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Abstract

Cores and borehole-geophysical logs collected on and around two tree islands in Water Conservation Area 3 have been examined to develop a stratigraphic framework for these ecosystems. Especially important is the potential for the exchange of ground water and surface water within these features. The hydrostratigraphic results from this study document the lithologic nature of the foundation of the tree islands, the distribution of porous intervals, the potential for paleotopographic influence on their formation, and the importance of low-permeability, subaerial-exposure horizons on the vertical exchange of ground water and surface water.

Figure 1. Location of Tree Islands 3AS3 and 3BS1. [larger image]

Results from this hydrostratigraphic study indicate that subtle differences occur in lithofacies and topography between the on-island and off-island subsurface geologic records. Specifics are described herein. Firstly, at both tree-island sites, the top of the limestone bedrock is slightly elevated beneath the head of the tree islands relative to the off-island core sites and the tail of the tree islands, which suggests that bedrock "highs" acted as "seeds" for the development of the tree islands of this study and possibly many others. Secondly, examination of the recovered core and the caliper logs tentatively suggest that the elevated limestone beneath the tree islands may have a preferentially more porous framework relative to limestone beneath the adjacent areas, possibly providing a ground-water-to-surface-water connection that sustains the tree island system. Finally, because the elevation of the top of the limestone bedrock at the head of Tree Island 3AS3 is slightly higher than the surrounding upper surface of the peat, and because the wetland peats have a lower hydraulic conductivity than the limestone bedrock (Miami Limestone and Fort Thompson Formation), it is possible that there is a head difference between surface water of the wetlands and the ground water in underlying limestone bedrock.

Introduction

Cores from eight test coreholes from two tree islands in Water Conservation Area 3 (WCA-3) were described and spatially correlated based on lithology, subaerial-related discontinuity horizons, and gamma-ray wireline logs. Four of the cores and geophysical logs were from Tree Island 3AS3, and four of the cores and geophysical logs were collected from Tree Island 3BS1 (Figure 1). The purpose of this hydrostratigraphic study was to characterize the shallow subsurface geology and provide stratigraphic correlation in order to: (1) identify key zones of porosity that may provide linkage between ground-water and surface-water systems which help to sustain the tree island habitats; and (2) identify any anomalous geologic features that may have contributed to the initial formation and continued sustainability of the tree-island systems.

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey.

Related Links:

Evolution of Everglades Tree Islands Project Page

Tree Islands of the Everglades Book

Tree Islands Symposium (July 1998)