EARLY DIAGENESIS AND ITS RELATIONSHIP TO DEPOSITIONAL ENVIRONMENT ANDRELATIVE SEA-LEVEL FLUCTUATIONS (UPPER CRETACEOUS MARSHYBANK FORMATION, ALBERTA AND BRITISH-COLUMBIA)

Early diagenesis of the Upper Cretaceous (late Coniacian to early Sant onian) Marshybank Formation was controlled by depositional environment (composition of depositional water, Fe and organic content of the sed iment, sedimentation rate, proximity to the shoreline) and influx of m eteoric water related to relative sea-level fall. Five depositional en vironments, each characterized by a distinct early diagenetic mineral assemblage, have been recognized. Offshore shelf sediments that were d eposited in a dysaerobic environment are characterized by abundant fra mboidal pyrite and rare septarian concretions, composed of 'early' cal cite and siderite. Intense sulphate reduction, promoted by the dysaero bic depositional water, was the primary influence on early diagenesis. Offshore shelf sediments deposited under aerobic conditions are chara cterized by abundant concretions, composed of two generations of sider ite (S1 and S2). In this environment, methanogenesis, rather than sulp hate reduction, was more important. Early diagenesis of the inner shel f sands was generally limited. However, in sands deposited proximal to the shoreline, mixing of marine and meteoric waters promoted crystall ization of Fe-rich chlorite and siderite. The shoreface was characteri zed by dissolution of detrital minerals in the upper portion, and prec ipitation of kaolinite or illite/smectite in the lower portion. In the coastal plain environment, brackish water and early reducing conditio ns resulted in formation of abundant euhedral pyrite. Ankerite, rather than siderite, is the typical early diagenetic carbonate. The delta(1 8)O values of the earliest cements (i.e. 'early' calcite, siderite S1, inner shelf siderite) indicate crystallization from a low-O-18, marin e-derived porewater. Assuming crystallization at 25 degrees C, a delta (18)O value of about - 7 parts per thousand (SMOW) can be estimated fo r the seaway during Marshybank Formation time. Similar calculations fo r the overlying Dowling Member (Puskwaskau Formation) suggest that the delta(18)O value of the seaway increased to about - 4% (SMOW), consis tent with its transgressive nature. Very low delta(18)O values are exh ibited by siderite S2. These results indicate crystallization during i ntermediate diagenesis (greater than or equal to 60 degrees) from mete oric water (greater than or equal to - 15 parts per thousand SMOW) tha t entered the Marshybank Formation during sea-level lowstand.