\subsubsection{Sedimentary Facies Comparison}

The sedimentary facies of the Ningaloo Reef exhibit both commonalities and distinctions when compared to other reef systems such as Bora Bora \citep{Gischler2011Bora}, the Maldives \citep{Gischler2006}, Roatan \citep{Schmitt2017}, the Belize-Yucatán system \citep{Gischler1999}, and the Kuwait ramp \citep{Gischler2004Kuwait}. These reefs represent a variety of reef types—including fringing reefs, barrier reefs, atolls, and carbonate ramps—situated in different oceans and influenced by diverse environmental conditions.

\paragraph{Similarities}

A common pattern among these reef systems is the transition of facies with increasing depth from coral-rich facies in shallow waters to molluscan-dominated facies at greater depths. In the Ningaloo Reef, shallow areas (0--10 meters) are characterized by coral-rich grainstones, which transition to molluscan coral grainstones and molluscan wackestones at depths greater than 80 meters. This trend reflects the change in dominant carbonate-producing organisms due to variations in light availability and hydrodynamic energy.

Similarly, in Bora Bora (Pacific Ocean), coral and coralline algae fragments are abundant in marginal reef sediments, while deeper lagoonal areas contain mollusk shells and foraminiferal tests \citep{Gischler2011Bora}. In the Maldives (Indian Ocean), coral grainstones dominate the reef areas, transitioning to mollusk packstones and wackestones in the lagoonal settings \citep{Gischler2006}. Roatan (Caribbean Sea) also exhibits algal-rich rudstones and coralgal grainstones in reefal environments, with foraminiferal-molluscan wackestones in the lagoon \citep{Schmitt2017}.

These similarities suggest that depth-related facies transitions are a common feature in reef systems, influenced by factors such as light penetration, hydrodynamic energy, and the distribution of carbonate-producing organisms.

\paragraph{Differences}

Despite these general similarities, significant differences exist in the sedimentary facies between the Ningaloo Reef and other reef systems, particularly concerning the abundance of non-skeletal grains.

In the Ningaloo Reef, non-skeletal grains such as peloids and ooids are generally scarce. Aggregate grains are found primarily in specific samples from Yardie Creek, and overall, non-skeletal grains constitute a minor component of the sediments. This low abundance aligns with observations from other Indo-Pacific reef systems, such as the Maldives, where non-skeletal grains are also rare \citep{Gischler2006}.

In contrast, non-skeletal grains are abundant in reef systems located in the Atlantic and Caribbean regions. In Bora Bora, non-skeletal grains, including peloids and aggregate grains, are significant components of the shallow lagoonal sediments, with peloid abundances reaching up to 50\% in certain facies \citep{Gischler2011Bora}. Similarly, in Roatan, non-skeletal grains are common in protected areas near the reef-lagoon transition zone, attributed to reduced skeletal grain formation and decreased sedimentation rates \citep{Schmitt2017}.

The Kuwait carbonate ramp in the Arabian Gulf (Indian Ocean) presents a different scenario. Here, non-skeletal grains, specifically ooids and aggregate grains, are abundant in the inner ramp ooid-skeletal grainstone facies \citep{Gischler2004Kuwait}. This abundance is linked to high depositional energy and the availability of carbonate-saturated waters favorable for inorganic precipitation.

These differences highlight the influence of oceanic conditions and reef types on sediment composition. The scarcity of non-skeletal grains in the Ningaloo Reef and the Maldives, both located in the Indian Ocean, may be attributed to higher water depths and lower rates of water circulation in lagoons, as suggested by \citet{Milliman1974}. In contrast, the higher abundance of non-skeletal grains in the Atlantic and Caribbean reefs is associated with shallower, more open lagoons with higher water circulation rates, promoting the precipitation of carbonate cements and the formation of non-skeletal grains.

\paragraph{Factors Influencing Facies Differences}

Several factors contribute to the observed differences in sedimentary facies among these reef systems:

1. **Reef Type and Morphology**: The Ningaloo Reef is a fringing reef along a continental margin, lacking extensive lagoonal environments where fine sediments and non-skeletal grains can accumulate. In contrast, barrier reefs and atolls like Bora Bora and the Maldives possess protected lagoons that favor the deposition of fine-grained sediments and the formation of non-skeletal grains.

2. **Hydrodynamic Energy**: The high-energy environment of the Ningaloo Reef, influenced by the Leeuwin Current and strong wave action, promotes the dominance of coarse skeletal grains and winnows away fine sediments. This contrasts with the more sheltered conditions in lagoons of other reef systems, where lower energy levels allow for the accumulation of fines and non-skeletal grains.

3. **Oceanographic Conditions**: Oceanic settings play a crucial role. In the Indian Ocean, reefs like the Ningaloo Reef and the Maldives exhibit lower abundances of non-skeletal grains, possibly due to differences in water chemistry, temperature, and circulation patterns compared to Atlantic and Caribbean reefs.

4. **Biological Communities**: Variations in the abundance and types of carbonate-producing organisms influence sediment composition. For example, the prolific growth of organisms like \textit{Halimeda} in certain reefs contributes significantly to sediment production, affecting facies development.

\paragraph{Expanded Comparative Table}

An expanded comparative table incorporating the new information is presented in Table~\ref{tab:comparative_facies_expanded}. This table highlights key aspects of sedimentary facies across different reef systems, considering reef type, ocean location, facies distribution, mineralogical trends, and the abundance of non-skeletal grains.


\paragraph{Interpretation}

The comparison reveals that non-skeletal grains are generally scarce in Indo-Pacific reef systems like the Ningaloo Reef and the Maldives, both located in the Indian Ocean. In contrast, Atlantic and Caribbean reefs tend to have higher abundances of non-skeletal grains. An exception within the Indian Ocean is the Kuwait carbonate ramp, where non-skeletal grains such as ooids and aggregate grains are abundant in the high-energy inner ramp environments \citep{Gischler2004Kuwait}. This suggests that local environmental conditions, such as high depositional energy and carbonate saturation levels, can override broader oceanic trends.

The occurrence of non-skeletal grains is influenced by factors including:

- **Water Circulation and Depth**: Higher water circulation rates and shallower depths promote the precipitation of carbonate cements and the formation of non-skeletal grains. This is evident in the Kuwait ramp and Atlantic reefs but less so in the deeper lagoons of Indo-Pacific reefs.

- **Sedimentation Rates**: Low sedimentation rates provide time for cementation processes that favor the formation of non-skeletal grains. In the Ningaloo Reef, high sedimentation rates and energetic conditions may limit this process.

- **Water Chemistry**: Variations in carbonate saturation states and magnesium/calcium ratios can influence the precipitation of non-skeletal grains.

\paragraph{Conclusion}

The facies comparison highlights that while there are commonalities in facies transitions with depth across reef systems, significant differences arise due to variations in reef type, oceanographic setting, and local environmental conditions. The Ningaloo Reef's sedimentary facies are distinct in their low abundance of non-skeletal grains and the influence of high-energy conditions on sediment composition. These findings underscore the importance of considering multiple factors when interpreting sedimentary records and caution against generalizations based solely on reef type or ocean basin.

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