Geological context, biostratigraphy and systematic revision of late early Cambrian olenelloid trilobites from the Parker and Monkton formations, northwestern Vermont, U.S.A.
|Title||Geological context, biostratigraphy and systematic revision of late early Cambrian olenelloid trilobites from the Parker and Monkton formations, northwestern Vermont, U.S.A.|
|Publication Type||Journal Article|
|Year of Publication||2016|
|Authors||Webster, M, Landing, E|
|Journal||Australasian Palaeontological Memoirs|
In northwestern Vermont, upper lower Cambrian (Dyeran Stage) dysoxic deposits of the Franklin Basin are separated by 7 km from coeval, shallow-shelf deposits of the Monkton Formation on the Middlebury platform. This report presents a systematic revision of olenelloid trilobites from three localities within the basinal facies of the Parker Formation (including the Parker quarry Lagerstätte) and three localities within the Monkton Formation. The study reveals a previously underestimated diversity of olenelloid species in both formations. Olenellus agellus, O. crassimarginatus, O. transitans and O. aff. transitans are presently known only from the basinal facies; Bolbolenellus hermani, B. cf. brevispinus and several indeterminate olenelloid species are presently known only from the shelf facies; and O. thompsoni, O. aff. fowleri and Mesonacis vermontanus occur in both facies. The revisions demonstrate that (1) the Parker and Monkton formations share several species, thus strengthening previous claims for at least partial age-equivalence of the units; and (2) the regression(s) represented by the Monkton Formation began after the formation of the Franklin Basin but prior to the deposition of the Parker quarry Lagerstätte. Some newly documented elements of the Parker and Monkton formation faunas are possibly conspecific with olenelloids from Nevada, which permits a provisional correlation of the Parker quarry Lagerstätte with the Bolbolenellus euryparia Zone or overlying uppermost Dyeran Nephrolenellus multinodus Zone of the Laurentian Cordilleran margin. The Monkton Formation regression(s) were broadly contemporaneous with one or more of the four late Dyeran shallowing-up events that have been identified in the southwestern United States. This indicates that the regression(s) were a eustastic rather than an epeirogenically controlled phenomenon.