Abstract of talk given to: Palaeomagnetism and Diagenesis in Sediments, Geological Society of London, London, October 1997.

Palaeomagnetic constraints on the nature and timing of subsurface fluid movement in the Jurassic sandstones of the Paradox Basin, Utah

Guscott, S. C.¹, Garden, I. R.^2,3, Burley, S. D.^1,3, Van Alstine, D. R.⁴, Foxford, K. A.⁵, Walsh, J. J.⁵ & Watterson, J.⁵
1 - Diagenesis Research Group, Dept. of Earth Sciences, University of Manchester
2 - Reservoir Description Research Group, Dept. of Petroleum Engineering, Heriot-Watt University
3 - British Gas Research, Ashby Road, Loughborough, Leicestershire
4 - Applied Paleomagnetics, Inc., Redmond, WA
5 - Fault Analysis Group, Dept. of Earth Sciences, University of Liverpool

Continental red bed sandstones of Pennsylvanian to Late Jurassic age outcrop in the footwall to the Triassic-early Tertiary Moab Fault, Utah. The majority of the sandstones retain their characteristic red colour, but those of the Lower to Middle Jurassic Wingate, Navajo, Page and Entrada formations are commonly reduced to a greyish-white colour. This colour change diminishes both southwards along the fault trace and westwards into the footwall, although it is still present up to 7km from the fault. The spatial extent of the white sandstones is controlled by proximity to the Moab Fault and by sandbody connectivity.

Palaeomagnetic studies were undertaken to constrain the timing and conditions of the iron reduction after petrographical observations indicated reduction occurred late in the burial history of the sandstones. Reduction post-dates the initiation of quartz overgrowth cementation, and exhibits a complex relationship with fault-related calcite cementation. The palaeomagnetic data show that the fluid migration responsible for the reduction occurred in two stages separated by a period of regional tilting in the early Tertiary.

The first stage, dated at 58-53Ma, is recognised in tan to greyish sandstones. It was a natural chemical remagnetisation event responsible for the remobilisation and removal of >99% of the iron from the sandstones whilst imposing a chemical remanent magnetization (CRM) on the residual ferromagnetic minerals. The second stage of remagnetisation, dated at 53-47Ma, post-dates westward regional tilting and resulted from a change in fluid composition. This change, probably involving a rise in pH, halted the CRM process, and resulted in calcite precipitation which helped preserve the CRM signature by preventing flushing by modern groundwaters. The second stage remagnetisation is observed exclusively in white sandstones. It is characterised by a distinctive magnetic mineral assemblage comprising magnetite and pyrrhotite. Thermal demagnetisation was used to determine the magnetic mineralogy and to estimate the abundance of magnetite relative to pyrrhotite using a proxy SIRMT363/SIRMT203 ratio. This ratio falls within a narrow range (0.55±0.04) suggesting that the stage 2 remagnetisation occurred in an environment with a high fluid to rock ratio. The timing of the reduction event corresponds closely to the timing of maximum burial, as determined by fission track analysis, indicating that the reduction event was a consequence of large-scale fluid migration at depth. The reduced strata record the migration pathway of these fluids in the subsurface.