Casa Grande Ruins National Monument: Geologic resources inventory report

Kellerlynn, Katie

This GRI report synthesizes discussions from a scoping meeting held in 2006 for Casa Grande Ruins National Monument (referred to as the “monument” throughout this report) and a follow-up conference call in 2018 (see Appendix A). Chapters of this report discuss the monument’s geologic setting and significance and draw connections between geologic and cultural resources, outline the geologic history leading to the present-day landscape, describe geologic issues facing resource managers, suggest future geologic investigations pertinent to the monument’s resources, and provide information about the previously completed GRI map data. This report, and the interpretation of the monument’s geologic history provided here, is supported by GRI GIS data of three compiled geologic maps of the Blackwater and Coolidge 7.5-minute quadrangles. The boundary of these two quadrangles runs through the monument. These geologic maps were produced by the Arizona Geological Survey (AZGS) and converted to the GRI GIS data model by the GRI team. A poster in the pocket of this report displays the GRI GIS data draped over a shaded relief image of the monument and surrounding area; it is the primary figure of this report. More information about the GRI GIS data is provided in the “Geologic Map Data” chapter. The GRI GIS data comprise 16 bedrock and 14 surficial map units. Source maps provided descriptions for all of the surficial map units but only two of the bedrock map units. Thus for this report, descriptions for the bedrock map units without them were compiled from mapping projects in proximity to the Blackwater and Coolidge quadrangles, for example, the Superior quadrangle, Gila River Indian Reservation, the Goldfield and the northern part of the Superstition Mts. SW quadrangles, and the Santan and Sacaton Mountains. Descriptions of all of the map units are provided in table 1. Figure 1 of this report is a geologic time scale based on the International Commission on Stratigraphy (ICS) international chronostratigraphic chart (International Commission on Stratigraphy 2018); the figure shows the associated geologic eras, periods, and epochs. Similarly, table 1 displays the map units of the GRI GIS data in a context of geologic time. Epochs listed in table 1 are from the GRI GIS data, with the exception of epochs associated with Tertiary (“T”) map units, which are from the US Geologic Names Lexicon (“Geolex”), a national compilation of names and descriptions of geologic units. Table 1 includes numeric ages from the ICS international chronostratigraphic chart as well as numeric ages for specific map units provided by source maps and other references. As discussed in the “Geologic Setting and Significance” chapter, a single map unit—river terrace and alluvium deposits (map unit Qi3r)—underlies the entire monument. This map unit is a late Pleistocene stream terrace (one of a series of flat or gently sloping surfaces in a stream valley, flanking the stream channel; a terrace is above the level of the stream and represents a former floodplain). The age of the monument’s terrace (Qi3r) indicates that the middle Gila River was flowing across the monument area approximately 130,000–10,000 years ago (Klawon et al. 1998; Richard et al. 2006). The terrace now sits 3–6 m (10–20 ft) above the Holocene floodplain (Qyr) (Huckleberry 1992), which in turn is above the modern stream channel (Qycr). Today, the middle Gila River is characterized by a wide streambed with a braided pattern of sandy and gravelly bars and channels. The streambed is entrenched and flanked by a floodplain (Qyr) and terraces (e.g., Qi3r). The Gila River is the unifying geomorphic feature of the monument and surrounding region (Huckleberry 1992). The seasonal flows and floods of the river set the rhythms of life for the Ancestral Sonoran Desert People (National Park Service 2011) who built the Great House and other prehistoric structures in the monument.

Casa Grande Ruins National Monument: Geologic resources inventory report

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