Geology

Lineament Analysis - Rock Fractures

The following is reproduced by permission of the United States Army Corps of Engineers, from a 2004 report on Lineament Analysis in South Florida.

Studies of lineaments (photolinears) discernable on aerial photographs have revealed that these linears are abundant natural, and are systematically oriented in four principal groups of external stresses in the WNW, NNW, NNE, and ENE directions. Fractures are formed predominately by terrestrial or earth tides due to gravitational effects of the moon and sun, tectonic forces, diagenetic, or weathering processes (Blanchet, 1957).

Numerous investigations have shown that most of the fractures and faults identified as photolinears are vertical or near-vertical zones of fracture concentrations (Kaiser, 1950; Blanchet, 1957; Lattman, 1958; Lattman and Matzke, 1961; Lattman and Parizek, 1964).

The following paragraphs present excerpts of a good summary on fracture-trace mapping principles by Diodato (1999):

In 1973, the U.S. GEOLOGICAL SURVEY completed the Water-Resources Investigations Report, I-73, GEOHYDROLOGY OF THE CROSSFLORIDA BARGE CANAL AREA WITH SPECIAL REFERENCE TO THE OCALA VICINITY (Prepared in cooperation with: U.S. DEPARTMENT OF THE ARMY CORPS OF ENGINEERS). The Foundation is indebted to the author, Glen L. Faulkner, for illustrating the importance of understanding the geological influences upon ground water flows in an aquifer system. The two figures 17 and 22 from the report are included to underline the relationship between potentiometric flows and conduit flows along fracture sets and fault lines which are discussed below.

Mapping the fracture lineaments in the Ocala vicinity aids in determining principal directions and routes of ground-water in the Floridan Aquifer. The writer and others have observed that most caverns and solution channels in the limestone are oriented along near-vertical fractures having trends of fracture systems mapped at the surface. The logical inference is that water moving through the aquifer tends to follow the line of least resistance or greatest permeability, which in this case is along the fractures. In general, the greatest solution of limestone at shallow depths below the water table takes place where the greatest amount of water moves through. Thus cavities are developed as the walls of fractures are dissolved away by recently recharged ground water with a high carbon dioxide (CO2) content. Refer to the I – 73 report p43.

The fault system, (as also is the case with Crystal River/Kings Bay, see NOTE below), apparently controls the rectilinear course of the Oklawaha River, and it lowered the geologic section east of Ocala and Silver Springs. Poorly permeable strata in the Hawthorn are preserved in the structurally lowered river valley, but were removed by erosion from the higher areas to the west. Therefore, the Floridan Aquifer is confined in the down-faulted area, and is more or less unconfined in the structurally high area to the west.

Structural dip of the strata in the Barge Canal area, even though very low, is thought to play an important role in the occurrence of Rainbow Springs. The Springs issue from points at or near the contact between the lower member of the Ocala Limestone and the underlying Avon Park Limestone just off the crest of the Ocala Uplift down the northeast flank of the structure. Although faulting may be significant among the reasons for the springs’ occurrence (fig. 17), the writer believes that the permeability of the upper part of the Avon Park is sufficiently low to produce a barrier at the base of the highly permeable Ocala Limestone. Thus, as the contact between the Avon Park and Ocala approaches the surface up the northeast flank of the Ocala Uplift, the large volume of ground water moving southward through the highly permeable Ocala Limestone [Foundation's emphasis], is too great to be conducted by the less permeable Avon Park as the overlying Ocala wedges out and the flow is discharged at the surface. On the other hand, faulting is one of the most important geologic factors causing Silver Springs. [Foundation's Emphasis] Apparent downfaulting east of the springs has placed poorly permeable beds of the Hawthorn Formation in position as a barrier to eastward flow in the Floridan Aquifer, thus maintaining a high enough potentiometric surface at the spring site to cause overflow from open limestone caverns and sinkholes. The geohydrologic relationships at the two springs sites are illustrated in part in section X-X’, Refer to Figure 10 in the I – 73 report, p46.

From p 60 of the I-73 report, referring to Figure 22: Ground water moves downgradient from the potentiometric highs toward the saddle and to the peninsular coasts along flow paths approximately perpendicular to the equipotential lines [Foundation's emphasis]. Ground-water drainage basins in the upper part of the aquifer may be outlined on the map by drawing lines along potentiometric divides. Thus, it is possible to demonstrate the spatial relationship of the canal route to the ground-water flow pattern and to delineate the drainage areas of Rainbow and Silver Springs. The mapped potentiometric surfacean imaginary surface defined by the level to which water in an aquifer would rise in a well due to the natural pressure in the rocks. is assumed to represent that part of the aquifer supplying the springs. A knowledge of the size and shape of the drainage areas of the two springs is essential to the quantitative analysis of aquifer characteristics along critical reaches of the canal.

The general configuration of the potentiometric surface changes little from one(dry/wet) season to the other, even though the surface is several feet higher and gradients tend to be somewhat steeper in the high-water period. The most interesting change is in an area northwest of Ocala where the ground-water drainage divide between Silver and Rainbow Springs appears to shift several miles westward during the change from the low- to the high-water period.

In addition to showing general directions and preferential routes of ground-water flow, the maps of the potentiometric surfacean imaginary surface defined by the level to which water in an aquifer would rise in a well due to the natural pressure in the rocks. help to demonstrate important geohydrologic relationships in the area. For instance, the configuration of the potentiometric surface tends to confirm routes of concentrated groundwater flow suggested by the fracture trace maps [Foundation's emphasis] , thickness of cover map, and top-of-rock map (figs. 15, 18, and 19 of the I-73 report). Conversely, the geologic maps are useful interpretive tools in estimating the potentiometric surfacean imaginary surface defined by the level to which water in an aquifer would rise in a well due to the natural pressure in the rocks. in areas of sparse water-level control, and for interpreting the anomalies on the potentiometric surface. The foregoing relationships are discussed later in this report.

[See I - 73 pp 68/9 and Vernon, R.O., 195l, Geology of Citrus and Levy Counties, Florida: Florida Geological Survey Bulletin 33, 256 p].

"Here's the figure that inserts at page 48 in the R.O. Vernon bulletin, it looks to be a much smaller scale than in the Faulkner report, so it shows all of Citrus County and regions to the south. Again, these are only the major lineament features interpreted to be bedrock fractures or jointing, and joint spacings can be much closer together, although there are probably dominant fractures that act as significant conduits for groundwater movement." Comment by David DeWitt P.G. ,Southwest Florida Water Management District

NOTE :

It is clear from the late R.O. Vernon's paper of 1951, (Figure 40, and page 242) that a convergence of potentiometric (piezometricA measure of pressure of water, especially in an aquifer. Shows altitude at which water would have stood in a tightly cased well relative to a given sea level. See also potentiometric surface.) flows from the north and the south tending towards the Gulf Coast, occurs in Citrus County. The fracture sets converging in Citrus County, in essence, assist these convergent flows. This is particularly and uniquely the case where the springs serving Crystal River/Kings Bay are concerned.

With regard to the source of fresh water to Crystal River/Kings Bay, it is more than likely that the NNE to SSW fracture set shown in Figure 17, which runs from Ocala, extends all the way to Hunter Spring run to feed the spring group in that region of Kings Bay. (It is thought that the shown early termination of this fracture might well coincide with the lack of aerial photographs referred to by Vernon on page 47 of his 1951 paper, and being obscured by the overburden of the Brookesville Ridge). This would be the case if that fracture set were to bear the same characteristic length as those other orthogonals depicted on the Figure 11 mapping on which the Figure 17 is based.

The fracture sets and equipotential lines (piezometric lines) pre-date the formation of Lake Rousseau by many centuries. Potentiometric flow from the high to the north of the Lake Rousseau would appear to merge with the flow originating to the east and south of the Rainbow River in the area of northeast Citrus County. This would also appear to strengthen the ground water stream along the fracture set passing through that area leading southwestward to the Hunter Spring / Three Sisters Spring group.

The angular transitions of the general flow line of Crystal River would appear to follow the fracture sets along its path, as predicated by Vernon. Flowing NNW from its origin with the springs in the south region of Kings Bay, turning more westerly as it is joined by flows from Hunter Spring run, then more northerly as joined by the fracture set running parallel to and set to the west of the initial Kings Bay direction, and on out to the Gulf.

On page 111, of the I-73 report, Dr Faulkner writes: "Additional geological information should be gathered by examination of good sets of lithologic samples available from test holes and wells. Various remote sensing methods, such as aerial infrared and color photography, should be investigated and utilized where possible to aid in the delineation of certain stratigraphic and structural characteristics of the area, the knowledge of which may help to further define routes of preferential ground-water flow, and thereby aid in preventing pollution of water in the aquifer. [Foundation's emphasis]"

Click for a discussion of Karst features.