Abundant fossil spores, pollen and dinoflagellates have been recovered from shale xenoliths, clastic fragments, and post-eruptive crater-fill deposits, in the Lac de Gras region, Slave Craton, Northwest Territories, Canada (65° N).

The chaotic mix of clastic xenoliths and finely dispersed clastic material mixed in with the kimberlite is the only remaining record of the now eroded sedimentary cover. The study was undertaken to determine depositional environments and ages, and place constraints on the thickness of the now-eroded cratonic sedimentary cover. The lithologic character and thickness of the pre-eruptive sedimentary cover and the environment (marine versus terrestrial) at the time of the eruption is considered to have affected the style of the eruptive process and the shape of the resulting diatreme (expanded near-surface kimberlite pipe). The ages of the eruptions has been found to be a predictor of the abundance of diamonds in the Lac de Gras region.

Assemblages interpreted as middle to late Albian, (~100-105 Ma), mid Cenomanian? to Turonian (~90-95 Ma), late but not latest Campanian (~73-80 Ma), and late Maastrichtian to early Paleocene? (61 to 69 Ma) were recovered. Recognition of these ages allows the correlation of inferred sequences in the Lac de Gras region with other similarly aged stratigraphies in western Canada, including those along the Manitoba Escarpment (49.5° N), which provide a valuable analog as they are located along depositional strike. The total maximum thickness of the Manitoba Escarpment section is 1000 m, setting a likely maximum thickness for the sedimentary cover over the Slave Craton.

Plant microfossils characteristic of latest Turonian through early Campanian, latest Campanian through early Maastrichtian, and late Paleocene ages have not been recognized in the Lac de Gras xenoliths suggesting depositional hiatuses of these ages occurred within the now eroded sedimentary cover (Sweet et al., 2003). During these hiatuses, erosion may have removed strata leaving behind only enhanced thermal maturity profiles (Stasiuk et al., 2003) as evidence for their former presence.

Near shore conditions during the Albian are indicated by mixed miospore and dinoflagellate assemblages. The presence of diverse and abundant dinoflagellates in Cenomanian to Turonian and, late but not latest Campanian samples provides evidence for the mid-continental seaway extending across the craton in the early Late Cretaceous and again, at least partially, in the late Campanian. Late Campanian samples also contain species (Expressipollis spp., Fibulapollis and Loranthacidites pilatus) indicating an Arctic source area to the north and northeast. During this time the late Campanian interior seaway probably provided a barrier to the input of spores and pollen from the west and southwest.

The exclusive presence of spores and pollen in Late Maastrichtian and Early Paleocene assemblages implies the establishment of a terrestrial landscape in the latest Cretaceous. This is consistent with the latest Cretaceous withdrawal of the interior seaway. Late Maastrichtian assemblages include species (Aquilapollenites conatus, A. reductus, Mancicorpus notabile, Myrtipites scabratus, Porosipollis porosus, Wodehouseia octospina, and W. spinata) known from the Western Canada Basin indicating a shift to a west to southwest source. These youngest assemblages from the kimberlites provide a maximum Early Paleocene age for many of the diatremes.

The kimberlite in the Giraffe diatreme has been dated at 47.4 Ma providing a maximum middle Eocene age for the associated crater-fill sediments, establishing their correlation with the fossil forests of Axel Heiberg Island (MacMillian , 2003). Such crater-fill sediments contain pristinely preserved miospore assemblages with those from Giraffe including a diverse angiosperm pollen flora dominated by oak (Hamblin et al., 2003).

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References

Hamblin, A.P., Stasiuk, L.D., Sweet, A.R., Lockhart, G. D., Dyck, D.R., Jagger, K. & Snowdon, L.R. 2003. Post-kimberlite Eocene strata within a crater basin, Lac de Gras, Northwest Territories, Canada. 8th International Kimberlite Conference, Victoria B.C., June 22-27, Long Abstract, (published in CD format).

Lockhart, G., Grutter, H. & Carlson, J. 2003. Temporal and geomagnetic relationship of Ekati economic kimberlites. 8th International Kimberlite Conference, Victoria B.C., June 22-27, Long Abstract, (published in CD format).

MacMillian, Stasiuk, L.D., Sweet, A.R., Isller, D.R., Kivi, K., Lockhart, G.D. & Dyck, D. 2003 Pre- and post-kimberlite emplacement thermal history of Cretaceous and Tertiary sediments, Lac de Gras, Northwest Territories. Canada. 8th International Kimberlite Conference, Victoria B.C., June 22-27, Long Abstract, (published in CD format).

Sweet, A.R., Stasiuk, L.D., Nassichuk, W.W., Catuneanu, O. & McIntyre, D.J. 2003. Paleontology and diamonds: geological environments associated with kimberlite emplacement, lac de Gras, Northwest Territories, Canada. 8th International Kimberlite Conference, Victoria B.C., June 22-27, Long Abstract, (published in CD format).

*Natural Resources Canada, 3303 33rd St., Calgary, Canada, T2L 2A7

**Manuka Palynologic Consulting, Calgary, Canada