ABSTRACT

The Devonian strata exposed along Peace River in the immediate vicinity of Peace Point include the evaporites of the Fort Vermilion Formation and the limestones and dolostones of the Slave Point Formation. An unconformity marks the boundary between the two formations. In many areas the strata are interrupted by extensive brecciation. Such breccias, informally designated as blocky breccia, shale breccia, gypsum breccia, dolomite breccia, mixed breccia, limestone breccia and sandstone breccia, incorporate material from the present exposed sequence and sequences no longer present in the area. For example, the shale breccia includes shale from the Waterways Formation, while the sandstone breccia includes Cretaceous sandstones. Maximum breccia development coincides with the topographically low areas of the unconformity that separates the Fort Vermilion Formation from the Slave Point Formation.

The Devonian strata exposed around Peace Point record a complicated depositional history that involved repetitive cycles of sediment deposition and subaerial exposure. Karst terrains developed during periods of subaerial exposure, which led to diagenetic alteration of the underlying strata as well as extensive but localized brecciation. At least three cycles of deposition, tectonism and subaerial exposure occurred: 1) The first phase of folding, tilting (or doming) and erosion occurred before deposition of the limestones and dolostones of the Slave Point Formation. As a result, the Fort Vermilion/Slave Point formational boundary is now marked by an angular unconformity. 2) The second phase of karst development, after deposition of the sediments of the Slave Point Formation, involved the formation of sinkholes as well as brecciation. 3) The third phase of karsting occurred after deposition of the shales of the Devonian Waterways Formation but before deposition of the Cretaceous sandstones and siltstones. Most of the Waterways Formation and virtually all the Cretaceous strata have since been removed by erosion. Nevertheless, the presence of the Cretaceous rocks in the area is recorded by the occurrence of clasts of this material in the breccia.

At least three phases of brecciation have affected the strata around Peace Point. Collapse 1 produced solution breccias before deposition of the Waterways Formation. Collapse 2, the major brecciation event, produced evaporite-solution-collapse breccias after deposition of the Waterways Formation but before Cretaceous sandstone deposition. Collapse 3 produced a sandstone breccia and occurred after deposition of the Cretaceous sandstone but before glaciation.

Cretaceous sandstones, siltstones and shales in the sandstone breccia contain faunal evidence of the Devonian-Cretaceous unconformity as well as lithologies and faunas typical of the McMurray Formation. The occurrence of these clastic rocks indicates that the area around Peace Point was once covered with Cretaceous rocks that were later eroded.

NATURE ET GENESE DE BRECHES EN STRATES DEVONIENNES, REGION DE PEACE POINT. PARC WOOD BUFFALO, ALBERTA

RESUME

Les couches devoniennes le long de la riviere La Paix au voisinage de Peace Point comprennent des evaporites de la formation Fort Vermillion, et des calcaires et dolomies de la formation Slave Point. Une disconformite separe les deux formations. Les strates sont interrompues en de nombreux endroits par des breches fortement developpees. Celles-ci sont decrites, de maniere non-formelle, comme breches en blocs, d'argile, de gypse, de dolomie, mixte, calcaire, ou de gres, et comprennent des materiaux de la suite presentement exposee ainsi que de celles aujourd'hui disparues. Les breches a argile comprennent, par example, des argiles de la formation Waterways, et les breches de gres des gres cretaces. La formation maximum de breches coincide avec les depressions des disconformites entre les formations Fort Vermillion et Slave Point.

Les lits devoniens qui affleurent aux environs de Peace Point enregistrent une complexe histoire de sedimentation cyclique et d'affleurement subaerienne. Des karsts furent formes en exposition subaerienne, qui resulta en une alteration diagenetique des couches sousjacente, ainsi qu'une formation de breches vastes et cependant isolees. Il y eut au moins trois cycles de deposition, tectonisme, et affleurement subaerienne. 1) La premiere phase de plissement et inclinaison de couches (ou formation de dome) et d'erosion eut lieu avant la deposition de calcaires et dolomies de la formation Slave Point. Il en resulta une disconformite separant les formations Fort Vermillion et Slave Point. 2) La deuxieme phase de developpement de karst, post-sedimentation Slave Point, resulta en des dolines et breches. 3) La formation de karsts de la troisieme phase eut lieu apres la deposition de la formation Waterways devonienne, et avant celle des gres et microgres cretaces. La plus grande partie de la formation Waterways et la presque totalite des lits cretaces ont ete erodees depuis. L'existence passee de roches cretacees est neanmoins temoignee par la presence de leurs constituants detritiques dans les breches.

Au moins trois phases de formation de breches affecterent les strates devoniennes de Peace Point. Le premier affaissement produisit des breches de dissolution avant le depot de la formation Waterways. Le second affaissement, celui-ci primordial, produisit des breches de dissolution d'evaporites apres la deposition de la formation Waterways, et avant celle des roches cretacees. Le troisieme affaissement produisit une breche a gres apres la deposition de gres cretaces et avant les glaciations.

Les gres, microgres et argiles cretaces des breches a gres comprennent un temoignage faunal de la disconformite devono-cretacee, ainsi que des roches et des faunes typiques de la formation McMurray. Ces roches detritiques indiquent que les environs de Peace Point furent autrefois couverts de roches cretacees, qui ont ensuite ete erodees.

Traduit par A. Zolnai

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