Sequence stratigraphy, biotic change, 87Sr/86Sr record, paleoclimatic history, and sedimentation rate change across a regional late Cenozoic unconformity in Arctic Canada

Abstract

Eustasy, tectonics, and climate contributed to a remarkable Miocene–Pliocene regional unconformity in the Beaufort–Mackenzie area of Arctic Canada. The unconformity extends from beneath deep basin turbidites on the continental rise, upslope across an erosional paleocontinental shelf, onto the cratonic margin as a regional paleosurface (peneplain) in the Mackenzie Delta area, and into pediment surfaces cut into the orogenic highlands of the Richardson Mountains. The unconformity was initiated by shelf exposure during latest Messinian or earliest Pliocene eustatic lowstand and was accentuated by tectonic uplift from the culmination of a major Late Miocene compressional pulse on the basin margin. Palynomorph, benthic foraminiferal, strontium isotopic, paleomagnetic, and radiometric data document the climatic and chronological events surrounding the unconformity. A widespread hardground (K-59 limestone) occurs at the unconformity and caps the Late Miocene Akpak Sequence. The hardground yields the benthic foraminifera Cibicides grossus, a regional marker in the Arctic Pliocene, and the bryozoan Adeonella sp. aff. A. polystomella, previously known from temperate North Atlantic environments. The 87Sr/86Sr data and new biostrati gra phic data indicate that the C. grossus Zone in the Beaufort–Mackenzie area may be younger than previously estimated, ranging into the earliest Pleistocene. Late Miocene regional uplift across the cratonic margin, coupled with eustatic lowstand followed by Early Pliocene tectonic quiescence and dry cool climatic conditions, combined to produce widespread erosion (pediments and peneplanation). Rapid erosion contributed to the >4 km-thick, Pliocene–Pleistocene Iperk Sequence and a 23-fold increase in sedimentation rates relative to the Early and Middle Miocene.