The Palaeomagnetism of the Bloomsburg Red Beds and its Possible Application to the Tectonic History of the Appalachians

Thermal analysis of the fossil magnetism of the Silurian Bloomsburg red beds of Pennsylvania indicates that there are two magnetic components; a thermally-discrete component of very great stability which is unchanged in temperatures up to 650 °C and which disappears between 650 and 700 °C, and a thermally-distributed component whose properties are defined by a series of blocking temperatures mostly in the range 300 to 550 °C. The latter is roughly parallel to the supposed direction of the Permian geomagnetic field, and is considered to have been induced by moderate heating during burial at about the time of the Appalachian orogeny. The former has apparently been unaffected by these events and is considered to be of Silurian age. The discrete component has a mean declination 6° W of N diverging by 29½° from that observed by Graham (1949) in the Silurian Rosehill Formation which is situated some 300 km to the SW. The divergence is equal to the angular change in tectonic trend between the two areas. This result may be explained by supposing that this part of the Appalachian geosyncline was straight in Silurian time and that the change in trend is a subsequent strain; that is, the Appalachians at this point are an orocline (Carey 1958). This is not to say that alternative hypotheses will not explain the observations, but it is clear that the question can be decided by further palaeomagnetic studies along this mountain chain.