1 - 4 Carbonates Flashcards
What are the main differences between carbonate and siliclastic depositional systems?
Carbonates are produced in FACTORIES, of which there are three kinds: warm-water, cool-water and pelagic. Carbonates are often deposited where they are produced, in contrast to siliclastics which are transported.
When they are subaerially exposed, sea-water within the pores of carbonates is replaced with acidic rainwater, which dissolves calcite. This calcite is reprecipitated in lower facies, leading to early lithification (over a period of several thousand years). The lithified rocks are relatively resistant to erosion. In humid climates, dissolution may result in karst landscapes, in which the dissolved calcite is lost to groundwater.
In contrast to siliclastic systems, carbonate production is greatest during relative sea-level rise, during which time the greatest extent of platform is flooded. Production is much lower during sea-level fall, potentially restricted to fringing reefs.
South Florida (carbonate platform)
A modern example of an ATTACHED rimmed platform.
- A sheltered LAGOON of limited fauna (MOLLUSCS, FORAMINIFERS, GREEN ALGAE), in which BIOCLASTIC MUDSTONE AND WACKESTONE is deposited; geochemical analysis demonstrates that this comes from BIOEROSION of calcareous skeletons rather than chemical precipitation.
- In places, these muddy sediments are swept by storm waves to form shallow-water MUD MOUNDS which are stabilised by THALASSA SEA-GRASS. These mounds MIGRATE landwards and, if they rise above sea-level, are colonised by MANGROVES. Shells accumulate on the beaches to form gravel-sized molluscan GRAINSTONES AND PACKSTONES.
- The FLORIDA KEYS are formed of LITHIFIED PLEISTOCENE SAND SHOALS and coral reefs (Key Largo Limestone). They ISOLATE the lagoon from the backreef.
- The BACKREEF contained PATCH REEFS (FRAMESTONES) which are BIOERODED to form surrounding SKELETAL GRAINSTONE SANDS and gravels.
- The REEF forms FRAMESTONES.
- The REEF FRONT contains SPURS AND GROOVES formed by the outward growth of corals.
- The FOREREEF contains TALUS deposits.
Great Bahama Bank (carbonate platform)
This isolated rimmed platform has THREE main depositional environments:
- CORAL REEFS develop in the HIGH ENERGY conditions of the windward margin, forming FRAMESTONES.
- OOLITES develop in the shallow TURBULENT waters of ELEVATED SALINITY (~38) along the LEEWARD margin, producing OOLITIC SANDS.
- CARBONATE MUDS are deposited in the CALM and HIGHLY SALINE (40 - 45) waters of the platform INTERIOR.
Sequence stratigraphy of a rimmed platform
Carbonate production is proportional to the area of platform flooded to the OPTIMUM DEPTH OF PRODUCTION (to 50m depth but greatest between 10 - 30m). Rimmed platforms have FLAT TOPS and STEEP SIDES (due to sub-vertical growth of coral reefs); production is high when sea-level rises to flood the platform top, but very low when the top is exposed and only a narrow area of slope lies in the productive depth range. At such a time, a FRINGING REEF will develop, its size controlled by the ANGLE OF SLOPE, and will deposit a thin wedge.
Llucmajor Platform, Mallorca
The LLUCMAJOR PLATFORM of MALLORCA, exposed along cliffs between CAP BLANC and CALA PI, demonstrate a highly productive MIOCENE rimmed platform. We can see two sequences. In the first sequence, a thin FSST develops low on the slope of the previous HST, a thick TST shows upwards-thinning, and a prograding HST is eroded by the second sequence boundary. The second sequence resembles the first, with a low thin FSST and LST and a thick TST. Through the section there is NO RETROGRADATION and NO MFS, highlighting the need to modify siliclastic models for highly productive carbonate systems.
We can trace the shallowest coral reef zone (PORITES) and use this as a proxy for sea-level, revealing an amplitude of about 30-40m.
Furtermore, at the end of the Miocene, falling sea-levels isolated the Mediterranean from other oceans; high evaporation led to the precipitation of evaporites and the resultantly high Mg/Ca ratio led to DOLOMITISATION which is seen in this section.
Sediment partitioning in carbonate systems
In mixed CARBONATE-SILICLASTIC systems we see SEDIMENT PARTITIONING. As relative sea-level rises, carbonates are productive while siliclastic deposition is restricted to coastal plains. As relative sea-level falls, carbonate production falls and siliclastic supply increases, but siliclastics BYPASS the platform and are deposited on the shelf slope. A clear example of this is seen in CAINOZOIC deposits on the GREAT BARRIER REEF.
In mixed CARBONATE-EVAPORITE systems we see similar partitioning. As relative sea-level falls in arid, isolated basins, precipitation of evaporites in the basin can increase the RATIO OF MG/CA leading to DOLOMITISATION. This process formed the magnesian limestones of County Durham.
Carbonate parasequences and stacking
As in siliclastic systems, parasequences are METRE-SCALE SHALLOWING-UPWARDS CYCLES.
Thick deep marine deposits represent rapid rise in sea-level.
The THICKNESS OF SUCCESSIVE PARASEQUENCES reflects longer-term cycles; e.g. thickening upwards reflects increase in the rate of creation of accommodation space.
Carbonate ramps
The southern ARABIAN GULF provides a modern example of a carbonate ramp. The OUTER RAMP is characterised by MARLS; the MID-RAMP (SWB to FWWB) by muddy and then CLEAN MOLLUSCAN SANDS. The INNER RAMP contains OOID BARRIER ISLANDS backed LAGOONS (restricted fauna, chemically precipitated lime mud, PELOIDAL WACKESTONES), intersected with TIDAL CHANNELS (cuts into bedrock, LITHOCLASTS), behind which are SABHKAS (EVAPORITES such as ANHYDRITE, GYPSUM, DOLOMITE). The lagoon and sabkha is a feature of arid-climate ramps, and is not representative of cool-water factories.
Carbonate ramps have SHALLOW RELIEF, which leads to fewer differences between HST, LST and FSST. Relatively small changes in sea-level can shift the LOCUS OF SEDIMENTATION considerable distances so, for example, we may see distal starvation and condensed layers with TSTs. This is seen in the Jurassic deposits at ZARGOZA.
Because they do not have highly productive reef communities, carbonate ramps are LESS PRODUCTIVE overall and with fewer differences in PRODUCTIVITY WITH DEPTH.
Sequence stratigraphy resembles siliclastic systems, in that we see RETROGRADATION during flooding, and a clear MFS. This is clear in the Jurassic deposits at ZARGOZA. Although there is potential for steepening of the ramp during HST, all of the tracts maintain SIMILAR DIP PROFILE.
How can computer modelling aid our understanding of carbonate platforms?
Models such as SEDTEC2000 use ALGORITHMS to PREDICT the development of carbonate stratigraphy according to user-set ENVIRONMENTAL PARAMETERS. They are used TEST HYPOTHESES and objectively RECONSTRUCT UNKNOWN PARTS of stratigraphic sections.
The algorithms are partially based upon EMPIRICAL DATA taken from study of Holocene platforms, for example patterns of deposition or erosion. However, some environmental parameters such as RATES OF CARBONATE PRODUCTION, RATES OR EROSION and SEA-LEVEL CHANGE can be altered by the user, to obtain different outputs.
The model runs for a set number of TIME STEPS, in each of which sediments are deposited, transported or eroded accordingly. The final ouput is a STRATIGRAPHICAL CROSS-SECTION
However, models can only demonstrate what is FEASIBLE. For example, the model found that two different hypotheses of the origin of carbonate mud in the mid- and outer-ramp at ZARGOZA (1. pelagic, 2. reworked inner-ramp) matched the record.
