A case for the growth of ancient ooids within the sediment pile

Abstract

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.

Three‐Dimensional Morphometry of Ooids in Oolites: A New Tool for More Accurate and Precise Paleoenvironmental Interpretation

“…While there is evidence for collisional abrasion, the predominance of frictional abrasion suggests that the primary mode of transport occurs between the threshold for motion and the threshold for saltation (Novák-Szabó et al, 2018). In many cases, the Δ sphericity is very close to that predicted by SNG, which could be the result of infrequent transport or growth in the sediment pile (sensu Anderson et al, 2020). Laboratory and natural experiments have shown that when the particle size is large, frictional abrasion dominates at low velocities, and at high velocities fragmentation occurs, instead of the chipping that occurs in smaller particles (see Figure 7 in Novák-Szabó et al ( 2018)).…”

“…Instead, the presence of fragments suggests that the ooids grew to their large size under calmer conditions (i.e., bed‐shear velocities below the threshold for saltation of the giant ooids), but were occasionally exposed to bed‐shear velocities high enough to cause fragmentation. Several scenarios exist that could explain both the size of the ooids and the occurrence of fragmentation including (1) the giant ooids grew in a calmer environment and were transported to a higher‐energy environment, (2) ooids grew within the sediment pile causing ooids to be oversized (Anderson et al., 2020), or (3) the ooids predominantly grew on the seafloor under calmer conditions where the dominant processes were surface normal growth and frictional abrasion from rolling, but occasional storms led to high bed‐shear velocities and fragmentation. Option one fails to explain why there are laminations on large, fragmented ooid cortices (Figure 9e).…”

“…Anderson et al. (2020) also suggest that if giant ooids are growing in the sediment pile, there may be discontinuities on each growth band representing grain‐to‐grain contacts. However, Anderson et al.…”

“…The 3D reconstruction of the GBG oolite shows that these ooids tend to be more prolate than oblate (Figure 6), which is consistent with rolling, but it is possible that the oblateness from forming in the sediment pile was removed by abrasion during subsequent transport. Anderson et al (2020) also suggest that if giant ooids are growing in the sediment pile, there may be discontinuities on each growth band representing grain-to-grain contacts. However, Anderson et al (2020) recognize that these discontinuities rarely are seen because the ratio of the total surface area of a sphere to the area of grainto-grain contacts is very small, and there is a low probability that any cross-sectional cut would intersect the points of contact.…”

“…This growth history may contain information about how sediment transport changes as the grain size and mass increase (Heller et al., 1980). Understanding how the transport mode changes as a function of size can provide clues about the paleohydraulic conditions of ooid‐forming environments and perhaps elucidate the formation of giant ooids (>2 mm in diameter), which must have formed under uniquely carbonate‐saturated and/or turbulent conditions (Anderson et al., 2020; Heller et al., 1980; Sipos et al., 2018; Sumner & Grotzinger, 1993; Trower, 2020).…”