Introduction

The erosion products of igneous, sedimentary or metamorphic rocks are sources of detrital particles that go to make new sedimentary rocks. The detrital and post-depositional remanent magnetizations (DRM and PDRM) acquired when sediments are deposited and consolidated are no more than a reconstitution of the NRM's of detrital magnetic grains from the source rock (or rocks). Depending on the size and remanence mechanisms of grains in the source rocks, DRM may inherit the temperature and time stability and AF hardness of TRM, CRM, TCRM or VRM of SD, PSD, or MD grains.

DRM and PDRM are inherently weak, often < 10−3 A/m (10−6 emu/cm3). There are two reasons for this low intensity. First, dense oxide grains are less readily transported than silicate grains of similar size. Secondly, DRM represents only a partial realignment of original NRM vectors. With the advent of cryogenic magnetometers (Collinson, 1983), measuring the weak magnetizations of sediments and sedimentary rocks is no longer a major problem.

Red sedimentary rocks – so-called red beds – possess, in addition to depositional remanence, a CRM (10−3–10−1 A/m) carried principally by hematite pigment and cement. The CRM is useful paleomagnetically if the time at which the hematite formed is known.

The great attraction of sedimentary rocks is the comparative continuity of their magnetic record. Major igneous activity and metamorphism occur intermittently, sometimes at long intervals.