Paleokarstic Features in Mississippian Limestones, New Mexico

Abstract

Paleokarst features in Mississippian limestones of southern New Mexico occur beneath the sub-Penn- sylvanian unconformity over an area greater than 20,000 km². These paleokarst features pervade the upper few meters to few tens of meters of the crinoidal calcarenites of the Lake Valley and Kelley formations, and occur in a vertical profile that shows progressively more intense dissolution upward. This profile consists of, in ascending order: (1) etching of pre-Pennsylvanian freshwater phreatic syntaxial cements; (2) micrite and microspar plugging of in- tergranular pores, filling around crinoids, bryozoans, and pre-Pennsylvanian freshwater syntaxial cements; (3) clay and detrital quartz silt plugging intergranular pores and filling around etched crinoids and etched pre-Pennsylvanian freshwater syntaxial cements; (4) fragmented host limestone forming rubble-and-fis- sure fabrics composed of nodules of host rock that are surrounded by anastomosing veinlets and fissures filled with “weathering calcarenite,” clay, and detrital quartz silt; and (5) chert breccia composed of chert fragments derived from the underlying Lake Valley/ Kelley formations within a matrix of clay, quartz silt, and sand-sized chert fragments.

The paleokarst profile is interpreted as having developed mainly in the vadose zone on crinoidal limestones that were partly cemented but had high intergranular porosity and permeability. The chert breccias are interpreted as insoluble residues from intense weathering of Lake Valley/Kelley formations limestones plus extraformational clay and detrital quartz. The rubble-and-fissure fabrics resulted from dissolution fragmentation, and the intergranular clay and quartz comprise eluviated sediment carried downward by percolating vadose waters. The intergranular micrite and microspar are interpreted as a combination of neomorphic microspar and eluviated carbonate sediment. This eluviated sediment was possibly derived from chemical disintegration of host rock carbonate in the rubble-and-fissure zone and from remobilized primary depositional mud.

Karstification was most intense in the western and northwestern parts of the study area, a regional trend that correlates with intensity of other diagenetic features. These regional trends attest to a chemically more aggressive groundwater system in the west and northwest than in the east during pre-Pennsylvanian freshwater diagenesis and karstification. The restriction of much of the alteration in the Lake Valley/Kelley formations to intergranular pores and relatively small dissolution cavities within a relatively thin interval (meters to tens of meters), contrasts with the large bedding- and joint-controlled caves and passages extending over thick intervals (hundreds of meters or more) characteristic of conventional extant karst. This difference is due in part to the high intergranular permeabilities and porosities of the Lake Valley/Kelley formations crinoidal calcarenites during karstification. As such, the Lake Valley/Kelley formations may provide a model applicable to karstification of other carbonates during early stages of their diagenetic histories while they still have high intergranular permeabilities.