RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES6001, doi:10.2205/2006ES000206, 2006


Magnetic Characteristics of Maastrichtian and Danian Sedimentary Rocks

[37]  Paleomagnetic research of many years allowed the authors and other researchers to gather and summarize a large amount of data on scalar magnetic characteristics of rocks in reference sections of the Maastrichtian and Danian of various regions [Ellwood et al., 2003; Molostovsky, 1986; Molostovsky and Khramov, 1997; Yampolskaya et al., 2004 and others].

[38]  In all known sections, Maastrichtian sediments are characterized by very low magnetization with its general growth in the Paleogene base. Jump of magnetization varies in a wide range and depends on the concrete geological situation. Generally two types of sequences are noted.

2006ES000206-fig04
Figure 4
[39]  The first type is abundant in Transcaucasian mountainous area, in southeastern Caucasus and in eastern Ciscaucasia (Figure 4 A, B, C).

[40]  In Transcausasia in Adzhi-dere section (Nagornyi Karabakh), Upper Palaeocenen terrigenous-carbonate deposits overlay Maastrichtian light gray limestone with stratigraphic unconformity. Upper Cretaceous sediments are weakly magnetic (k=1-6cdot10-5 SI units), and the Paleocene top and the Eocene are distinguished in the section for their increased magnetization (k=20-140cdot10-5 SI units). Cretaceous - Paleogene boundary is considerably marked if less contrasting in the magnetic susceptibility of Yunusdag Range in Northeast Azerbaijan, where Maastrichtian weakly magnetic limestone and marl (k=3-5cdot10-5 SI units) are overlain by more magnetic variegated marl of the Paleocene (k varies from 20cdot10-5 SI units to 162cdot10-5 SI units). In the carbonaceous flysch of the Caucasus north-western termination (Novorossiysk-Anapa) susceptibility increase was established [Guzhikov et al., 1998], which is minor but statistically significant at the Cretaceous-Paleogene boundary (k=1-10cdot10-5 -10-20cdot10-5 SI units in Maastrichtian and 20-42cdot10-5 SI units in the Paleogene).

[41]  A feature of the first type of sections is sharp erosional boundary between systems and a considerable time gap. Susceptibility increase is caused by the ash delivery into Paleogenic basin from Transcaucasia during explosions or input of fragmental magnetic product supply from wash-down new sources.

[42]  The second type of sequences is characterized by magnetic susceptibility increase, which is minor but noted everywhere near the Maastrichtian-Danian boundary; locally it is a short-time jump of susceptibility (k-peak) and in other places it is a prolonged process. Sections of Cis-Caucasus, lower Povolzh'e, Central Asia, West Europe and others (Figures 2 and 4 D, E, F, G) belong to this type.

[43]  This slight magnetization increase near K/T boundary is caused by a change in sedimentation conditions and is determined by a supply of finely dispersed terrigenous material. This general manifestation of susceptibility increase is most likely caused by erosion activity revival in wash-down areas as a result of wide regression at the end of Cretaceous-the beginning of Paleogene. This manifestation is most pronounced in the sections of the shelf and the upper continental slope. From data of oceanic sediments columns [Pechersky and Garbuzenko, 2005], K/T boundary is frequently marked by the magnetic susceptibility peak (k-peak) but it is noted only in 30% of columns of continuous sequences including K/T boundary, i.e. for oceanic sediments it is not a characteristic of the Mesozoic-Cenozoic boundary. The distribution and value of k-peak do not depend on the distance to the nearest land, i. e. from the distance to a wash-down area. On the contrary, sediments are less magnetic in the columns that are closer to continents and susceptibility increase (k-peak) is not noted there. The value of k-peak ranges widely in agreement with lithological characteristics of sediments. The largest values of k-peak (from 60times10-5 SI units to 120-250times10-5 SI units) are located near the epicenters of active plumes Kergelen, Hawaii, and Whale Range. It should be emphasized that k-peaks near K/T boundaries are not unique; it is a common feature of oceanic sediments, specifically of the Upper Cretaceous and the Paleocenic. The width of k-peaks varies significantly, reflecting different time of relative enrichment of sediments with magnetic material from less than 10 thousand years to ~0.4 million years. K/T boundary and consequently k-peak close to it are within a magnetochron of reversal polarity C29R and occupies different positions in it though they show similarity of lithology and thicknesses, i.e. biostratigraphic Maastrichtian-Danian boundary is not synchronous in the basin of the World ocean and the difference amounts to ~0.7 million years. Non-synchronous increase of susceptibility (k-peak) and its different duration can be seen in epicontinental sediments columns (Figures 2 and 4).

[44]  Thus the aforesaid on oceanic epicontinental sediments susceptibility and their lithological characteristics suggests a considerable time interval, during which magnetic minerals were accumulated at the Maastrichtian-Danian boundary and biota changes took place, rather than an abrupt "instant" jump of sedimentation changing conditions, specifically magnetic minerals accumulation at the Maastrichtian-Danian boundary. Such "duration" rules out a relation between the enumerated processes and impact events, which undoubtedly are short-lived.

[45]  Let us discuss the results of detailed studies of magnetic characteristics of rocks of sections Klyuchi and Teplovka that throw light on magnetization nature of the region sediments near K/T boundary.

2006ES000206-fig05
Figure 5
Klyuchi.
[46]  Material on a number of magnetic characteristics reflecting the composition and texture of magnetic minerals in Klyuchi section is uniform, which is evident from thermomagnetic analysis uniform curves (Figure 5), practically similar in Danian samples beginning with 30 cm above K/T contact and from very close values of remanent coercive force Hcr =33-38 mT. Thus from thermomagnetic analysis data, both Maastrichtian and Danian sediments have:

[47]  a) phase with Curie point Tc =120-140oC, which disappears after the first heating. Evidently these are ferric hydrated oxides of goethite type; their contribution in magnetization amounts to approximately 10%,

[48]  b) phase with T c =200-250oC; it is most likely hemoilmenite; its contribution in magnetization is less than 20%; when a sample is heated, hemoilnmenite undergoes partial homogenization and as a result TMA curve attains hyperbolic form,

[49]  c) irreversible drop of magnetization at 300-320o C that is typical of transition from maghemite to hematite,

[50]  d) phase with Tc =560-590oC; it is magnetite; its contribution in magnetization is 10-30% and it is higher in Danian samples as compared to Maastrichtian sediment samples; when heated magnetite is oxidized completely or partially; maghemite disappearance and magnetite oxidization result in magnetization decrease after heating up to 800oC and it makes 0.75-0.9 of the initial value,

[51]  e) phase with Tc =710-740oC; it is metallic iron with small admixtures (pure iron Tc =769oC). The latter is reliably established in Maastrichtian sediments and is practically lacking in Danian sediments. These results were supported by finds of spherules of 1-10  m m in Maastrichtian rock heavy fraction near K/T boundary (Figure 3), which were not noted in other stratigraphic layers. Microprobe analysis confirmed that it was iron.

2006ES000206-fig06
Figure 6
[52]  Magnetic susceptibility (Figure 2a) very low and even negative (at the expense of diamagnetic calcite) in Maastrichtian rocks and gradually increases in Danian sediments beginning with K/T boundary to 4-5times10-5 SI units. The jump is even more vivid in total magnetization including magnetization of magnetic and paramagnetic materials of K/T boundary: from 4.6-5.3times10-3 Am2 kg-1 in Maastrichtian rocks to 11.6-17.6times10-3 Am2 kg-1 in Danian rocks. It occurs first of all at the expense of considerable contribution of paramagnetic component. Thus in Danian rocks it amounts to 11.2-17.4times10-3 Am2 kg-1 (i.e. it makes a great contribution of the total magnetization). In Maastrichtian rocks near K/T boundary it makes less than 3times10-3 Am2 kg-1, and at 10-20 cm below the boundary it passes into small negative values where diamagnetic component prevails and as K/T boundary has been approached against the background of this paramagnetic component increases up to its evident predominance in Danian sediments. Paramagnetic component predominance in magnetization is evident from TMA curve form resembling hyperbola (Figure 5). Paramagnetism of rocks is determined by total iron content in them. From the direct comparison [Grachev et al., 2005], it follows that in upper Maastrichtian iron content in the form of Fe 2 O 3 is less than 0.5%, where as in Danian base it is 2-3.5%. Since composition and coercivity of magnetic minerals are close on the whole section, we may assume that the saturation remanent magnetization of rocks in the section is only determined by magnetic mineral concentration. In Maastrichtian rocks it varies from 0.014-0.015times10-3 Am2 kg-1 to 0.029-0.042times10-3 Am2 kg-1, the jump is not great but it is significant and it gradually increases up the section (Figure 6). Thus judging by magnetic characteristics Maastrichtian sediments differ from Danian sediments first of all by the total iron content as well as by a small change in the total content of magnetic minerals and presence of small concentrations of metallic iron in the top of Maastrichtian sediments.

Teplovka.
[53]  The material is considerably uniform by magnetic characteristics, even more uniform than in Klyuchi section in both composition of magnetic minerals (thermomagnetic analysis data, Figure 5) and their structure (remanent coercivity varies in a narrow range from 39 mT to 42 mT). It is evident from susceptibility behavior as well (Figure 2).

[54]  From thermomagnetic analysis data, Maastrichtian and Danian sediments of Teplovka section contain similar minerals as in Klyuchi section:

[55]  a) ferric hydrated oxides of goethite type (Tc =110-140oC, which disappear after the first heating of the sample), their contribution in magnetization is approximately 10%,

[56]  b) hemoilmenite (Tc =210-270oC), its contribution in magnetization is less than 20%, when samples are heated hemoilmenite undergoes partial homogenization and as a result TMA curve takes the hyperbolic form,

[57]  c) irreversible drop of magnetization at 300-320o C, typical of maghemite transition into hematite,

[58]  d) magnetite (Tc =560-590oC), its contribution in magnetization is 20-40%, when heated, it is partially or completely oxidized, disappearance of maghemite and magnetite oxidization result in magnetization decrease when it is heated up to 800o C (0.76-0.95 of initial value),

[59]  e) metallic iron (Tc =730-770oC), the latter is established in Maastrichtian sediments and is not revealed in Danian sediments, iron contribution in magnetization ranges from 5% to 50% (Figure 5), maximum is at 30 cm below the contact with Danian sediments. Value of saturation magnetization in this point is 6times10-3 Am2 kg-1, iron saturation magnetization is ~200 Am2 kg-1, and correspondingly metallic iron content is 0.003%. Numerous magnetic spherules of metallic iron of 1-10  m m (Figure 3) were extracted from heavy fraction of Maastrichtian rocks samples near K/T boundary, which is supported by microprobe analysis data.

2006ES000206-fig07
Figure 7
[60]  As distinct from Klyuchi section, magnetic susceptibility does not increase gradually in the transition from Maastrichtian sediments to Danian sediments. In section Teplovka above the K/T contact, a narrow peak of magnetic susceptibility is noted; above and below it, magnetic susceptibility value is similar on the whole section (Figure 2a) and it is somewhat higher than in section Klyuchi. Total and paramagnetic magnetization behave in the same way: the former varies in a small range from 11.2times10-3 Am2 kg-1 to 13.4times10-3 Am2 kg-1, against it a peak up to 40times10-3 Am2 kg-1 is noted above the contact and a quick drop to background level at the first tens of cm; paramagnetic magnetization "background" is 6-7times10-3 Am2 kg-1 in Maastrichtian sediments and 9-11times10-3 Am2 kg-1 in Danian sediments and the peak reaches 35.7times10-3 Am2 kg-1. That means that similar to Klyuchi section, the paramagnetic component makes a considerable part both of the peak and background value of magnetization, which is manifested in the form of TMA curves, which is close to hyperbolic (Figure 5). But the general level of magnetization is considerably higher than the rocks of Klyuchi section. In Teplovka section, iron content (Fe2O3 ) is 1-1.5% in Maastrichtian "background", approximately 2% in Danian "background" and "the peak" of content is 7%. A higher "background" iron content influenced the Teplovka rock susceptibility higher level. As distinct from the general (paramagnetic) iron content, magnetic mineral content behaves in a different way. It resembles Klyuchi section, which can be seen from the behavior of saturation remanent magnetization (Figure 7) depending on magnetic mineral content. Generally, magnetic mineral content in Teplovka section is considerably higher than in Klyuchi section. In Maastrichtian sediments, Jrs =0.023-0.04times10-3 Am2 kg-1, in Danian sediments, near the contact, it is 0.045times10-3 Am2 kg-1, and up the section, 30 cm higher, J rs rises up to 0.076times10-3 Am2 kg-1 and then gradually increases with some variation (Figure 6) actually similar to Klyuchi section.

[61]  Thus the impression is formed that the flash of magnetic susceptibility near K/T boundary was not caused by a single event but resulted from close in time but not synchronous events of iron accumulation in paramagnetic minerals in sediments that is ferric hydroxide and clayey minerals containing iron. It resembles the formation process of metal-bearing sediments and ferrous microconcretions, which is a result of volcanic and hydrothermal activity [Gurvich, 1998]. This process differs essentially from terrigenous accumulation of magnetic minerals; the series is identical in both Maastrichtian top and Danian low part; their concentrations somewhat vary from the Maastrichtian to the Danian.


RJES

Citation: Molostovsky, E. A., V. A. Fomin, and D. M. Pechersky (2006), Sedimentogenesis in Maastrichtian-Danian basins of the Russian plate and adjacent areas in the context of plume geodynamics, Russ. J. Earth Sci., 8, ES6001, doi:10.2205/2006ES000206.

Copyright 2006 by the Russian Journal of Earth Sciences

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