Summary

Dinoflagellate cysts, largely Nannoceratopsis gracilis and Nannoceratopsis senex, Tasmanites algae and acritarchs, sphaeromorph forms only, have been identified. The lower sample has a stratigraphic position in the Early Toarcian, as already indicated by ammonite stratigraphy. The upper sample is dated as Late Toarcian by ammonites and has only a poor record of dinoflagellate cysts.

Introduction

Two samples from Graham Island of the Queen Charlotte Islands have been investigated palynologically by Niels E. Poulsen in order to determine if dinoflagellate cysts are present in late Early Jurassic strata of the northeastern Pacific Coast area. W. A. S. Sarjeant has previously unsuccessfully investigated samples from this area, but did not identify any dinoflagellate cysts (Howard Tipper, personal commun., 1991). The samples were collected by Giselle Jakobs, who also determined the ammonites. In this investigation large samples (50 - 60 grams) were prepared in order to have sufficient organic material for the palynological investigation. The material was prepared following the preparation method of Poulsen et al. (1990).

Material

Two samples from Yakoun River, Graham Island of the Queen Charlotte Islands, Canada.

Sample C-158005: 6.9 m at section 9, Whiteaves Formation, Lower Toarcian.

Sample C-157735: 59.3 m at section 10, Whiteaves Formation, Upper Toarcian.

Ammonite biostratigraphy

Toarcian strata are widespread in the North American Cordillera, however the tectonic complexity of this area has, until now, prevented the construction of a regional ammonite zonation. The Queen Charlotte Islands have yielded well preserved and abundant ammonites in a relatively uncomplicated tectonic setting, which has allowed the construction of a Toarcian ammonite zonation for North America (Jakobs 1992; Jakobs et al. 1994).

Sample C-158005 occurs with a poorly preserved ammonite fauna that includes such taxa as: Harpoceras cf. H. exaratum (Young and Bird), Harpoceras sp., and Hildaites sp.. This interval (Zone 2 of Jakobs et al. 1994) correlates with the Northwest European Falciferum Zone (Early Toarcian).

Sample C-157735 occurs with a well preserved ammonite fauna that includes: Grammoceras thouarsense (d'Orbigny), Podagrosites cf. P. latescens (Simpson), and Phymatoceras n. sp.. This interval (Zone 5 of Jakobs et al. 1994) correlates with the uppermost part of the Northwest European Thouarsense Zone (Late Toarcian).

Palynology

Sample C-158005 is very rich in Nannoceratopsis senex (more than 150 specimens on a slide). Nannoceratopsis gracilis is common (about 35 specimens on a slide). A single specimen of Nannoceratopsis triceras (badly preserved), and two specimens of Lithodinia serrulata are recorded. The sample is also very rich in sphaeromorph acritarchs (more than 1000 on a slide). Sphaeromorph acritarchs are regarded as Spheripollenites spp. (miospores) by some (see Dybkjær (1991) for discussion). Sphaeromorph acritarchs become abundant in most of Europe at the earliest Toarcian, top of the Tenuicostatum Zone.

Sample C-157735. Two specimens of N. senex are recorded together with three specimens of Tasmanites (green alga).

Palynological biostratigraphy

In northwest Europe the first occurrence of Nannoceratopsis spp. is at the Early - Late Pliensbachian boundary. N. senex has its first occurrence just below the boundary, whereas N. gracilis and N. triceras have their first occurrence just above the boundary.

In the Sverdrup Basin, Arctic Canada, Davies (1983) found the first occurrence of N. senex in the Pliensbachian? or Toarcian Oppel-Zone A, and the first occurrence of N. gracilis in the Toarcian Oppel-Zone B. Lithodinia serrulata is only recorded in Davies's Oppel-Zones A (Pliensbachian? or Toarcian) and C (Toarcian - Early Bajocian). It is missing in the Oppel-Zone B (Toarcian). Parvocysta and Susadinium spp. occurring in the Middle Toarcian - Early Aalenian of northwest Europe, Spitsbergen, and Arctic Canada were not recorded in sample C-158005, which was rich in dinoflagellate cysts. Nor were they recorded in sample C-157735, however, this sample had a low content of dinoflagellates, and it cannot be excluded, that they did not exist in the studied area. The absence of Parvocysta spp. in the Early Toarcian sample, C-158005, confirms that these species did not occur before the Middle Toarcian.

Sample C-158005 is correlated to the boundary between Davies (1983) Oppel-Zones A and B based on the presence of L. serrulata together with N. gracilis. The high occurrence of sphaeromorph acritarchs (miospores?) is noticed as this seems to coincide with the Early Toarcian acme of these acritarchs (miospores?) in Europe. Sample C-157735 has too few dinoflagellate cysts to make reliable correlations.

References

Davies, E. H., 1983. The dinoflagellate Oppel-Zonation of the Jurassic - Lower Cretaceous sequence in the Sverdrup Basin, Arctic Canada. Geological Survey of Canada Bulletin 359. 59 pp.

Dybkjær, K., 1991. Palynological zonation and palynofacies investigation of the Fjerritslev Formation (Lower Jurassic - basal Middle Jurassic) in the Danish Subbasin. Geological Survey of Denmark Series A 30. 150 pp.

Jakobs, G. K., 1992. Toarcian (Lower Jurassic) Ammonite Biostratigraphy and Ammonite Fauna of North America. Doctoral Thesis, University of British Columbia, Vancouver, B.C., Canada.

Jakobs, G. K., P. L. Smith, and H. W. Tipper, 1994. Towards an ammonite zonation for the Toarcian of North America. (3ème Symposium International de Stratigraphie du Jurassique, Poitiers, France, 22-29 Septembre 1991). Geobios 17 (1): 317-325.

Poulsen, N. E., L. Gudmundsson, J. M. Hansen, and Y. Husfeldt, 1990. Palynological preparation techniques, a new macerationtank-method and other modifications. Geological Survey of Denmark Series C 10. 22 pp.

Woollam, R. and J. B. Riding, 1983. Dinoflagellate cyst zonation of the English Jurassic. Institute of Geological Sciences Report 83/2. 44 pp.

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  This article first appeared in CAP Newsletter 15(2):25-27, 1992. The reference list has been updated.