IMPLICATIONS OF THE PETUNIABUKTA SYNCLINE FOR THE TIMING OF BILLEFJORDEN BASIN DEVELOPMENT ON SPITSBERGEN, NORWAY

The Billefjorden basin, a well studied example of a suite of Carboniferous rifts on the Barents Shelf and NE Greenland, consists of a half-graben with the Billefjorden fault zone (BFZ) to the W. The Hultberget, Ebbadalen and Minkinfjellet formations are clearly rift fill, constraining rift development as Bashkirian. The overlying Moscovian to Asselian Wordiekammen Fm. shallow marine carbonates were deposited across the BFZ, and are commonly considered as post-rift. However, Wordiekammen Fm. strata are clearly involved in the large scale, basin axial, Petuniabukta syncline. This syncline has a steep western limb (40-50 degrees dip), and a long eastern limb (10- 20 degrees dip), a shallow southerly plunge, and 800-1000 m of structural relief. The western limb is immediately underlain by deformed evaporites and faults of the BFZ, indicating the syncline is a result of normal fault offset and tri-shear zone development as part of the half-graben architecture. Possible models for the syncline development consistent with the regional geologic history are: 1) it represents continued Paleozoic rift development, or 2) it is a result of Tertiary reactivation, or 3) it is a result of both. We suggest that a significant later Paleozoic phase exists. Wordiekammen Fm. strata thicken basinwards across a monocline cored by the Lovehovden fault, an antithetic, sub-basin forming, conjugate fault to the BFZ. Further, compressional, east-side up Tertiary reactivation structures deforming younger strata are well exposed along strike of the BFZ, in direct contrast to the east-side down movement needed to form the syncline. Structures in deformed evaporites under the western syncline limb that are consistent with Tertiary compressional, E-side up, movement clearly overprint the dominant fabric associated with E-side down movement. Also, later anhydrite porphyroblasts overprint deformed gypsum, a history inconsistent with Tertiary reactivation. Finally, tilting appears to have developed pre breccia pipe development, which is likely Paleozoic in age. Continued rift development during Wordiekammen Fm. times implies fault submergence, in a shallow, carbonate dominated setting. Slow rifting and/or compaction/solution in underlying rift fill may have contributed to accommodation space and syncline development.