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CSPG Special Publications

Abstract


Petroleum Geology of the Cretaceous Mannville Group, Western Canada — Memoir 18, 1997
Pages 1-29

Sedimentology, High-Resolution Allostratigraphy, and Key Stratigraphic Surfaces in Falher Members A and B, Spirit River Formation, West-Central Alberta

Richard Rouble, Roger Walker

Abstract

The Spirit River Formation (late early Albian) of west-central Alberta is comprised of a series of northward-prograding shoreline deposits punctuated by marine transgressions. Falher Members A and B of the Spirit River Formation have been studied using 76 cores and 321 well logs, Eight facies have been identified and interpreted; 1, swaley cross stratified sandstones (in places passing up into conglomerates) interpreted as storm dominated prograding shoreface deposits; 2, brackish to marine mudstones and sandstones interpreted as transgressive lagoonal deposits; 3, brackish to non-marine mudstones and sandstones interpreted as coastal plain deposits; 4, hummocky cross-stratified sandstones and interbedded bioturbated mudstones interpreted as open shelf storm deposits; 5, thin pebble conglomerates interpreted as transgressive lags; 6, bioturbated very fine sandstones interpreted as estuary fill; 7, cross-bedded sandstones interpreted as subaerial channel fills; and 8, planar laminated sandstones interpreted as barrier island washover deposits. The prograding shoreface deposits of Falher Members A and B each split northward and are separated by tongues of mudstones and HCS storm deposits, implying transgression during shoreface progradation. These transgressive surfaces have been traced southward as far as Twp. 68, implying that Falher Members A and B can be divided into four distinct allomembers, B1 and B2, and A1 and A2.

The bulk of each allomember consists of a prograding shoreface sandbody. The shoreface successions commonly coarsen upward, with gravel in the upper shoreface and beach. Each shoreface is truncated by a marine flooding surface (MFS), with some preservation of thin transgressive sediments. These consist of lagoonal mudstones, lag conglomerates, and rarely, marine mudstones. The absence of transgressive marine mudstones is probably due to erosion during the progradation of a subsequent shoreface sand body, which has a regressive surface of erosion (RSE) at the base. The key surfaces that separate the allomembers are MFSs, modified to RSEs, and designated MFS/RSE. In the southern part of the area, transgressive lagoonal deposits are overlain by the next prograding shoreface, implying that the southern limit of the shoreface was an immediately preceding barrier sand body. Recognition of four MFS/RSEs allows a high-resolution subdivision of Falher A and B. It places the reservoir conglomerates in a better-defined stratigraphic position, and allows the recognition of both shoreface and transgressive lag conglomerates. The four MFS/RSEs also provide a better picture of relative sea level fluctuation during deposition of Falher Members A and B.


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