Stromatolitic Buildups and Associated Microbial Facies of the Neoproterozoic Beck Spring Dolomite, Southern California

Controls on microbial buildups are not well understood, and the roles of individual depositional processes in formation of specific stromatolites are poorly constrained. The Neoproterozoic Beck Spring Dolomite (BSD) preserves abundant microbial deposits, including stromatolitic buildups, that reflect both biological and environmental influences. Bioherms and associated facies of the BSD formed in intertidal to basinal environments. The absence of purely physical sedimentary structures (i.e. ripple cross stratification) and the abundance of biogenic structures suggest strong microbial influences. However, bioherm and stromatolite morphology vary with depositional environment.

Microbial buildups, with columnar, branching, composite-domal, conical, and cuspate stromatolites, are a significant feature of the BSD. These bioherms and biostromes are diverse in geometry and stromatolite morphology, with variations corresponding to differences in depositional conditions. Small domed bioherms, surrounded by shale, developed in quiet water, whereas larger bedded buildups surrounded by breccia developed under higher energy conditions, suggesting that buildup geometry is influenced by flow speed and/or water depth. Environmental conditions also influence stromatolite morphology throughout buildups. For instance, recumbent columnar and asymmetric domal stromatolites are oriented in response to currents. Stronger currents, indicated by abundant microunconformities, also resulted in the formation of thin branching columns, demonstrating that current variability significantly influenced stromatolite morphology and bioherm development.

Intraclastic megabreccia at the base of the BSD represents large-scale fragmentation and downslope transport of large stromatolitic buildups. Blocks up to ~5 m long, with ~35-50 cm tall columnar stromatolites, make up the megabreccia deposit. Stromatolite centers are occasionally eroded and filled with micrite. The orientation of geopetal structures is consistent with regional bedding, suggesting that micrite filled voids after blocks were transported. Although previous studies have interpreted the BSD to have been deposited in subtidal to intertidal environments, the megabreccia deposit suggests that the BSD was partially deposited basinward in quiescent, deeper water.