Paleoenvironments and the Precambrian-Cambrian transition in the southern Great Basin: Implications for microbial mat development and the Cambrian radiation

The Precambrian-Cambrian transition (∼542 Ma) is a pivotal time in Earth’s history and is notable for marking the first skeletonized occurrences of many major groups of marine organisms. Concurrent with the appearance of these taxa are significant changes to the marine substrate, shifting from a seafloor dominated by microbial mats to one dominated by metazoan bioturbation. In the southern Great Basin, United States, these changes, however, appear gradual and the early Cambrian is unlike either the Precambrian or the subsequent Phanerozoic in terms of the unusual coexistence between seafloor microbial mats and marine organisms. Essential to understanding these interactions is a strong paleoenvironmental framework to work within to examine which environments microbial mats occur in and whether these are or are not the same environments that metazoan fossils and trace fossils occur in. The purpose of this dissertation is to develop a rigorous depositional model of Precambrian-Cambrian strata within the southern Great Basin to document the paleoenvironmental distribution of fossil taxa, trace fossils, and microbial mat structures to examine the nature of this coexistence during the early Cambrian. This may shed light on whether metazoans and microbial mats existed in similar environments or if each occupied a preferred environmental niche apart from the other.;The Precambrian-Cambrian transition of the southern Great Basin, United States, consists of mixed-carbonate siliciclastic sedimentary rocks. Prior studies have emphasized that carbonate rocks were deposited offshore to siliciclastic rocks; however, reevaluation shows that these mixed carbonate-siliciclastic sedimentary rocks reflect the interaction of shallow shelf and shoreface siliciclastic sediments with marginal marine carbonate sediments of a barrier island coastline. The highest abundance of skeletal material in the form of shell beds, bioherms, and reefs occurs within the nearshore carbonates of these deposits. These mixed carbonate-siliciclastic strata are arranged into depositional units termed ‘Grand Cycles’, consisting of a siliciclastic half-cycle overlain by a carbonate half-cycle. The siliciclastic-carbonate transitions of these depositional units reflect a shift in lateral depositional environments, rather than a complete change in the depositional system, and the progradation events recorded by these Grand Cycles allow for assessment of paleoecological patterns over short timescales. Notably, the early Cambrian archaeocyathans expand their ecological niche from their first occurrence as level bottom forms to bioherms to reefs over the course of a single progradational episode—less than 6 million years in duration—despite remaining in an onshore position, within the shoreface or landward of it.;Lower Cambrian wrinkle structures within siliciclastic strata show a preference for occurring within heterolithic deposits, primarily of offshore transition and mixed tidal flat environments. These microbial mat features reflect the absence of a surface mixed layer, which is in accord with previous observations that early Cambrian bioturbation was shallow-penetrating and not extensive within siliciclastic shelf environments. In contrast, siliciclastic tidal flat environments from the middle member of the Wood Canyon Formation record deep-penetrating burrows associated with wrinkle structures; however, bioturbation levels are generally low (ii = 2). Lagoonal environments of the upper member of the Wood Canyon Formation record similar deep-penetrating burrows that are attributable to anemones and exhibit higher levels of bioturbation (ii = 4).;The nature of the lower Cambrian trace fossil and microbial mat record shows that nearshore environments were characterized by deep-penetrating burrows in lagoonal and tidal flat environments, while shallow shelf environments exhibit shallow-penetrating levels of bioturbation and a prevalence of microbial mat structures. These distributions—coupled with the high abundance of shell beds, bioherms, and reefs in onshore environments—parallel previous observations that evolutionary novelties within Cambrian-Ordovician communities show a preference for onshore origination. Paleoenvironmental distributions of microbial mats and metazoans in the earliest Cambrian in the southern Great Basin suggest that metazoan taxa and deep-penetrating bioturbation dominated in the onshore position throughout much of this interval, while microbial mat substrates and shallow penetrating bioturbation were typical for shelf environments, a distribution opposite to that found across shelf and nearshore environments today.