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Triassic-Jurassic stratigraphy and paleontology of the Takwahoni and Sinwa Formations at Lisadele Lake, Tulsequah map-area, northwestern British Columbia Shirmohammad, Farshad

Abstract

A southwestern outlier of the Whitehorse Trough basin strata in the central Tulsequah map area includes interbedded fossiliferous conglomerate, sandstone, siltstone, and mudstone of the Lower to Middle Jurassic Takwahoni Formation (Laberge Group) which unconformably overlies Upper Triassic limestone of the Sinwa Formation. Ammonite collections record all stages from Pliensbachian to the Bajocian except for the Aalenian. Middle Toarcian ammonite faunas include the first record of the Tethyan genus Leukadiella in the Tulsequah map-area. The refined age and provenance of several episodes of coarse clastic input into the basin show that the character of the dominant clasts in the conglomerates changes up-section. Immediately above the unconformity, breccias and conglomerate contain sedimentary clasts derived from the Sinwa Formation. Clast dominance changes to volcanic near the base of the section, plutonic (in Pliensbachian-Toarcian strata), metamorphic (in uppermost Toarcian rocks), and finally, after an interval of fine-grained sedimentation, to chert in the Middle Jurassic strata of Early Bajocian age. The uppermost chert pebble conglomerates are tentatively placed in the Bowser Lake Group. The relative proportions of lithic fragments, feldspar, and quartz in the Lisadele Lake sandstones, plotted on the ternary tectonic discrimination diagram, indicate a complex arc-basin evolution. The biochronological age control of three recognized sandstone petrofacies illustrates strong temporal trends and rapid tectonic and magmatic events. The trends indicate a very rapid change from transitional to dissected arc and finally a recycled orogen during the evolution of the northern Stikinia in Early and Middle Jurassic time. Geochronological studies of samples of detrital zircons and plutonic clasts from Lower Jurassic strata in the Lisadele Lake area indicate a very rapid uplift, exhumation, and deposition. Comparison of the isotopic ages of the zircons and granite clast ages with the biochronologically constrained ages of the enclosing strata suggests that Early Jurassic intrusion, uplift, unroofing, and deposition probably occurred within five million years.

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