Abstract: Biofacies and Sequence Stratigraphy in an Intracratonic Seaway, Middle to Upper Jurassic, East Greenland
PIASECKI, STEFAN and MICHAEL LARSEN; Geological Survey of Denmark and Greenland (GEUS), Thoravej 8, DK-2400 Copenhagen NV, Denmark
Middle to Late Jurassic deposition in East Greenland took place in a north-south elongate, intracratonic rifted seaway 800 km long and 140 km wide. Sediment input was controlled by large river systems entering low gradient ramp-type basins from the north and west and redistributed within these basins by tidal and longshore currents. The basin bathymetry changed due to accelerated rifting in the Upper Jurassic leading to development of a pronounced shelf-slope basin configuration.
Two depositional systems, each with a distinct marginal to basinal development, are analysed within a sequence stratigraphic and biofacies framework: 1) Bajocian-Callovian coarse-grained shallow marine sandstones, and silly sandstones and mudstones of the inner shelf; 2) Oxfordian-Kimmeridgian deltaic sandstones and deep marine basin floor mudstones. The Middle Jurassic depositional system formed during sea-level lowstand followed by an overall rise. The Upper Jurassic succession was deposited during a marked sea-level highstand and shows pronounced differentiation into shallow marine and basinal biofacies assemblages.
Biofacies are controlled by basin configuration and ocean circulation patterns as well as by the balance between elastic sediment input and biogenic production. These parameters are closely related to changes in relative sea-level so that the identified biofacies may be coupled to specific systems tracts. Marine fauna and plankton associations, terrestrial versus marine palynomorph assemblages, composition and preservation of organic matter, grain-size of coaly particles and Leco and RockEval data are used to characterise bio-and depositional facies within a sequence stratigraphical framework.
The Middle Jurassic fauna is very restricted in sediments deposited at basin margins, but ammonites in particular occur in distinct stratigraphic horizons in the distal deposits. The ammonite fauna varies stratigraphically from strictly Boreal to Subboreal assemblages when oceanic barriers to the south were flooded during high sea-level stands. Terrestrially derived organic material dominates all over the basin and TOC is constantly low. In proximal settings, OM is only represented in thin mudstone beds orelated to maximum flooding events with dominantly terrestrial material and abundant dinoflagellate cysts. In distal settings, marine palynomorphs are most abundant close to maximum flooding surfaces and highest diversity coincides with intervals of Subboreal ammonite faunas. Laminated mudstones are rare and facies characterised by OM of amorphous kerogen have not been identified in this succession. All the organic material is classified as Type III.
The Upper Jurassic fauna of bivalves, brachiopods and ammonites, is abundant both in proximal and distal deposits. Changes in affinity of the ammonite assemblages and the content of organic matter follow the outlines described for the Middle Jurassic. Proximal facies are strongly bioturbated and have a low organic content. The organic content in distal facies is high and dominantly terrestrial in composition. However, amorphous algal material dominates in the Kimmeridgian distal facies. The organic material is classified as type II and III.
Significant source potential for oil in the Middle-Upper Jurassic succession of Jameson Land is restricted to the Kimmeridgian, a time of marked sea level highstand recognised all over the North Atlantic region. Integration of biofacies and sequence stratigraphy has strengthened existing geological models and established a clear relation between facies and large scale geological events, which can be used for analysis of the petroleum systems of the Jurassic Jameson Land basin.
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah