Carbonate Depositional Systems

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Geol 370: Sedimentology and Stratigraphy Topic 17: Carbonate Depositional Systems

Transcript of Carbonate Depositional Systems

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Carbonate Systems

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Any questions?

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Carbonates are pretty

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Especially when covered by snow

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or carved by glacial processes

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Carbonate Factory

Carbonate sediment is formed mostly in place through biological processes that are controlled by temperature, salinity, water depth, basin geometry, and degree of clastic sediment influx.

“Carbonate sediments are born, not made.”

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Modern Carbonate Settings

Modern carbonates are found in three major groups, warm-water, cool-water, and pelagic. Each group is composed of a distinct set of organisms and processes. Warm- and cool-water carbonates can occur at similar latitudes on opposite sides of the same ocean basin due to circulation patterns associated with the coriollis-effect.

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Ancient Carbonate Settings

During periods of high eustatic sea-level, vast areas of continents became flooded, leading to widespread development of shallow-water carbonate systems. Pelagic carbonates did not develop until coccoliths became abundant in the Cretaceous.

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Sediment Production: Foramol/bryomol Assemblage

Cool water carbonate sediment composed mostly of benthic forams, molluscs, barnacles, bryazoa, and calcareous red algae.

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Sediment Production: Chlorozoan Assemblage

Warm water carbonate sediment composed of hermatypic corals and calcareous green algae, in addition to foramol organisms.

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Warm-water Carbonates

Warm-water carbonates form in shallow, tropical settings that support photosynthetic-dependent organisms which rapidly produce calcitic skeletal structures, such as corals and calcareous green and red algae. Waters are commonly supersaturated with respect to calcium carbonate that precipitates to form carbonate grains, including ooids, and lime mud. Most reefs form in warm-water settings.

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Modern Reef Occurrence

Carbonate reefs form primarily through organic processes that require warm, shallow, clear water. These conditions are best met along the margins of continents and islands located in a narrow band north and south of the equator.

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Cool-water Carbonates

Cool-water carbonates form in shallow to moderate-depth arctic, temperate, and some tropical shelf settings that support non-photic calcifying benthic invertebrates, such as molluscs, bryozoa, forams, and barnacles, as well as photic calcareous red alage.

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Pelagic Carbonates

Pelagic carbonate organisms, such as coccoliths and forams, exist in shallow photic zones of the open ocean. Upon death, their skeletons settle to the seafloor and accumulate to form intermediate-water carbonate oozes, mostly on shelves and ocean-ridge flanks. Below the carbonate compensation depth (CCD), usually between 3000- and 4000-m depth, cold water tends to be undersaturated with respect to calcium carbonate leading to the dissolution, rather than accumulation of these tests.

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Controlling Factors: Shelf Geometry

The development and lateral distribution of carbonate facies is dependent, in part, on the width and shape of the shelf upon which they form.

Belize shelf

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Controlling Factors: Water Depth

Carbonate production occurs primarily within the photic zone, typically to maximum depths of 80 to 100 m. Deep-water carbonates can form to depths of over 350 m. Water depth strongly influences both the physical processes at work and the organisms present in the carbonate system.

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Because carbonate platforms have flat tops and relatively steep slopes and primary production is restricted to depths of around 50 m, highstands allow a broader area to be productive than lowstands.

Water Depth and Geometry

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Controlling Factors: Siliciclastic Influx

Carbonate systems tend to be poorly developed in regions of significant siliciclastic input due to turbidity, which blocks sunlight and clogs filter-feeding mechanisms.

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Controlling Factors: Water Temperature

Colder water can absorb more CO2 than warmer water, therefore, temperature affects carbonate saturation potential, making it more likely for carbonate-secreting organisms to exist in equatorial regions than in polar regions. However, some carbonate-producing organisms do prefer cooler (below180C) water conditions

Carbonate saturation levels

Warmer temperatures increase chemical reaction rates, leading to a greater likelihood of precipitation of chemical grains.

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Associated with ocean volume and, therefore, ionic concentrations, glacial episodes favor formation of chemical grains and cement in the form of aragonite and high-Mg calcite; whereas, interglacials tend toward precipitation of calcite, which is more stable (weathers more slowly) under subaerial conditions.

Controlling Factors: Water Chemistry

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Controlling Factors: Sunlight

The vast majority of carbonate-producing organisms are phototropic and/or photosynthetic or symbiotic with other organisms that are so and, therefore, require shallow, clear water to thrive. Thus, carbonates decrease poleward because of less winter sunlight and near clastic sources because of turbidity.

Sunlight can penetrate to around 100 m depth but is most effective in the upper 10 to 20 meter of the ocean.

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Controlling Factors: Salinity

Salinity is a measure of the amount of dissolved solids contained within the water mass, dominated in seawater by the cation Na+ and anion Cl-. Most carbonate organisms prefer salinites between 32 and 400/00. Salinities too high kill most organisms, while those too low lead to dissolution. Higher salinity enhances the production of chemical grains.

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Controlling Factors: Substrate

Different organisms prefer different types of substrates, depending upon feeding habits, methods of locomotion, and needs for defense.

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Organisms are often measured in terms of body size, diversity, and total abundance. High energy environments tend to favor high diversity and large body size; whereas, quiet water settings seem to foster high abundance. Energy conditions impact water circulation, which, in turn affects oxygenation.

Controlling Factors: Water Energy

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Controlling Factors: Organic Compounds (nutrients)

Coastal runoff and upwelling rich in organic nutrients favor production of benthic and planktonic algae; whereas, reef-building organisms prefer more clear waters.

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The rate of carbonate production for shallow, warm-water reefs is rapid and can usually outpace the rate of accommodation production through 3rd-order cycles. Glacio-eustatic rises and fault-related basin subsidence are among the few exceptions. Uplift can place deeper water facies into shallower water settings. Once accommodation space is filled (production reaches sea-level), it shuts off or progrades leeward as sediment is washed off the platform top (highstand shedding).

Controlling Factors: Accommodation

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The types of organisms forming organic build-ups have changed through time.

Controlling Factors: Evolution

Cretaceous Rudist Reef Modern Coral Reef

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Unlike clastics, grain size is not necessarily an indicator of energy conditions. Rather, in carbonates, energy regime is demonstrated by the amount of matrix that has or has not been winnowed from the deposit.

Differences with Clastics: Grain Size

Low energy High energy

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Carbonate Platforms

Carbonate platforms consist of thick accumulations of shallow water carbonate sediment separated into a number of zones based on water depth and energy conditions. As carbonates build to sea-level, platform tops tend to be very flat.

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Types of Carbonate Platforms

Carbonate platforms can be attached to or detached from the mainland and may or may not have a rimmed margin isolating the main platform from marine processes.

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Ramp Model

Ramps are platforms that slope gently (<10) basinward. Carbonate sediment is produced by organisms or chemical processes. Some of this material remains in place (in situ), and some is transported short distances by waves, tides, and oceanic currents to produce a variety of environments. Carbonate ramps are morpholigically similar to clastic shoreface and barrier island systems.

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Rimmed Platform Model

A rimmed platform has a reef or sand shoal near its outer (typically windward) edge, which absorbs wave energy and creates a broad restricted zone of low energy between the rim and the mainland or inner platform. Rimmed platforms require a major break in slope to produce the necessary energy requirements.

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Wilson Ramp Model

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Detached Platform/Bank

Detached platforms, also called carbonate banks, are flat-topped accumulations of carbonate sediment with steep slopes on both sides.

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Great Bahama Bank

The Great Bahama Bank began forming in shallow, warm water during the Jurassic over thinned continental crust as rifting began forming the Atlantic Ocean. Carbonate sedimentation has continued to keep pace with basin subsidence, forming a platform over 5-km thick that is completely isolated from clastic influx.

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Epeiric Platform

An epeiric platform is a shallow sea dominated by carbonate sedimentation that covers a large portion of a craton. These have been common and long-lived in the geologic past.

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Reef Model

Reefs are commonly defined as topographic build-ups formed dominantly by the growth of calcareous organisms. Reef systems are typically very-steep fronted rimmed platforms composed of three major components from land to see, a shallow quiet water lagoon, the reef itself (reef flat/top and reef front subfacies), which absorbs most of the wave energy, and quiet deep water fore reef zone.

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Fringing Reefs & Atolls

Reefs that form along the margins of an island are referred to as fringing reefs. Fringing reefs that form around subsiding volcanic islands become atolls.

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Reef Organisms

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Tidal Model

Carbonate tidal systems are relatively simple and structurally similar to clastic tidal systems, dominated by fine-grained sediment that is often rippled, bioturbated, and crossed by small channels.

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Laminated Mud

One of the most common facies in tidal, lagoonal, and deeper water environments is laminated mud produced by tidal or wave reworking of peloidal or bioclastic material. Calcareous algae is a common source of carbonate mud.

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Ripple-bedded Mud

Ripple-bedded mud is formed by reworking of carbonate mud by tides and waves and is common on tidal flats and lagoon floors.

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Structureless Mud

Structureless mud, composed mostly of peloidal and bioclastic material, accumulates in quiet water settings.

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Bioturbated Mud

Bioturbation can be abundant, particularly on tidal flats and the floor of lagoons.

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Thallasia/Hallimeda/Penicillus

Calcareous algae and grass are major components of carbonate mud.

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Bioclastic Packstone

Bioclastic packstone commonly represents material broken from a reef and can be found in lagoonal washover fans and as reef-front “talus”.

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Oncoidal Packstones

Peloidal packstones form in relatively quiet-water zones that occasionaly experience storm surges, such as lagoons or the foreslope/deep ramp. Mud accumulates and algal binding takes place during quiet water episodes. Oncoids are rolled over during storms.

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Intraclast Breccia (packstone)

Intraclast breccia is common in carbonates and has two common origins, reworking during a lowering of base-level and diagenetic dissolution.

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Edgewise Conglomerate (packstone)

Edgewise conglomerate represents storm deposition. Grains are rip-up clasts and can be imbricated.

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Bioclastic Grainstone

Bioclastic grainstone is common in high-energy zones, particularly along wave-dominated shorelines. Grainstones can also be produced by storms and are frequently cross-bedded.

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Ooidal Grainstone

Ooidal Grainstones form in warm, high-energy shoreline settings. Ooids are the most common form of coated grain.

Bahamas oolite shoal

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Organic Buildup/Boundstone

Organisms are the major source of sediment for carbonate systems. Organic buildups have a framework of organisms, such as corals, that bind other sediment together.

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Algal Structures

Algal structures, such as stomatolites, ar common on tidal flats and in shallow, subtidal environments.

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Cherty Limestone/dolostone

Chert is a common diagenetic alteration component of both limestone and dolostone.

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Great Barrier Reef, Australia

Reef

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Great Barrier Reef, Australia

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Barrier Reef, Lagoon, and Tidal Flat

Carbonate systems often include a reef, or other type of organic build-up, that is detached and separated from the mainland by a lagoon. The reef absorbs nearly all of the wave energy, while quiet water conditions exist in the lagoon. Portions of the reef can be “washed over” into the lagoon during storms. Tidal processes typically dominate the primary shoreline landward of the lagoon.

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Shark Bay, Australia

Tidal Flat

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Idealized Vertical Profiles

Most carbonate successions shoal (coarsen)-upward from deeper, quiet-water sediment to shoreline deposits.

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Large-scale Architecture

Carbonate sediment is produced primarily by organisms. Some of this material remains in place (in situ), and some is transported short distances by waves, tides, and oceanic currents to produce a variety of carbonate environments.

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A significant difference between carbonates and clastics is that facies tend to build vertically, rather than laterally, as organisms strive to maintain their relationship to base-level, producing thick, but often narrow, facies belts.

Differences with Clastics: Facies Belts

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Dolostones form primarily through diagenetic replacement of Ca in limestones with Mg. The processes associated with dolomitization are poorly understood, but four models have been proposed.

Dolomitization

Evaporite brine residue/seepage reflux model

Meteoric-marine/groundwater mixing model (obsolete)

Burial compaction/formation water model

Sea water/convection model