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

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 P(2)O(5) 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. Al(2)O(3), K(2)O, Fe(2)O(3), TiO(2), Rb, Co, Zr, Nb) and characterized by high TOC (6-21 weight percent). Excess SiO(2) (i.e. silica which cannot be accounted for by the aluminosilicate phase) accounts for up to 94% of the SiO(2) 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, P(2)O(5) 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.