This study deals with the weathering of basement, resulting particle formation, depositional conditions, provenance and mode of transportation of Triassic/Jurassic strata in evolving rift basin setting in Stord Basin. Data from sedimentary core logging, XRD and petrographical analysis of cutting and core intervals from well 17/3-1 have been integrated to elucidate the depositional transition from basement to Triassic/Jurassic.
Fracture filling calcitic veins and striations are observed on basement indicating local basement has undergone different post Caledonian tectonic events. The low XRD% of kaolinite and high XRD% of plagioclase in studied section above basement indicates an absence of weathering profile. Lower part of the Smith Bank Formation is sourced by local basement identified on the basis of high amount of quartz and feldspar grains in mudclasts, high plagioclase XRD%, and high striated calcite percentage, deposited in alluvial fan settings. Mineralogical change observed by introduction of K-feldspar indicates another source than amphibolitic basement in middle part of the Smith Bank Formation. Mudclasts having more clayey content indicate sediments originated from distal source. High clay percentages, Gamma ray trends show deposition as channel sediments and floodplains by axially flowing meandering river. Structural settings of Utsira High makes it improbable provenance for sediments in study area. Uplifted hinterland of Fennoscandian shield is main probable provenance for middle part. The progressive decrease in feldspars in uppermost part of Triassic sediments depicts that basin has achieved tectonic maturity.
On the basis of observed structures Bryne Formation is interpreted to be deposited in delta plain/tidal flat settings. Sandnes Formation is interpreted to be deposited dominantly in shallow marine settings. It is overlying the non-marine Bryne Formation, suggesting a rise in sea level. Feldspars have been transformed mostly to clays by late diagenetic alteration noticed by XRD results and thin section analysis. The presence of Saponite rich clay intervals indicates altered basic, ultra basic rocks as one of probable source for this formation or by alteration of nontronite.. The possible source for these formations can be volcanic rocks in south of study area or from mainland Norway.
Keywords: Stord Basin, Smith Bank Formation, provenance, depositional conditions, XRD, thin section.