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.
Highlights
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”
An emerging body of evidence suggests that this global cooling and the onset of the “Early Palaeozoic Icehouse” (EPI) may have started much earlier than previously assumed[3], i.e. during the early Katian[4,5,6], before the Sandbian[7] or even during the Early-Middle Ordovician[3]
The sequence stratigraphic architecture of various Ordovician packages, such as those treated herein[8], the Peninsula Formation (Floian to Darriwilian) in South Africa[9] and the Darriwilian Hiswah and Dubaydib formations in Jordan[10], which have been suggested to record 3rd and 4th order sea level changes corresponding to glacio-eustatic cycles; (3) microfossil evidence with relatively steep graptolite and chitinozoan equator-to-pole faunal gradients that suggest cooling towards the Hirnantian glacial maximum was already underway in the Sandbian[7,11]; and (4) General Circulation Models (GCMs) for the Early-Middle Ordovician[12] that suggest a long-term cooling trend through the Ordovician
Summary
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”. The sequence stratigraphic architecture of various Ordovician packages, such as those treated [8], the Peninsula Formation (Floian to Darriwilian) in South Africa[9] and the Darriwilian Hiswah and Dubaydib formations in Jordan[10], which have been suggested to record 3rd and 4th order sea level changes corresponding to glacio-eustatic cycles; (3) microfossil evidence with relatively steep graptolite and chitinozoan equator-to-pole faunal gradients that suggest cooling towards the Hirnantian glacial maximum was already underway in the Sandbian[7,11]; and (4) General Circulation Models (GCMs) for the Early-Middle Ordovician[12] that suggest a long-term cooling trend through the Ordovician Each of these methodologies is inconclusive on its own. Other scenarios[23] consider that the nodules are the result of bacterial activity where the sulfate reduction and fermentation by the bacteria led to the production of bicarbonate and sulfide eventually turned into carbonate and pyrite, commonly found in the rhythmites
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