Reconstructing time and diagenesis of limestone-marl alternations from the selective compaction of colonies of the tabulate coral Halysites

Abstract

rhythmically alternating lithologies for analysing and dating time span and causes of changes in the depositional environment. The approximate time span of individual beds and couplets are a basic requirement for finetuned cyclostratigraphy based on lithological changes. Limestone-marl alternations are often used for this kind of approach, even though correlation of individual limestonemarl couplets over long distances have been questioned in the last 25 years by studies investigating their specific diagenetic processes (among others Munnecke & Samtleben 1996; Böhm et al. 2003; Westphal et al. 2010, 2015; Bádenas et al. 2012; Gygi 2012; l’Heureux 2018). As the precise duration of the deposition of a single bed normally cannot be determined, the temporal range of the whole succession is usually divided by the number of couplets (Schlager et al. 1998). The resulting time span of single beds/couplets thus varies from several 1000 (e.g. millennial cycles) to several 100,000 (Milankovitch cycles) years (Hilgen et al. 2003; see compilation in Strasser et al. 2006). Several problems, however, arise through this method. Apart from hiatuses and the difficulty of defining the precise age of a couplet, fluctuating sedimentation rates are a further limiting factor for any fine-tuned resolution (Sadler 1981). Sadler (1981) and later Schlager et al. (1998) formulated a dependence of the calculated sedimentation rate on the length of the observed interval, with a reduced rate from larger observation intervals (Sadler effect). Or, in other words: “We invariably find that the rock record requires only a small fraction, usually 1 to 10 per cent, of the available time, even if we take account of all possible breaks in the sequence” (van Andel 1981), which means that more than 90% of the time is not recorded in the respective sediments, not even in the deep sea. So how do we know if this missing time is still reflected in the cyclicity (e.g. only 10% of every climatic cycle is recorded)? Or is the missing time rather reflected in a lack of an unknown