Open-file Report - 548
Preliminary geologic map of the Carrizozo area, Lincoln County, NM

The Carrizozo area is mostly underlain by alluvium consisting of sand, gravel, silt, and clay that are less than 2.6 million years old. This alluvium mantles sedimentary bedrock that is 300-70 million years old. Sedimentary layers in the bedrock dip gently to the east towards the north end of the Sacramento Mountains. Fault lines offset these layered rocks, but the exact locations of some of these faults are poorly constrained due to the alluvial cover. During a time of intense volcanic activity, about 37 to 27 million years ago, the sedimentary bedrock was intruded by molten rocks; this produced a variety of igneous geometries that include stocks, sills, and dikes. Because igneous rocks are crystalline, we infer that groundwater flow through them is mainly along cracks and fractures.

The sedimentary bedrock was deposited by water in a variety of environmental settings -- including deep marine, immediately offshore of a beach, and rivers (in general, from oldest to youngest). The Mancos Shale was deposited in a deep marine setting. Its clay-rich composition inhibits groundwater flow and it is widely considered to be a poor to very poor aquifer. Conformably above the Mancos Shale lies the Gallup Sandstone, which was generally deposited immediately offshore of a beach and composed of sandstone. However, interbedded in the near-beach sandstones are deeper-marine shale layers that could act as barriers to groundwater flow within the unit. Accordingly, we postulate that the Gallup Sandstone may not be as productive of an aquifer as the overlying Crevasse Canyon Formation -- unless the two are displaced against one another along fault(s). This fault scenario seems to have occurred west of Carrizozo near Anchor Spring, where the Gallup Sandstone is in fault-juxtaposition against the Crevasse Canyon Formation and ground water is flowing out of the Gallup Sandstone. Northeast of Anchor Spring lie a series of aligned seeps that terminate at a northwest-trending dike (illustrating how a dike can act as a groundwater barrier). The Gallup Sandstone is more indurated in the Willow Hill area than at Carrizozo and this likely reduces the permeability of the formation there. The Crevasse Canyon Formation was deposited by rivers. Springs discharge from this formation in the map area (e.g., Bloom Spring and Lower Coyote Spring) and to the south (e.g., Milagro Spring and Jake Spring northeast of Oscura). Evidently, sandstone channel-fills in much of the Crevasse Canyon Formation (especially the mid-upper part) are suitably connected so that they can transmit groundwater. Although younger in age, the Cub Mountain and Sanders Canyon Formations were also deposited by rivers and locally host springs and seeps. In a practical sense, the Crevasse Canyon, Cub Mountain, and Sanders Canyon Formation may function as a single aquifer in which groundwater flows primarily in channel-fill sandstones (the mudstone-rich floodplain deposits being less transmissive). Based on inspection of well records, in some areas the basal part of the geologically young alluvium (unit Qa in the cross section) is saturated and screened by many wells in the Carrizozo area. The spatial distribution of this saturated alluvium may be controlled by upwelling of groundwater from underlying sedimentary bedrock of the Crevasse Canyon Formation.

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