The Cretaceous Jackass Mountain Group (JMG), within the Camelsfoot Range of south-central B.C., is a >2 km thick succession of marine and nonmarine volcanic-rich, feldspathic litharenite and lithic arkose sandstone, lesser mudstone, and minor conglomerate, ranging early Aptian to Cenomanian/Turonian age. The JMG was deposited in the Jura-Cretaceous Methow-Tyaughton Basin, a piggyback basin that formed atop the accreting Bridge River, Cadwallader, and Shulaps terranes, between west coast North America and impending Insular Belt terranes. It is likely that most JMG sediment was derived from first cycle weathering of nearby source rocks dominated by a mix of moderately weathered volcanic and plutonic complexes. Three facies associations were identified that essentially represents two-thousand metres of sand-rich submarine fan turbidites, delta front or prodelta deposits, and nonmarine sedimentation. Combined, the facies associations represent an overall regression in the Methow-Tyaughton Basin as accreting terranes eventually closed the basin and amalgamated it onto the continent.
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