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Lithostratigraphy, palynostratigraphy, and sedimentology of the Northern Skeena Mountains and their implications to the tectonic history of the Canadian Cordillera

University of British Columbia

The Skeena Mountains of north-central British Columbia contain exposures of more than 4500 m of Jura-Cretaceous strata deposited in the Bowser Basin. These strata were deposited as a result of tectonism that formed the Canadian Cordillera, and serve as a record of the tectonic history of the Cordillera. The strata of the north-central Bowser Basin have been the subject of an integrated lithostratigraphic, palynostratigraphic and sedimentologic study in order to better understand the depositional history of the basin and the tectonic history of the Cordillera. The study area is in the northern Skeena Mountains, approximately coincident with boundaries of the Groundhog coalfield. Four hthostratigraphic units are exposed in the study area. The upper three units, consisting of over 2000 m of strata, are well exposed, and are formally named herein, from oldest to youngest, the Currier, McEvoy and Devils Claw Formations. Underlying the Currier Formation is the informally named Jackson unit, which comprises the oldest exposed strata in the study area. Together, the Jackson unit and Currier Formation comprise the Bowser Lake Group. The McEvoy and Devils Claw Formations unconformably overlie the Bowser Lake Group and correlate with the Skeena Group. Palynostratigraphy has been used successfully to correlate and date the sediments. Marine macrofossils are rare above the Jackson unit, necessitating use of palynomorphs to date the strata. Recovery of palynomorphs from anthracite rank coal measures has proven possible by extended treatment in Shulze's solution. The Currier Formation is suggested to be Late Jurassic (Oxfordian to Kimmeridgian or Tithonian) in age. The overlying McEvoy and Devils Claw Formations are entirely Cretaceous in age. The McEvoy Formation extends from the late Barremian or Aptian to the middle or late Albian, and the Devils Claw Formation spans the middle or late Albian to the Cenomanian. Two sedimentologically distinct deltaic sequences have been interpreted from the lithofacies associations. Both delta sequences probably accumulated in relatively shallow water, suggesting shallow shelf deposition. The older deltaic sequence occurs in the Currier Formation, and is analogous to facies of the modern Mississippi delta. The younger deltaic sequence encompasses strata of the McEvoy and Devils Claw Formation, and is more analogous to the coarse grained delta of the Copper River. The two delta sequences are separated by a hiatus of 20 to 35 million years. Sediment provenance, interpreted from paleocurrent data and clast lithologies, appears to have remained the same during deposition of both deltaic sequences. Paleocurrent indicators point to a source to the east-northeast, and the dominance of chert suggests a dominantly sedimentary source terrane, likely the Cache Creek Group. North American provenance is consistent with paleocurrent indicators and clast lithologies. The tectonic history of the Cordillera is related to the depositional history of the Bowser Basin. Subsidence, as indicated by sediment accumulation, occurred during the Middle and Late Jurassic and probably decreased near the end of the Jurassic, leading to a period of tectonic quiescence in the Cordillera. Subsidence resumed near the end of the early Cretaceous, leading to another period of deltaic sedimentation. This second deltaic sequence, represented by strata of the McEvoy and Devils Claw Formations, is coarser than the Late Jurassic delta, which suggests a more rugged and proximal source area.

Text

2010-08-16

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http://hdl.handle.net/2429/27409

Geological Sciences

University of British Columbia

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