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Sediment geochemistry of the Lower Jurassic Gordondale Member, northeastern British Columbia

Department of Geological Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4

R. Marc Bustin

Department of Geological Sciences, University of British Columbia, 6339 Stores Road, Vancouver, BC V6T 1Z4

Paleoenvironments during the Jurassic Gordondale Member sedimentation have been investigated using detailed geochemistry. Four offshore marine litho-stratigraphic units occur within the Gordondale in the Peace River Embayment area of northeastern British Columbia (NE B.C.). The basal mudstone conglomerate/breccia unit (unit A) represents the initiation of a transgression across the basin. The overlying phosphatic marlstone (unit B) was deposited during a time of high organic productivity, characterized by enrichments in the productivity proxying element P₂O₅ and total organic carbon (TOC) contents averaging 5.5 weight percent. Under these conditions, bio-productivity elements (V, Cr, Cu, Ni, Zn) were effectively incorporated with the organic fraction (matrix bituminite). The co-occurrence of the marine phosphate francolite and pyrite suggests the dysoxic-anoxic redox boundary oscillated vertically in the sediment with time.

A shift in geochemistry occurs at the onset of the overlying unit C deposition. Unit C sediments are highly siliceous, low in detrital proxying elements (e.g. Al₂O₃, K₂O, Fe₂O₃, TiO₂, Rb, Co, Zr, Nb) and characterized by high TOC (6–21 weight percent). Excess SiO₂ (i.e. silica which cannot be accounted for by the aluminosilicate phase) accounts for up to 94% of the SiO₂ in the organic-rich sections, suggesting a biosiliceous source. A productive water column is also inferred by relatively high Ba concentrations. The low concentrations of all other major oxides and trace metals in unit C reflect silica dilution in the depositional basin. Carbon-S-Fe plots show a sulfidic bottom-water environment persisted for much of unit C deposition where S was concentrated into the organic fraction (alginite and matrix bituminite) due to the lack of Fe available for pyrite formation.

Unit D represents two different stages of sedimentation. The contact between units C and D is marked by a phosphatic rich-marlstone, depleted in all major and trace elements except CaO, P₂O₅ and Mn. The enrichment of Mn in the carbonate suggests dissolved oxygen was present in bottom waters. Thereafter, unit D is dominated compositionally by detrital proxying elements (clays and associated elements such as Ti, K and Fe) reflecting an increase of terrigenous input. However, despite the sediment-dilution by aluminosilicate phases, TOC contents are comparable to unit B, averaging 5.6 weight percent.

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