ABSTRACT

Laminated, lacy, and pisolitic carbonate crusts have been found on and within some outcrops of Pleistocene limestones which rise above the modern sabkha surface in Abu Dhabi. These sediments are similar in many ways to typical caliche but have unusual mineralogies and cement textures. Internally, the pisolites are uniformly aragonitic and are cemented together by fibrous aragonite and micritic and fibrous high-Mg calcite in isopachous, meniscus, and microstalactitic textures. It is suggested that caliche crusts with unstable carbonate mineralogies, together with marine or brine associated isotopic ratios and trace element concentrations, yet showing dominantly vadose textures, can develop in marine influenced areas where vadose and phreatic pore waters have unusually elevated saliniti s and high concentrations of Sr and Mg. Thus, in strongly evaporitic, low relief settings such as the Persian Gulf sabkhas, supratidal vadose areas are characterized, not by "fresh water," but by hypersaline brines which allow the precipitation of cements of marine chemical character with vadose textures. Similar examples are known from Bonaire, another evaporite locality. Thus, very clear distinctions should be drawn between the terms "vadose" and "fresh water"; ancient vadose textures do not immediately imply fresh-water pore fluids. One can have "hypersaline vadose" as well as "meteoric vadose" conditions.

The Abu Dhabi pisolitic crusts with their "unusual" mineralogies (including some dolomite) appear to be quite analogous to the pisolitic facies of the Permian reef complex of Texas and New Mexico and may provide a model for their formation and diagenesis. The initial presence of aragonite and high-Mg calcite in the pisolitic and microstalactic crusts of the Permian back-reef sediments would be more compatible with the observed penecontemporaneous dolomitization than would a stable low Mg calcite caliche.