Sequence Development over the Peripheral Bulge of a Foreland Basin: Stratigraphy of the 1.96 Ga Rifle Formation, Kilohigok Basin, Northwest Canada

John P. Grotzinger

The Rifle Formation developed as a clastic wedge that prograded from the cratonic side of a foreland basin and was strongly affected by variable subsidence associated with flexure over the peripheral bulge. The Rifle is an overall coarsening-up unit of marine and non-marine shales, siltstones, sandstones, and minor carbonates, and comprises three sequences. Each sequence (designated I, II, & III) is bounded above and below by sequence boundaries (SB) (designated I, II, III, & IV). SBs I and IV form lower and upper formation boundaries of the Rifle and correlate for 400 km across the Slave Craton. SBs II and III are not correlated as far due to removal by extensive erosion on Sequence Boundary IV. SB I exhibits some erosion but has the Sequence I transgressive surface superimposed on it Sequence I has no lowstand deposits but includes distal-shelf deposits of the transgressive systems tract, a 1 m-thick condensed-section carbonate, and highstand deltaic and fluvial deposits. SB II is characterized by subaerial exposure, paleosols, and erosion. The Sequence II lowstand has marine shales and siltstones, but no sandstones in contrast with the transgressive and highstand tidal, deltaic, and fluvial shales, siltstones, and sandstones. A thin condensed-section carbonate is also found in Sequence II. SB III is similar to SB II. Sequence III has no lowstand deposits but does have transgressive fluvial sandstones a few meters thick, with highstand tidal, deltaic and fluvial deposits composing most of the sequence. SB IV exhibits extensive erosion and paleosol development It is has incised valleys filled with fluvial deposits and by the condensed-section carbonate of the fourth sequence.

The Rifle Formation contrasts with sequences described from the hinterland side of other foreland basins, which are supplied by abundant sediments generated through unroofing mountainous source regions. Transgressive systems tracts of the Rifle Formation sequences are characteristically sediment starved, which results in the superimposition of sequence boundaries, transgressive surfaces and condensed intervals. This reflects a limited sediment supply due to the low gradient and small size of the source area (peripheral bulge) developed on the cratonic side of the basin.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California