Petrology, petrochemistry, and stromatolites of the Middle to Late Proterozoic Beck Spring Dolomite, eastern Mojave Desert, California

Abstract

The Beck Spring Dolomite is the medial unit of the Middle to Late Proterozoic Pahrump Group, the oldest sequence of sedimentary rocks in eastern California. Stratigraphic sections of the Beck Spring Dolomite examined in the eastern Mojave Desert and Death Valley regions consist of four members. These are, in ascending order, a lower cherty member, a lower laminated member, an oolitic–pisolitic member, and an upper cherty member. More than 80% of the Beck Spring Dolomite is algal-laminated dolomite with a possible Middle to Late Riphean stromatolite assemblage characterized by cf. Conophyton, eroded, irregular columnar forms similar to Kussiella or Baicalia, and several types of stratiform Stratifera. Petrographic, X-ray diffraction, and atomic absorption spectroscopic analyses indicate that the formation is composed of well-ordered replacement dolomite with less than 25% acid-insoluble residue. Concentrations of Fe and Mn are two to six times higher in the algal-laminated members than in the oolitic–pisolitic member, whereas the concentrations of Ca, Mg, Ba, Sr, Na, and K show no systematic variations. Stratigraphic relationships, primary and secondary sedimentary structures, petrology, and stromatolite assemblages suggest deposition during Middle to Late Proterozoic time on a platform that most likely included offshore shoals, restricted lagoons, and broad tidal flats with ponds, channels, and levees.