Abstract

A crucial fold test for magnetizations measured in Bajocian through Kimmeridgian limestones from the Jura Mountain region shows that rare blue-gray limestones found in this sequence preserve a prefolding magnetization. In contrast, tan limestones, which predominate in this sequence, contain secondary magnetizations acquired after folding in the late Miocene. The postfolding magnetizations in tan limestones appear to be carried in goethite and, because reversed as well as normal polarity directions are present, the magnetization in these limestones is probably a post-Miocene, pre-Brunhes (>700,000 yr) chemical remanent magnetization (CRM). Field relations between tan and blue-gray limestones, which are present in some cases in the same rock unit (for example, the Hauptrogenstein), suggest that the tan limestones may have formed from alteration of blue limestones. Whereas goethite pseudomorphs after pyrite are found in tan limestones, the occurrence of pyrite in the blue limestones supports this model, indicating that goethite could have formed from oxidation of pyrite. Abundant meteoric water probably was available for such an oxidation process during uplift of these strata subsequent to deformation.

Natural remanent magnetization in blue limestones appears to be carried in magnetite. The positive fold test for the characteristic magnetization in these rocks is significant at the 99% confidence level (k values for 24 site-mean directions before and after tilt correction are 9.3 and 43.2, respectively). The simplest interpretation for the origin of observed remanence in magnetite in the blue limestones is a primary magnetization of depositional or biogenic origin. Characteristic directions in these rocks yield a mean paleopole position of 77.7N, 148.4E (K = 26.0, A95 = 5.9). This pole is in reasonable agreement with one other Upper Jurassic pole from western Europe and with other Middle to Upper Jurassic poles from the Atlantic-bordering continents after rotation of these continents to a reconstructed configuration for Middle to Late Jurassic time.

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