Abstract

Petrographic, rock magnetic, and paleomagnetic studies of fine-grained red sediments of late Cenozoic age in Baja California show that the sediments have variably acquired chemical remanent magnetization (CRM) overprints that have obscured the original magnetization of the deposits. The chemical remanence in the sediments is carried predominantly by three authigenic minerals — hematite, goethite, and a Mn · Ba compound, herein called “hydropsilomelane” — that occur as pigments and as concretions. Remanence directions generally associated with each mineral are goethite, reverse; hydropsilomelane, normal; and hematite, normal or reverse. Some samples possess a remanence that is strong and normal or strong and reverse; these generally contain only one of the authigenic minerals in abundance. Many samples, however, are weak in intensity and random in direction. When such samples are split into parts and measured, it is generally found that each part is strongly magnetized but that some parts are normal and others are reverse in direction. In such cases, the magnetization of the whole sample is resultant of the multiple components that are generally carried by two or more of the authigenic minerals.

The following conclusions can be drawn concerning the acquisition of CRM in the Baja California deposits: (1) The sediments contain chemically unstable iron- and manganese-bearing minerals, such as hornblende and biotite, that are susceptible to postdepositional alteration, and they have provided the parent material for the authigenic magnetic minerals. (2) Authigenic magnetic minerals, growing from crystallites, generally acquired a remanence that was parallel to the Earth's field when they surpassed the critical grain size. (3) The rate of CRM acquisition has not been uniform in these sediments, probably because the processes of alteration and formation of authigenic magnetic minerals depend on an interplay of highly variable factors such as the chemistry of the interstitial water, hydraulic gradients, and mineralogy of the sediments. (4) Complex variability in acquisition of CRM has led to a remanence stratigraphy that bears little discernible correlation with the geomagnetic field at the time of deposition.

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