What are sparry allochemical carbonate rocks? Comment on their depositional environment.

Defining Sparry Allochemical Carbonate Rocks

Sparry allochemical carbonate rocks are sedimentary rocks composed of carbonate grains (allochems) bound together by sparry calcite cement. The term 'sparry' refers to the crystalline texture of the calcite cement, appearing as clear, euhedral crystals, typically formed during later diagenetic stages. ‘Allochem’ refers to allochthonous chemical sediments, meaning grains that have been transported from their site of origin. These rocks are distinct from micritic carbonates, which are composed of finely crystalline carbonate mud.

Components of Sparry Allochemical Carbonate Rocks (Allochems)

Allochems are the building blocks of these rocks and can be broadly categorized as follows:

• Grains of Biogenic Origin: These include shell fragments (bivalves, gastropods, foraminifera), coral fragments, algal grains, and peloids (spherical carbonate grains of uncertain origin, often formed by microbial activity).
• Grains of Intraclast Origin: These are fragments of already lithified carbonate material, broken up by wave action or other physical processes.
• Grains of Lithic Origin: These are fragments of non-carbonate rocks, such as quartz or chert, transported into the carbonate environment.

Depositional Environments

Sparry allochemical carbonate rocks typically form in high-energy, shallow marine environments. The specific depositional environment influences the type of allochems present and the sedimentary structures observed. Key depositional environments include:

1. Reef Environments

Coral reefs are prolific producers of carbonate sediment. Broken coral fragments, algal grains, and shell debris accumulate around the reef structure. These allochems are often well-sorted and rounded due to the constant wave action. The resulting rocks exhibit a diverse range of allochem types and often contain evidence of in-situ reef growth. Example: The Great Barrier Reef, Australia, provides a classic example of a reef environment contributing to extensive allochemical carbonate deposits.

2. Carbonate Platforms and Shoals

Shallow carbonate platforms and shoals are characterized by high rates of carbonate production and accumulation. Wave and current action winnow out fine-grained sediment, leaving behind a concentration of allochems. These environments often exhibit well-developed cross-bedding and ripple marks, indicating strong current activity. Example: The Bahamas are a prime example of a modern carbonate platform with extensive allochemical carbonate deposits.

3. Grainstone Shoals & Beaches

High-energy beaches and shoals are ideal for the accumulation of well-sorted allochems. The constant wave action and currents create a winnowing effect, removing finer sediment and concentrating coarser grains. These deposits often exhibit large-scale cross-bedding and are typically composed of rounded, well-sorted allochems.

4. Lagoonal Environments (with tidal influence)

While generally lower energy, lagoons experiencing strong tidal currents can also accumulate allochemical carbonates. Tidal currents can transport allochems into the lagoon and deposit them as shoals or bars. These deposits may exhibit tidal laminations and a mix of allochem types.

Diagenesis and Cementation

The sparry calcite cement that binds the allochems together is typically formed during diagenesis – the physical and chemical changes that occur after deposition. The formation of sparry calcite cement is often associated with the dissolution of aragonite (a less stable form of calcium carbonate) and the precipitation of calcite in pore spaces. This process is influenced by factors such as temperature, pressure, and the chemical composition of pore fluids. The sparry texture indicates a relatively late stage of cementation, often involving multiple phases of precipitation and dissolution.

Sedimentary Structures

Sparry allochemical carbonate rocks often exhibit distinctive sedimentary structures that provide clues about their depositional environment. These include:

• Cross-bedding: Indicates current or wave action.
• Ripple marks: Also indicate current or wave action.
• Graded bedding: Suggests deposition from a waning current.
• Bioturbation: Evidence of organism activity.