Abstract

The middle Eocene Seeb Formation represents deposition on one of the earliest "modern" style carbonate platforms, i.e., influenced by seagrasses and mangroves, and presents an opportunity to explore controls on the preservation of Cenozoic carbonate lithofacies. In the study area, the Seeb Fm. is dominated by an anomalously thick (approximately 250 m) and uniform package of shallow marine, platform-interior sediments. These nodular, indistinctly bedded shallow-subtidal sediments display no evidence for relative sea-level change (such as subaerial exposure features), and lack the shallowing-upwards cyclothems that have characterized carbonate sediments deposited in platform-interior settings throughout much of the rock record (particularly during greenhouse periods). We conclude that this is a consequence of thorough bio-retexturing of the sediment by burrowing organisms and the roots of marine vegetation, which destroyed primary fabrics, facies diversity, evidence for cyclicity, and "missing time" horizons such as cycle-bounding exposure surfaces. It seems possible that the remarkable thickness of apparently acyclic Seeb Fm. sediment that built up may reflect increased bio-disturbance of the shallow marine environment following the Late Cretaceous expansion of seagrasses and mangroves. A similar lack of peritidal cyclothems may be common to many Cenozoic shallow marine carbonate deposits.

"Missing time" horizons in limestones, reflecting erosion or nondeposition, have been identified by some authors to explain discrepancies between low calculated accumulation rates of ancient sediments and higher rates measured in comparable modern environments. However, our studies of the Seeb Fm. suggest that the significance of such "missing time" horizons in ancient sediments may have been overstated, and that apparent differences between modern and ancient accumulation rates are a consequence of the extrapolation of unrepresentative, localized high rates of modern sediment production to large (platform-scale) areas. Our study reinforces the idea that modern sediment production and accumulation rates may be much lower than previously thought because they are typically measured in highly productive areas such as seagrass beds and do not take into account the highly variable nature of carbonate production, storage, erosion, and destruction across platforms. Therefore, far less "missing time" needs to be inferred for ancient sediments to account for any imbalance between modern and ancient sediment accumulation rates.

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