ABSTRACT

The Beck Spring Dolomite is a middle to late Proterozoic formation exposed in the Death Valley region of eastern California. It is a platform carbonate sequence dominated by cryptalgal laminites with some stromatolites and grainstones and is inferred to have been deposited in shallow subtidal to intertidal environments. Original fabrics are well preserved in the Beck Spring Dolomite, in contrast to many Phanerozoic dolomites. The diagenetic pattern is similar to that of many shallow-water limestones: early fibrous cements (dolomite) are followed by later sparry cements (also dolomite), and several phases of internal sedimentation are evident. Cavity structures, where the cements occur. include birdseyes, laminoid fenestrae, stromatactis, sheet cracks, and neptunian dikes.

Four types of synsedimentary-early diagnetic fibrous dolomite are distinguished; they show features comparable to those of fibrous calcite, together with some new fabrics. Type A forms isopachous fringes consisting of length-slow acicular to columnar pseudopleochroic crystals with fine, substrate-parallel color banding. The dolomite is mostly nonluminescent with some bright blotches, and inclusion trains define acicular crystallites within the columnar crystals. Type B consists of length-slow columnar crystals with distinctive bladed relic structures, defined by inclusions and nonluminescence, surrounded by inclusion-free and brightly luminescent dolomite. Type C consists of bladed to wedge-shaped pseudopleochroic crystals with undulose extinction (mostly fascicular optic); inclusion rains define acicular crystallites within crystals. Type C commonly forms mammillated crusts and botryoids. Type D fibrous dolomite forms thin layers of acicular crystals, with a distinctive geometry of the cement layers thickening into thecorners of cavities. Fibrous carbonates frequently are interpreted as replacements, and types B and C contain relic structures indicating such an origin, but types A and D have fabrics which could be primary.

The later diagenetic dolomite spar has all the features of a direct precipitate: increase in crystal size cavity-ward and delicate growth zonation and terminations (revealed by cathodoluminescence). However, truncation surfaces indicate phases of dissolution-reprecipitation. and silts within the spar mosaic represent internal sedimentation of crystal fragments.

The depositional components (pisolites and micrite) and some fibrous dolomites have oxygen isotopic values around -2 versus PDB, compatible with a marine origin. However, Na (around 220 ppm) and Sr (around 65 ppm) contents are very low, suggesting precipitation from dilute waters. Taking a uniformitarian approach that early dolomite is always a replacement, the geochemical data are best interpreted in terms of meteoric-marine mixing-zone dolomitization of CaCO₃ sediments and early cements. An alternative, however, is primary marine precipitation of poorly ordered calcian dolomite and early diagenetic stabilization to stoichiometric dolomite. The dolomite spar has very negative values of ¹⁸O (-9.6 versus PDB) and is more depleted in Na (34 ppm) and Sr (47 ppm), reflecting precipitation from meteoric waters of very low ionic strength during burial diagenesis.

A replacement origin for the dolomite of the depositional components and fibrous cements can be reconciled with the lack of obliterative replacement fabrics by invoking an original high-Mg calcite mineralogy.

This study, demonstrating that Precambrian dolomites can be treated in the same way as Phanerozoic limestones, shows that both possess similar petrographic, trace element, and stable isotope characteristics. However, in this Precambrian formation, the principal diagenetic mineral is dolomite, whereas in most Phanerozoic carbonate formations it is low-Mg calcite.