The basinal geochemical signature and platform margin geometries in the Upper Devonian mixed carbonate-siliciclastic system of western Canada Le système mixte carbonaté/silicoclastique de l'Ouest do Canada : Teneurs géochimiques du bassin et géométries de la bordure de la plate-forme

In order to understand fully the factors that determine the stratigraphic architecture of shallow water carbonate system, the intergration of information found in the adjacent basinal sediments is essential. With this purpose a buildup margin-to-basin transition of Upper Devonian (Frasnian) age has beem investigated in outcrops in the Canadian Rocky Mountains. The study combined the platform margin geometry, as an indicator of relative sea leval change, and the geochemical characteristics of the adjacent basin sediments to povide a more complete picture of the evolution of this mixed carbonate/siliciclastic sedimentary system. The Frasnian represents a 2nd order transgressive/regressive sequence, which can be subdivided in six 3rd order sequences, each of which is characterised by specific platform margin geometrics. Vertical measured sections show a distinct pattern in the cross plots of the carbonate organic carbon for each 3rd order sequence. Particularly striking is the inverse linear relationship oberved in sequences 3 and 4, between the carbonate content, and the organic matter and clay content. This is interpeted as a dilution relationship, whereby low carbonate input and dysaerobic conditions led to the accumulation of large amounts of organic matter in the basin. Maximum accumulation of organic matter occured in two 3rd order sequences precedinh the 2nd order maximum flodding surface (sequences 3 and 4), and highest concentariation are recorded in the lowstand of squence 4. In addition, rapid lateral changes in sediment compositon have also been observed around the carbonate buildup. Four sedimentologic phases in the Fransian basin infill pattern are defined. Sequence 1 is characterised by regional shallow-water carbonate ramps amd platforms. In sequence 2, platforms retrograde and buildups are initiated; topography is low, and the sediment composition is carbonate dominated. Sequences 3 and 4 show buildup aggradation and basin starvation, which caused the development of a pronounced topography. In this phase a dilution relationship apperas between carbonate versus organic matter and calys. Carbonate input it low, and a large amount of organic matter accumulated in the dysacrobic basin. In sequnces 5 and 6, clays fill in the basin, and isolated platforms prograde. The topography is reduced, and sediment composition is domianted by carbonate and clay. The stratigraphic architecture is interpreted to be primarily the result of different orders of (eustatic) fluctuations inrelative sea level. In addition, dysaerobic conditions at the sea floor enhanced organic matter accumaulation in sequences 3 and 4, while at yet undefined environmental factors probably played a role in the change from regional platforms to isolated buildups (sequence 1 to 2). Finally, circulation patterns in the basin contributed to increased sedimentaion of (fine-grained) siliciclastics towards the end of the Frasnian (sequnces 5 and 6). Since there is strong evidence for a relative sea level control of the sedimentation pattern, the here proposed model will be predictive for the stratigraphic architecture in the alberta shale basais, and greatly facilitate correlation with the adjacent subsurface.