2. Analyzed Material

[6]  The time interval (Barremian-Cenomanian) has been chosen because of the following reasons.

[7]  1. According to the classical ideas [Khain and Lomidze, 1995], in the Barremian and Aptian ages the stretching processes (of the riftogenesis epoch) prevailed on the planetary scale. In Albian-Cenomanian, there occurred an activation of orogenesis processes (the Austrian epoch of the Alpine geotectonic cycle). We think that studying of this temporal interval makes it possible to compare the behavior of the paleointensity during the epochs of predominant stretching and predominant compression of the Earth crust.

[8]  2. A temporal prevailing of long magnetopolar intervals of direct polarity over intervals of inverse polarity was noted in Barremian-Cenomanian [All-Russian Scientific Research Geological Institute (VSEGEI), 2000]. Excluding from consideration intervals of inverse polarity, one can obtain a relatively simple (not complicated by the polarity changes) and fairly complete picture of the paleointensity behavior.

[9]  3. Until recent time, there has been carried out almost no determination of the paleointensity in the studied part of the Cretaceous. The information on the paleointensity of the lower Cretaceous available in publications is based on the results of studies of Valangine and Goterive [Solodovnikov, 1995; Tanaka and Kono, 2002]. A correctness of interpolation of these results to Barremian, Aptian, and Albian is not substantiated. We plan to fill in this gap in the data of the magnetic field intensity partially by the results of this paper.

[10]  The sediments used in this paper have been formed approximately in similar conditions of Cretaceous epicontinental seas of the Russian platform and adjacent territories. All the sediments had similar aleurite-clay structure. The main bearer of magnetization in all cases was detrital magnetite, whereas the natural magnetization was of an orientation nature. Orientation nature of the magnetization is manifested in low values of the Keniksberger factor, low compactness of the initial magnetization directions, and visual studies of the separated sediment grains containing the bearer of the residual magnetization. The sediments of the same sediment strata (polar interval), except one case, had slightly changing by power petromagnetic parameters. For example, in the Albian-Cenomanian sediments the magnetic receptivity K varied within (5-10) 10^-5 SI units, and the residual saturation magnetization was Irs ~(0.1-0.3) A m^-1. In the Aptian sediments, K was (13-20) 10^-5 SI units, and Irs was (1.0-3.0) A m^-1; however, several lower samples had Irs more than 100 A m^-1. In the Barremian sediments, K and Irs were (15-25) 10^-5 SI units and (0.5-1.0) A m^-1, respectively.

[11]  The data on the paleointensity of Cenomanian and Albian and upper Barremian were taken from Kurazhkovskii et al. [2002] and Kurazhkovskii et al. [2003], respectively. These results have been obtained in studies of marine gray magnetite-containing aleurite-clay sediments in the eastern part of the Russian platform. The paleointensity of the lower Barremian was determined from the magnetite-containing gray sediments of north Caucasus [Guzhikov et al., 2002]. The data on the paleointensity of Aptian are being published for the first time. The data have been obtained in studies of marine aleurite-clay magnetite- and hematite-containing gray sediments sampled in the Crimea (Simferopol, Mar'ino village). In the same way as in the previous publications [Kurazhkovskii et al., 2002, 2003] the values of the geomagnetic field intensity H/H₀ were calculated in the following manner: H/H₀= Rns₃₅₀/Acp₃₅₀, where H is the field intensity of the ancient magnetic field, the Rns ₃₅₀ parameter is equal to the ratio In₃₅₀/Irs₃₅₀ (In is the natural remanent magnetization, Irs is the remanent saturation magnetization, the 350 index is the temperature of magnetic cleansing), A₃₅₀ is the coefficient equal to Ird₃₅₀/Irs₃₅₀ (Ird is the mean laboratory orientation magnetization of one sample obtained as a result of several redepositions). The average value of this coefficient was calculated separately for each sedimentational strata. To determine it, the results of redeposition of samples from all layers of one studied cut of sedimentations were used. This method of H/H₀ evaluation with performing of a grading suggested by Kurazhkovskii and Kurazhkovskaya [2001] (the values of the A ₃₅₀ coefficients should not be less than 2 10^-3, their difference in the thickness of the sedimentation strata should not exceed 20%) the random error does not exceed 13%. To determine the paleointensity, such sediments were used that their initial magnetization was revealed at temperatures not higher than 350^o.

[12]  Kurazhkovskii and Kurazhkovskaya [2001] showed that the usage of the Rns parameter to create the paleointensity dynamics reduces the probability of random errors. Conducting of a reprecipitation makes it possible to exclude errors due to the variations in magnetomineralogical and granulometric composition and also to evaluate the absolute values of the paleointensity.

[13]  In all cases the sediment strata had a thickness of a few tens of meters and nearly similar aleurite-clay structure. Thus one may suggest that the studied paleointensity fragments had approximately the same duration. The intervals between the levels of Cenomanian-upper Barremian depositions where samples had been collected were about 1 m. The lower Barremian samples of the northern Caucasus were collected from the strata separated by 2-3 m. This magnetopolar interval is studied in less detail.