CAMBRIAN DEEP MARINE COARSE-GRAINED CHANNEL-FILL SUCCESSIONS IN EASTERN QUEBEC; THE RECORD OF EUSTATIC AND TECTONIC EVENTS ALONG THE IAPETAN RIFTED MARGIN

Lower Cambrian to Lower Ordovician successions in eastern Quebec consists of slope to toe-of-slope deposits that are exposed laterally for over 600 km. These deposits were accumulated after the Ediacaran rifting of Rodinia, on the “passive” continental margin of Laurentia. The succession reaches up to 5 km in thickness and is dominated by mudstone. Three major sandstone and conglomerate intervals are recognized and can be linked with either eustatic events or tectonic readjustment of failed Proterozoic rift arm. The first deep marine coarse-grained deposit is late Early Cambrian in age and coincides with a global sea level lowstand that occurred shortly after the inception of the drift phase. These deposits consist of coarse sandstone and conglomerate beds, arranged in thickening and coarsening-upward decametre-thick intervals. This succession ranges between 500 to 600 meters in thickness and is recognized along the entire length of the Quebec Reentrant. The second coarse-grained event occurred at the end of the Cambrian Grand Cycle B but the facies architecture and nature of the deposits suggest tectonic activity along a failed rift graben. The thickest and coarsest interval is found adjacent to the Montmorency second-order promontory. The succession fills metre- to decametre-deep channels with abandonment mud facies and lateral switching; a common fining-upward trend is present within individual channel. The sediment consists of Cambrian limestone and sandstone, Late Proterozoic rift basalt and Proterozoic metamorphic fragments. These clasts are well rounded and up to 2 metres in diameter. The last event on the continental slope is latest Cambrian to earliest Ordovician and coincides with the end of the Cambrian Grand Cycle C at a time of a major sea level lowstand. The deposits consist of well sorted, medium-grained quartz arenite. The sandstone is massive with rare graded bedding and few current ripples on the top of some beds. These discontinuous deposits can reach thickness of 200 meters and form lenses of various widths within the background mudstone sediments. This regionally constrained chronostratigraphic framework demonstrates depositional variability on the scale of a passive margin and provides evidence that at least locally and periodically, the Cambrian margin was not so passive after all.