D I A G E N E S I S O F S A L I N E P A N H A L I T E: C O M P A R I S O N O F P E T R O G R A P H I C F E A T U R E S O F M O D E R N, Q U A T E R N A R Y A N D P E R M I A N H A L I T E S t

Petrographicstudies of modern saline pan halites (SalineValley,CA; Salina Omotepec, Baja California, Mexico)and Quaternaryshallow-buffed(0-200 m) halites(SalineValley,CA; BristolDry Lake, CA; SearlesLake, CA; Qarhan salinepan, Qaidam Basin, China; Lake Uyuni, Bolivia) show that the diagenetic modification of halite begins contemporaneouslywith deposition, is most intense within the upper few meters of burial, and is essentiallycomplete within the first 45 m of burial. Halite crusts from modern saline pans that have undergonerepeatedepisodesof flooding,evaporative concentration,and desiccationcontain abundant syndepositional diagenetic features. These "mature" modern halites are dominated by dissolution textures and fabrics (formed during flood stages) and cementation textures (formed during desiccation stages). Interlayeredmud beds contain varying amounts of displacive halite crystals. At shallow burial depths, halites retain many textural features of "mature" modern saline pan halite. Halites below the first few meters are no longer susceptibleto dissolution from floodwatersbut continue to be cemented by clear halite. Within the first 10 m of burial, cementation reduces the porosity of halite crusts to less than 10%. The remaining pore spaces are completely filled by burial depths of approximately45 m. Displacivegrowth of halite in muds continuesat shallow-burialdepths and is probablylimited to the first few tens of meters of burial depth. The mechanisms for cementation and clisplacivegrowth of halite at shallow-burial depths probably include (1) evaporative concentration of groundwaterbrines and (2) coolingof surface brines when they sink below the sediment surface. Undeformed halites from the Permian Salado and Rustler Formations of New Mexicocontain delicate syndepositionaltextures and abundant clear halite cements that are comparable to those observed in modern saline pan halites and shallow-buriedhalites. The Permian halites are interpreted to have undergone a depositional and early diagenetic history similar to the modern and Quaternary analogs. Completecementation of saline pan halites at shallowburial depths has important implicationsfor the origin of saline formation waters in sedimentary basins. Parent evaporite brines may not be stored in the pores of halite rocks and later expelledduring burial compaction if the rocks are cemented early, and tightlycrystallizedhalite rocks may also impede the downward migration of dense syndepositionalbrines.