Carrier of Remanent Magnetization in Certain Red Sandstones

Abstract

MANY of tho palaeomagnetic data which have accumulated during the past 10 years have resulted from extensive measurements on red sandstones, which often possess a stable and measurable component of remanent magnetization. At the present time, the process by which these rocks acquire their magnetization is not fully understood. Thermal demagnetization experiments show that haematite (α-Fe₂O₃) is generally responsible for the stable natural remanent magnetism, and this material occurs in two distinct forms in red sediments. One form is the red pigment, which gives the rocks their distinctive colour and is in the form of a coating on the quartz and feldspar grains and sometimes fills the interstices of the rock; the other form is black crystalline particles (‘speeularite’). Both these varieties of haematite are capable of carrying the natural remanent magnetism, through the acquisition of either depositional or chemical magnetization. The former process seems most likely to be associated with specularite¹, but the latter could be acquired by the pigment, if it were deposited from circulating iron-bearing solutions, or by the specularite if diagenetic alteration of magnetite to haematite occurred.