The nature of Ordovician limestone-marl alternations in the Oslo-Asker District (Norway): witnesses of primary glacio-eustasy or diagenetic rhythms?

Abstract

Ordovician limestone-marl alternations in the Oslo-Asker District have been interpreted as signaling glacio-eustatic lowstands, which would support a prolonged “Early Palaeozoic Icehouse”. However, these rhythmites could alternatively reflect differential diagenesis, without sedimentary trigger. Here, we test both hypotheses through one Darriwilian and three Katian sections. Our methodology consists of a bed-by-bed analysis of palynological (chitinozoan) and geochemical (XRF) data, to evaluate whether the limestone/marl couplets reflect an original cyclic signal. The results reveal similar palynomorph assemblages in limestones and marls. Exceptions, which could be interpreted as reflecting palaeoclimatological fluctuations, exist at the species level: Ancyrochitina bornholmensis seems to be more abundant in the marl samples from the lower Frognerkilen Formation on Nakkholmen Island. However, these rare cases where chitinozoans differ between limestone/marl facies are deemed insufficient for the identification of original cyclicity. The geochemical data show a near-perfect correlation between insoluble elements in the limestone and the marls, which indicates a similar composition of the potential precursor sediment, also in the Frognerkilen Formation. This is consistent with the palynological data. Although an original cyclic pattern could still be recorded by other, uninvestigated parameters, our palaeontological and geochemical data combined do not support the presence of such a signal.

Glacial onset predated Late Ordovician climate cooling

“…It is also still unclear how glacial onset could have possibly occurred in the midst of a prolonged period of hot climate. Evidence for warm shallow seas in the tropics (up to 37°C) [ Finnegan et al , ], as suggested by most recent Ordovician geochemical data [ Trotter et al , ; Finnegan et al , ], is in apparent contradiction with Middle to Upper Ordovician deposits where glacioeustatic events are identified [ Loi et al , ; Turner et al , ; Elrick et al , ; Dabard et al , ; Rasmussen et al , ] or suggested [ Nielsen , ] (see discussion in Amberg et al []). These eustatic cycles support the presence of ice sheets since the Middle Ordovician Darriwilian (467 Ma) rather than a sudden emplacement of a massive ice sheet restricted to the Hirnantian stage (445–444 Ma).…”

Detecting initial aragonite and calcite variations in limestone–marl alternations

“…Based on these two well‐documented observations, it follows that a constant element ratio will be also retained across a succession where aragonite or calcite input in the precursor sediment varied. This theoretical prediction has been corroborated empirically (Westphal e t al ., 2008, 2010; Amberg et al ., 2016).…”

Le dévonien supérieur du sahara nord-occidental (algérie) : faciès, environnements et signification géodynamique des calcaires griottes

“…Les plaques d'échinodermes observées au sein du faciès sont peu usées, et probablement exotiques, apportées sous forme d'éléments flottés. L'effet de la diagenèse précoce dans le développement des alternances rythmiques à partir de sédiments préalablement homogènes a été démontrée par certains auteurs (MUNNECKE, 1997 ;MUNNECKE & SAMTLEBEN, 1996 ;WESTPHAL & MUNNECKE, 2003 ;RIDING & LIANG, 2005 ;WESTPHAL, 2006 ;AMBERG et al, 2016), distinguant un flux des eaux interstitielles soit dirigé vers le bas (cf. JENKYNS, 1974), soit entraîné et dirigé vers le haut par compactage (cf.…”