THE BASINAL GEOCHEMICAL SIGNATURE AND PLATFORM MARGIN GEOMETRIES IN THE UPPER DEVONIAN MIXED CARBONATE-SILICICLASTIC SYSTEM OF WESTERN CANADA

In order to understand fully the factors that determine the stratigrap hic architecture of shallow water carbonate systems, the integration o f information found in the adjacent basinal sediments is essential. Wi th this purpose a buildup margin-to-basin transition of Upper Devonian (Frasnian) age has been investigated in outcrops in the Canadian Rock y Mountains. The study combined the platform margin geometry, as an in dicator of relative sea level change, and the geochemical characterist ics of the adjacent basin sediments to provide a more complete picture of the evolution of this mixed carbonate/siliciclastic sedimentary sy stem. The Frasnian represents a 2nd order transgressive/regressive seq uence, which can be subdivided in six 3rd order sequences, each of whi ch is characterised by specific platform margin geometries. Vertical m easured sections show a distinct pattern in the cross plots of the car bonate and organic carbon for each 3rd order sequence. Particularly st riking is the inverse linear relationship observed in sequences 3 and 4, between the carbonate content, and the organic matter and clay cont ent. This is interpreted as a dilution relationship, whereby low carbo nate input and dysaerobic conditions led to the accumulation of large amounts of organic matter in the basin. Maximum accumulation of organi c matter occurred in two 3rd order sequences preceding the 2nd order m aximum flooding surface (sequences 3 and 4), and highest concentration s are recorded in the lowstand of sequence 4. In addition, rapid later al changes in sediment composition have also been observed around the carbonate buildup. Four sedimentologic phases in the Frasnian basin in fill pattern are defined. Sequence 1 is characterised by regional shal low-water carbonate ramps and platforms. In sequence 2, platforms retr ograde and buildups are initiated; the topography is low, and the sedi ment composition is carbonate dominated. Sequences 3 and 4 show buildu p aggradation and basin starvation, which caused the development of a pronounced topography. In this phase a dilution relationship appears b etween carbonate versus organic matter and clays. Carbonate input is l ow, and a large amount of organic matter accumulated in the dysaerobic basin. In sequences 5 and 6, clays fill in the basin, and isolated pl atforms prograde. The topography is reduced, and sediment composition is dominated by carbonate and clay. The stratigraphic architecture is interpreted to be primarily the result of different orders of (eustati c) fluctuations in relative sea level. In addition, dysaerobic conditi ons at the sea floor enhanced organic matter accumulation in sequences 3 and 4, while as yet undefined environmental factors probably played a role in the change from regional platforms to isolated buildups (se quence 1 to 2). Finally, circulation patterns in the basin contributed to increased sedimentation of (fine-grained) siliciclastics towards t he end of the Frasnian (sequences 5 and 6). Since there is strong evid ence for a relative sea level control of the sedimentation pattern, th e here proposed model will be predictive for the stratigraphic archite cture in the Alberta shale basins, and greatly facilitate correlation with the adjacent subsurface.