MULTISCALE CONTROLS ON THE FORMATION OF LACUSTRINE TUFAS FROM THE MIOCENE BARSTOW FORMATION, MOJAVE DESERT, SOUTHERN CALIFORNIA

Meter-scale tufa (carbonate) mounds occur in the lacustrine sediments of the Miocene Barstow Formation and reveal multi-scale and multi-process controls on tufa formation. Tufa samples examined here originate from several decimeter-scale nodular sub-units of larger meter-scale mounds. The subunits display tubular (~2mm in diameter) molds interpreted as growth around aquatic plants. Like many lacustrine carbonates, the samples are strongly banded on the cm/mm scale; mesofabrics alternate between a dense banded carbonate and a porous weakly laminated carbonate.

Microscopically, the dense banded carbonate is characterized by (1) laterally continuous fibrous spar/micrite couplets and (2) micritic laminae characterized by an irregularly laminated clotted texture. The porous regions consist of microclots, peloids, and filamentous microfossils in a matrix of microspar. Diatoms also occur enclosed within pore spaces. The spar laminae can be traced unambiguously from sub-unit to sub-unit, whereas the micritic fabrics do not maintain a constant thickness across sub-units.

The tufa mounds of the Barstow Formation are products of multi-scale controls that each left behind a unique imprint on their morphology: m-scale localization caused by spring activity, cm-scale sites for nucleation provided by aquatic plants, and sub-mm-scale changes in texture. The occurrence of filamentous microfossils and a ubiquitous clotted micritic texture present evidence for a significant microbial component. Interestingly, the micritic textures vary in thickness from sub-unit to sub-unit, suggesting microbial influence was “locally” controlled at the cm-scale. In contrast, the spar laminae maintain constant thickness between sub-units across the entire tufa mound, suggesting control at or beyond the m-scale mound, likely representing larger scale changes in lake chemistry/climate/spring activity.

Given the common use of lacustrine carbonates as environmental archives and partial analogs of early Earth microbialites, conducting multi-scale level of analyses allows us to more thoroughly evaluate the different factors that control their formation.