ABSTRACT

Solution pockets in the Mississippian Lake Valley Formation of New Mexico occur below the sub-Pennsylvanian unconformity. Large masses of columnar calcite spar form within the solution pockets. The calcite shows fine growth banding, stalagmitic and stalaclitic fabrics, coarse detrital inclusions, and truncation of crystal terminations indicating a vadose zone or speleothem origin for the spar. The calcite is twinned and cross-cut by both stylolites and forroan calcite-filled fractures indicating deeper burial after precipitation near the surface.

The mean ¹³C of - 7.29 and ¹⁸O of -3.06 PDB of the speleothem calcite defines the light carbon end member of an inverted J-shaped curve of isotopic compositions of pre-Pennsylvanian components in the host limestone. The isotopic relationship between speleothems and pre-Pennsylvanian components suggests a pre-Pennsylvanian age for the speleothems. The isotopic composition and low-magnesium content of speleothem calcite reflects precipitation from meteoric water altered little by stabilization of components of the host rock.

Primary fluid inclusions are either one-phase, all liquid which indicate calcite precipitation below about 40-50°C, or two-phase with variable ratios of vapor to liquid. The two-phase inclusions formed by high-temperature reequilibration of low-temperature fluid inclusions, and entrapment of air and water in the vadose zone. The petrographic characteristics and one atmosphere pressure of some of the fluid inclusions suggest precipitation of speleothems in the vadose zone. Some secondary fluid inclusions are two-phase with rather consistent ratios of vapor to liquid. These inclusions indicate a late history of relatively high temperatures.

Of the primary fluid inclusions, 74% of the two-phase population have been refilled with brines of deeper burial, and 26% retain fresh water. For the one-phase fluid inclusions, 100% are fresh water. Therefore, the one-phase all-liquid inclusions in partially reequilibrated low-temperature fluid inclusion populations should be viewed as reliable indicators of temperature and salinity of cement precipitation in this and other diagenetic studies.