- © 2004 by AASP Foundation
Reworking of Devonian, Carboniferous and Permian palynomorphs into the Lower Triassic of western Canada, Yukon, the Canadian Arctic Archipelago and Alaska has been documented by a number of workers. The phenomenon occurs in other northern circumpolar localities, such as East Greenland and the Barents Sea, and was probably widespread. Examples are given from western and eastern Europe, Russia, the Middle East, Pakistan, China, Brazil and Australia.
The abundance and diversity of reworked palynomorphs from a number of stratigraphic units of different ages into the Lower Triassic is an important palynostratigraphic phenomenon of chronostratigraphic value. It may be the result of a major regional regression in the late Permian, followed by a widespread marine transgression in the Early Triassic, or it may be due to tectonic activity resulting in eustatic sea-level rise. Whatever the mechanism, the advancing transgression resulted in erosion of exposed Devonian, Carboniferous and Permian rocks, but by Mid-Triassic times most pre-Triassic rocks had been covered by sediment, and the supply of reworked material much reduced.
Reworked Devonian taxa belong to a variety of suprageneric groups including cavate trilete spores (lycopsids), acavate trilete spores (ferns) and monolete spores; Lower Carboniferous taxa include cingulate trilete spores. Possibly reworked Upper Carboniferous and Permian pollen includes that of Gymnosperms—Cordaites, conifers, pteridosperms (taeniate bisaccate and polyplicates), and cycads and gnetophytes. Lower Triassic palynomorphs, possibly in situ, may include (sometimes abundant) acanthomorph acritarchs such as Micrhystridium breve, M. setasessitante, M. fragile and Wilsonastrum colonicum.
Lack of recognition of reworking has many implications concerning chronostratigraphy, palynostratigraphy, paleoenvironments, paleoclimates, coal petrography, thermal maturity, geochemistry, and chemostratigraphy. For example, if the Upper Permian and Lower Triassic Densoisporites playfordii, D. complicatus, D. nejburgii, Lundbladispora obsoleta and Aculeisporites variabilis are not in situ, but reworked specimens of the Devonian progymnosperm cavate morphon Geminospora lemurata, one has to question the commonly held view that a lycopsid dominated macroflora, with a large biomass, re-colonised the land in the Early Triassic. Lower Triassic assemblages in which reworking occurred but has not been recognised may give the impression of palynofloral diversity, whereas low diversity and low abundance would be more consistent with the hostile arid environment of the Lower Triassic. One Upper Permian and Lower Triassic plant entity, well-adapted to harsh conditions, was the algal cyst Chordecystia chalasta. Nevertheless, some higher plants must have survived the Permian–Triassic crisis as witnessed by the presence of gymnosperm pollen in the Mesozoic. This is Geological Survey of Canada (Calgary), Contribution no. 2004237.