Petrology and Diagenesis of Upper Triassic and Lower Jurassic Graben-Fill Sandstones of Jameson Land Basin, Central East Greenland
Kevin W. Davee, Charles F. Mansfield
Sands of the Upper Triassic-Lower Jurassic Kap Stewart and Lower Jurassic Neill Klinter Formations were deposited during the early development of a major Jurassic rift basin. Depositional environments have been interpreted as alluvial fan, fluvial, and deltaic for the Kap Stewart, and as tidal estuary for the Neill Klinter.
Most of these sandstones are fine to medium-grained quartz and sub-arkosic wackestones. Clay matrix is abundant and comprises mostly diagenetic kaolinite, illite, and complex clay mixtures derived from the breakdown of labile framework grains, especially plagioclase and biotite. Early emplacement of carbonate cement in some sandstones prevented development of diagenetic clay matrix. The carbonate cement is poikilotopic and constitutes up to 30% of some samples.
The current high level of maturity indicated by the quartz-feldspar-lithic composition for these sandstones results from selective diagenetic decomposition of labile grains. The original, more feldspathic compositions reflect less maturity and an earlier stage of basin fill than the Middle Jurassic, high-porosity arenites, which also contain abundant quartz and up to 25% feldspar. Because the Upper Triassic and Lower Jurassic sands were deposited in continental and low-energy marine environments, they likely underwent greater in-situ alteration but less winnowing of labile grains than the overlying Middle Jurassic sands deposited in higher energy marine settings.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.