BIOSEQUENCES, THE PALEO-BIOTIC RESPONCE TO CLIMATE CHANGE IN THE QUATERNARY DEPOSITS OF THE CANADIAN BEAUFORT SHELF.
Davies, Ed . Branta Biostratigraphy Ltd..
BLASCO, STEVE . Geological Survey of Canada.
The Quaternary section of the Canadian Beaufort Shelf consists of a thick sediment wedge that accumulated in the subsiding Tertiary age Beaufort-Mackenzie sedimentary basin (Blasco et al, 1990). In 1988 Gulf Canada Resources Limited completed a 500m deep borehole in 32m of water on the central Canadian Beaufort Shelf near the Quaternary depocentre of the Beaufort-Mackenzie sedimentary basin. The Pleistocene to Holocene core samples from the Gulf Amauligak 3F-24 deep borehole and a series of shallow subseabed boreholes located on the inner shelf of the Beaufort Sea have been analyzed to model deep core biostratigraphy. Recovered core samples were analyzed for palynology, botanical macrofossils, arthropods and micropaleontology.
Palynological distributions exhibit strong cyclicity in the marine dinoflagellates, euryhaline algae, terrestrial pollen and spores, organic walled faunal tests and reworked palynomorphs. The distributions of the aquatic palynomorphs allows for the recognition of fluvial, transitional to nearshore depositional environments. Determination of the paleovegetation can be made utilizing the association of the terrestrial derived pollen and spores. Botanical macrofossil distributions from the Amauligak 3F-24 core samples exhibit zones of increased abundance downcore with scattered occurrences between zones. These macrofossil assemblages are similar in basic composition comprising predominately spruce needles and sedge seeds, boreal and tundra shrubs and herbs along with diverse moss fragments comprising both arctic and boreal forms. Arthropod segments are common comprising a diverse fauna similar to that of the Mackenzie Delta today. Samples analyzed for micropaleontology were often barren, however, abundant low diversity benthic elphidioid foraminiferal and ostracodes assemblage define marine intervals while euryhaline cyprid ostracode assemblages identify transitional environments.
Biostratigraphically defined depositional environments range from lacustrine or fluvial delta top to middle neritic delta front but are predominately estuarine/delta top to shoreface. The paleovegetation consists of boreal forest to high arctic tundra with forest-tundra or littoral tundra being the most prominent. The expanding boreal forest and changing tundra biomes coupled with sea level changes and variations in freshwater influx result in the generation of sequential palynological assemblages, which reflect the stadial-interstadial and glacial-interglacial cyclicity in the Mackenzie Delta/ Beaufort Sea. This paleo-biome succession can be described and interpreted as a series of biosequences where a biosequence is defined as:
The stratigraphic interval delimited by the development and deterioration of a fossil biome or paleobiome represented by a series of fossil assemblages in succession.
The concept of a biosequence allows for the construction of a comprehensive paleontological facies model which facilitates the description of the cyclical succession of fossil assemblages. The preliminary ideas were presented by Davies & Bujak (1987) from studies on the Plio-Pleistocene of northern Gulf of Mexico. Palynological cyclicity reflected the changing floral development responding to glacial-interglacial climatic cycles of North America. This technique has been adapted and applied to the Quaternary strata of the Canadian Beaufort Shelf.
The idealized biosequence (figures 1 and 2) reflecting a complete development and collapse of a paleo-biome has been constructed in order to express the changes in floral and faunal development and climate (warmth and precipitation) during the time of deposition. The six main interdependent driving forces for this cyclicity are interpreted to be: glacial advancement (terrestrial niche destruction), sediment influx (turbidity), freshwater influx (including salinity and nutrients), temperature (both air and sea), precipitation and isostatic/eustatic sea-level changes (the development of marine niches). The succession of marine and freshwater fossil communities and of terrestrial floral communities follows through an idealized biosequence divided into four phases and ten assemblages discerned primarily by the varying fossil diversities and abundances. The resulting succession of assemblages produces a biosequence log reflecting a glacial-interglacial-glacial cycle. The complete, idealized palynological biosequence proceeds upward from left to right, with the next cycle subsequently starting from the far left in figure 1.
In the section penetrated by the Beaufort/Mackenzie boreholes seven biosequences comprising a total of 19 subsequences can be defined. These correspond closely to described coarse and fine-grained sedimentological units. However, the boundaries between the biosequences often occur within the bottom of the coarse-grained units rather than at the boundary between the coarse-grained and fine-grained units. The biosequences can be further grouped into three larger trends which correspond to major periods of sediment accumulation and development of marine conditions.
The integration of a biosequence stratigraphy with sedimentological and geochemical data will result in a much clearer understanding of the depositional history of ice-bearing Quaternary sediments of the Beaufort Shelf.
Blasco, S.M., Fortin, G., Hill, P.R., O'Connor, M.J. and Brigham-Grette, J., 1990. The late Neogene and Quaternary stratigraphy of the Canadian Beaufort continental shelf, in The Arctic Ocean Region, The Geology of North America, vol L, ed. A. Grantz, L. Johnson and J. F. Sweeney, p 491-502.
Davies, E.H. and Bujak, J.P., 1987. Petroleum exploration and the application of palynological assemblage successions in the Flexure Trend, Gulf of Mexico. In Innovative Biostratigraphic Approaches to Sequence Analysis: New Exploration Opportunities. Eighth Annual Research Conference, Gulf Coast Section, Society of Economic Paleontologists and Mineralogists Foundation, Houston, December 6-9, 1987, pp. 47-51.
Figure 1. Idealized Biosequence BIOME Succession.
Figure 2. Biosequences and Sedimentary Cycles.
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