Diagenesis of Lime Mud, Mississippian-Age Bioherms, Sacramento Mountains, New Mexico: ABSTRACT

Abstract

Samples from six Waulsortian bioherms were examined from the Mississippian Lake Valley Formation, Sacramento Mountains, New Mexico, to determine the timing and mechanisms of lime-mud cementation and to evaluate the role of this cementation in biohermal stabilization and growth. Petrography and cathodoluminescence of bioherm and interbioherm muds defined distinct diagenetic mud types. Each mud type is characterized by a distinct grain size, mud color, and cathodoluminescence. Bioherm type 1 mud is dark gray, fine-grained, End_Page 559------------------------------ (Y = 3.7 µm) and has a dull luminescence. Type 2 mud, which coexists with type 1 mud, is coarse grained (Y = 5.7 µm), lighter gray, and has a brighter luminescence. Type 3 mud is darker gray, coarser-grained (Y = 6.7 µm), and is nonluminescent. Type 4 mud is gray brown, and is the coarsest grained (Y = 8.8 µm) and most brightly luminescent of all bioherm mud. For comparison, interbioherm mud has the finest mean grain size (Y = 3.36 µm and the brightest luminescence of all mud types examined. It is proposed that synsedimentary marine cementation of biohermal muds and within synsedimentary stomatactis is the causative factor for bioherm stabilization and growth. Dark gray nonluminescent bioherm muds reflect a significant component of synsedimentary marine cements while lighter gray luminescent bioherm muds reflect a higher proportion of post-burial intergranular cements. This interpretation is supported by the oxygen and carbon isotopic composition of the lime-mud types. For example, synsedimentary stromatactis cavities lined by fibrous marine cements are constructed from nonluminescent type 3 muds which possess the heaviest isotopic signatures (-1.8 to -2.7 ^pmil^dgr18O; + 4.81 to +4.1 ^pmil^dgr13C). In contrast, cavities infilled by post-burial granu ar cements are associated with luminescent mud types 1, 2, and 4, which possess lighter isotopic signatures (-2.3 to 14.4 ^pmil^dgr18O; +2.0 to +4.2 ^pmil^dgr18O). Interbioherm muds, interpreted to have lithified at a later stage than bioherm muds, possess lighter isotopic signatures (-3.8 to -6.7 ^pmil^dgr18O; +2.3 to +2.5 ^pmil^dgr13C). Because of the heterogeneous distribution of synsedimentary cemented muds within the bioherms, the formation of the stromatactis cavities characteristic of Waulsortian bioherms is interpreted to occur from the slumping and subsequent removal of uncemented grains from beneath patches of early marine cemented mud. As such, this study emphasizes the role of early marine cements in fine-grained sediments as an active process which merits careful consideration in determining original porosities and diagenetic histories in ancient fine-grained carbonate facies. End_of_Article - Last_Page 560------------