Limestone–marl alternations as environmental archives and the role of early diagenesis: a critical review

Abstract

Limestone-marl alternations and other micritic calcareous rhythmites have long appealed to sedimentologists, as they appeared to directly reflect high-frequency environmental change. In particular, when orbital forcing gained popularity amongst sedimentologists and paleoclimatologists in 1980s, such rhythmites seemed to offer an ideal tool for high-resolution chronostratigraphy and environmental reconstruction. However, in spite of the fact that orbital forcing has become a routine interpretation of calcareous rhythmites, and that the processes of formation of calcareous rhythmites are considered well understood, research in the past 10 years again has questioned their primary origin and their direct interpretability. Detailed petrographic, paleontological, and geochemical data from numerous successions through geological time provided the basis for testing whether or not the regular alternation of limestone beds and marl or shale interlayers represents bimodally fluctuating environmental conditions in a direct way. In particular, these data, supplemented by box model simulations, imply that post-depositional alteration (diagenesis) has the potential to not only seriously distort primary environmental signals, but also to mimic primary signals. This questions the use of micritic calcareous rhythmites for high-resolution chronostratigraphy and for environmental interpretations where independent data of diagenetically inert parameters are not available. Diagenetic changes appear to have a yet widely underestimated influence on the appearance of limestone-marl alternations and other calcareous rhythmites. The aim of the present review is to summarize new approaches and give an overview of our research results in this field of the past decade. This review also aims at pointing to still enigmatic aspects that need to be addressed before the interpretation of micritic calcareous rhythmites can be considered a reliable tool for high-resolution chronostratigraphy and paleoenvironmental interpretation.

Lacustrine microporous micrites of the Madrid Basin (Late Miocene, Spain) as analogues for shallow-marine carbonates of the Mishrif reservoir Formation (Cenomanian to Early Turonian, Middle East)

“…Clays are totally absent; thus they could not be involved as a factor of compaction (Ehrenberg 2004;Ehrenberg and Boassen 1993;Lambert et al 2006). The strontium content does not indicate aragonite enrichment (Lasemi and Sandberg 1993) in tight layers that may speak in favor of a differential diagenesis as in limestone-marl alternations (Munnecke and Samtleben 1996;Westphal 2006). The stable isotopes present too little variations (Dd 18 O = 0.8% and Dd 13 C = 1.2%) to be significant (Bellanca et al 1992;Eugster and Kelts 1983;Leng et al 2006;Lerman 1978).…”

Cretaceous-Paleogene transition at the Paraíba Basin, Northeastern, Brazil: Carbon-isotope and mercury subsurface stratigraphies

“…3) overlies the Itamaracá Formation and, as typical in the Maastrichtian worldwide, it is composed of limestone-marl alternation (rhythmites; Milankovitch-range to millenial scale) deposited on 100e150 m deep carbonate platform that as environmental archives directly reflect high-frequency environmental changes (Westphal, 2006). This formation displays characteristics of a high stand tract and in its upper portion it presents traces of a forced regression, just before the transition to the Paleogene, caused by tectonic uplift (Barbosa et al, 2003;Barbosa, 2007).…”

Climatic cycles recorded in the Middle Eocene hemipelagites from a Dinaric foreland basin of Istria (Croatia)

“…5) is suggestive of a possible diagenetic imprint. However, if diagenetic carbonate reorganization was the cause of the approximately 11% difference in CaCO 3 between LRM and MRM, one would expect MRM to display more negative δ 18 O values due to higher amounts of secondary carbonate cement (Thierstein & Roth 1991;Frank et al 1999;Westphal 2006). The subtle and consistent differences in isotope values (Fig.…”