Refined magnetostratigraphic position of the Shyrokyne unit in loess sequences from Central Ukraine

D V Hlavatskyi

doi:10.15421/111930

Abstract

The youngest geomagnetic polarity reversal, the Matuyama–Brunhes boundary (MBB), which occurred 780 kyr ago, is a “golden spike” in the age calibration of sediment sequences. The use of palaeomagnetic method as a stratigraphic tool in the study of loess sequences from Ukraine originated some 50 years ago. One major problem in using the available data is the contradictory position of the MBB in different stratigraphic units, which affected historic evolution of thechronostratigraphic models of the Quaternary in Ukraine. The most important units in this regard are the Shyrokyne and Martonosha units, in which the MBB had been defined most often. This paper provides the careful analysis of the previous magnetostratigraphic data and new preliminary results from key loess-palaeosol sections in Central Ukraine. Shyrokyne palaeosol complex in four loesspalaeosol sections located in the Middle Dnieper and Podolia regions has been palaeomagnetically studied. It is shown that the transition zone of the Matuyama–Brunhes palaeomagnetic reversal is most likely located at the base of the soil complex. In the Vyazivok section the MBB has been found in the lowermost part of Shyrokyne palaeosol sh1. Preliminary palaeomagnetic studies of theStari Kaydaky section reveal that the MBB cannot be defined at least above sh1 subunit. Medzhybizh and Holovchyntsi sections were deposited after the Matuyama–Brunhes reversal; however, the palaeomagnetic informativeness of the part of studied strata is doubtful. Magnetostratigraphic position of the Shyrokyne unit below the MBB in some previous studies is explained by methodological reasons and inconsistent chronostratigraphic models. The paper substantiates that normal magnetic polarity zone in the Pryazovya loess and upper part of the Shyrokyne soil is not associated with the influence of secondary processes on the palaeomagnetic record.

Highlights

Loess sequences, alternating loess and palaeosol horizons, are unique continental formations of the Quaternary
For magnetologists the most important unit in the loess sequences is the Shyrokyne horizon (hereinafter the stratigraphic terminology is used in accord with the Stratigraphic Framework of the Pleistocene of Ukraine (Veklich et al, 1993)) – palaeosol complex consisting of three subunits: brown clayey soils of earlier optimum sh1 and later optimum sh3, and uncommon loess-like loam subunit sh2 (Gozhik and Gerasimenko, 2011; Sirenko, 2017)
According to their generalized Pleistocene Magnetostratigraphic Scale of Ukraine (Tretyak and Vigilyanskaya, 1994), which is based on the palaeomagnetic studies of almost 60 sections in Ukraine, Moldavia and Pryazovya, the Matuyama–Brunhes boundary (MBB) is located in the Shyrokyne horizon (according to stratigraphic nomenclature of (Veklich et al, 1984)), the age of which was estimated at 730 kyr

Summary

Introduction

Loess sequences, alternating loess and palaeosol horizons, are unique continental formations of the Quaternary. 2) the MBB in the Vyazivok section had been defined based on only one sample with anomalous polarity at the base of Sula loess

Results

Conclusion