Abstract

The Cenozoic paleogeography of Arizona is interpreted from the sedimentary and volcanic strata throughout the state. Paleogeographic features such as mountains, plateaus, depositional basins, and drainage systems are interpreted within the constraints of known structural features of Laramide age, and in some places, the depth of erosion into the pre-Cenozoic basement rocks. The absence of Paleocene sediments and the pronounced unconformity beneath Cenozoic rocks attests to an epeirogenic uplift of the region during the Laramide orogeny. This uplift was most pronounced in central and western Arizona. The resultant erosion produced a beveled surface that cut deeply into Precambrian rocks of central Arizona and removed much of the Mesozoic cover of the future Colorado Plateau. Paleocene drainage systems transported the sediments of that age northward to the Uinta basin of Utah and eastward to the San Juan basin of New Mexico, along the downwarped Coconino and Baca-Eager basins that were formed as a synclinal trend adjacent to the Mogollon Highlands. The northward regional drainage was controlled by the Sevier foreland basin in Utah.

During the Paleocene and Eocene, compressional deformation of the Laramide orogeny uplifted the Rocky Mountains, which interrupted eastward drainages and converted the Colorado Plateau to a very large internally drained basin during the Eocene. It was bounded on the west by the Sevier thrust belt, on the north by the Uinta uplift, on the south by the Mogollon Highlands and by the Rocky Mountains to the east. That same deformation created numerous north-trending anticlinal and synclinal folds, thereby converting the Colorado Plateau into a synclinorium. These interior folds controlled the development of drainage patterns throughout the remainder of the Tertiary.

Eocene strata on the southern Colorado Plateau are typically coarse-grained, well-sorted conglomerates, with clasts of Precambrian igneous and metamorphic rock whose provenance was the Mogollon Highlands of central and southern Arizona. These strata were deposited in elongate synclinal and erosional valleys north of the Mogollon Highlands by streams that flowed northward into the early Tertiary Coconino and Baca-Eager basins on the southern margin of the Colorado Plateau. The coarse clastics were deposited at the southern margins, and fine-grained clastic and carbonate sediments accumulated in the more distal, northern parts of the basins. Drainage patterns were controlled by Laramide anticlinal uplifts and synclines.

In southern and western Arizona, Paleocene volcanic rocks are common, but no sediments of Paleocene or Eocene age are known. Eocene volcanic rocks are rare in Arizona. Oligocene sediments and volcanics occur in uplifted fault blocks of present-day mountain ranges and in the subsurface of several sedimentary basins. These sedimentary rocks include coarse-grained, angular, poorly-sorted redbeds that were deposited as fanglomerates adjacent to local highs that were formed by Laramide and middle Tertiary orogenesis. Fine-grained clastic, carbonate, and evaporite sediments are intercalated with those basin margin fanglomerates in some areas, suggesting deposition in the center of closed basins. Late Oligocene-Early Miocene strata include fanglomerates and fine-grained fluvial and lacustrine sediments with clasts and flows of silicic to intermediate volcanic rocks reflecting the onset of extensive volcanism and structural warping of the mid-Tertiary orogeny. Middle Miocene sediments and volcanics were affected by low-angle normal faulting, subsequent folding, and unroofing of metamorphic core complexes.

Late Miocene-Pleistocene sedimentary and volcanic rocks are generally confined to basins that coincide with modern valleys throughout the state. Late Miocene to Holocene strata in the Basin and Range Province were deposited in normal-fault bounded extensional basins initiated by the Basin and Range disturbance. In southern Arizona basin deposits typically exhibit gradation from marginal fanglomerates to fine-grained clastic, carbonate, and evaporite sediments in the centers of closed basins. By late Miocene time, the integrated drainage system of the Gila, Salt, Verde and other tributaries to the lower Colorado River had developed, and dissection of the higher basins (e.g., Chino Valley, Verde, Tonto, and Safford basins) had begun in response to the lowered base level established by the opening of the Gulf of California in the late Miocene. The Pliocene-Pleistocene paleogeography of Arizona was similar to present topography except for a few Holocene volcanic mountains and increased dissection of eroded stream valleys.