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Provenance constraints on Early Jurassic evolution of the northern Stikinian arc : Laberge Group, Whitehorse Trough, northwestern British Columbia Johannson, Gary G.


The Lower to Middle Jurassic Inklin Formation forms part of the Laberge Group in northwestern B.C. which links the Stikine and Cache Creek terranes along much of the length of the Whitehorse Trough. Ammonite biochronology of the Inklin Formation in the Atlin Lake area constrain the age of the succession there to a range of Early Sinemurian to Late Pliensbachian and provide the temporal framework for analyzing petrofacies and interpreting basin history using paleocurrent data, ternary plots, and conglomerate clast age and provenance. Inklin Formation sediments record a complex arc-basin evolution during the Early Jurassic characterized by episodic flank uplifts, rejuvenated volcanism, and strike-slip influence on sedimentation. Strong temporal trends in both paleocurrent patterns and sandstone-conglomerate petrofacies are evident and allow definition of three discrete phases in basin-fill history. QFL, QmPK, and QpLvLs sandstone ternary plots, conglomerate clast trends and provenance, paleocurrent patterns and measured sections together indicate a regime of stable tectonics characterized by relative volcanic quiescence and low sedimentation rates prevailed through most or all of the Sinemurian. Sinemurian sandstone and conglomerate petrofacies record a transitional arc provenance derived from an older volcanic pile, flanking coastal sediments, and arc roots to the southwest. Progressive incision of the Late Triassic arc initiated in latest Triassic or earliest Jurassic time transported predominantly Upper Triassic elastics of mixed volcanoplutonic and sedimentary provenance initially eastward into the basin via turbidity currents where sediments were dispersed in a generally northwesterly direction, reflecting longitudinal fan-systems prograding north/northwesterly along the axial trend of the basin. Pliensbachian petrofacies record two episodes of distinct provenance and indicate the establishment of a new sedimentation regime. Lower Pliensbachian sandstone and conglomerate petrofacies indicate progressive arc dissection was reversed by a major magmatic episode accompanied by widespread rejuvenated volcanism in the arc segment, causing a strong provenance shift to volcanogenic sources. The volume of broadly coeval volcaniclastic debris deposited during this sub-stage delineates a distinct depositional episode in Early Jurassic basin-fill history and demonstrates provenance from a nearly undissected magmatic arc. Early Pliensbachian paleocurrent patterns record a fundamental change in paleoflow systems from predominantly southwest-derived longitudinal paleoflow to opposed bidirectional radial or transverse paleoflow systems. The initiation of southwest-directed paleoflow systems during the Early Pliensbachian indicates a change in basin morphology with the development of an outer basin margin and may represent an ephemeral or episodic ridged forearc phase in Early Jurassic basin history. Late Pliensbachian sediments record a new phase of arc evolution in which active tectonics replace volcanism as the first-order controls on sedimentation. Sandstone and conglomerate petrofacies record an abrupt shift from a volcanogenic provenance in Early Pliensbachian time to a mixed volcanoplutonic provenance dominated by granitic material in the Late Pliensbachian, indicating a rapid transition to a dissected arc provenance. Accelerated uplift of segments of the arc massif accompanied major intra-arc strike-slip faulting that led to rapid arc dissection and exhumation of comagmatic plutons. The resulting influx of granitic detritus delineates another distinct depositional episode in Early Jurassic basin-fill history and includes a significant component derived from Pliensbachian plutons (U-Pb: 186.6 +0.5/-1.0 Ma) to the southwest, underlining dramatic rates of uplift and arc incision during this time. High sedimentation rates prevailed and provided the impetus for a major progradational-aggradational pulse towards the northeast which subdued outer margin topography and changed former (Early Pliensbachian) bidirectional paleoflow systems to southwest-derived paleoflow systems.

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