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.