National Park Service geologic type section inventory: Sonoran Desert Inventory & Monitoring Network
Authors: Henderson Tim , Santucci Vincent , Connors Tim , Tweet Justin 
Published: September 2022

Type sections are one of several kinds of stratotype. A stratotype is the standard (original or subsequently designated), accessible, and specific sequence of rock for a named geologic unit that forms the basis for the definition, recognition, and comparison of that unit elsewhere. Geologists designate stratotypes for rock exposures that are illustrative and representative of the map unit being defined. Stratotypes ideally should remain accessible for examination and study by others. In this sense, geologic stratotypes are similar in concept to biological type specimens; however, they remain in situ as rock exposures rather than curated in a repository. Therefore, managing stratotypes requires inventory and monitoring like other geologic heritage resources in parks. In addition to type sections, stratotypes also include type localities, type areas, reference sections, and lithodemes, all of which are defined in this report. The goal of this project is to consolidate information pertaining to stratotypes that occur within NPS-administered areas, in order that this information is available throughout the NPS to inform park managers and to promote the preservation and protection of these important geologic heritage resources. This effort identified six stratotypes designated within four park units of the Sonoran Desert Inventory & Monitoring Network (SODN): Chiricahua National Monument (CHIR) has three type areas; Coronado National Memorial (CORO) has one type area; Gila Cliff Dwellings National Monument (GICL) has one type area; and Saguaro National Park (SAGU) has one type area. Table 1 provides information regarding the six stratotypes currently identified within SODN parks. There are currently no designated stratotypes within Casa Grande Ruins National Monument (CAGR), Fort Bowie National Historic Site (FOBO), Montezuma Castle National Monument (MOCA), Organ Pipe Cactus National Monument (ORPI), Tonto National Monument (TONT), Tumacácori National Historical Park (TUMA), or Tuzigoot National Monument (TUZI). However, CHIR, MOCA, SAGU, and TUZI contain important rock exposures that could be considered for formal stratotype designation as discussed in the “Recommendations” section. The inventory of geologic stratotypes across the NPS is an important effort in documenting these locations so that NPS staff may recognize and protect these areas for future studies. The focus adopted for completing the baseline inventories throughout the NPS has centered on the 32 inventory and monitoring (I&M) networks established during the late 1990s. Adopting a network-based approach to inventories worked well when the NPS undertook paleontological resource inventories for the 32 I&M networks and was therefore adopted for the stratotype inventory. The Greater Yellowstone I&M Network (GRYN) was the pilot network for initiating this project (Henderson et al. 2020). Methodologies and reporting strategies adopted for the GRYN have been used in the development of this report for the SODN. This report includes a recommendation section that addresses outstanding issues and future steps regarding park unit stratotypes. These recommendations will hopefully guide decision-making and help ensure that these geoheritage resources are properly protected and that proposed park activities or development will not adversely impact the stability and condition of these geologic exposures.