--> Abstract: Definition of Middle Frasnian Woodbend Sequences, Surface and Subsurface Alberta, by J. M. Weissenberger, M. Gilhooly, K. Potma, and P. Wong; #91012 (1992).

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ABSTRACT: Definition of Middle Frasnian Woodbend Sequences, Surface and Subsurface Alberta

WEISSENBERGER, JOHN, M. GILHOOLY, K. POTMA, and P. WONG, Esso Resources Canada Ltd., Calgary, Alberta, Canada

The middle Frasnian (Upper Devonian) Woodbend Group includes shallow water carbonate and basinal sediments preserved in the Alberta Rocky Mountains and the adjoining subsurface. Three third-order sequences can be defined within this lithostratigraphic unit, here informally termed the lower, middle, and upper Leduc sequences.

The lower and middle Leduc sequences are characterized by relatively high accommodation in the basin, resulting from their position at the transgressive heart of a larger, second-order sequence (the Gilwood-Graminia). The style of carbonate platform development was transgressive-skewed and therefore primarily back-stepping or upbuilding, with a corresponding opening and deepening of the basin. Black, organic-rich shales of the Duvernay Formation were deposited in deeper, poorly circulated parts of the basin. The Duvernay is now a prolific hydrocarbon source rock.

The upper Leduc sequence is more regressive-skewed. Locally, shallow water carbonates rapidly prograde into the basin, depending on current/wind movement of sediment. Lobes of calcareous shale having a fine siliciclastic component, the Ireton Formation, filled much of the basin during the late regressive phase of the upper Leduc sequence.

Boundaries between sequences are typically conformable owing to high accommodation rates. Such surfaces are identified in outcrops along the Alberta Front Ranges and inferred from log and core data in the subsurface. Regional correlation of third-order sequences was achieved using the distinctive stacking pattern of component fourth-order cycles and their stratigraphic position within the second-order sequence and corroborated with conodont biostratigraphy. This correlation has allowed the establishment of a precise time-stratigraphic relationship between the Grosmont platform, the Ireton basin-fill, Duvernay source-rock, Leduc reefs, and lower Fairholme Group carbonates in outcrop.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)