What is a reef? In your answer provide a definition, general diagnostic features and explain how the reef fauna and flora aid in the precipitation of calcium carbonate.
Reefs are self-generated limestone deposits, created by the organisms living in them. They have topographic relief over the surrounding environment, and are formed in the wave-zone out of a wave-resistant framework. Reefs are interesting because they preserve an entire ecological community with its detrius, because they are indicators of very narrow environmental conditions (those that have sufficient nutrients and are favourable to carbonate precipitation), and because they are stratigraphic oil traps.
Flora & Fauna
The reef ecology consists of framework-builders creating the reef, bioeroders weakening it, and critters living within the reef environment. Framework-builders are the creatures that build wave-resistant structures and baffles against strong currents and sediments. The exact creatures involved change over the growth of the reef, with pioneering, colonizing, diversifying, and domination stage colonies. The pioneering community are creatures like sea grass and crinoids that stabilize sediments through encrusting or driving roots into the sand. These organisms thrive in turbulent, unstable environments. The colonizing community are the main framework builders like branching corals, building the major reef structure. The diversifying stage is the increase in organism variety to fill the newly-created environmental niches as the reef builds up to sea level, while the domination stage is the increase in laminating, encrusting forms covering a solid reef core after the reef reaches the turbulent sea level.
Bioeroders are the boring algae, worms, sponges, and mollusks that weaken the reef structure so that storms topple existing reef into debris, allowing new reef to grow. This is an important process for maintaining the reef at the surface when sea level is changing. The toppled material, lime mud, and skeletal debris make up the interstitial materials of the reef, infilling cavities and eventually cementing to the framework so the reef core becomes a solid, massive limestone unit.
The degree of organism diversity is an indication of how ideal the conditions are for reef formation. In optimal conditions with plentiful nutrients and few stressors, diversity is high. In sub-optimal conditions, when temperature or salinity has rapid changes, low light, or intense wave activity diversity is low.
A reef’s structure is categorized into facies by the environmental conditions. The change in energy level and sedimentation rate impacts the form of framework-builders, thus the structure of the reef. The reef core is the main bulk of the reef, a solid, unbedded, infilled framework. The crest of the core has the greatest wave activity with an extremely low sedimentation rate, with a proliferation of encrusting organisms. The flank of the reef below the wave base has low light, low energy, and high sedimentation rate, leading to the growth of delicate but fast-growing branching corals. The reef flat has moderate current in shallow water, and is full of mostly debris with irregular patches of creatures. The backreef is protected by carbonate sands, mud, and globular forms. The reef flank is mostly debris from slumps and storm sands, grading from a steep slope near the reef down to a progressively more shallow slope to the interreef facies. The interreef region is thinly bedded with mud and skeletal sand, distinguished from shallow, subtidal carbonate banks by the presence of reef debris tossed by extreme storms. In areas of restricted circulation, layers of evaporite may form.
In using reef deposits to interpret geologic history, the key features are:
- Tectonic Setting – Reefs occur at the edge of carbonate banks with upwelling water to carry nutrients. Reefs occur within a narrow range of depths, temperatures, salinity, and nutrient conditions, so are indicative of shallow, low-latitude, passive margins or eperic seas with low clastic input.
- Geometry – Reefs can be meters to hundreds of meters thick, and meters to thousands of meters long.
- Typical Sequence – Reefs have a clearly-defined succession of lifeforms, and strong lateral relationships between adjacent facies, but little vertical stratigraphic patterns beyond those formed by shifting facies.
- Sedimentology – The reef framework is the dominant structure, deposited, grown, and bound in situ. Interstitial lime mud, skeletons, and rock breccias infill crevasses. Reefs originally form as calcite or aragonite depending on environmental conditions, and may dolimitize over time.
- Fossils – Reefs have a huge prevalence of fossils, which serve as indicators of environmental conditions.