PROGRESSIVE LOWERING OF THE WATER TABLE IN THE GRAND CANYON AS RECORDED BY CAVE AND MINE DEPOSITS

Speleothem and ore deposits in Grand Canyon (GC) caves and mines record the progressive lowering of the water table over time. The sequence of significant deposits and events in the GC is:

(1) Ore mineralization (Cu-U) episode. Sulfide ore mineralization, as exposed in the breccia pipes/mines of the GC, formed in the reduced zone, possibly during the Laramide when H2S migrated up deep basement faults and monoclinal structures. Uranium precipitated in the redox zone and calcite spar formed paragenetically with ore mineralization. Time: Paleocene to Eocene?

(2) Hematite/goethite/manganese episode. The oldest cave deposits are manganese and iron oxides (hematite/goethite) containing minor halite and trace-metals (e.g., As, Ba, Pb). These deposits fill small solution cavities in the Redwall Limestone exposed by cave passages. These metal-rich deposits formed when ascending warm saline waters mixed with descending oxidized cold waters in the deep phreatic zone. Time: Oligocene?

(3) Calcite spar episode. Calcite spar crystals are found lining the walls of a number of GC caves. Since they line cave passages, they must be younger than these passages. Large calcite spar crystals are known to form from low-temperature hydrothermal solutions under quiet phreatic conditions. Time: Miocene?

(4) Mammillary and replacement gypsum episode. Mammillaries, consisting of microcrystalline fibrous calcite, are a speleothem type that forms in the shallow-phreatic zone just below the water table. Replacement gypsum rinds form at or just above the water table where degassing H2S reacts with wet limestone. These two cave deposits can be used to determine past water table positions in the Redwall Limestone as well as incision rates for the GC. Time: Late Miocene-Pliocene in the western GC to Pliocene-Pleistocene in the eastern GC to the present in Marble Canyon.

(5) Subaerial speleothem episode. Speleothems such as stalactites and stalagmites record when GC caves became air-filled. Most of these speleothems are very old, surpassing the limit of U-series dating. Time: Pliocene-Pleistocene. U-Pb and U-Series dating of mine calcite, calcite-spar cave linings, water-table mammillary calcite, and subaerial speleothem calcite should provide an absolute time scale for the history of water table lowering in, and incision of, the GC.