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Post-Tectonic Landscape Evolution in Southeastern Arizona: When Did a River Start to Run Through It?

Article Author(s): 

Matthew C. Jungers

 

Arizona’s more evolved army of caterpillars…

In 1886, renowned American geologist Clarence Dutton described the western portion of North America as follows:

‘The great belt of cordilleras coming up through Mexico and crossing into United States territory is depicted as being composed of many short, abrupt ranges or ridges, looking upon the map like an army of caterpillars crawling northward. At length, about 150 miles north of the Mexican boundary, this army divides into two columns, one marching northwest, the other north-northeast. The former branch becomes the system of mountain ridges spread over the southern and western portions of Arizona, the whole of Nevada and the western portion of Utah and extending into Oregon and Idaho.’

Figure 1. Area of interest in southeastern Arizona. This region is the southeastern extent of the larger Basin and Range physiographic region of North America. The Gila River flows northwest through the Safford Basin within this region, and both the San Pedro River and Santa Cruz River flow into the Gila River northwest of the Galiuro Mountains. The deposits of interest for this study are: 1) The uppermost, undeformed basin fill surfaces adjacent to mountain range fronts common throughout the region’s main basins; 2) Upper most sedimentary basin fill sediment, best exposed in the Sonoita, Upper San Pedro, Lower San Pedro, and Safford Basins; 3) Quaternary terraces best preserved in the Sonoita, Santa Cruz, and Safford Basins.The western column of Dutton’s army is now more commonly referred to as the Basin and Range physiographic province of the North American Cordillera (Dickinson, 2004), and the northward crawling caterpillars are the topographic expression of extensional tectonics that define the region. Bedrock ranges have been uplifted along normal faults, and adjacent, intervening basins have subsided and filled with sediment. Many structural basins, particularly in the northern part of the Basin and Range, are still actively subsiding and internally drained, having no fluvial connection to adjacent basins. If we follow the western column of the caterpillar army south and east to where it spills across the modern United States-Mexico border, we’ll find ourselves nestled in the southeastern corner of Arizona at the southern extent of the broader Basin and Range province. Extensional tectonics are not very active on a regional scale in southeastern Arizona, and most basins that were formerly internally drained are now integrated into the Gila River system.

Southeastern Arizona’s Basin and Range is composed of high relief, rugged mountain ranges – e.g., the Santa Catalinas, Rincons, Galiuros, Pinaleños, Santa Ritas, Chiracahuas – that are separated by intervening tributaries of the Gila River (Figure 1). The rocks of the ranges can be quite distinct from one another - mid-Tertiary volcanics can happily sit across a basin from a mid-Tertiary metamorphic core complex - but they share a common history of deformation by steep angled normal faulting during the Basin and Range Disturbance ~12 to 8 million years ago (Menges and Pearthree, 1989). During this episode of normal faulting, the region would have looked much like the modern Basin and Range province to the northwest: bedrock islands in a sea of internally drained sedimentary basins. However, after regional rates of extension decreased dramatically in the last few million years, these isolated basins began to integrate with one another through a combination of headward stream capture and spillover events that combined to create the modern Gila River drainage system (Figure 2).

Figure 2. Final stages of the Basin and Range disturbance. (A) Structural basins were filled with sediment, and most basins were still internally drained. (B) Following the cessation of extensional tectonics in the region, basins continued to fill with sediment and faults were buried. Basins began to integrate with the mainstem Gila River via a combination of basin spillover and headward drainage capture. (C) Following integration with an adjacent basin, sedimentary fill was incised as its basin adjusted to a new, lower baselevel. (D) As a new, through-flowing drainage network was established, integrated basins graded to the Gila River. The shift to an oscillating climate in the Quaternary may be preserved in flights of terraces that record alternating periods of floodplain stability followed by rapid incision. Figure adapted from Menges and Pearthree, 1989.The goal of my current research at Arizona State University is to discern the pace and pattern of these late Tertiary or early Quaternary drainage integration events, and quantify the magnitude of incision and erosion that can be attributed to integration events alone. Additionally, since the Gila River integrated to its modern form, it has experienced periods of punctuated incision through the Quaternary that may have been driven by the more dramatically oscillating climate during that time. We also hope to discern whether Quaternary incision was driven by a regional climate signal, or, alternatively, in response to sedimentation perturbations unique to the distinct fluvial systems within the broader Gila River network. In achieving these goals, we may provide insight into the future evolution of the still internally drained Great Basin to the north and all the smaller basins and ranges that it contains.

How is the timing of Gila River drainage integration preserved?

Figure 3. (A) Northern edge of Davis Mesa, a basin high stand remnant on the east side of the Santa Catalinas in the Lower San Pedro Basin. The Davis Mesa remnant is composed of 10s of meters of coarse grained alluvium deposited on top of an erosional surface into older, deformed basin fill. The remnant preserves the original depositional slope and aspect of the basin’s high stand piedmont. (B) Standing on the proximal end of the remnant and looking east across the Lower San Pedro valley toward the Galiuro Mountains, one can get a sense of what the basin may have looked like prior to widespread dissection.When two adjacent basins, at different altitudes, undergo a drainage integration event (i.e., an event that joins two previously unconnected drainage networks), one basin experiences a sudden lowering of base level for its fluvial network. Streams incise and remnants of the basin’s uppermost surface get left behind atop exposures of late stage basin fill (Figure 3). Exceptionally well exposed and continuous sedimentary basin deposits in southeastern Arizona’s Basin and Range province preserves a record of landscape response during a more stable, wetter Pliocene climate and a subsequent shift to a drier, more cyclical climate in the Pleistocene. The age of high stand remnants should provide a maximum age for the timing of drainage integration and subsequent basin fill dissection in each basin.

Pleistocene terraces inset into incised basin fill deposits provide further insight into more recent rates of incision and how they might vary with climate cycles. Basin high stand remnants in southeastern Arizona are prominent and striking deposits throughout the region, and as such they have attracted the attention of geologists from the early 20th century through the present (e.g., Bryan, 1926; Davis and Brooks, 1930; Dickinson, 1991). However, the ages inferred for loosely correlative basin high stands have ranged from ~20,000 years ago (Melton, 1965) to 1.0-2.2 Ma (Menges and McFadden, 1981; Morrison, 1985). While it is unlikely that each basin’s high stand would have been preserved at the same time since the timing of tributary integration with the Gila River was not simultaneous (Dickinson, 1991), the potential for a regional similarity in climate driven drainage integration remains, as does the potential to identify basin high stands that have not yet experienced widespread incision (Melton, 1965; Dickinson, 1991).

An absolute timeline for the final stages of sedimentary basin aggradation and subsequent basin fill dissection is necessary to resolve the effects of Late Cenozoic climate on regional basin aggradation rates and drainage integration pace and pattern. Stratigraphic relationships and soil development support an age of 1 Ma or older for some of these basin fill surfaces (Menges and McFadden, 1981), but no absolute dates exist for these basin high stand remnants. If these undeformed surfaces and underlying basin fill do indeed date to the Early Pleistocene or Late Pliocene, then they represent a potential bridge in understanding between Late Miocene-Early Pliocene Basin and Range tectonics and post-tectonic landscape evolution. In short, establishing unambiguous ages for these deposits and the underlying strata may help provide just the constraints needed to untangle the complex coupling between climate and tectonic forcing of the landscape.

Matt Jungers

Ph.D. Candidate and ASU/NASA Space Grant Fellow, School of Earth and Space Exploration, Arizona State University, Tempe, Arizona

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