High-resolution magnetostratigraphy and micropalaeontology across the J/K boundary strata at Brodno near Zilina, western Slovakia: summary of results

This study summarizes the final results of magnetostratigraphic and micropa laeontological investigations of the Tithonian/Berriasian boundary (J/K) li mestones at the locality of Brodno near Zilina, Western Carpathians, West S lovakia. The aim is to prepare the background for correlation of a Late Tit honian and Early Berriasian biostratigraphic zonation with global magnetoev ents (manifested in derailed magnetostratigraphic profiles) between the Tet hyan realm and other regions, particularly the Boreal and Pacific realms. S uitable physical properties of the limestones studied, a favourable geologi cal setting and rich microfossil assemblages (calpionellids) at Brodno (mag netozones M21r to M18r) allowed a systematic collection of new data on the original synoptic section, which now has the character of a high-resolution profile across the J/K boundary. Two reverse subzones detected within magn etozones M20n and M19n are named 'Kysuca Subzone' and 'Brodno Subzone', res pectively. Both can be correlated with analogous subzones in the M-sequence of marine anomalies. Their existence can also be confirmed in the Bosso Va lley section in Umbria, central Italy. A detailed study of the interval bet ween magnetozones M20n and M17r in the Bosso Valley profile is presently in its final stage. Both magnetostratigraphic profiles, from the Brodno and B osso Valley localities, were thoroughly investigated in the intervals close to the J/K boundary and are still unique among continent-based profiles in the detection and precise definition of both reverse subzones within M20n and M19n. Samples collected from the boundary strata of the reverse Kysuca Subzone at Brodno indicated a transition from N (R) to R (N) polarity of th e Earth's palaeomagnetic field within a time interval of c. +/- 5 ka. Limes tone samples (total number n=368) were subjected to progressive thermal dem agnetization in 10-12 thermal fields and to subsequent multi-component anal ysis of remanence. The C-component, as the carrier of palaeomagnetic direct ions, was determined in the temperature interval of 300 degrees to 540 degr ees, or possibly 580 degrees C. Magnetomineralogical analyses of pilot samp les and unblocking temperatures determined for all samples showed that magn etite is the carrier of remanent magnetization. The positions of the indivi dual events of calpionellid stratigraphy relative to the global magnetic po larity timescale are precisely defined. The base of the Calpionella Standar d Zone, which is considered a provisional J/K boundary indicator in ammonit e-free sections in the Tethyan realm, lies within magnetozone M19n at the l evel of 35% of its local thickness. None of the boundaries in the calpionel lid zonation coincides precisely with any of those in the palaeomagnetic zo nation, but the first appearance of Calpionella grandalpina, indicating the base of the Intermedia Subzone, lies in close proximity to the base of mag netozone M19r. (C) 1999 Academic Press.