CHAMOSITE OOID HORIZONS OF THE CAROLINEFJELLET FORMATION, SPITSBERGEN, NORWAY

Cretaceous age ooids of Fe minerals have been identified at many locations globally, and have been linked to greenhouse conditions. The Carolinefjellet Formation of Spitsbergen is an Aptian-Albian marine sequence. Chamositic ooids in its Dalkjegla Mbr. have been identified from three separate locations. Presence of chamosite, pyrite and siderite indicates Fe rich, reducing conditions. Analysis of thin sections indicates the following: 1. A wide variety of ooid shapes, ranging from symmetric flaxseed and round, to more complex forms are present. Certain ooid shapes conform to original outlines of seed grains. Many composite forms show a history of multiple growth events and of deformation. 2. The ooids are generally found in association with large well rounded quartz grains and opaque minerals. Large planar cross sets, oscillation ripple marks, and the degree of sorting indicate a shallow marine setting. 3. XRD shows that the main mineral in the ooids is chamosite. 4. The inner portion of the ooids consists of a seed, surrounded by rings of tangentially oriented chamosite. Seeds are typically quartz, but can also be opaque minerals or carbonate clasts. This inner portion is finer grained and the rings are thinner. Outer layers show an overgrowth of calcite surrounded by an outermost layer of chamosite showing radial growth. Many grains in the samples other than the ooids show this final radial growth. These differing forms may be indicative of changing chemical conditions. 5. In many cases there is then a predominantly late stage calcite spar cement with well developed twinning. Four hypotheses for the formation of the ooids include; chemical precipitation, mechanical accretion, biologically mediated growth and a history of cycles of burial and excavation. The presence of both tangential concentric growth and radial growth shows that there was likely a combination of these factors at work.