Delineation of Cycles and Ideal Cycles

High-frequency cycles form the fundamental building blocks of the rocks of the Biscayne aquifer (Fig. 3, Fig. 4, and Fig. 6). The placement of vertical lithofacies successions between significant bounding surfaces defines these cycles (Fig. 3). Bounding surfaces are flooding surfaces. In some cases, a calcrete layer indicative of subaerial exposure delineates the flooding surface (Fig. 4). A flooding surface is a boundary that separates younger from older strata and across which there is a sharp upward increase in paleowater depth (cf. Van Wagoner et al., 1988). Flooding surfaces herein indicate a sharp upward deepening of paleomarine water depth or paleoflooding of a subaerial exposure surface by seawater or freshwater.

Three distinct recurring vertical lithofacies successions translate into three ideal high-frequency cycles: an upward-shallowing subtidal cycle, an upward-shallowing paralic cycle, and an aggradational subtidal cycle (Fig. 3; Table 1). Paralic depositional facies cap the upward-shallowing paralic cycles. The principal characteristic of paralic environments is that they occur at the transition between marine and terrestrial realms- estuaries, coastal lagoons, marshes, and coastal zones subject to high freshwater input (Debenay et al., 2000). Figure 3 shows the vertical stacking of lithofacies and associated interpretive depositional environments within the three ideal high-frequency cycles. In the Fort Thompson Formation, only the two upward-shallowing cycles were observed, and only aggradational subtidal cycles were observed in the Miami Limestone (Fig. 4).

Figure 6. Hydrostratigraphic correlation section A-A' between the S-3163 and S-3164 production wells, including six digital optical image logs from observation and injection wells at the Northwest Well Field (Fig. 1D). Black or very dark areas on the digital optical image logs commonly indicate large-scale vuggy porosity. For additional information about this figure, please contact Kevin Cunningham at kcunning@usgs.gov. [larger image]

Cycle Hierarchy

In the Fort Thompson Formation and Miami Limestone, two hierarchical levels of cyclicity were observed. The high-frequency cycles are the fundamental cycle type, but based on upward trends of progradation or aggradation, they group into two high-frequency cycle sets (Fig. 4; Table 2). The lower high-frequency cycle set (Fort Thompson Formation) displays a broad uniform upward-shallowing trend indicative of carbonate-shelf progradation. The singular depositional facies characteristic of the two high-frequency cycles of the upper high-frequency cycle set (Miami Limestone) is suggestive of carbonate-shelf aggradation (cf. Kerans and Tinker, 1997).

Orders of Cycles

Within the study area, we propose a hierarchical order to the cyclicity recognized in the Fort Thompson Formation and Miami Limestone: high-frequency cycles are fifth-order scale and high-frequency cycle sets are fourth-order scale (Table 2). The proposed scales for the cycle ordering are based on various ranges of ages proposed by Multer et al. (2002) for the five unconformity-bound Quaternary marine units or Q units defined by Perkins (1977). Perkins' (1977) Q1-5 units correlate to our new cyclostratigraphy shown in Figure 4, based on comparison of his descriptions of lithofacies and unconformities to those we observed. No lithofacies or unconformity observed in the study area reliably correlated to Perkins' Q1 unit (Fig. 4). Multer et al. (2002) assumed a maximum age of 420 ka for Perkins' Q1 unit (basal Fort Thompson Formation; Fig. 2) and reported that the Q5 unit of Perkins (1977) accumulated during the marine isotope substage 5e, which terminated ca. 114 ka (Shackleton et al., 2003). Our model of the ordering of cyclicity thus assumes a maximum duration of ~306 k.y. for accumulation of the 13 high-frequency cycles identified in the study area (Fig. 4), or an average cycle duration of ~23.5 k.y., which is consistent with fifth-order cyclicity (Table 2). Fourth-order scaling of the high-frequency cycle sets is in agreement with cycle-set durations based on Q-unit ages presented in Perkins (1977) and Multer et al. (2002).