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.’
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).
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?
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.