Structure and formation of recent limestone (Miami & Key Largo) of Southern Florida

This post provides a brief overview of the formation and characteristics of recent limestone deposits in Southern Florida, the Miami Limestone and Key Largo Limestone. Most of this material was collected as part of a presentation for EOS 404S: Geology of Tropical Marine Environments in Spring 2013, but has been expanded and re-formatted here.

I. Common Roots

Both the Miami and Key Largo Limestones formed during the Pleistocene due to sea level change, but they were created by different processes/organisms. Basically, sea level was 100 ft higher 125 thousand years ago. Then, 20 thousand years ago, glacier formation caused to sea levels dropped, exposing the reefs and creating limestone islands.

  • >100-125 kya
    • Interglacial ⇒ higher sea levels (100 ft) ⇒ CaCO3 producers!
  • <20 kya
    • Glacation ⇒ sea level drops (300 ft) ⇒ exposing reefs ⇒ fossilize

II. Miami Limestone

Miami Limestone Map

Image by Neal et al.

The Miami Limestone occurs in the southern part of main Florida and in the southernmost Keys. It consists of two layers, an oolitic facies and a bryozoan facies. The oolitic facies is located near the Atlantic Coast (think Miami), as ooids require energy to form. The bryozoan facies, which only occurs on the Florida main, is located more centrally (think Everglades) because bryozoans prefer quieter waters.

  • Oolite: spherical grains of limestone
    • Atlantic Coastal Ridge: longshore currents pushed ooids up
  • Bryozoans: “tiny moss animals”
    • Central: prefer quieter waters

IIA. Oolitic Faces

Miami Limestone Cross-bedding

Photo from SEPM STRATA.

Ooids are small (<2 mm), round, white grains that form when calcium carbonate precipitates around a nucleus (such as a shell fragment); they typically form areas of high wave energy, such as a tidal bar and channel system, where the ooids can be rolled back and forth.

  • Well-sorted ooids (grainstone)
  • Varying skeletal material (corals, echinoids, mollusks, algae)
  • Some quartz sand
  • Cross-bedded
  • White to orangish gray
  • 3-5 m thick (deeper seaward edge)
  • “Upper unit”

Ooids

  • Form in high energy environment
  • “Seed” washed around by waves
  • Collects CaCO3
  • Eventually buried
    • Bigger oolith =  “lived” longer

IIB. Bryozoan facies

Bryozoan facies of Miami Limestone

Image by Halley et al.

Bryozoans are colonial animals that excrete calcium carbonate skeletal structures (similar to coral, but they belong to a different phylum and are more complex, with organs). The species Schizoporella floridana created most of the bryozoan facies in Florida.

  • Schizoporella floridana with ooids
  • White to orangish gray
  • Grainstone & packstone
  • Average 10 ft thick, 1 ft on Gulf side
  • “Lower unit”

Schizoporella floridana

Schizoporella floridana bryozoan

Photo by GSU.

  • Habitat: harbors, bays, mangroves, temperate zone reefs to 50 m in tropical waters
  • Colony height 10-15 cm, width 15-30 cm
  • Different from coral: more complex (organs), smaller colonies

III. Transition from Miami to Key Largo

IV. Key Largo LimestoneKey Largo Limestone Map

  • Coral in calcarenitic matrix
    • Calcarenite: over 50% detrital (transported) sand-sized carbonate grains
  • Seaward edge: grainstones
  • Gulf edge: wackestones, packstones
  • Thickness varies (up to >60 m)
  • White to light gray
Brain coral at Windley Key Quarry by Cortney Cameron.

Brain coral at Windley Key Quarry by C. Cameron.

The Reef: Then vs. Now

  • 7 km (5 mi) inward
  • 7 m higher sea level
  • Main: Montastrea annularis
  • More: Halimeda
  • Less: Millepora, coralline algae
  • No: Acropora palmata
  • “Complex of shallow-water shelf-margin reefs & associated deposits along a topographic break”

ancient-key-largo-reefs

V. Summary

 Miami Limestone

 Key Largo Limestone

  • Pleistocene
  • Oolitic & bryozoan
  • Everglades & Lower Keys
  • Pleistocene
  • Coralline
  • Upper Keys

Miami Limestone

Photo by FL DEP.

Key Largo Limestone

Photo by FL DEP.

VI. Resources

  • The best concise but thorough resource for the structure and formation of recent Southern Florida limestones can be found in this paper: "Geology and Hydrogeology of the Florida Keys" by Halley, R. B., H.L. Vacher, H. L., and Shinn, E. A. (1997) in Geology and Hydrology of Carbonate Islands. Developments in Sedimentology.
  • A physical description of the Miami Limestone by the USGS.
  • A physical description of the Key Largo Limestone by the USGS.
  • This paper by Haley et al (1977) in the AAPG Bulletin discusses the formation of ooids and the Atlantic Coastal Ridge near Miami.
  • This paper by Neal et al (2008) in the Journal of Sedimentary Research has some neat 3D radar-generated maps of the Miami Limestone.
  • A discussion of geology in the Everglades can be found here.
Cortney

About Cortney

Geology lover. Proud owner of a 2014 Honda CRF250L. Grew up on NES, N64, & Gameboy. Collects maneki neko (lucky cats). Married to a gearhead. Email cortney@luckysci.com. Wishlist.

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