Abstract

Major discharge from springs issuing from a large carbonate-rock aquifer beneath the southwestern Colorado Plateau in the vicinity of the modern Grand Canyon, north western Arizona, deposited the Hualapai limestone along Grand Wash Cliffs at the Colorado Plateau-Basin Range boundary (herein proposed). Deposition of the limestone ended at 5 Ma, concurrent with the first appearance of surface water of the Colorado River at Grand Wash Cliffs. The River deposited its earliest gravel conformably on top of the Hualapai limestone. The spatial coincidence and chronologic succession of these events and deposits strongly suggests that the aquifer was a precursor of the modern Colorado River in its Grand Canyon reach. Evidence that a Late Miocene aquifer preceded the cutting of Grand Canyon is as follows:

(1) The Hualapai limestone, 11-5 Ma (M. A. Wallace, 1999), has the sedimentary characteristics of a carbonate, supersaturated, spring-discharge deposit. (2) Preliminary Sr isotope data suggest a similarity between the high radiogenic Sr content of the Hualapai limestone and that of modern aquifer water transmitted through Paleozoic carbonate rocks in and near the Grand Canyon (see abstract Part II, Schmidt, this volume). (3) Oxygen isotope data suggest a nonevaporative deposition of the Haulapai limestone. (4) In contrast, gypsum and halite, normally expected in solution in the paleoaquifer and spring discharge, are found as large evaporative salt deposits downslope from the Hualapai limestone.

(5) A large potential gradient favored a west-flowing aquifer after about 16 Ma when extensional deformation greatly lowered the Basin-Range relative to the Colorado Plateau. By about 11 Ma, this gradient was fully utilized by the paleoaquifer that deposited the Hualapai limestone, and at 5 Ma, its average gradient was a measurable 3–4 m/km over a distance of 200 km between the Kaibab Upwarp-Bidahochi basin and Grand Wash Cliffs. 6) Fractures and joints in the thick Paleozoic-carbonate strata of the southwestern Colorado Plateau seem adequate for efficient, fracture-controlled aquifer flow. (7) An adequate fracture network is also suggested by the probable existence of older regional aquifers that had previously utilized most fractures used by the Late Miocene aquifer. (6) By about 11 Ma after Oligocene-Miocene reversal of surface drainage on the southern Colorado Plateau, abundant new surface water from the upper Colorado River Basin flowed into the Bidahochi basin and recharged into the Kaibab limestone, earlier on the east side, and later on the west side of the Kaibab Upwarp.