RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 8, ES6001, doi:10.2205/2006ES000206, 2006
[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.
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Figure 4 |
[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=1-610-5 SI units), and the
Paleocene top and the Eocene are distinguished in the section for their increased magnetization
(k=20-140
10-5 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=3-5
10-5 SI units)
are overlain by more magnetic variegated marl of the Paleocene
(k varies from 20
10-5 SI units to
162
10-5 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=1-10
10-5 -10-20
10-5 SI units in Maastrichtian and
20-42
10-5 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 6010-5 SI units
to 120-250
10-5 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.
![]() |
Figure 5 |
[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.
![]() |
Figure 6 |
[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 610-3 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.
![]() |
Figure 7 |
[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.
Citation: 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.
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