New Mexico Geological Society Annual Spring Meeting — Abstracts

The earth fissures, which are located within a 60 sq mi area south of Deming, New Mexico (T.25 S., R.9 W., and T.24 S., R.9 W.), are tensional features occurring in unconsolidated Quaternary alluvium. Mapping of the fissures by surveying and by using aerial photographs revealed two types of fissure patterns; curvilinear and polygonal. Curvilinear fissures are up to 2000 ft in length. Polygonal to semi-polygonal fissures consist of a series of fissures interconnected at junctures. Individual polygons range from 100 to 600 ft across. The fissures open with 0.25 to 1.0 in. of displacement, and widen by piping and erosion from surface runoff. The average width of fissures along their length is about 2 to 2.5 ft, although some fissures are as much as 20 to 30 ft wide. The maximum measured depth is 42 ft. Earth fissures have damaged roads, streets, farmland, wells, irrigation-distribution systems, and are a safety hazard to people, livestock, and structures.

Increased pumping for irrigation since 1915 has removed water from storage, lowering the water table by as much as 120 ft, forming a circular cone of depression. The fissures are areally and temporally associated with water-level declines. A severe drought, from 1952 to 1957, coupled with increased pumpage, caused dewatering of fine-grained surficial sediments. Desiccation caused by dewatering of silty clays and clayey silts appears to be the main mechanism involved in the formation of polygonal fissures. Curvilinear fissures occur in silty sand and were probably formed by differential compaction of dewatered finer-grained sediments at depth.