U.S. Department of the Interior
U.S. Geological Survey
OFR-02-325

>Executive Summary Introduction Study Area & Objective Methods Results Summary & Recommendations Acknowledgements & References Statement of Work Appendix A Appendix B PDF Version

The Northwest Dade County Freshwater Lake Plan Area (commonly referred to as the Lake Belt Area) is vital to the future planning and development of southeastern Florida. This area is located within one of the most environmentally sensitive parts of the state - the eastern borders of the Everglades National Park (ENP). The Lake Belt Area and Water Conservation Area BB (WCA BB) provide half of the limestone mining resources used in the state every year. Starting in the mid-1800s canals and levees were built in the area to drain and help develop economic and water resources including protection from floods and droughts. These construction projects have changed the natural water flow (hydropattern and hydroperiod) through the hydrologic system. Changes to the hydropattern and hydroperiod of the area have also had an adverse impact by disrupting the normal breeding patterns of species within the Everglades ecosystem.

The last several years there has been much attention focused on the restoration of the natural hydropattern of the greater Everglades ecosystem including the Lake Belt Area. Water management is key to balancing the needs of Everglades Restoration, flood protection, and economics of the adjacent communities. The South Florida Water Management District (SFWMD) and US Army Corps of Engineers (USACE) will implement the Comprehensive Everglades Restoration Plan (CERP), including several major components that are located within the Lake Belt Area. Successful implementation of CERP will require a thorough understanding of the geology and hydrogeology of the Everglades and specifically the Lake Belt Area.

South Florida has a long history of geologic investigation and this report incorporates much information from previous studies. There has been a wealth of studies of the Quaternary geology of south Florida, but until recently the pre-Quaternary geology has been less studied. This study relies heavily on several of these studies including USACE Serial No. 20 (1953), Nemeth and others (2000), Reese and Cunningham (1999) and Cunningham and others (2001).

The Lake Belt Area consists of approximately 230 km² (89 mi²) located in north central Miami-Dade County, south of the Miami-Dade/Broward County line. Water Management structures for this area includes the East Coast Protective Level (ECPL) system and a series of drainage canals.

The objective of this study was to develop a better understanding of the geology underlying the Lake Belt Area by conducting a high-resolution, seismic reflection survey of area canals. High-resolution, seismic reflection profiling is a water-based towed geophysical technique that can be used to identify subbottom geologic features. More than 109 line-kilometers (68 linemiles) of data were collected from 8 major canals plus the canal adjacent to the ECPL. Quality of profile data varied between good to moderate and poor depending on numerous variations in canal structure and lithology. Acoustic reflections were recorded from depths that varied greatly from 3 to 79 m (10 to 260 ft), but generally usable data were recorded from to 30 m (100 ft) below sea level. Approximately 80 per cent (84 km; 52 mi) of the canals yielded usable data including C-9, C-6, L-33, L-30, Wellfield Recharge, L-31, and Black Creek. Three canals from which there was no usable data include Tamiami Trail, Bird Drive, and the Snapper Creek Canal extension. The usable data were integrated with information from reports, published and unpublished core sections, original core descriptions from the SFWMD files, and personal communication with other researchers familiar with the study area.

In all canals surveyed, the Holocene sediments and Pleistocene Miami Limestone were removed during canal construction. The bottom of most canals has an acoustically transparent layer of undifferentiated muck. The first solid reflection recorded is the original surface left from canal construction within the Pleistocene Fort Thompson Formation that is typically composed of marine limestone, minor gastropod-rich freshwater limestone, and sandy limestone. The Fort Thompson Formation as described from core descriptions (pers. comm., see Appendix B) is a vuggy, hard to weak limestone (hard is described as good induration or 'the foundation was hard, solution-riddled limestone' USACE Ser. 20; weak is inferred as less dense friable limestone or poor induration). The rock-fabric facies within the Fort Thompson Formation stratigraphic cycles is moderately variable, but is conformable around much of the Lake Belt study area. The thickness of this unit varies from ~1 to ~25 m (~3 to ~80 ft). Beneath the Fort Thompson Formation are irregular alternating layers of sand, silt and limestone of the Tamiami Formation.

An acoustically transparent layer of undifferentiated sediment overlies a coherent reflection, which is the dredge surface of the canal. This surface is within the Pleistocene Fort Thompson Formation. In the upper acoustically transparent section there are numerous vertical features that are characteristic of shallow solution pipes or vugs, which commonly penetrate through more than one horizon and may be conduits for vertical water flow through the formation. In the Tamiami Formation, there are numerous features seen in seismic profile that are inferred as solution pipes and collapse structures. In general, the reflections shown in the upper 15 m (50 ft) of section shown in seismic profiles are flat continuous reflections that are assigned to the Ft Thompson and Tamiami Formations. An exception is the C-6 Canal, where instead there is a seismic reflection within the upper 15 m (50 ft) of section that is irregular but fairly continuous. Evidence from core descriptions indicates this reflection is produced by a lithologic contact within the Fort Thompson Formation between a weak limestone (poor induration) above and a moderate to hard limestone (good to moderate induration) below. This is a distinctive reflection not clearly identified beneath other canals.

In general, it is inferred that the highly variable depositional lithology of the area does not impact water flow as much as post-depositional dissolution that provides pathways and conduits for water to flow between units both laterally and horizontally.

Disclaimer For Informational and Personal Communication Purposes Only. Not For Distribution. The use of trade, product and firm names used in this publication/presentation are for descriptive purposes only and in no way imply endorsement by the U. S. Government. The U. S. Geological Survey, in cooperation with the South Florida Water Management District, prepares this information "as is" for its own purposes and this information may not be suitable for other purposes. This Open File Report 02-325 has not been reviewed for conformity with U. S. Geological Survey editorial standards.