OVERVIEW OF STRATIGRAPHIC AND STRUCTURAL CONTROL OF GROUNDWATER FLOW IN THE SOUTHEASTERN IDAHO PHOSPHATE DISTRICT, USA

Phosphate ore is mined in open pits from the Permian Phosphoria Fm in southeastern Idaho. The Phosphoria Fm (~175 m thick), overlying fine-grained clastic and carbonate rocks of the Triassic Thanes and Dinwoody Fms (~1700 m thick), and the underlying carbonate rocks of the upper Paleozoic Wells and Brazer Fms (~2000 m thick) are part of the Meade Thrust Allochthon. The allochthon is a mountainous highland up to 40 km wide (east-west) and 80 km long (north-south). The sole of the thrust fault is 1200-3300 m bgs. Steeply dipping splays of the thrust crop-out along the eastern and southern edges of the allochton. The allochthon is truncated on the west by an extension fault with 900-1500 m of throw. Strata in the interior of the allochthon are folded into broad northwest anticlines and synclines that are locally cut by extension faults with <200 m of throw. The Phosphatic Shale m of the Phosphoria Fm forms a regional barrier to vertical groundwater flow. Groundwater systems above and below the shale are designated as Tiers 1 and 2, respectively.

Most Tier 1 groundwater occurs as bedding controlled local systems that discharge < 100 l/s, contain abundant ³H, are Ca²⁺-HCO₃^- type water that tend to be carbonate mineral undersaturated, and do not interact with underlying Tier 2 groundwater. Tier 2 groundwater circulates as much as 1900 m below land surface and discharge from splays of the Meade Thrust fault to the east and south and along deep seated extension faults at the base of the Aspen and Wooley Ranges to the west. Both fault splay and extension fault springs can discharge up to 450 l/s and have discharge temperatures up to 26 ^oC. Fault splay discharge waters are chemically similar to Tier 2 waters. Extension fault discharges are also Ca²⁺-HCO₃^- type, but have elevated TDS, pCO₂ ~ 1 atm, and are associated with extensive tufa deposits. Groundwaters discharging along fault splays are interpreted as flowing along bedding and thrust fault damage zones, whereas extension fault discharging groundwater is interpreted as flowing generally west and northwest along bedding surfaces until intercepted by the extension fault damage zones where flow is channeled to the surface. Elevated pCO₂ contents are attributed to thermal decomposition of carbonate rocks at depth independently of Tier 2 groundwater flow.