3 - Shallow Subtidal Carbonate Depositional Environments

What factors drive carbonate factories?

Carbonate production is largely bio-chemical, favoured by:

• Light

• Nutrients

• Warm water § Elevated salinity

• Protection from terrigenous influx

Production maximal in shallow, (sub) tropical areas: carbonate shelves

How are carbonate depositional environments and lithofacies reflected in stratigraphy?

• Distribution of modern sediments reflects physical environment: water depth, nutrient supply, energy

• Lithofacies: biogenic & abiogenic sediment composition

• Facies analysis (macro- and microfacies): reconstruct depositional environments & processes

What are the tidal zones of carbonate deposition?

The peritidal facies group may encompass all three zones:

• Subtidal

• Intertidal

• Supratidal

What are the features of peritidal facies?

• most peritidal facies develop where tidal range is low (<2m)

• different facies mosaics and diagenesis/hydrology under humid and arid conditions

• topography can influence development of islands and lagoons

• sediment source: subtidal

• channels rework sediment and focus onshore sediment transport during storms

• low faunal diversity

What are the features of peritidal facies supratidal zones?

• Presence of plant debris, rootlets Fenestrae (mangroves), algal mats

• Mostly muddy sediment

• Depositional processes and sedimentary structures:

  • Desiccation cracks

  • Reworking of lithified crusts by storms: mud chips, flat-pebble conglomerates

  • Fenestrae

What are the features of peritidal facies intertidal zones?

• Microbial mats

• Mangrove rootlets

What are the features of peritidal facies microbial mats & stromatolites?

• Presence of rootlets (mangroves)

• Low-diversity, high-abundance skeletal fauna

• Mostly muddy sediment

• Depositional processes and sedimentary structures:

  • Desiccation polygons, fenestrae

  • Crinkly (microbial) lamination

  • Bioturbation in lower intertidal

What are the features of peritidal facies subtidal zones?

• Includes ponds and tidal channels

• High-diversity, high-abundance skeletal fauna

• Depositional processes and sedimentary structures:

  • Bioturbation

  • Sediment transport via tidal channels: fining-upwards successions, levee deposits

What are the features of arid climate peritidal facies supra/intertidal zones?

• Depositional processes and sedimentary structures:

  • Halite crusts, gypsum, anhydrite

  • Dolomite

  • Algal mats

  • Desiccation cracks, teepee structures

  • Wind-blown/storm-deposited sand and mud

• Early diagenetic evaporite precipitation can disrupt/obscure depositional texture

Describe platform exposure

1. Rainwater reacts with soil to release CO 2 and form mild acid

2. Downward percolation may follow pre -existing structural trends

3. In humid setting create dense network of interconnected caves

4. Subtle karst, calcretes in (semi) arid climate

What are the features of shallow subtidal environment lagoons and open shelves?

• Moderate-/low-energy environments

• Muddy sediments, peloids, bioclasts (lime mud/wacke/packstones)

• Optimal conditions for fauna in shallow waters often lead to fossil-rich bioclastic limestones (float/rudstones)

• Fossil content will depend on salinity and substrate nature

• Micritization by cyanobacteria in illuminated shallow water

• Often pervasively burrowed

• Separation based on degree of connection to open ocean:

Open shelf

• No restriction, normal salinity

• Abundant & diverse fauna does not tolerate salinity variation (brachiopods, echinoderms, bryozoa…)

Lagoon

• Restriction, elevated salinity, storm recharge: salinity fluctuations

• Abundant but low-diversity fauna tolerates salinity variation (gastropods, algae, ostracods…)

What are the features of shallow subtidal environment carbonate sand bars/shoals?

• High-energy & high carbonate production rates: shoreface, shelf edge, areas of strong currents (tidal, storms)

• Grainy sediments – important reservoirs

• Analogous to siliciclastic shorelines & sand waves

• Usually very clean, well-sorted carbonate sands

• Lateral and vertical heterogeneous: finer horizons, cemented beds (caliche, beachrock)

What are the features of shallow subtidal environment shelf-margin shoals?

• Stable sand bodies, composed of oolitic and/or skeletal and peloidal carbonate sands

• Geometry and orientation strongly influenced by waves and tidal currents

What are the features of shallow subtidal environment shorefaces?

• Fairweather wave base to low tide

• Sand spits (longshore drift) and mobile sand shoals

• Beach-barrier island complex

  • Tidal channels

  • Stable, low-gradient coast

• Strandplain systems

  • Low/moderate topography, often rocky coastlines

• Depositional processes and sedimentary structures:

  • Wave-controlled sedimentation

  • High rates of carbonate sediment supply

  • Low-angle cross-stratification in shoreface oolitic/skeletal grainstones

  • Early lithification: beachrock