SOFIA - MGS-Bock 1969, Late Pleistocene Geology in An Urban Area

Late Pleistocene Geology in An Urban Area

Compiled by

Wayne D. Bock, Donald R. Moore, A. Conrad Neumann, and Peter R. Supko

INTRODUCTION

The geology of South Florida presents a picture of a slowly subsiding plateau, warm tropical waters, and a great accumulation of carbonate sediments. The structure of the area is comparatively simple, but very difficult to examine. Almost all we know is based on cores from oil or artesian well drillings.

Investigation of the surface features combined with well studies have also shown that the geology is not as simple as it would appear. Subsidence must have virtually ceased near the end of the Miocene. The Tamiami Formation (Upper Miocene) outcrops some forty miles west of Miami, and represents either a long period of nondeposition in the area or the erosion of Pliocene sediments.

The Pinecrest, a sandy facies with shallow water fossils, appears to show a period of lowered sea level near the end of the Miocene. During much of Pliocene time, however, sea level was evidently considerably higher. The crest of the Tamiami Formation was then too deep to produce large amounts of sediments of biological origin, and probably supported a very sparse flora and fauna. The environment was probably similar to depths of around 100 feet or more off the west coast of Florida today. The water in this environment was crystal clear, but there was not much light due to depth. Much of the bottom was hard, and supported a scattered fauna of sponges and coelenterates (sea whips, small corals, etc.) Mollusks and echinoderms were the most conspicuous hard shelled motile organisms.

A more recent formation, the Fort Thompson, overlies the Tamiami in much of South Florida. It contains fossils of the modern South Florida fauna, and is apparently mostly Pleistocene in age. In the Miami area it is about 150 feet thick, but is much thinner to the north and northwest.

Overlying and forming a unit of the Fort Thompson in Dade County is the Miami Limestone. Although called the Miami Öolite for most of this century, Hoffmeister et al. (1967) have shown that this formation is made up of two units, a bryozoan facies and, overlying the southeastern part, an öolite facies. Fossils are rather rare in the öolite, and are mostly bryozoa in the bryozoan facies.

The Miami Öolite was formed during the Pamlico sea stand when sea level was approximately 23 feet higher than today. Students of sea level stands have found another, the Princess Anne, at 14 feet, and the Silver Bluff sea level was about 8 feet above the present sea level. We have not found the Princess Anne shore line in the Miami area, but the Silver Bluff can be seen along South Bayshore Drive.

The close of the Pliocene saw a vast change in ocean current systems. With the end of the Panamic Seaway, the Gulf Stream began, and a great warm current began to sweep by the southern end of Florida. This current and the lowering of sea level brought favorable conditions for coral reef development in what is now the Florida Keys. This formation, the Key Largo Limestone, interfingers with the Fort Thompson in the Biscayne Bay area, and is found under the surface deposits of the islands on the eastern edge of the bay.

This trip, then, will allow us to see Pleistocene equivalents of phenomenal which are now taking place in the Recent sediments of the Bahamas. The main observable features will be those described from the Pleistocene of this area by Hoffmeister et al. (1967), and from the Recent of the Bahamas by Ball. Hoffmeister's Recent analogues are in the Bahamas sediment of today; Ball's Pleistocene analogues are to a large extent observable in the Pleistocene of South Florida. Both major studies are concerned with rock units whose recognizable criteria for interpretation of depositional milieu are very subtle and subject to very minor changes of environment and particularly, hydrologic conditions. These are the types of changes with which we are concerned in the Recent, and presumably also were in the Sangamon. They are instructive for a study of uniformitarianism over a short time basis; this is the single major advantage of this field trip.

The geology that we will see on our field trip may seem monotonous to those accustomed to seeing many formations revealed by erosion, faulting or other means. The clues to past events in the Miami area are concealed by vegetation, urban development, and the nature of the sediments. It takes careful study to work out the history of the region, but the results are well worth the effort.

(The entire report is available below.)