Mineralogical and Rb-Sr Isotope Studies of Low-Temperature Diagenesis of Lower Cambrian Clays of the Baltic Paleobasin of north Estonia

Abstract

Abstract--X-ray diffraction (XRD), Rb-Sr isotope analysis, transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and Fourier transform infrared (FTIR) methods were used to study diagenetic illite and illite-smectite (I-S) in Lower Cambrian unlithified clays of shallow depth of burial in the northern part of the intercratonic Baltic paleosedimentary basin of the East-European Platform. The studies focused on the <0.06-~m size fraction of the clay. This fraction consists of a highly illitic illite-smectite (I-S) and a poorly crystalline illite (PCI), with some traces of Fe-rich chlorite also present. Rb-Sr isotopic data for the <0.06-1xm size fractions suggest that the illitic I-S and PCI have different formation ages. No precise isotopic ages were derived directly owing to the composite illite mineralogy and retention of radiogenic St. This retention occurred because of imperfect isotopic homogenization at low water/rock ratios. The age of burial diagenesis is proposed to coincide with the time of maximum burial depth, which was achieved during the Middle to Late Devonian and eontirmed until Permian-Triassic erosion. Because of the shallow depth of burial (<2 km), diagenesis was probably a low-temperature (<50~ transformation process. The resident time of 100 150 million years at maximum burial had a major influence in the process.

Isotope geochemistry and origin of illite-smectite and kaolinite from the Seilitz and Kemmlitz kaolin deposits, Saxony, Germany

“…Shoji et al, 1981;Imbert & Desprairies, 1987) or alternatively by low-temperature illitization of smectite (e.g. Eberl et al, 1993;Tribble & Yeh, 1994;Kirsimäe & Jørgensen, 2000;Kirsimäe et al, 1999) that crystallized from the glass. In weathering environments, silicic glass usually transforms into kaolinite ('tonstein') at low pH, into smectite ('bentonite') at intermediate, near-neutral pH, or into zeolites and/or K-feldspar at high pH (e.g.…”

Mixed-layer illite-smectite in the Kinnekulle K-bentonite, northern Baltic Basin

“…The illitization of shallow buried and uncompacted Baltic Basin Lower Palaeozoic clayey sediments has drawn much attention in recent decades (e.g. Gorokhov et al, 1994;Chaudhuri et al, 1999;Kirsimäe et al, 1999a,b;Kirsimäe & Jørgensen, 2000;Lindgreen et al, 2000;Ś rodoń & Clauer, 2001;Clauer et al, 2003). The organic material thermal alteration indexes (CAI, TAI) of 41 (Nehring-Lefeld et al, 1997) and 20S/(20S+20R) epimer ratios of C29 and C28 steranes of 0.137 and 0.306, respectively (Talyzina et al, 2000) suggest that this sedimentary sequence is thermally very immature.…”

“…The clay mineral fraction of Palaeozoic claystones in the Baltic Basin is a physical mixture of detrital and diagenetic clay minerals, which is difficult to study for meaningful diagenetic information (e.g. Clauer & Chaudhuri, 1995;Kirsimäe & Jørgensen, 2000). K-bentonites, which are mostly composed of diagenetic illite and illitesmectite, are common in Ordovician and Silurian sequences of the Baltic Basin (for reviews see Bergström et al, 1992, 1998 andreferences therein) and could be used to study the clay mineral diagenetic evolution in the Basin.…”

Multiphase Silurian bentonites in the Baltic Palaeobasin

“…The shallow burial of the area is also suggested by the Fennoscandian apatite fission track data (Hendriks et al, 2007). Less than 2 km was suggested for the maximum burial depth for the Cambrian complex in northern part of the basin containing unconsolidated plastic clay beds (Kirsimäe and Jørgensen, 2000). Consequently, the maximum burial depth of the Silurian complex in Estonia has most likely been less than 1.5 km whereas the subsidence of the basin was most rapid during Silurian and Devonian times (Poprawa et al, 1999).…”