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

Abstract

Delineation of stratigraphic sequences and their component systems tracts in mud-stone-dominated successions is challenging due to the relatively homogenous, finegrained 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 (δ 13 C org ), grain size and CaCO 3 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 CaCO 3 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 δ 13 C 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. K E Y W O R D SLower Cretaceous, Lower Saxony Basin, mudstones, sequence stratigraphy, Weissert Event, X-ray fluorescence core scanning | 237 THÖLE ET aL.

Geochemical fingerprinting of key lithologies and depositional processes across the upper boundary of the Opalinus Clay (Aalenian, Middle Jurassic, northern Switzerland)

“…For instance, Thöle et al . (2020) applied the method to drill cores covering the Berriasian to Aptian interval within the Lower Saxony Basin (northern Germany). They identified several elemental ratio proxies allowing them to correlate proximal and distal basinal environments, and to characterise the studied successions in terms of grain‐size and carbonate content variations using Si/Al and Ca/Ti, respectively.…”

“…The present study differs from the work of Thöle et al . (2020), as it focusses on a limited stratigraphic interval, and on the detailed characterisation of one particular sequence boundary (i.e. discontinuity surface).…”

“…Based on petrographic criteria alone, especially when investigating subsurface successions that are limited to the study of drill cores, it may be difficult to assess the genetic origin of sedimentary discontinuities (given that they can be identified as such), as well as to understand their significance in terms of sequence stratigraphy. It has already been shown that the use of elemental proxies can be convenient for identifying and characterising stratigraphic sequences within mudstone‐dominated successions (Thöle et al ., 2020; Turner et al ., 2016; Ver Straeten et al ., 2011). The present study nonetheless highlights the potential of XRF‐CS as an additional and complementary tool for unravelling the depositional history of complex and mixed, carbonate‐siliciclastic transitions.…”

“…X‐ray fluorescence (XRF) core scanning, a non‐destructive logging technique, provides continuous information on relative changes in elemental concentration at very high spatial resolution (down to 200 µm). Downcore variability in elemental data sets has successfully been applied to decipher variations in environmental, sedimentological and diagenetic processes (Bloemsma et al ., 2012; Croudace and Rothwell, 2015; Cuven et al ., 2010; Deplazes et al ., 2019; Jaccard et al ., 2005; Vogel et al ., 2010; among many others), and assist in correlation studies (Chatellier et al ., 2018; Kaboth‐Bahr et al ., 2019; Thöle et al ., 2020).…”

Lithological correction of chemical weathering proxies based on K, Rb, and Mg contents for isolation of orbital signals in clastic sedimentary archives

“…LEFs were calculated for the K/Al, Mg/Al, and K/Rb element ratios using Al/Si, Zr/Rb, Ca, and/or Fe as independent variables. Al, Si, Rb, and Zr, in particular their ratios Al/Si and Zr/Rb are broadly applicable grain-size proxies (Chen et al, 2006;Bouchez et al, 2011;Lupker et al, 2011Lupker et al, , 2013Flood et al, 2016;Matys Grygar and Popelka, 2016;Thöle et al, 2019), although both Al/Si and Zr/Rb can also be influenced by sediment mineralogy and provenance (Profe et al, 2016). The principle of LEF in this study is the same as that of the enrichment factor EF of local EF (LEF) in environmental geochemistry (Matys Grygar and Popelka, 2016;Birch, 2017): the target element ratio is divided by the ratio calculated from a baseline function for the target sample using a suitable independent (explanatory) variable.…”