High-resolution single-channel seismic reflection surveys of Orange Lake and other selected sites of north central Florida

Abstract

The potential fluid exchange between lakes of north central Florida and the Floridan aquifer and the process by which exchange occurs is of critical concern to the St. Johns Water Management District. High-resolution seismic tools with relatively new digital technology were utilized in collecting geophysical data from Orange, Kingsley, Lowry and Magnolia Lakes, and the Drayton Island area of St. Johns River. The data collected shows the application of these techniques in understanding the formation of individual lakes, thus aiding in the management of these natural resources by identifying breaches or areas where the confining units are thin or absent between the water bodies and the Floridan aquifer. Orange Lake, the primary focus of the study, is a shallow flooded plain that was formed essentially as an erosional depression in the clayey Hawthorn formation. The primary karstic features identified in the lake were cover subsidence, cover collapse and buried sinkholes structures in various sizes and stages of development. Orange Lake was divided into three areas southeast, southwest, and north-central. Karst features within the southeast area of Orange Lake are mostly cover subsidence sinkholes and associated features. Many of the subsidence features found are grouped together to form larger composite sinkholes, some greater than 400 m in diameter. The size of these composite sinkholes and the number of buried subsidence sinkholes distinguish the southeast area from the others. The potential of lake waters leaking to the aquifer in the southeast area is probably controlled by the permeability of the cover sediments or by fractures that penetrate the lake floor. The lake bottom and subsurface of the north-central areas are relatively subsidence sinkholes that have no cover sediments overlying them, implying that the sinks have been actively subsiding with some seepage into the aquifer from the lake in this area due to the possible presence of the active subsidence and faulting. The largest and most important features in the lake are the collapse sinkholes found along the southwestern shore that provide conduits for exchange between the lake and subsurface aquifer. There are two basic differences between the southwest and other areas of the lake: (1) the features found towards the central part of the lake are smaller in scale (1to 10 m across) and tend to be singular structures compare to the southwest area where features combined to form larger sinkholes (>400 m), and; (2) the southwest area is the only site where collapse dolines were identified. These dolines are located along the southwestern shoreline adjacent to Heagy-Burry Park. The comparison of seismic profiles from the several other selected lake and river sites to the Orange Lake profiles showed that other study areas were constructed of one or two large subsidences or a combination of sinkholes to form one large sinkhole. Aside from the difference in scale the basic characteristics of the subsidence sinkholes were similar.