New Mexico Geological Society Annual Spring Meeting — Abstracts

The Mescal Limestone, part of the youngest sequence of Proterozoic rocks in Arizona south of the Mogollon Rim, crops out from Young southward almost to Tucson and from the Vekol Mountains eastward to near San Carlos. A lower member of cherty and noncherty limestone and an overlying algal member are found throughout the outcrop area of the Mescal, whereas the next-youngest members of basalt and argillite are restricted to northern exposures. Most of the carbonate strata ,though called limestone, are metamorphic products of dolostone produced by widespread intrusion of diabase sills about 1,100 Ma. Local unmetamorphosed remnants of the original carbonate beds are everywhere dolostone and cherty dolostone in sections as thick as 118 m. Careful examination has revealed no calcite in these unmetamorphosed strata.

The lower member of the Mescal rests on siltstone of the Dripping Spring Quartzite, the upper few meters of which contains molds of shortite and abundant halite in northern outcrops. The lower member of the Mescal within this northern area, from the Roosevelt Dam northward, contains a conspicuous bed of dolostone collapse breccia above a few meters of basal dolomitic sandstone and siltstone that locally is missing. Higher beds in the member in northern outcrops and all strata in the member south of the Roosevelt Dam consist mainly of alternating sequences of chert-free and chert-rich dolostone, much of which consists of cryptalgal dolomicrite. Some algal mats are highly crinkled from desiccation. Chert in the conspicuous collapse breccia and in the lower third to half of the lower member north of the dam contains abundant rounded to cubic and hopper-shaped molds from halite. Some dolostone beds above the collapse breccia contain dolomite pseudomorphs of trona or nahcolite in blade-shaped crystals that radiate from a nucleus, forming rossettelike nodules, as much as 40 cm in diameter. The nodules grew in carbonate mud with little deformation to the host algal laminations. Sparse stromatolites and ministromatolites are found in the member. The algal member of the Mescal in northern and southern exposures consists, at base, of
digitate and stratiform stromatolites that make up extensive bioherms comprising several genera. Above the bioherms and extending to the top of the member are nearly chert-free dolostone algal laminates.

Dolostone of the Mescal in northern outcrops previously has been interpreted as marine, with halite and possibly gypsum having formed in basal, presumably supratidal beds and the lower third to half of the lower member accumulating under highly saline conditions. We found no evidence of gypsum. The halite molds and the collapse breccia resulted from the leaching of salts, and the sodium bicarbonate (possibly with sodium carbonate) pseudomorphs indicate nonmarine conditions of strong evaporation and the concentration of saline solutions. However, the stromatolites of the algal member of Riphean forms recognized worldwide as probably marine. Available evidence suggests that Mescal deposition in northern areas, after local initial siliclastic accumulations, possibly on salt flats, was in a large saline lake that continued from late Dripping Spring time, eventually resulting in the development of dolomitic algal mats. Some early-formed mats were overprinted by saline minerals from evaporatively concentrated nonmarine waters. The onset of marine deposition is uncertain in southern outcrops but was sometime after accumulation of the sodium-rich nodules in northern area and certainly existed by the time stromatolitic
bioherms began to grow during deposition of the algal member and probably continued throughout the accumulation of that member.