from ferrihydrite and was too fine-grained to acquire stable CRM until heat raised the solubility of hematite allowing grain growth. Hypothesis I explains the timing of remanence acquisition better, but there is some evidence that oolitic goethites may be stable to considerably more than 80°C. Hypothesis II has some difficulty explaining preliminary paleomagnetic results from oolitic hematite-bearing ironstones of the Silurian Clinton Group, New York State. We prefer hypothesis I but both hypotheses remain plausible. Both hypotheses warn that continental red beds may also acquire CRM during diagenesis induced by deep-burial heating, long after deposition but before folding.