Chapter 3: Geology of the Santa Fe Group Aquifer System
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Chapter 3: Geology of the Santa Fe Group aquifer system The Santa Fe Group aquifer system, which supplies the ground- An aquifer is defined as “a rock unit water resources for the Albuquerque metropolitan area and surrounding that will yield water in a usable quantity communities within the Middle Rio Grande Basin, is composed chiefly of to a well or spring” (Heath, 1983). An sand and silt with lesser amounts of clay and gravel. Most of the sediment aquifer system is two or more aquifers was transported into subsiding fault-bounded basins of the Rio Grande Rift that are separated (at least locally) by impermeable rock units but function by ancient streams and rivers that drained the surrounding regions. Addi- together as an aquifer with regional tional deposition was a result of windblown sediment (eolian deposits), extent. The impermeable or low- intermittent streams, and downslope creep from mountain uplifts. Because permeability rock units that bound these geologic processes have been active in the Middle Rio Grande Basin aquifers are confining beds. for millions of years, the Santa Fe Group locally is thousands of feet thick. The varied processes and the available sediment combine to form a complex, three-dimensional framework of truncated and overlapping sedi- mentary units of contrasting hydrologic properties. (See Box B.) The Rio Grande Rift Rock units may be classified and mapped on the basis of many different criteria including lithology, magnetic The geologic processes at work in this part of New Mexico have polarity, age, and depositional environ- guided and controlled formation of the Santa Fe Group aquifer system. ment. The most common method is to More than 25 million years ago, tectonic forces operating on the western classify rock strata of about the same age and similar physical characteristics North American continent began to pull apart the brittle upper crust, and into formations. Formations may be the continent began to spread and extend toward the Pacific Ocean basin subdivided into members and beds or (Chapin and Cather, 1994; Pazzaglia and others, 1999). In New Mexico aggregated into groups. By convention, and Colorado, these forces created a more-or-less continuous north-south the formal name of the unit is a structural zone called the Rio Grande Rift (fig. 3.1). This rift formed in geographic feature near the type expo- sures (or outcrops) of the rock unit. northern Mexico and southern New Mexico and slowly continued to split Geologic maps typically portray the and extend toward the north as the Colorado Plateau block on the west surface extent and structure of such separated from the North American continental interior block on the east. stratigraphic units. However, young Since its inception, the floor of the rift has subsided, sporadically and unconsolidated deposits are generally unevenly, to local depths greater than 14,000 feet as the continental crust mapped and classified primarily by depositional environment and material east and west of the rift continues to slowly pull apart (Grauch, Gillespie, properties. and Keller, 1999). The Rio Grande Rift is a zone of faults and sediment-filled basins extending along its length. Some faults appear to follow ancient lines of weakness in the brittle crust that are related to older geologic events, but many of the faults cut through older rock along new, crosscutting traces. These faults caused pieces of the brittle crust to pull apart and slide past each other; blocks on the inside of the rift zone generally dropped down under the influence of gravity relative to (uplifted) blocks on the outside A rift is an elongated valley in the edges of the rift. In this way, the rift has created a series of valleys along its Earth’s crust that forms in response to length that have influenced the flow of streams and sediment deposition. the crust extending or spreading apart. Faulting, uplift, volcanic activity, subsidence, and deposition rates have The Rio Grande Rift forms one of the varied through time in the rift. Nonetheless, the overall geologic processes great rift valleys of the world, on a scale have remained fairly constant. Geologic and geophysical data show that similar to the East African rift and the Lake Baikal rift in Russia. the rift consists of numerous discrete structural basins that mark the loca- tions where faulting and subsidence rates and displacements have been greatest overall (see Box C). 23 B Structural, sedimentologic, and climate effects on the Middle Rio Grande Basin aquifer system James C. Cole1 and Byron D. Stone2 The interplay of faulting and sedimenta- 106˚45' 106˚40' 106˚35' tion within the Rio Grande Rift has controlled major aspects of aquifer geom- 35˚10' Albuquerque etry. Basin-fill sediment thickens substan- volcanic field tially toward the center of the basin. COORS BLVD Outcrop and drill-hole studies show that basin-fill sediments thicken abruptly Rio Grande 25 Llano de 4TH ST Inner across major faults, indicating that faults Albuquerque valley 2 BLVD SAN MATEO were active while sediment was accumu- 1College 40 lating (fig. B.1). Rising fault blocks were 98th Street eroded, and the detritus was deposited on A A' 2 35˚05' CENTRAL AVE the sinking fault blocks in a process that 3 4 1 West recycled older sediment within the rift or Mesa 2 stripped material off the rift-flank moun- 1 Don VD tain ranges. The rising and falling fault 1 Leavitt Map 2 COORS BL blocks also shifted the internal stream 3 area drainage in the subbasins and caused various depositional environments (such A 0 1 2 3 MILES as river channel, overbank flood plain, 0123 KILOMETERS alluvial fan, and pediment) to migrate laterally over time (Cather and others, well 1994; Stone, 2001). In this dynamic well well setting, the types of sediment that accu- A A' 98th Street well West Mesa no. 1 College no. 1 College no. 2 Land mulated across the rift basins were highly surface varied, and the location of their deposition changed as the basin filled. Erosion on the uplifted blocks created breaks in the sedi- ? Atrisco beds mentary record that make it difficult to Approximately Atrisco beds 380 feet thick correlate sequences of strata from one Approximately location to another. 740 feet thick The Middle Rio Grande Basin consists of INCREASING DEPTH three discrete subbasins distinctly sepa- rated by zones where the rocks underlying the basin-fill sediment (basement rocks) are high (fig. 3.2). Each subbasin has a B Modified from Allen and others (1998); unique structural and depositional history. Stone and others (1998) Drainage was internal to each subbasin EXPLANATION 1 Fault; bar and ball Production well, 98th Monitoring wellA A' Line of section during the early history of the rift valley, on downthrown side Don well-field name, Street as indicated by the type of sediment and well number Figure B.1.—Generalized section showing substantial thickening of the Atrisco silt- and clay-rich beds in the downthrown fault block east of the 1U.S. Geological Survey, Denver, 98th Street well location (A) and index map (B). All wells except the Colorado. 2 98th Street well are projected to the line of section, parallel to the northwest- U.S. Geological Survey, Storrs, trending fault trace. Connecticut. 24 deposited; thus, no continuous hydrostratigraphic deposit in the deeper cobble gravel are common in the younger parts of the Santa Fe Group aquifer system extends across all three deposits, indicating that streams trans- subbasins. porting these materials had greater The youngest sediments that were deposited during roughly the last 5 discharge and flowed at greater velocities. million years can be traced across the buried basement highs and the deep The change from fine- to coarse-grained subbasin centers. These deposits differ from the older rift-fill deposits in sediment is evident on a regional scale ways that reflect significant changes in rift history and climate. First, these and is consistent with regional and world younger deposits are thinner (for a specific interval of time), indicating evidence that rainfall and runoff increased that the rates of subsidence began decreasing at least 5 million years ago. substantially beginning about 5 million Second, the younger deposits are more tabular in form across all three years ago (Krantz, 1991; Thompson, subbasins and show less variation in thickness across faults, indicating that 1991). These younger deposits locally are faulting was less active during deposition. Some subhorizontal younger productive zones of the aquifer in the sedimentary layers were deposited above erosion surfaces on top of Middle Rio Grande Basin, but they inclined older deposits, consistent with a decline in the rate of tilting. compose only a small percentage of the Third, the younger deposits are only more than a few hundred feet thick rift-fill deposits and are not laterally along the central axis of the subbasins (Hawley, Haase, and Lozinsky, extensive. These deposits are currently 1995; Connell, Allen, and Hawley, 1998), showing that active faulting and being eroded in the Rio Grande drainage. subsidence occurred in a much narrower part of the rift than earlier in the Rift faults remain active today and rift's history. These younger deposits show that the supply of sediment was continue to influence sedimentation. greater than subsidence; thus, stream deposition was able to fill the sepa- Outcrops along the western and northern rate subbasins to form a connected, broad basin (Cather and others, 1994; basin margins show sequences of alluvial Chapin and Cather, 1994; Pazzaglia and others, 1999). and windblown sediment that are thickest The youngest sedimentary deposits also consist of coarser grained mate- on top of the tilted, downdropped blocks rials than most of the earlier rift-fill deposits.