Complex origin of magnetic susceptibility in the Eifelian dolostones of Zachełmie Quarry (Holy Cross Mts, Poland): implications for stratigraphical and palaeoenvironmental interpretations

Zache?mie Quarry (northern Holy Cross Mts.) exposes a ca. 100 m thick succession of the
Eifelian restricted-marine marly dolomite mudstones and wackestones grading upwards into
fossiliferous crystalline dolomites of a more open-marine carbonate platform (Narkiewicz &
Narkiewicz, 2010). Recently, the earliest tetrapod trackways were found in the lower part of the
section (Nied?wiedzki et al., 2010). The current follow-up project aims to constrain the depositional
environment of the track-bearing sediments, in order to better understand the ecological context of
the evolutionary fish-to-tetrapod transition. Within the framework of the project we performed a set
of rock magnetic analyses in the lower part of the section (ca. 30m) to evaluate the suitability of
petromagnetic parameters as tools for stratigraphic and palaeoenvironmental applications. The
analyses include field magnetic susceptibility (MS) measurements in close steps of ca. 10 cm, as well
as laboratory measurements of low field MS, anhysteretic remanent magnetization (ARM),
isothermal remanent magnetization (IRM) and frequency dependence MS, performed on 30 samples
taken from the section with ca. 1 m resolution. A good agreement is found between results from
field and laboratory measurements of MS. Magnetite and hematite occur in the rocks in various
proportions and they both contribute to the MS. In samples with magnetite predomination, a
relatively low S-ratio, and a good correlation of MS with ARM are observed. Conversely, where
magnetite was virtually absent, a high S-ratio and a good correlation of MS with IRM in 1T occurred.
The question if MS might be used as a proxy for detrital input into the basin is not yet fully resolved.
Two subsections from distant parts of the quarry correlate quite well but there are some MS peaks
which possibly result from late diagenetic hematite redistribution. Preliminary spectral analyses of
the MS signal reveal the presence of a strong and persistent cyclicity, characterized by a period of 1.1
- 2 m which might be interpreted as the result of astronomical forcing by climatic precession, with a
period of ~18 kyr (Berger et al., 1992). Moreover, a low-frequency spectral peak was identified,
tentatively interpreted as the result of 100kyr eccentricity forcing. This would indicate that primary
depositional MS record was essentially preserved and not affected by a pervasive oxidation of
unknown age. Further geochemical tests, focusing on the verification of the primary nature of the MS
signal, are planned.
Financial support of Polish Ministry of Science and Higher Education (research grant No. N
N307 323439 to MN and JG) and the Research Foundation - Flanders (FWO) for a Ph.D. fellowship (to
DDV).
References:
BERGER, A., LOUTRE, M. F. & Laskar, J., 1992. Stability of the Astronomical Frequencies over the Earth's History
for Paleoclimate Studies. Science 255, 560-565.