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publications > paper > new Tertiary stratigraphy for the Florida Keys and southern peninsula of Florida > characterization of the Arcadia Formation > lithostratigraphy and sequence stratigraphy
Characterization of the Arcadia Formation in the Florida Keys and Southern Peninsula
Lithostratigraphy and Sequence Stratigraphy of the Arcadia Formation
The lithostratigraphy of the Arcadia Formation is based mainly on examination and analysis of the Stock Island, Long Key, and Everglades Cores (Fig. 1). The mineralogy of the Arcadia Formation is principally low-magnesium calcite with subordinate amounts of dolomite and aragonite (Fig. 7). Aragonite is most abundant in the Stock Island Core (Fig. 7). Quartz grains are very minor components of cores from the Keys but increase northeastward in the upper Arcadia of the Everglades Core, where they may constitute as much as 20% of the rock (Fig. 7). The Stock Island Core contains as much as about 3% black phosphorite grains in the Arcadia. Phosphorite grains also increase in a northeastward direction with as much as 10% black phosphorite grains contained in thin beds of the Long Key and Everglades Cores. Principal grains of the Arcadia Formation are skeletal fragments, mollusks, benthic foraminifers, red algae, and echinoids. This assemblage indicates a foramol association as defined by Lees and Buller (1972). The assemblage is consistent with deposition during temperate conditions and possibly episodic upwelling across the Florida Platform.
The Arcadia Formation is a composite sequence composed of four high-frequency sequences that consist of multiple high-frequency cycles (Fig. 12; cf. Kerans, 1995). Correlation of gamma-ray signatures between the Stock Island core hole and Long Key core hole suggests that exposure surfaces that cap high-frequency sequences HFS1 and HFS2 in the Stock Island Core are correlative to hardgrounds at the tops of HFS1 and HFS2 in the Long Key Core (Fig. 12). The top of HFS3 is an exposure surface in the Stock Island Core; however, correlation of gamma-ray signatures suggest that this surface is truncated by erosion in the Long Key Core (Fig. 12). The upper surface of HFS4 is an erosion surface in the Stock Island, Long Key, and Everglades Cores. In the Long Key Core subaerial exposure is suggested; however, lithologic evidence suggests erosion during subaerial exposure or flooding or both for the Stock Island and Everglades Cores (Fig. 12). High-frequency cycles most commonly coarsen upward, but some fine upward (Fig. 12). High-frequency cycles capped by laminated crusts or auto-brecciated calcretes are only found in the Stock Island Core (Fig. 12). High-frequency cycles capped by hardgrounds are common in both the Stock Island and Long Key Cores (Fig. 12). The top of the Arcadia is bounded by a regional unconformity. In the Stock Island Core, a relatively shallow-marine carbonate facies of the uppermost Arcadia Formation is unconformably overlain by a deep-water carbonate facies of the Stock Island Formation (Fig. 13A). Typically the uppermost Arcadia is replaced by a thin layer of black phosphorite (Fig. 13A). In the Everglades Core the upper 0.46 m of the Arcadia contains black (N2 to N3), phosphorite clasts up to small-cobble size, in a matrix of very finely crystalline, moldic, fossil-fragment dolomite. Here, two irregular erosion surfaces within the upper 4.1 m of the Arcadia may indicate an amalgamated upper sequence boundary defining the top of the Arcadia Formation.
In southernmost Florida, the Suwannee-Arcadia contact can be identified in well cuttings by the last down-hole occurrence of phosphorite grains and the first down-hole occurrence of light brown (5YR 6/4) to dark yellowish orange (10YR 6/6) calcretes. Also, floral and faunal assemblages change down-hole from a foramol assemblage in the Arcadia to a chlorozoan assemblage in the Suwannee. This is the most important distinction between the Arcadia Formation and the Suwannee Limestone. In the past, many workers discerned the Arcadia-Suwannee contact as the first down-hole occurrence of Oligocene-Miocene and Eocene-Miocene benthic foraminifers, such as Miogypsina spp. and Lepidocyclina spp.; respectively. These foraminifers were at one time considered to be Oligocene, at youngest (e.g., Cole, 1938; 1944). More recently, it has been demonstrated that the first down-hole occurrence of Miogypsina spp. and Lepidocyclina spp. occurs in the lower Arcadia of southern Florida (Peacock, 1983; Cunningham and Rupert, 1996a, 1996b). Consequently, the first down-hole occurrence of Miogypsina spp. and Lepidocyclina spp. cannot be used as criteria for recognition of the Arcadia-Suwannee boundary (Peacock, 1983; Cunningham and Rupert, 1996a, 1996b). In southwest Florida, Brewster-Wingard et al. (1997) have shown that Arcadia deposition began during the late-early Oligocene. Various authors have mistakenly placed the Arcadia- Suwannee boundary within the Arcadia (Meyer, 1971; Puri and Winston, 1974; Miller, 1986; Meyer, 1989; Reese, 1994), thus complicating the stratigraphy of southern Florida.
U.S. Department of the Interior, U.S. Geological Survey
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