Pleistocene Sediments of southern Florida

Trevor Anson

Pleistocene Aged Rock UnitsMost of southern

Florida is underlain by marine sedimentary sequences.

These sequences are punctuated by pockets of freshwater limestones, and subaerial exposure surfaces.

This “wedge” of Pleistocene aged sediment is approximately 200 ft thick in the lower Keys.

Key Largo Limestone (KLL)Longest period of

deposition of these three rock groups.

Is mostly found as bedrock on the main chain of the Florida Keys.

Consists of wave resistant elements that were trapped in coral reefs.

Deposition occurred 120000, 130000 ybp

Fort Thompson Formation (FTF)

Can be recognized as an equivalent to the lower facies of the KLL.

Has both marine and freshwater facies.

Has a lower porosity than the KLL.

Miami Limestone (ML)

Recognized as the equivalent to the upper facies of the KLL.

Has two sub groups, a bryozoan and a oolitic facies.

Has a lower porosity than the KLL.

Deposition of oolitic facies occurred 120000-130000 ybp.

The “Quaternary Units”R.D. Perkins was

the first person to realize that the FTF, and the ML could be subdivided into 5 different units.

Q1-Q5, Q1 is the oldest unit.

Ages of Q Units

Perkins did not actually give dates for each of the Q Units, but based his assumptions off of data in a report by Richard Mitterer. The five units recognized by Mitterer (1975) are believed to correlate with the 5 Q Units of this study.

Mitterer recognized five distinct marine units in the Pleistocene, the upper four of which he dated as:Unit 5- 134,000Unit 4-180,000Unit 3-236,000Unit 2-324,000Unit 1- not dated.

Q1, Q2, and Q3 unitsQ1 is characterized by an abundance of quartz

grains. Mollusk fragments and benthic forams occur frequently. The common lithology of this unit contains quartz grainstones, mixed quartz/mollusk grainstones and mixed quartz/mollusk wackestones.

Q2 is characterized by grainstones and packstones containing mollusks and forams, however, red corralline algae is more common in Q2. This unit has a similar lithology to Q1.

Q3 is characterized by thick sequences of coral boundstones. Also contains wackestones and packstones with fragments of mollusks and forams. Multer et al. only found quartz grains in one core sample.

Q4 and Q5 units

Q4: Packstones and grainstones consisting of mollusk and foram fragments. Less coral boundstone than in Q3.

Q5: Corresponds to oxygen isotope stage 5. Coral and coral boundstone occurs, red algae is abundant as well as packstones and grainstones of mollusk and forams.

Unconformities between Q Units

Perkins noted that these five sequences are “bounded by discontinuity surfaces indentified by features including (a) vadose sediment, (b) land-plant root structures, (c) laminated crusts, (d) diagenetic soilstones, (e) soils and soil breccias, (f) solution surfaces, (g) bored surfaces, and (h) freshwater limestones.

These discontinuity surfaces were generally noted when they occurred in combination with each other.

Vadose Sediments: internal sediments that consist of lime mud and detrital quartz. During the Pleistocene was most likely introduced into cement-lined voids in the bedrock during periods of subaerial exposure. Pleistocene vadose sediment represents the last stage in the filling of these voids.

Land Plant Root Structures: The evidence of these strucutures created by calcification of roots are interpreted to have occurred during periods of subaerial exposure.

Laminated Crusts: Commonly found in Pleistocene sequences of south Florida, similar to caliche crusts. The laminated crusts found in southern Florida bear a resemblance to “algally” laminated sediments, but it has been shown they were created during periods of subaerial exposure.

Diagenetic Soilstone: Commonly occurs with root calcification and laminated crusts. Caused by diagenetic alteration of previously existing “textures” to a dense, mottled mudstone.

Soil Breccias: Commonly composed of poorly sorted, subangular fragments of host rocks. Occur as pockets of hardened soils or as paleosols.

Solution Surfaces: In southern Florida, these discontinuities are characterized by solution pipes. Believed to be initiated by leaching of limestones by plant activity.

Freshwater Limestones: Pockets of freshwater caused by low sea-level led to the formation of freshwater limestones.

Topography

The topography of southern Florida has had an important impact on sedimentation patterns and biotic distribution.

Sinkholes are common features in the Florida Keys. Sinkholes that developed in the KLL and the ML have been dated at approximetely 110000 +/- 20000 ybp.

Conclusions

Research of the Pleistocene aged rock groups of southern Florida is important for several reasons:The changes in sea-level that have been observed

are significant because they show how the edges of continents are impacted.

Understanding the hydrogeology of southern Florida bedrock allows for understanding of modern environmental concerns.

Works Cited Daley, G.M., 2002: Creating a Paleoecological Framework for Evolutionary and Paleoecological Studies: An

Example from the Fort Thompson Formation (Pleistocene) of Florida. Palaios, v. 17, p. 419-434.

Dodd, R.J., C.T. Siemers, 1971: Effect of Late Pleistocene Karst Topography on Holocene Sedimentation and Biota, Lower Florida Keys. Geological Society of America Bulletin, v. 82, p. 211-217. Hickey, T.D., 2010: Pleistocene Carbonate Stratigraphy of South Florida: Evidence for High-Frequency Sea-Level Cyclicity. Journal of Coastal Research, v. 26, p. 605-614.

Hickey, T.D., 2010: Pleistocene Carbonate Stratigraphy of South Florida: Evidence for High-Frequency Sea-Level Cyclicity. Journal of Coastal Research, v. 26, p. 605-614.

Hoffmeister, J.E., H.G., Multer, 1968: Geology and Origion of the Florida Keys. Geological Society of America Bulletein, v. 79, p. 1487-1502.

Hoffmeister, J.E., K.W., Stockman, H.G., Multer, 1967: Miami Limestone of Florida and Its Recent Bahamian Counterpoint. Geological Society of America Bulletein, v. 78, p. 175-190.

Mitterer, R.M., 1974: Pleistocene Stratigraphy in southern Florida Based on Amino Acid Diagenesis in Fossil Mercenaria. Geology, v. 2, p. 425-428.

Multer, H.G., E. Gischler, J. Lundberg, K.R. Simmons, E.A. Shinn, 2002: Key Largo Limestone Revisited: Pleistocene Shelf-edge Facies, Florida Keys, USA. Facies, v. 46, p. 229-272.

Neal, A., M., Grasmueck, D.F., McNeill, D.A. Viggiano, G.P., Eberli, 2008: Full-Resolution 3D Radar Stratigraphy of Complex Oolitic Sedimentary Architecture: Miami Limestone, Florida, U.S.A. Journal of Sedimentary Research, v. 78, p. 638-653.

Perkins, R.D., 1977: Depositional Framework of Pleistocene Rocks in Southern Florida. Geological Society of America Bulletein, v. 147, p. 131-198.

Schmoker, J.W., T.C. Hester, 1986: Porosity of the Miami Limestone (Late Pleistocene), Lower Florida Keys. Journal of Sedimentary Petrology, v. 56, p. 629-634.

http://academic.emporia.edu/aberjame/ice/lec09/lec9.htm

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