MIAMI FLORIDA STRUCTURE (CRATER)
by: Charles O’Dale
- Type: Central peak?
- Location: N 25° 45’ W 80° 07’
- Age ma: <40 Geological Estimatea – CRETACEOUS
- Diameter: ~1 km
a Dating Method: Impact after the deposition of the Florida Plateau
Florida Continental Shelf
Geologists estimate the age of the Earth at about 4.6 billion years. An understanding of the geologic processes of Florida begins over 40 million years ago when the region was at the bottom of the ocean. The Florida peninsula is actually the emerging portion of a tectonic platform called the Florida Plateau; where the Florida peninsula and adjacent continental shelf occupies this plateau region. It has been suggested that the Florida plateau, since it is composed of a thick section of mostly undeformed carbonate rocks, that this carbonate platform that has been developing since the late Triassic opening of the North Atlantic which occurred about 180 million years ago. The development of the platform was controlled by regional subsidence of the passive margin and eustatic sea level changes which allowed the deposition of a thick section of carbonate rock over many millions of years. NOAA Nautical Chart 11468 indicates the structure is in less that 20′ of water.
Discovered by Cory Boehne using NOAA Bathymetric Viewer, 5/11/2012 The current assumptions are as follows:
- The structure appears to be roughly 1km in diameter.
- Non-Karst related (evidence: No major karst features in area, 1km would be quite large for a karst structure, karst structures do not feature a central peak)
- Young feature (Soft sedimentary rock, heavy erosion zone – this means that the feature is most likely quite young)
- Feature shows ‘text-book’ impact structure, with central peak surrounded by concentric rings, along with what appears to be an ejecta field to the north-west of the feature.
- Additionally, there is a very clear outflow of sediments from this feature, running roughly east-north-east from the feature.
Currently, this has been ran past two geologists with relevant credentials:
- Dr. Peter Draper (FIU) – confirmed that was very unlikely to be karst related
- Dr. Andrew Glikson (Climate Change Institute) – agrees that it is probable this is an impact feature – confirmed expectation of young feature
Comments by Cory Boehne:
The Florida shelf is a carbonate platform that is approximately 100 km in width off of St. Augustine tapering to less than 2 km by West Palm Beach to the south
The East coast shelf is a higher energy coast influenced by the Gulf Stream in the Atlantic and comprised of siliciclastic dominated sediments to the north, grading into a mixed siliciclastic-carbonate sediment composition at approximately the latitude of Ft. Lauderdale/Miami. Rapid population growth to the coast has impacted shelf habitats, including offshore reefs—recent observations of macro algae blooms off of Broward County shelf is suspected to be the result of nutrient rich submarine groundwater discharge of anthropogenic origin (runoff, wastewater point and non-point sources). I’ve been working for a couple of years on a structure in Miami which I have yet to be able to find good reason to disprove. Having just gotten a very positive e-mail from Dr. Andrew Glikson, a month or so ago, I feel as though it is time to proceed to sampling and testing. At this point, I feel we have several challenges, and would like to get input on the matter. As this is an underwater structure, although shallow, this does present a different set of problems in sampling and analysis than those I am used to. This is in carbonate material, I’m very familiar with typical impacts in quartz-rich materials, but I’ve not seen much in the way of 100% carbonate impact zones. Questions about the expected effects in such a material are, to my knowledge, not well answered. For example, should we try to find planer deformation of grains, or will we be better advised to look for melt/breccia zones and shatter cones?
I would expect any iron to have disintegrated/oxidized due to interaction with seawater, but I intend to examine any samples for secondary mineralization that may have occurred as a result of this weathering process and interaction between the iron and seawater/carbonate country rock. There does appear to be a relatively substantial ejecta field to the north-west of the structure, and I’ve considered sampling this, but am unsure as to the viability of that material, especially given the likelihood of coral development on these rocks and my reluctance to disturb any living animals for sampling purposes, ethically and legally this could present significant challenges. As the structure is underwater, in a heavy erosion zone, my assumption would be that it is probably partially buried, but given the extreme weather that affects this area, that can likely change drastically and quickly. Theoretically, a major storm could completely unbury the feature, or could drop 5-10 feet of sediment on it. I’ve not been able to find further mapping data – this is essentially a VERY homogeneous area geologically, so there hasn’t been a great deal of remote sensing done here from what I can tell, especially at scales appropriate to this structure. If you know of any further remote sensing data or maps, they would be most welcome. Further complications involve proximity to both Fisher Island and the Government Cut, along with the fact this lies alongside a ‘dump’ site. Fortunately, it is NOT within the nearby protected marine sanctuary. To my knowledge no one has yet dived and explored the site or collected material. I currently have no samples, however, it is my intention to attempt to gather some hand samples that might reveal the character of this material. I’m also seriously considering the possibility of a core-sample, despite the serious costs associated with that investigation. One consideration is having a small ‘collar’ sub-surface support structure built and sunk within a few weeks, although this could present problems with laws meant to protect coral reefs. Although our first attempt will be simply hand-sampling with simple tools using SCUBA equipment. If anyone is interested in helping with this, I would be very glad of advice or actual hands-on assistance.
Once we have the samples we will proceed with macro and micro level examinations and characterization, hopefully revealing at least several diagnostic features that would be expected in a crater of this size.
Sinkholes are a common feature of Florida’s landscape. They are only one of many kinds of karst landforms, which include caves, disappearing streams, springs, and underground drainage systems, all of which occur in Florida. Karst is a generic term which refers to the characteristic terrain produced by erosional processes associated with the chemical weathering and dissolution of limestone or dolomite, the two most common carbonate rocks in Florida. Dissolution of carbonate rocks begins when they are exposed to acidic water. Most rainwater is slightly acidic and usually becomes more acidic as it moves through decaying plant debris.
Limestones in Florida are porous, allowing the acidic water to percolate through their strata, dissolving some limestone and carrying it away in solution. Over eons of time, this persistent erosional process has created extensive underground voids and drainage systems in much of the carbonate rocks throughout the state. Collapse of overlying sediments into the underground cavities produces sinkholes. When groundwater discharges from an underground drainage system, it is a spring, such as Wakulla Springs, Silver Springs, or Rainbow Springs. Sinkholes can occur in the beds of streams, sometimes taking all of the stream’s flow, creating a disappearing stream. Dry caves are parts of karst drainage systems that are above the water table, such as Marianna Caverns.
Other subterranean events can cause holes, depressions or subsidence of the land surface that may mimic sinkhole activity. These include subsurface expansive clay or organic layers which compress as water is removed,collapsed or broken sewer and drain pipes or broken septic tanks, improperly compacted soil after excavation work, and even buried trash, logs and other debris. Often a depression is not verified by a licensed professional geologist or engineer to be a true sinkhole, and the cause of subsidence is not known. Such events are called subsidence incidents.
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Lane, E., 1987, A guide to rocks and minerals of Florida; Florida Geological Survey Special Publication 8 (revised), 61 p.
Lane,E. (editor), 1994, Florida’s Geological History and Geological Resources, FGS Special Publication 35, 76 p.
Lane, E. ed. 1994. Florida’s Geological History and Geological Resources Special Publication No. 35, Department of Environmental Protection, Florida Geological Survey, Tallahassee, FL.
Randazzo, A.F. and D.S. Jones, eds. 1997. The Geology of Florida, University Press of Florida, Gainesville, FL.