Carbon isotope data from Upper Cambrian sections in three Laurentian basins in northern Utah, central Iowa, and western Newfoundland record a large positive ␦ 13 C excursion (SPICE event) of up to ؉ 5‰. Peak ␦ 13 C ratios are well dated by trilobite collections to the middle of the Steptoean Stage (Dunderbergia Zone) and occur during maximum regression associated with formation of the Sauk II-Sauk III subsequence boundary on the North American craton. Maximum regression was marked by an influx of quartz sand into carbonate-platform settings in all three widely separated basins. In northern Utah, this quartz sand formed a thick sequence known as the Worm Creek Quartzite, which marks a conspicuous interruption of carbonate deposition during the Middle to Late Cambrian in the region. In western Newfoundland, the thickness of the quartz sand unit is much reduced but still marks a brief shutdown of the carbonate factory that is unique to the Cambrian shelf succession of the area. In the central Iowa area of the cratonic interior, an upward-shallowing carbonate succession culminates in cross-stratified trilobite grainstones at the peak of the SPICE in Dunderbergia Zone time, and the lowest point on the relative-sea-level curve is associated with the occurrence of coarse quartz sand derived from the encroaching shoreface.Although it is difficult to determine precisely the departure from baseline ␦ 13 C that marks the beginning of the SPICE excursion in the stratigraphic successions analyzed, our results are consistent with a rise and subsequent fall in ␦ 13 C tracking a major regressive-transgressive event recorded across northern Laurentia. The correlation of a major ␦ 13 C excursion with regression is similar to that described for the Late Ordovician, for which the pattern has been attributed to either increased carbonate relative to terrigenous weathering rates as ice sheets covered up organic-matter-containing silicates at high latitudes or high productivity and organic-carbon burial driven by oceanic overturn. The lack of known Steptoean-age ice sheets that could have affected the ratio of carbonate to silicate weathering rates suggests that organiccarbon burial was the likely cause of the SPICE event. We suggest that increased weathering and erosion rates during relative sea-level fall (Sauk II-III) increased the burial fraction of organic carbon in an expanded region of fine-grained siliciclastic deposits in shelf and upper slope environments during the Steptoean.

Upper Cambrian limestones and dolostones of western Newfoundland, Canada, display conspicuous synsedimentary mud cracks. Cracks occur in carbonate mudstone interbedded with ooid and peloid grainstone (unwashed oolite and ribbon rock lithofacies). The traditional interpretation is that these are desiccation cracks. The weight of evidence supports an alternative explanation: cracks resulted from the differential mechanical behaviour under stress of stiff mud interlayered with loose ooid/peloid sand. The processes envisaged to cause such diastasis cracks may be applicable to a wide variety of both carbonate and terrigenous clastic deposits composed of interlayered sediments of contrasting material properties, and may be a viable alternative to synaeresis. Diastasis cracks are not depth limited, and may form in any subtidal environment from the beach zone to below wave base. If this interpretation is correct, there may not be nearly as many intertidal lithofacies in the rock record as are presently assumed.

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In modern ooid-forming environments in the Caribbean, aerobic respiration of organic matter below the sediment–water interface drives an increase in pCO2 and a corresponding decrease in carbonate saturation state (Ω) that creates shallow sediment porewater that is neutral or slightly caustic to carbonate. The locus of ooid growth, therefore, is presumed to be in the water column during suspension, where supersaturation with respect to calcium carbonate is the norm. In the past, however, during conditions of low aqueous O2, high Ω, or low organic-matter input, the shallow sub-sediment marine burial environment was conducive to carbonate precipitation. Here we present petrographic and electron probe microanalyzer (EPMA) data from exquisitely preserved oolites through time that suggests that some ancient ooids may have grown within the sediment pile. We propose that each increment of ooid cortical growth originated as incipient isopachous marine cement formed during shallow burial within migrating ooid dunes. After a period of burial (∼ weeks to months), ooids were remobilized and rounded during bedload transport. This “bedform model” for ooid growth explains: 1) why ancient ooids are not limited by the precipitation–abrasion balance that appears to prohibit modern tangential Caribbean ooids from achieving grain sizes larger than coarse sand, 2) the radial crystal fabric that defines the internal structure of many ancient ooids, and 3) the first-order correlation of the abundance of large and giant ooids in the rock record to periods with predicted high porewater Ω. This model implies that photosynthetic microbes were unimportant for growth of large and giant ooid but it remains agnostic to the effect of other microbes. The physical and chemical milieu of modern marine ooid-forming environments is perhaps not the best analogue for ancient ooid-forming environments; this should be considered when using ancient ooids to reconstruct secular trends in ocean chemistry.