SPATIAL AND TEMPORAL METEORIC DIAGENETIC PATTERNS OBSERVED IN PENNSYLVANIAN ALGAL MOUNDS, HOLDER FORMATION, NEW MEXICO

The Holder Formation has been intensively studied from a petrologic and sedimentologic perspective with these studies focusing largely on the cyclic shelf sedimentation and the presence of large phylloid-algal bioherms (e.g. Toomey et al., 1977; Wilson, 1967). Although geochemical analyses of this unit have also been undertaken (e.g. Fernberg, 1987; Goldstein, 1991), these studies have primarily examined diagenetic alteration associated with paleosols and zones affected by subaerial exposure. This study examines material obtained from a stratigraphic section through one of the unit's main bioherms, Yucca Mound, as well as interbiohermal and flanking material lateral to the bioherm to document the multiplicity and stratigraphic extent of meteoric alteration events.

Diagenetic alteration associated with subaerial exposure of the mounds has been determined by δ¹³C and δ¹⁸O analysis coupled with elemental analysis of four discrete phases of meteoric calcite cement delineated through cathodoluminescence petrography. Stable isotopes define two distinct meteoric calcite lines (Lohmann, 1988): one cement phase lies at -5.5‰ δ¹⁸O_VPDB and corresponds to values reported by Goldstein (1991); the second cement phase lies at -4.9‰ δ¹⁸O_VPDB. In each cement phase the Mg/Ca and Sr/Ca ratios increase from 0.003-0.03 and 0.00015-0.0014 (mol/mol) respectively as distance increases down-section from the exposure surfaces.

Combined petrographic, isotopic, and elemental analyses suggest that mound carbonate experienced at least two separate phases of meteoric diagenesis caused by waters of differing initial compositions, which correlate with the two episodes of exposure described by Wilson (1967). Furthermore, elemental analyses have led to a model that helps to constrain which portions of the mounds were altered under closed system versus open system diagenesis, and during which intervals of their depositional histories such conditions were the dominant diagenetic processes. Cements with the highest Sr and Mg contents suggest a closed system that retained a record of these incompatible elements within meteoric calcite. This model also provides insight into the dynamics of fluid flow and reaction kinetics during alteration by meteoric waters.