Physical Controls on Carbonate Intraclasts: Modern Flat Pebbles From Great Salt Lake, Utah

Abstract

In carbonate-forming environments, authigenic minerals can cement surface sediments into centimeter-sized intraclasts that are later reworked into "flat-pebble" or "edgewise" conglomerates. Flat-pebble conglomerates comprise only a small portion of facies in modern marine environments but are common in ancient strata, implying that seafloor cements were more widespread in the past. Flat-pebble conglomerates nearly disappeared after the Ordovician radiation, yet it is unclear if this decline was due to changing seawater chemistry or if increased infaunalization and bioturbation simply worked to break down nascent clasts. We discovered a process analog that produces flat-pebble conglomerates around the Great Salt Lake, Utah, USA, and studied these facies using field observations, wave models, satellite imagery, petrography, and microanalytic chemical data. Clasts were sourced from wave-rippled grainstone that cemented in situ in offshore environments. Lake floor cements formed under aragonite saturation states that are lower than modern marine settings, suggesting that physical processes are at least as important as chemical ones. Results from our wave models showed that coarse sediments near the field site experience quiescent periods of up to 6 months between suspension events, allowing isopachous cements to form. Using a simple mathematical framework, we show that the main difference between Great Salt Lake and modern, low-energy marine settings is that the latter has enough bioturbating organisms to break up clasts. Observations from Great Salt Lake demonstrate how geologic trends in flat-pebble abundance could largely reflect changes in total infaunal biomass and ecology without requiring regional-to-global changes in seawater chemistry. Plain Language Summary Calcium carbonate is a common sedimentary mineral that precipitates quickly under Earth surface conditions. If carbonate sediments are at rest for long periods of time, they may solidify via cementation into flat, pebble-sized clasts. Geologic trends show that flat pebbles are anticorrelated with burrows, suggesting they may tell us about animal evolution and ecology. However, it was unknown if animals disrupt clasts through chemical means, leading to slower rates of mineral growth, or if they physically push sediments apart, breaking up nascent cements. We studied flat pebbles forming in Great Salt Lake, Utah, where the water is too salty for burrowing animals. The composition of lake water should lead to slower growth rates for carbonate minerals, yet clasts still form. We conclude that pebbles form during quiescent intervals with little physical energy to move grains, either from waves or from burrowing organisms. As a result, ancient flat pebbles are probably not unique markers for seawater chemistry.

Arid Coastal Carbonates and the Phanerozoic Record of Carbonate Chemistry

“…Although seawater carbonate chemistry should affect trends in these facies as well, their temporal trends are closely tied to Phanerozoic diversity, such that changes in these facies are most recognizable around radiations and extinctions, such as the mid-Ordovician radiation (Sepkoski, 1982) and the end-Permian extinction (Wignall & Twitchett, 1999). These patterns highlight the importance of burrowing organisms on seafloor carbonate precipitation by both altering porewater geochemistry (Aller, 1994) and physically disrupting incipient cements (Smith et al, 2020). In contrast, tepee and pisoid facies may act as indicators of seawater chemistry not only because elevated salinity enhances carbonate precipitation, but also because it reduces competing signals tied to the evolution of burrowing organisms which strongly affect open marine facies.…”

Extensive Early Marine Seafloor Cementation in a Modern Epeiric Sea Induced by Seawater Properties and a Shallow Redox Boundary Below the Seafloor

“…In terms of formation mechianism, the shallow redox boundary may be favored by shallow, restricted marine settings, where weak physical disturbance influences the seafloor showing negligible topography herein, and O 2 diffusion in the surficial sediment at the seafloor. Moreover, higher seawater salinity (up to 49 g/kg; Figure 6b) under hot and arid climate may hinder burrowing activities of marine biota at the seafloor that promote O 2 downward diffusion (Hu & Burdige, 2008; Smith et al., 2020), and therefore help build a shallow redox boundary. The weak burrowing activities are evidenced by the planar upper surfaces of firm‐ and hardground with few borings and encrustations in the landward and restricted lagoonal areas (Figure 3a), and the stable existence of a thin layer of microbial mats at the seafloor surface (Ge, Pederson, et al., 2020).…”

“…It seems reasonable to assume similar formation scenarios of early marine cementation like this study occur in their ancient analogs. Further, the break‐up of firm‐ and hardground of modern landward lagoons, for example, by big waves or storms, can produce very similar fabrics to flat pebbles or flakestones (Figure 3a) commonly occurring as early marine cemented features in Precambrian and early Paleozoic (Smith et al., 2020; Wignall & Twitchett, 1999). This supports similar mechanisms like this study may also work in deep‐time Earth records.…”

Atypical ooid diversity in the Upper Cretaceous Yacoraite Formation, Argentina

“…Many of the recorded textures in the Yacoraite Formation are strikingly similar to those described for ooids from the Great Salt Lake (Eardley, 1938, Carozzi, 1962; Halley, 1977; Chidsey et al ., 2015; Paradis et al ., 2017; Paradis, 2019; Trower et al ., 2020; Ingalls et al ., 2020; Smith et al ., 2020). In addition, some features are comparable to the incipient growth stages observed in Lake Geneva ooids (Davaud & Girardclos, 2001; Plee et al ., 2008; Ariztegui et al ., 2012).…”