An Upper Ediacaran Glacial Period in Cadomia: the Granville tillite (Armorican Massif) – sedimentology, geochronology and provenance

Abstract

In the Cadomian orogenic belt a package of glacigenic sedimentary deposits have been recently described in the Armorican Massif (Normandy, France). The Granville Tillite Member, the middle part of the upper Granville Formation, is late Ediacaran in age. Maximum depositional ages of the pre- and syn-glacial sedimentary deposits obtained by LA-ICP-MS U–Pb detrital zircon dating indicate a maximum age of 561 ± 3 Ma. Combined with geochronological data on the previously described glacial deposits in Cadomia, West Africa, Arabia and Iran, the Granville Tillite Member appears to represent an Upper Ediacaran Glacial Period in northern peri-Gondwana, clearly younger than the c. 580 Ma old Gaskiers glaciation. Detailed mapping and analysis of the depositional regime of two sections near the city of Granville are indicative of two independent glaciomarine lower and upper tillite deposits separated by a distinct conglomeratic marker horizon, evidently a massive gravel beach horizon deposited during an interglacial stage. Age spectra of detrital zircon U–Pb ages constrain the palaeogeographical position of the upper Granville Formation to the periphery of the West African Craton. Post-Gaskiers aged glaciations in Cadomia and in West Africa should be grouped into an Upper Ediacaran Glacial Period dated at c. 565 Ma. This glacial period seems not to be related to the negative δ13C Shuram–Wonoka anomaly. Sedimentary deposits formed during the Upper Ediacaran Glacial Period show a scattered distribution along the marginal orogens of the Gondwana supercontinent independent of palaeolatitude and are coupled most likely to contemporaneous orogenic processes and uplift.

Sea level controls on Ediacaran-Cambrian animal radiations

“…Low total rock volume throughout the ~565to 550-Ma interval coincides with polar glaciations that may have extended to midlatitudes at ~555 to 550 Ma, which is supported by glacial deposits that are radiometrically constrained on multiple cratons [e.g., (57)(58)(59)]. This therefore potentially records a protracted interval of generally low sea level, particularly during ~555 to 550 Ma, when carbonate rock volume reaches its nadir (see the Supplementary Materials for expanded discussion).…”

“…δ 238 U carb data are scarce through the ~569to 551-Ma interval, coincident with the dominant White Sea assemblage, and δ 13 C carb data are particularly sparse or entirely absent throughout the ~555to 551-Ma interval. However, on the basis of the available data, the areal extent of reducing marine conditions may have increased between 565 to 557 Ma, potentially coincident with late Ediacaran high to mid-latitude regional glaciations [e.g., (57)(58)(59)]. A second major shallow marine oxygenation event can then be inferred on the basis of peak δ 13 C carb values at ~557 Ma, with subsequent oxygenation of deeper environments culminating in maximum oxygenated seafloor area at peak δ 238 U carb values recorded during the nadir of the subsequent negative δ 13 C carb excursion (BANE) at ~551 to 550 Ma.…”

“…These are exemplified by correlation uncertainty in the interval of ~538 to 533 Ma and the paucity of carbonatedominated successions with δ 13 C carb data that are radiometrically constrained in the interval of ~565 to 550 Ma (and especially ~555 to 550 Ma). Correlation uncertainties between approximately 565 and 550 Ma may be partly explained by the far-field effects of a long-lived glacioeustatic sea level lowstand [discussed further in the Supplementary Materials; e.g., (57)(58)(59)]. Given these numerous uncertainties, the global stratigraphic correlation framework presented here, while inherently incomplete, represents the most parsimonious framework in chronostratigraphic terms, when considering all available published data, including radiometric (zircon U─Pb CA-ID-TIMS and Re─Os), chemostratigraphic, and biostratigraphic information.…”

“…Notably, the ~555to 550-Ma interval may preserve few or no carbonate rocks. Therefore, it is possible that calcified cloudinids first appeared >550.5 Ma (or even at 562 Ma, if preservation in dolostones is precluded) and that the apparent global onset of biomineralization inferred by the appearance of calcified biota at ~550.5 Ma may be entirely constrained by availability of carbonate host rocks, with 550 Ma marking the end of a global lowstand and a massive flooding event possibly related, in part, to deglaciation of high-to mid-latitude ice sheets [discussed further in the Supplementary Material, e.g., (57)(58)(59)]. As calcareous biomineralization of cloudinids is inferred to have been under minimal biological control (69), it is also possible that this was not facilitated until widespread highly saturated seawaters with respect to carbonate became available.…”

Cryogenian and Ediacaran integrative stratigraphy, biotas, and paleogeographical evolution of the Qinghai-Tibetan Plateau and its surrounding areas

“…The geological record of the Gaskiers glaciation mainly occurs in Newfoundland and is dated as ~580 Ma (Pu et al, 2016). Whereas, the late Ediacaran glaciogenic deposits have been recorded from multiple regions, including North China, Qaidam, Tarim, West Africa, Kalahari blocks, as well as Iran, and Europe (Zhu and Wang, 2011;Etemad-Saeed et al, 2016;Pang et al, 2021b;Linnemann et al, 2021). The Luoquan Formation distributed along the southern margin of the North China Block represents the most well-preserved record of a late Ediacaran glaciation (Zhou et al, 2019b).…”