Triassic-Jurassic biochronology of the eastern Iskut River map area, northwestern British Columbia
Creators: name: Nadaraju, Genga T.
Publisher: name: University of British Columbia
Year: 2008

description: Norian to Early Callovian faunas in the eastern half of the Iskut River map area can be divided into three suites: a benthonic fauna consisting of bivalves, corals, and gastropods; a planktonic fauna of radiolarians; and a nektonic fauna of coleoids and ammonoids. The ammonoids (27 genera, 47 species) and radiolarians (14 genera, 18 species) are described. Other taxa are identified and listed in Appendix III. Ammonoids, bivalves, and radiolarian of the following stages are recognized: Norian (Cordilleranus Zone and Amoenum-Crickmayi Zones), upper Hettangian - Lower Sinemurian (Canadensis Zone), Upper Pliensbachian (Kunae and Carlottense Zones), Lower Toarcian (Assemblage Zones 2-3 of Jakobs), Upper Toarcian (Assemblage Zone 6 of Jakobs), Aalenian (Erycitoides howelli assemblage zone), Lower Bajocian, Upper Bathonian, and Lower Callovian. In the eastern Iskut River map area: i) the Stuhini Group is Norian, ii) the Jack "formation" and the basal sedimentary sequence of the Unuk River Formation are upper Hettangian to Lower Sinemurian, iii) the upper volcanic sequence of the Unuk River Formation ranges from the Lower Sinemurian to the Pliensbachian, iv) the Betty Creek Formation ranges from the Upper Pliensbachianto Lower Toarcian?, v) the Mt. Dilworth "formation" is restricted to the Upper Pliensbachian to possibly Lower Toarcian (at John Peaks), vi) the Salmon River Formation ranges from middle? Toarcian to Lower Bajocian, vii) the Eskay Creek and Troy Ridge facies of the Salmon River Formation are Aalenian to Lower Bajocian, and viii) the Ashman Formation ranges from Lower Bajocian to Lower Callovian. Regional correlations with strata elsewhere in the Bowser Basin are as follows: i) the Betty Creek and Mt. Dilworth formations are coeval with the Wolf Den Formation (Spatsizi Group) and the Nilkitkwa Formation (Hazelton Group); ii) the Salmon River Formation is coeval with the Melisson, Abou, and Quock formations of the Spatsizi Group and with the Smithers Formation of the Hazelton Group. At the Snippaker Ridge, strata previously described as Toarcian, are now shown to be Norian inage. At the Treaty Ridge section, the Aalenian fauna below a felsic unit provided the first evidence of ayounger felsic horizon in the map area; this unit represents a higher stratigraphic level than the felsicunit that constitutes the type Mt. Dilworth "formation". Age of the stratabound mineralization at Eskay Creek is confined to Aalenian to possibly Early Bajocian. The rocks that host the Treaty Glacier alteration systems are shown to be Aalenian to Early Bajocian in age. Jurassiomineralization was therefore restricted to the Aalenian - Lower Bajocianinterval. Ammonites are locally common in siltstone and shale, benthonic faunas are abundant in coarse sandstone, and the planktonic forms are locally common in siliceous shale and limestone. The benthonic and nektonic faunas were deposited in a wide spectrum of near shore to off-shore shallow shelf to slope/basinal environments represented by the Composite assemblage depth zones B-D ofTaylor (1982). Nassellarian-spumellarian radiolarian ratios suggest that within the Salmon River Formation at Eskay Creek, both the contact zone sediments hosting the stratabound mineralization and the felsic volcanics were deposited in deeper water than the overlying andesite/pillow basalt sequence.