Upper Ordovician facies in the Lac Saint‐Jean outlier, Québec (eastern Canada): palaeoenvironmental significance for Late Ordovician oceanography

Abstract

Upper Ordovician (Caradocian) carbonates of eastern North America were deposited along the Iapetus continental margin and record a transition from warm-to cool-water settings despite this margin having been within the southern hemisphere tropical belt. This event has been documented from Virginia (USA) to southern Que Âbec (Canada) although, not previously from areas close to the palaeoequator.Field, petrographic and major element geochemistry data have been gathered from the poorly-known Upper Ordovician carbonate succession outcropping in the Lac Saint-Jean outlier in central Que Âbec. The succession consists of a lower siliciclastic formation (Tremblay) overlain by three limestone formations (Simard, Shipshaw and Galets) and capped by shales (Pointe-Bleue Shale). From macro-and microfaunal evidence, carbonate sedimentation occurred during the late Caradoc and is younger than the early-to mid-Caradoc carbonate succession present farther south. Relative sea level¯uctuations recorded in the sediments suggest an overall sea level rise brie¯y halted by a minor end-Caradocian sea level fall.The lower limestone formation (Simard) consists of muddy sediments with algalcoral-stromatoporoid boundstones; green algae are abundant. This unit re¯ects low energy sedimentation on a shallow warm-water carbonate ramp colonized by a diverse chlorozoan fauna. The upper limestone formation (Galets) is typi®ed by coarsegrained bioclastic sediments punctuated by numerous phosphate-rich hardgrounds with evidence for high energy shallow marine conditions. Faunas were dominated by crinoids and bryozoans. This unit represents high energy sedimentation on a cool shallow water carbonate ramp colonized by a brynoderm faunal association. Between both units, a deeper marine (outer shelf) limestone formation (Shipshaw) was developed.In the Lac Saint-Jean area, a transition from warm-to cool-water carbonate ramps occurred in latest Caradoc times and is litho-and biofacies-wise, similar to what is documented for lower Caradocian limestones present farther south. Upwelling of nutrient-rich cool bottom oceanic waters was a probable cause for this transition.

Temporal variations of methane concentration and isotopic composition in groundwater of the St. Lawrence Lowlands, eastern Canada

“…These Upper Ordovician shales were deposited in a fast subsiding tectonic foredeep with the carbonates being produced from a coeval but distant shallow marine carbonate platform that was backstepping on the craton (Lavoie and Asselin, 1998;Lavoie, 2008). The Utica Shale is 2-km deep in the Saint-Édouard Talisman A267/A275 wells, but shallows to about 500 m underneath the Lotbinière Formation at the northwest limit of the study area (Lavoie et al 2016) (Fig.…”

Could land‐based early photosynthesizing ecosystems have bioengineered the planet in mid‐Palaeozoic times?

“…Periodicity could have resulted from exhaustion of terrestrial, and hence marine, phosphate, with subsequent increased phosphate input from volcanicity, and from glaciation followed by deglaciation. Midde–Late Ordovician phosphate‐rich limestones on Laurentia associated with upwelling and high δ 13 C (GICE; Lavoie and Asselin ; Pope and Steffen ; Cherns and Wheeley ), as also seen at each end of the Proterozoic (Papineau ), deserve further attention in this respect.…”

Forced regressive deposits of a deglaciation sequence: Example from the Late Quaternary succession in the Lake Saint‐Jean basin (Québec, Canada)

“…It is located at the north‐western end of the Saguenay graben. Overlaying the Proterozoic igneous basement (Dimroth et al ., ; Rivers, ; Hebert & Van Breemen, ), sedimentary rocks consist of Ordovician limestones (Desbiens & Lespérance, ; Lavoie & Asselin, ). Above, no sediment older than the Late Pleistocene (Late Wisconsinan) has yet been identified (Tremblay, ).…”