Rock-magnetic characteristics of the Cryogenian–Ediacaran volcano-sedimentary section of the northwest Siberian Platform (Igarka Uplift)

We present the rock-magnetic results obtained from the sedimentary and magmatic rocks of the Igarka Uplift (northwestern margin of the Siberian Platform). This part of the study is an integral stage of paleomagnetic research that is extremely important for a better understanding of the Siberian geological history in the Neoproterozoic. The studied objects are located in the Lower Yenisei River near the Sukharikha River mouth and comprise red-colored siliciclastic rocks (sandstones, gravelites) of Gubinskaya (Cryogenian) and Izluchina (Ediacaran) formations  (Kochnev et al., 2020, 2021) and three dolerite sills intruded in the Gubinskaya formation.

We have analyzed the primary rock-magnetic parameters – magnetic susceptibility (K), anisotropy of magnetic susceptibility (AMS), natural remanent magnetization (NRM), and Koenigsberger ratio (Q). K in dolerites is 20–45*10^-3 SI unit and NRM is 100–500 mA/m that is two orders of magnitude higher than in the sedimentary rocks studied. Q is low in both types of rocks (0.1–1) but noticeably rises to 2–3.7 in the baked contact rocks indicating an impact of chemical magnetization. Among the magnetic minerals, magnetite and hematite are identified according to the temperature demagnetization curves.

The AMS data (degree of anisotropy P, the AMS-ellipsoid shape and distribution of the axes) estimate preservation of the primary magnetic texture, the amount of deformation and cleavage in rocks, and suggest hydrodynamic conditions and paleoflow directions during accumulation of the sediments.

Most of the studied rocks have a low degree of anisotropy – less than 5%, mostly at 1–2%. Sedimentary rocks of the Gubinskaya Formation have the oblate form of the AMS ellipsoid. In the planar-bedded fine-grained sandstones, minimal axis (K3) is normal to the bedding planes that is typical for the calm or slow-current conditions. In the cross-bedded coarse-grained sandstones and gravelites  the maximum axis K1 is steeply inclined to the  bedding surfaces (50-70 degrees) that supports intense hydrodynamics. It should be noted that the anisotropy values do not exceed 1% in the coarse-grained rocks. As for the Izluchina Formation rocks the degree of anisotropy is higher (6–8%) and the orientation of the axes is rather related to the cleavage and the direction of stress.

In the dolerite sills, the degree of anisotropy is 3–6% that suggests weak deformation of the magnetic texture. Distribution of the ellipsoid axes is not in accordance to the shape and strike of the magmatic bodies and probably mirrors a stress impact. In the baked contact zone, AMS ellipsoid is flattened along the contact plane with the K3 being perpendicular to it.

So far, we can conclude that in the most cases the primary magnetic texture is preserved or slightly disturbed but further paleomagnetic studies will be carried out taking into account the data obtained.

This work was financially supported by the Russian Science Foundation grant no. 21-17-00052.