JOURNAL OF SEDIMENTARYPETROLOGY, VOL. 48, NO. 2, P. 475"-488 Fits. 1-5, JUNE 1978 Copyright © 1978, The Society of Economic Paleontologists and Mineralogists MICRODOLOMITE-RICH SYNTAXIAL CEMENTS: PROPOSED METEORIC-MARINE MIXING ZONE PHREATIC CEMENTS FROM MISSISSIPPIAN LIMESTONES, NEW MEXICO I WILLIAM J. MEYERS ANDKYGER C. LOHMANN 2 Earth & Space Sciences Department State University of New York Stony Brook, New York 11794 ABSTRACT: Inclusion-richechinoderm-syntaxialcementscomprise upto38% oftheintergranularcements in Mississippian skeletal calcarenites of the Lake Valley Fro., N.M. These cements have crystal morphology, substrate selectivity, and luminescence characteristics identical to previously interpreted fresh water phreatic cements, yet contain microdolomite inclusions, implying that they were originally Mg calcite. These petrographic characteristics, plus the restriction of the microdolomite-rich syntaxial cements to the lower part of the Lake Valley Fro., argue for their having precipitated in the zone of mixing between meteoric and marine phreatic groundwaters. Petrography of these cements, their association with interpreted fresh water phreatic cements, and their regional restriction to the south, suggest that the mixing zone cements were formed mainly daring the slowing down stages of a late Mississippian global sea level drop. These cements plus other microdolomite-rich calcites reported on elsewhere indicate that coarse crystals of Mg calcite commonly act as closed or semi-closed systems during re-equilibration to low-Mg calcite, rather than undergoing complete loss of Mg through conventional incongruent dissolution. INTRODUCTION Modern research on carbonate cements has focussed on understanding processes of ce- mentation in marine and fresh water dia- genetic environments. An integral part of this broader goal has been development of criteria for identifying ancient cements pre- cipitated in the maj or marine and fresh water diagenetic settings. These studies have yield- ed important fabric criteria for identifying marine (e.g., Shinn, 1969), vadose (e.g., Dunham, 1971), and fresh water phreatic (e.g., Land, 1970) cements. Studies describ- ing distinctive petrographic criteria for recognizing cements precipitated from the mixing zone between fresh and marine phreatic waters are relatively rare. Those described from Quaternary carbonates are essentially marine cements (Schmalz, 1971; Moore, 1973) formed as shallow (a few meters) subsurface beach rock with mineral- ogy and fabrics identical to those of surface marine cements. Studies on interpreted mix- ~Manuscript received July 7, 1977; revised November 13, 1977. 2 Present address: Department of Geological Sciences, Brown University, Providence, R.I. ing zone cements from pre-Quaternary lime- stones are even rarer and have relied on Mg content (Oglesby, 1976) rather than on petrographic features for interpreting the cements. The purpose of this paper is to add to the catalogue of cement characteristics now used by carbonate petrologists to interpret diagenetic environments. Specifically we de- scribe echinoderm-syntaxial cement rich in microdolomite inclusions from the Missis- sippian Lake Valley Fm. of New Mexico. Furthermore, we propose a mixing zone origin for the cement and integrate its char- acteristics and distribution with a regional cementation model. Furthermore, we suggest that the criteria and interpretive approach may be widely applicable to coarse grained cements of other geologic ages. PHYSICAL STRATIGRAPHY OF MISSISSIPPIAN STRATA The stratigraphy and llthofacies of the Mississippian Lake Valley Fm. of South- western New Mexico, (Fig. 1) has been described by Laudon & Bowsher (1949), Armstrong (1962), and by Meyers (in press). Mississippian rocks within the study area comprise the Caballero Fm. (Kinderhoo-