Basinal Marine Dolomicrite from the Pennsylvanian/Wolfcampian Horquilla Formation, New Mexico

Abstract

ABSTRACT A 6-m-thick sequence of black dolomicrite occurs within a shaley, basinal portion of the Pennsylvanian-Wolfcampian Horquilla Formation, New Mexico. Deep-water lithologic associations, absence of hypersaline and freshwater indicators, elevated Na and Sr contents, and marine-derived 18O values all indicate that the dolomite precipitated from marine-derived water during burial beneath the sediment-water interface. Trace and major element contents and oxygen isotopic compositions of the dolomites exhibit significant covariance. Sr, Na, and 18O all covary positively with excess Ca in the dolomite structure, and Fe and Mn covary negatively with excess Ca in the dolomite structure. Such variations are suggestive of a diagenetic alteration trend ranging from primary marine compositions preserved in calcian dolomite to diagenetic compositions in more stoichiometric dolomite. Moreover, dolomite which preserves primary 18O values exhibits only minor variation in 13C. This is in contrast to the highly variable 13C values observed in Tertiary deep-water dolomites where sulfate reduction during organic diagenesis plays a key role in promoting dolomitization. Thus, depletion of sulfate was not a prerequisite for dolomitization of basinal carbonates of the Hor uilla Formation. Rather, dolomitization was promoted by the metastability of the precursor mineralogy. While such a mechanism has been postulated for dolomitization of marginal slope facies of oceanic atolls, this study extends its application to burial-marine dolomitization of deep-water micritic sequences.