Geochemical signature of Ordovician Mn-rich sedimentary rocks on the Avalonian shelf

Abstract

wt.% MnO) sedimentary layers (coticules, Strohschiefer) occur in Ordovician siliciclastic sedimentary rocks that were deposited on the Avalonian shelf and subsequently metamorphosed between 340 and 380 Ma. The Mn-rich layers formed during periods of restricted clastic input, when Mn concentrated by a redox-controlled dissolutionprecipitation process near the sediment-water interface, comparable to recent Mn-crusts in the boreal settings of the Baltic Sea, the Barents Sea, and the Kara Sea. Enhanced clastic sedimentation brought the process to a halt and buried the Mnrich layers. Formation of Mn-rich layers resumed upon renewed reduction in clastic input. Nd and Sr isotope data exclude a volcanic or hydrothermal derivation of Mn. Instead, Mn input occurred via rivers from the continental crust. The Nd isotopic composition and geochemical fingerprints of Mn-rich and hosting Mn-poor sedimentary rocks suggests that the clastic input was dominated by material derived from Gondwana rather than from Avalonia. The occurrence of coticules, however, seems to be restricted to the Avalonian shelf.Résumé : Des couches sédimentaires (coticules, Strohschiefer) riches en Mn (3,26-15,6 % poids MnO) se retrouvent dans des roches sédimentaires ordoviciennes qui ont été déposées sur la plate-forme avalonienne et par la suite métamorphisées il y a 340 à 380 Ma. Les couches riches en Mn se sont formées durant des périodes d'entrée limitée de sédiments clastiques, lorsque le Mn se concentrait par un processus de dissolution-précipitation contrôlé par oxydoréduction à proximité de l'interface sédiments-eau, ce qui est comparable aux récentes croûtes Mn dans les environnements boréals de la mer Baltique, la mer de Barents et la mer de Kara. Une sédimentation clastique rehaussée a fait cesser le processus et a enfoui les couches riches en Mn. La formation de couches riches en Mn a repris lorsque l'entrée des sédiments clastiques a de nouveau été ré-duite. Les données provenant des isotopes du Nd et du Sr excluent une provenance volcanique ou hydrothermale pour le Mn. L'entrée de Mn a plutôt été effectuée par les rivières à partir de la croûte continentale. La composition isotopique du Nd et les empreintes géochimiques des roches riches en Mn et les roches pauvres en Mn qui les contiennent suggèrent que l'entrée de clastes ait été dominée par du matériel dérivé du continent de Gondwana plutôt que du continent d'Avalon. La présence de coticules semble toutefois limitée à la plate-forme avalonienne.[Traduit par la Rédaction]

Cambrian-Ordovician acritarchs in the Meguma terrane, Nova Scotia, Canada: Resolution of early Paleozoic stratigraphy and implications for paleogeography

“…Stratigraphic differences between the northwestern and southeastern parts of the Meguma terrane are consistent with previous interpretations (Waldron et al, 2009) that the Meguma terrane formed in a rift environment. The present study confi rms a Cambrian Epoch 3 age for manganese-bearing units, consistent with the stratigraphy in north Wales, and contrary to Romer et al (2011), who considered these rocks to be an example of widespread Ordovician manganiferous beds formed on Avalonian shelves. This work will facilitate better comparisons of Lower Paleozoic sedimentary successions in the Meguma terrane with other northern Gondwanan successions and hence will contribute to resolving the problem of its paleogeographic origin.…”

“…In contrast, Romer et al (2011) considered these rocks to be an example of widespread Ordovician manganiferous beds formed on Avalonian shelves. Hence, as emphasized by Waldron et al (2011), manganese-rich beds in the Meguma terrane are of comparable age to manganiferous beds in north Wales.…”

“…Waldron et al (2011) suggested that the Cambrian successions of the Meguma terrane and north Wales were part of a common deep-water basin that they named Megumia, not directly related to the Avalonian shelf. This interpretation was questioned by Romer et al (2011), who claimed that the manganese-rich strata of both Meguma and north Wales are of Ordovician age.…”

Discussion of “Geochemical signature of Ordovician Mn-rich sedimentary rocks on the Avalonian shelf”1Appears in Canadian Journal of Earth Sciences, 2011, 48(4): 703–718 [doi: 10.1139/e10-092].

“…1. Despite the lack of location maps or coordinates, sites C2 and C3 of Romer et al (2011) can be located on the maps of Horne et al (2009) andHill (1986), which show that they represent the pelitic Beaverbank formation, assigned to the top of the Goldenville Group in the revised stratigraphy of White (2010b). Despite the lack of location maps or coordinates, sites C2 and C3 of Romer et al (2011) can be located on the maps of Horne et al (2009) andHill (1986), which show that they represent the pelitic Beaverbank formation, assigned to the top of the Goldenville Group in the revised stratigraphy of White (2010b).…”

“…The paper "Geochemical signature of Ordovician Mn-rich sedimentary rocks on the Avalonian shelf" by Romer et al (2011) provides welcome new data on Mn-rich metasedimentary rocks from Nova Scotia, Belgium, and Germany. However, the title of the paper, the abstract, the interpretations, and the conclusions imply that the Nova Scotian samples are (i) Ordovician, (ii) Avalonian, and (iii) were deposited in a shelf environment.…”