Subsurface remobilisation and intrusion of sand : case studies from the Faroe Shetland Basin and SE Utah

• Main Author: Shoulders, Simon
• Published: Cardiff University 2005
• Subjects: 553.6
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.583528

This study has explored the phenomenon of basin-wide sand intrusion. Two case studies from regions with very different tectonic and depositional histories, host rock successions and intrusion geometries have been used to examine sand intrusion in the subsurface. Striking first-order similarities between the two contrasting case studies are apparent 1) the basin-wide nature of the sand intrusion event 2) the strong temporal link between the intrusion of sand and major tectonic events within the basin. It therefore appears likely that sand remobilisation in the subsurface and subsequent sand intrusion was triggered by earthquake activity. The case studies are presented as four papers produced for publication, supported by a review of previous work and a discussion of the case study results. A broad-based approach using a range of geological techniques was used to examine problems in our understanding associated with basin-wide fluid escape and sand intrusion. The first of the two case study areas consists of a region (in excess of 10,000 km<super>2 </super>) of large-scale conical sandstone intrusions hosted within a deep-water Eocene-Oligocene succession consisting of claystone and bio-siliceous ooze in Tranche 6 of the Faroe-Shetland Basin. The emplacement of the conical sandstone intrusions occurred during the Late Miocene, coincident with a phase of basin inversion. Polygonal faults were exploited as conduits to feed the conical intrusions, however intrusion propagation occurred along new fractures rather than exploiting the pre-existing polygonal fault network. The second case study is a region of sandstone pipes hosted within the sand-dominated rocks of the Middle Jurassic Carmel Formation and Entrada Sandstone in SE Utah. Pipes formed before the deposition of the Middle Jurassic Cannonvillle Member of the Entrada Sandstone across the region. Pipe formation was coincident with increased tectonic shortening and the inception of eastward tilting of the basin due to uplift to the west. Fluid-flow velocity during pipe formation is constrained to around 1.6 cms"1 based on calculations of settling velocity of grains present within the pipe fill.