Diagenesis of Strawn limestone, South Carlsbad Field, Eddy County, New Mexico

Strawn bioherms in South Carlsbad Field represent carbonate mud mounds formed by sediment trapping and baffling organisms, which colonized topographically higher areas of the sea floor. Communities of fenestellid and cyclostome bryozoa created wave-resistant shelters along the Early Strawn shelf margin, behind which dense populations of the phylloid algae Ivanovia thrived. Crinoids, blastoids and siliceous sponges inhabited slightly deeper regions along mound flanks. The mound core was the site of deposition for micrites abundant in aragonitic and calcitic skeletal grains. Skeletal grainstones were deposited along mound flanks and in intermound channels. Relatively soon after deposition biohermal carbonates were subaerially exposed during a eustatic lowering of sea level and were subjected to mineralogic stabilization in vadose and phreatic fresh water environments. Porosity development and preservation reflect interaction of mixed mineral assemblages with vadose-phreatic groundwater dynamics. Porosity within mound flank and intermound grainstones was rapidly occluded by epitaxial cement growth on monocrystalline echinoderm components. Development of secondary moldic and fracture porosity occurred during dissolution of skeletal aragonite. Distribution of remaining primary fossil and sheltered porosity and secondary moldic porosity resulted from differential cementation rates, which in turn reflects (1) characteristics of the substrate and (2) length of exposure to vadose and phreatic diagenetic environments. Stabilization reactions in the vadose zone released calcium carbonate, which migrated downward and enriched phreatic solutions. Accelerated rates of stabilization and porosity occlusion in phreatic zones forced the upward movement of the groundwater table, and phreatic diagenetic fabrics were superimposed on earlier vadose features. Diagenetic fabrics in Strawn carbonates record a slowly rising groundwater table, exhibiting numerous fluctuations due to humid climatic conditions. Sediments subjected to predominantly phreatic diagenesis have retained little or no porosity and permeability, and these intervals are generally non-productive. Throughout the zone of groundwater table fluctuation, porosity preservation is strongly influenced by nature of the substrate for cement precipitation. Upper intervals were subjected to prolonged vadose diagenesis, and porosity in these intervals was preserved because no overlying donor of calcium carbonate was available.