Abstract

The study of the structure and origin of remanence in five sections comprising the Rillo Mudstones and Sandstones and Torete Multicoloured Mudstone and sandstone Formations (Middle Triassic, Central Spain) reveals the presence of several components of magnetization of different ages. Detrital hematite and ilmenohaematite carry a primary magnetization of Triassic origin in the form of a DRM or PDRM. In addition, it is likely that an early CRM also contributes to the primary remanence. Goethite and several textural phases of authigenic hematite are responsible for a recent overprint in the form of a late CRM. Thermal demagnetization allowed the isolation of a characteristic Triassic remanence on which a reliable magnetostratigraphy for Anisian-Ladinian times could be based, and correlated with sections of similar age in North America and Asia. However, it did not succeed in entirely removing the CRM associated with the latest component of the overprint which resulted in the primary remanence being biased by residual components of Recent origin.

Remanence acquisition was controlled by the sedimentary environment and the diagenetic processes that took place during burial and uplift. The sedimentary environment controlled the conditions of deposition and facies distribution which limited the occurrence of a particular diagenetic phase within the rock volume. Acquisition of secondary components of magnetization, and the consequent pervasive Holocene magnetization (in the form of a late CRM) is primarily related to coarser grained horizons. Sand body interconnectivity has also played an important role in the process. Primary remanence is better preserved in the finer-grained horizons. A PTRM associated with the uplift is easily removed during thermal demagnetization. Calculation of mean vectors by integrating data from adjacent magnetozones allows correction for the overprint bias. The corrected directions can then be closely compared with results from stable Europe.