Hettangian ammonite succession and analysis of the Triassic/Jurassic boundary sequence at Black Bear Ridge, Williston Lake, British Columbia

• Autorzy: Hall R. , Pitaru S. , Henderson C.
• Konferencja: International Congress on the Jurassic System (7 ; 06-18.09.2006 ; Kraków, Poland)
• Języki publikacji: EN

Abstrakty

EN

The basal Fernie Formation at Black Bear Ridge consists of 22 m of concretionary, brown weathering siltstones containing abundant ammonites which prove the presence of lower, middle, and upper Hettangian strata. Based on over 300 specimens we have recognized 13 ammonite horizons which may be grouped into six local zones. Lower Hettangian faunas are dominated by strongly ribbed psiloceratids associated, in the upper part of their range, with small Kammerkarites; no smooth psiloceratids of the tilmannipacificum-planorbis group have been found. The Middle Hettangian is marked by the appearance of Kammerkarites hircinum associated with Kammerkarites spp., Discamphiceras, and Pleuroacanthites mulleri. Upper parts of the Middle Hettangian yield Sunrisites sunrisense, Saxoceras portlocki, Kammerkarites spp., Discamphiceras and Alsatites. First appearance of schlotheimiids, which marks the base of Upper Hettangian strata, is above the last occurrences of Sunrisites sunrisense. Faunas in higher strata are dominated by Laqueoceras nigroursus, which ranges up into beds containing Badouxia oregonensis; near the top of the section, Pseudaetomoceras doetzkirchneri appears. Lower Hettangian psiloceratid faunas from Black Bear Ridge can best be correlated with European zonations. Elsewhere, first occurrences of Kammerkarites usually mark the base of the Middle Hettangian, and they are not usually associated with psiloceratids as at Black Bear Ridge. However, strata with Kammerkarites in both Alaska and Queen Charlotte Islands have been placed in the Lower Hettangian. Sunrisites here is associated with both Saxoceras portlocki and Alsatites subliasicus, indicating its Middle Hettangian age, while in both Nevada and South America it occurs above intervals with Kammerkarites and Saxoceras, and is associated with Schlotheimia. At Black Bear Ridge the latter genus only appears above the last Sunrisites. In both Nevada and South America, Laqueoceras is also associated with Sunrisites, not succeeding it as here. Occurrences of Badouxia and Pseudaetomoceras clearly indicate correlation of the highest parts of the sequence with the Upper Hettangian. Correlation of the ammonite associations recorded at Black Bear Ridge with a regional zonation scheme proposed for North America is only partially successful due to differing local ranges of key taxa, and the absence of some genera (Euphyllites, Mullerites, Franziceras, Fergusonites, and Eolytoceras) in this section. Between the "Monotis beds", dense coquinas comprising the uppermost Pardonet Formation (Norian), and the first Jurassic ammonites in the base of the Fernie Formation, is a sparsely fossiliferous interval of flaggy-bedded, brown siltstones 2.3 m thick. Fragmented pectiniform and ramiform conodont elements of Norigondolella sp. and Epigondolella sp., obtained by us from two beds 1.5 and 1.7 m above the "Monotis beds", indicate a probable Rhaetian age for part of this interval. Positive carbon- and nitrogen-isotope excursions at the top of the "Monotis beds" have been correlated with the Norian/Rhaetian boundary (Sephton et al. 2002); they interpreted the Rhaetian/Hettangian (Triassic/Jurassic) boundary to occur 9.0 m higher in the section, though the anticipated negative carbon-isotope excursion was not found at this level. Our new ammonite data confirm that strata just 2.3 m above the top of the "Monotis beds" are already of Hettangian age.

Słowa kluczowe

EN

Fernie Formation; British Columbia; Hettangian; ammonites; biostratigraphy; conodonts; Rhaetian; Triassic/Jurassic boundary; carbon isotopes

• Wydawca: Państwowy Instytut Geologiczny - Państwowy Instytut Badawczy
• Czasopismo: Volumina Jurassica
• Rocznik: 2006
• Tom: Vol. 4, no. 1
• Strony: 285--286
• Opis fizyczny: Bibliogr. 1 poz.

Twórcy

autor

Hall R.

autor

Pitaru S.

autor

Henderson C.

• Department of Geology and Geophysics, University of Calgary, Calgary, Alberta, T2N 1N4 Canada

Bibliografia

• 1. Sephton M., Amor K., Franchi I. A., Wignall P. B., Newton R. and Zonneveld J.-P. 2002 Carbon and nitrogen isotope disturbances and an end-Norian (Late Triassic) extinction event. Geology, 30, 12: 1119-1122.