Integrated Stratigraphy and Fluvial Cyclicity of the Westphalian B in the southern North Sea Basin

Abstract

The upper Carboniferous in the southern North Sea, especially the Westphalian B and lower Westphalian C stage is characterized by the deposited cyclothems that are a series of coarsening-upward fluvial sediments cycles. These cyclothems are often coal-bearing and no more than 15 m. Studying the fluvial cyclicity has a good potential for improved stratigraphy because the cyclothems show cyclic signals and provide a possible way of high-resolution correlation. An integrated approach using multiple approaches to come to a solid outcome also enables improving the stratigraphy. The previous studies mainly focused on large-scale stratigraphy with a vertical resolution of a few hundred meters. This study has analyzed the fluvial cyclicity and constructed an improved stratigraphic framework of Westphalian B and lower Westphalian C to better understand the sedimentary system. An integrated approach including lithostratigraphy, biostratigraphy and Integrated Predictive Error Filter Analysis (INPEFA) is applied to the series.
Six intermediate-scale biozones with a thickness of tens of meters to around 150 m are applied. Six intermediate-scale units named stratigraphic packages were recognized by INPEFA curves of the gamma-ray log. The correlations of the biozones and the stratigraphic packages are consistent with each other, and the offset between the boundaries of these two zonations is generally between 2-15 m. Individual intermediate-scale units have a highly variable thickness and the standard deviation of one unit can be up to 42 m. Constrained by the biozones and INPEFA stratigraphic packages, four main coal seams are correlated and their lateral extent can reach 15 km.
The small-scale cyclothems that range from 7 to 15 m can be extracted by INPEFA of potassium content log, referring to the small-scale INPEFA cyclicity. The cyclothems and the corresponding INPEFA cycles are thicker and better developed in the southern area. The average thickness of small-scale cycles is 12 m and these cycles are related to obliquity with a duration of approximately 35 kyr. The duration of Westphalian B calculated by the number and the time span of obliquity-scale cycles is nearly 1.2 Myr. No long-period, 100 & 400 kyr eccentricity cycles were recognized in this study.