Key Largo Limestone vs. Miami Limestone:
Biscayne Bay is located near the start of the Florida keys and where the Miami River deposited sediment. As a result, this area contains both Key Largo and Miami limestone. Note that in the image below, the northern part of Biscayne Bay is solely composed of Miami Oolite (limestone) while the south half is Key Largo limestone.
Key Largo limestone is typically light gray in color. It is composed of fossilized coral and contains CaCO3 (also known as calcium carbonate). Key Largo limestone was formed in a warm, shallow environment, very similar to the conditions of present-day Biscayne Bay. Key Largo limestone is developed due to the process of permineralization. Over time, hard coral skeletons become replaced with minerals, converting them to rock. Deposits of Key Largo limestone found in the Florida Keys extend from Soldier Key to Dry Tortugas National Park. Bryozoans and corals are two common fossils found in Key Largo limestone. Bryozoans are a large complex of individual animals. Modern-day sea fans are similar to Fenestrellina fossils pictured below.
Coral fossils come in all different varieties. Although brain, tabulate, and horn coral fossils appear slightly differently, fossil hunters can decode coral fossils from bryozoans due to the presence of fossilized polyps. The image below is a diagram of a polyp.
Miami limestone is dark gray in color. It is found in southeastern Florida and is the bedrock for the Everglades. Unlike Key Largo limestone, Miami limestone was formed due to depositional forces due to the low sea level about 100,000 years ago (during Early Wisconsin). The deposition has allowed this sedimentary rock to be composed of fossilized coral, shells, and quartz ooids. Ooids are a geological term that refers to large spherical debris.
Logging Tasks:
At the GZ, you should be standing on a boardwalk overlooking Biscayne Bay. With your back towards the Bay, you should notice limestone rocks embedded in the wall that lines the walkway and large limestone boulders slightly farther away in the grass. The rocks that are a part of the wall are considered Key Largo limestone, while the boulders are classified as Miami limestone.
- Compare the Key Largo limestone to the Miami limestone. What are some noticeable differences in texture, appearance, and composition?
- Do the examples of Key Largo limestone contain mostly fossilized bryozoans or corals? Explain your rationale.
- A few feet away from the walkway is a boat launch. What sediments are found on this tiny strip of beach? What does this infer about the underlying bedrock of Biscayne Bay?
- (optional) In your log, provide a photo of yourself, your GPSr, or a personal item at this site.
Sources:
https://mrdata.usgs.gov/geology/state/sgmc-unit.php?unit=FLPSk%3B0
https://www.floridastateparks.org/learn/geology-windley-key-fossil-reef-geological-state-park
https://www.thesevenseas.net/wp/wp-content/uploads/2017/06/indonesian-fossil-coral.pdf
http://www.geosciences.fau.edu/events/virtual-field-trips/miami-limestone/index.php#:~:text=Miami%2DDade%20%26%20Broward%20counties%2C%20Florida&text=Osmond%2C%20Carpenter%2C%20and%20Windom%20
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