A comparison of glauconite K-Ar dates and 87Sr/86Sr dates from Rupelian and Chattian strata, Campine area, Belgium
|Category||Paleontology and Stratigraphy|
|Location||International Geological Congress,oslo 2008|
|Author||Harris, W. Burleigh۱; De Man, Ellen۲; VanSimaeys, Stefaan۲; Vandenberghe, Noël۳; Wampler, J.M.۴|
|Holding Date||08 October 2008|
The classic lower and upper Oligocene unit-stratotypes of the Rupelian and Chattian Stages were defined on successions in the southern North Sea Basin. The outcrop area of the Boom Clay Formation along the river Rupel in northwestern Belgium is the location of the international unit-stratotype for the Rupelian, but these surface sections only include lower Rupelian deposits. The youngest Rupelian occurs in boreholes in eastern Belgium and only recently has been studied in detail. The Doberg section near Bünde, Germany, is the neo-stratotype for the Chattian Stage, and is divided into Chattian A and B (Eochattian Substage) and Chattian C (Neochattian Substage) based on pectinid ranges. Chattian A, B, and C are identified as North Sea Chattian sequences Ch-1, Ch-2, and Ch-3, respectively.
The basal Chattian sequence Ch-1 is characterized by the distinct benthic foraminifer Asterigerinoides guerichi guerichi, known over the entire North Sea Basin as the Asterigerina Horizon; it defines the base of the Chattian Stage. Sequence Ch-1 is considered to be within calcareous nannofossil zone NP24 and sequences Ch-2 and Ch-3 within calcareous nannofossil zone NP25. To date the Rupelian-Chattian boundary in the type region, conventional K-Ar age values for well-ordered, high K2O (>6%) glauconitic mica from the Boom Clay and Voort Sand Formations in boreholes from the Belgium Campine area (MOL-1, Hechtel, and Helchteren) were determined and compared to 87Sr/86Sr dates from the Hechtel borehole and the nearby Weelde borehole.
Three K-Ar glauconite dates from the youngest Rupelian sediments in MOL-1 and Helchteren range from 31.2 ± 0.4 Ma to 29.2 ± 0.4 Ma and are consistent with 87Sr/86Sr dates (29.3 ± 0.5 Ma to 28.6 ± 0.6 Ma) for equivalent sediments in Weelde and Hechtel. Three K-Ar dates from the Chattian sequence Ch-1 in MOL-1 are 27.0 ± 0.3 Ma, 30.5 ± 0.4 Ma (31.7 ± 0.7 Ma in a duplicate determination), and 28.2 ± 0.3 Ma (28.5 ± 0.3 Ma in duplicate). The smaller value of 27.0 ± 0.3 Ma is consistent with 87Sr/86Sr dates (27.5 ± 0.7 Ma to 26.6 ± 0.6 Ma) from the same sequence in Weelde and Hechtel.
The two larger values are from approximately the same horizon and are considered to be invalid for dating that horizon. Chattian sequence Ch-2 provided three K-Ar dates of 28.3 ± 0.4 Ma, 26.9 ± 0.4 Ma, and 26.1 ± 0.4 Ma (same in duplicate) from MOL-1, Hechtel, and Helchteren. The two smaller values are consistent with each other and with 87Sr/86Sr dates from underlying strata in Weelde and Hechtel, thus the larger value fails to correctly date the strata.
Chattian sequence Ch-3 provided K-Ar dates from MOL-1 of 26.5 ± 0.4 Ma and 26.5 ± 0.3 Ma; these are considered to be reliable as they are consistent with dates for the older sequences. These data suggest that the Rupelian-Chattian boundary in the southern North Sea Basin lies between 28.6 Ma and 27.5 Ma, and may be closer to 27.5 Ma rather than the 28.45 Ma date placed on the boundary by the 2004 Time Scale.