Petrography, geochemistry, and diagenesis of the Mississippian carbonate mud-mounds and inter-mound limestones from the Alamogordo Member of Lake Valley Formation, Sacramento Mountains, New Mexico

The origin of carbonate mud comprising mud-mounds has long been debated, i.e., is the mud derived from inter-mound deposits or precipitated in situ. The carbonate mud in the mounds is petrographically similar to inter-mound mud. Thus, the major purpose of this dissertation was to determine whether geochemical distinctions exist between the mound and inter-mound mud and to decipher the origin of mud comprising the mounds.; Mud-mounds and inter-mound limestone are well exposed in the Alamogordo Member, Sacramento Mountains, New Mexico. Stable isotopic analyses demonstrate that the microspars (i.e., carbonate mud) comprising the inter-mound, mound, and neptunian dikes within the mounds have average δ13C values of 2.6, 3.4, and 4.8‰PDB, respectively. The average δ 13C values of well-preserved brachiopods from the inter-mound strata and Muleshoe mud-mound are 2.7 and 3.9‰PDB, respectively.; The similarity in δ13C values between the inter-mound brachiopods and intermound microspars as well as between the mound brachiopods and mound microspars indicates that the δ13C values of inter-mound and mound microspars are essentially primary. This interpretation is supported by the narrow range of δ13C values and wide range of δ18O values exhibited by the microspars. The inter-mound microspars are interpreted to have deposited in normal Mississippian seawater. The high δ13C values of the mound and neptunian dike microspars are attributed to precipitation in the Mississippian seawater mixed with the methanogenic (bacterially fermented organic matter) fluids. The fluids rose from the erosionally truncated surfaces of the underlying strata into the mounds. Within the mounds the 13C-enriched fluids also moved upward through neptunian dikes. The loci of mounds, therefore, are related to the seepage of methanogenic fluids.; Carbon and oxygen isotopes indicate that diagenetic fluids ascended into the intermound strata of the Alamogordo Member from an underlying paleoaquifer rather than directly descended from the overlying pre-Pennsylvanian exposure surface.; The Sr/Ca ratio of Paleozoic seawaters generally is believed to have been similar to modern seawater, however, the extremely low Sr concentrations in the well-preserved mound and dike microspars indicate that Mississippian seawater probably had a Sr/Ca ratio considerably lower than the present oceans.