Using high‐resolution XRF analyses as a sequence stratigraphic tool in a mudstone‐dominated succession (Early Cretaceous, Lower Saxony Basin, Northern Germany)

Hauke Thöle,Friedrich Wilhelm Luppold,André Bornemann,Ulrich Heimhofer,Martin Blumenberg,Reiner Dohrmann,Jochen Erbacher

doi:10.1002/dep2.83

Abstract

AbstractDelineation of stratigraphic sequences and their component systems tracts in mudstone‐dominated successions is challenging due to the relatively homogenous, fine‐grained nature of the strata. High‐resolution elemental intensity data from X‐ray fluorescence core scanning is used in order to develop a sequence stratigraphic framework for the Lower Cretaceous monotonous mudstone succession in the eastern Lower Saxony Basin. The study is based on four drill cores covering the Berriasian to Aptian interval. In addition, carbon isotope (δ13Corg), grain size and CaCO3 analyses were carried out on discrete samples. The studied cores represent both proximal and distal basinal environments of the eastern Lower Saxony Basin and can be reliably correlated by utilizing variations in selected X‐ray flourescence elemental ratios, K/Ti data have proven to be particularly suitable in this regard. The core correlation shows that chemostratigraphic variability within the studied succession is laterally reproducible in the eastern Lower Saxony Basin, and can be used to establish a sequence stratigraphic framework. Further, Si/Al and Ca/Ti ratios have been applied to characterize the cores in terms of variation in grain size and CaCO3 content, respectively. Vertical grading trends inferred from Si/Al changes were used to identify transgressive and regressive systems tracts within the studied succession. An important regression in the uppermost lower Valanginian coincides with the onset of the Valanginian Weissert Event, as indicated by the well‐known positive δ13C shift, and, thus, supports the idea that the initial interval of this event corresponds to enhanced supply of terrigenous material. The results of this study are also in agreement with previously recognized transgressive–regressive trends in the Lower Saxony Basin and adjacent areas. This clearly shows that systematic geochemical variations recorded in mudstone‐dominated basinal settings are suitable to establish sequence stratigraphic frameworks.

Highlights

Mudstones are the most common sedimentary rocks (Picard, 1971; Wedepohl, 1971; Stow, 1981) and some of the most challenging to study, analyse and understand
Because mudstone‐ dominated sedimentary successions are being increasingly targeted as unconventional hydrocarbon reservoirs and as host rocks for geological disposal, there has been a significant advance in our understanding of the sedimentology and stratigraphy of mudstones over the last decade, especially concerning the pervasive facies variability present within these fine‐grained rocks (Bohacs and Schwalbach, 1992; Macquaker et al, 2007; Macquaker et al, 2010; Hammes et al, 2011; Abouelresh and Slatt, 2012)
This study aims at developing a sequence stratigraphic framework for the Lower Cretaceous succession in the eastern Lower Saxony Basin (LSB), and associating facies variability of the studied sediments with sequences and systems tracts

Summary

Introduction

Mudstones are the most common sedimentary rocks (Picard, 1971; Wedepohl, 1971; Stow, 1981) and some of the most challenging to study, analyse and understand. An approach that has recently been used to establish sequence stratigraphic frameworks in mudstone‐dominated basinal settings without clear ties to coastal records, is to determine systematic stratigraphic variations in the inorganic and organic geochemistry of sedimentary rocks (Ver Straeten et al, 2011; Turner et al, 2015, 2016). These variations are subsequently used to characterize and define stratigraphic sequences. Such approaches rely upon the fact that even apparently homogeneous mudstones show changes in their chemical composition

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