PALEOENVIRONMENTS OF EARLIEST PHANEROZOIC BIOCLASTIC ACCUMULATIONS AND THE EMERGENCE OF CAMBRIAN SHELL BEDS, SOUTHERN GREAT BASIN, USA

The Precambrian-Cambrian transition marks a clear turning point in Earth history in which bioclasts became the dominant component of carbonate rocks and trace fossils emerged that show a vertical component to bioturbation. Microbial mat features, such as wrinkle structures, persisted in siliciclastic environments throughout the Cambrian, but became scarce in non-stressed marine settings thereafter. While the timing of these significant events is well-constrained, their paleoenvironmental distribution during this transition and relation to each other is not.

At Emigrant Pass, California, the Lower Cambrian upper member of the Wood Canyon Formation preserves the oldest Phanerozoic bioclastic carbonate rocks of the region in association with the oldest assemblage of deep-penetrating vertically-dominated trace fossils. While older rocks in the southern Great Basin may preserve Cambrian-age scattered fossil elements, rocks of the upper member of the Wood Canyon Formation represent the first interval in which bioclast accumulation of a Phanerozoic assemblage was considerable enough to form a shell bed. Facies analysis reveals that bioclast accumulation, in association with a vertically-dominated trace fossil assemblage, occurred first in flood tidal delta and lagoonal environments of a wave-dominated tidal inlet setting, back-barrier to a siliciclastic-dominated shoreface and shelf. Older wrinkle structures from the Lower Cambrian middle member of the Wood Canyon Formation at the Salt Spring Hills, California, in addition to contemporaneous examples from the upper member, show that wrinkle structures formed across a mixed tidal flat environment, as well as within the offshore transition—between fair-weather wave base and storm wave base.

While this study provides a brief snapshot in time for a particular region, it does reveal that the main focus of bioclastic carbonate production and accumulation coincided environmentally and temporally with the emergence of deep and vertically-dominated trace fossil assemblages, while environments that preserved wrinkle structures were contemporaneously barren of bioclasts and vertically-dominated assemblages of trace fossils.