Bulletin No. 64


 Oldsmar Formation

 Much of the interbedded mudstones, wackestones, packstones and dolostones of the upper Oldsmar have sedimentary structures and vertical facies variations that are indicative of tidal flat deposition (Shinn, 1983). The most representative sequence of probable tidal flat origin was cored in the Merritt Island well from 1,820 to 1,830 feet BLS. This section consists of interbedded dolostones, mudstones, wackestones, and packstones (Appendix A). Common allochems include peloids, foraminifera, highspired gastropods (molds and casts) and echinoderm fragments. The dolostones are laminated and contain root molds filled with dolospar. Some laminations have been partially disturbed by burrowing. Contacts between the dolostones and limestones vary from extremely sharp and unconformable to gradational. One unconformable contact in this interval has a highly porous and permeable packstone overlying a dense, laminated dolostone containing dolospar-filled root molds. The upper surface of the dolostone is irregular with curved to domal algal laminated structures ("tepee" structures). Deposition of the packstone on top of the tidal flat sequence probably occurred during a brief transgressive phase. The gradational contacts are diagenetic in nature and are the result of either increasing or decreasing degrees of dolomitization. At 1,821 feet BLS, irregular masses or clumps of wackestone with abundant root traces floating in a matrix of dolomite may represent a caliche horizon developed during subaerial exposure.

 Andros Island tidal flats serve as an excellent modern analog for the depositional environment of upper Oldsmar carbonates. Many of the sedimentary structures and sequences found in tidal flat sediments on Andros Island (Shinn et al., 1969) are present in upper Oldsmar Formation core. Laminated dolostones of the Oldsmar Formation may be comparable to recent supratidal dolomitic crusts that occur on Andros Island tidal flats (Shinn et al., 1969). Wackestones and


packstones containing root casts, pelloids, and high-spired gastropods are similar to intertidal zone sediments of western Andros Island (Shinn et al., 1969). Stratigraphically equivalent intervals of the upper Oldsmar Formation in other injection wells in Brevard County have lithologic sequences very similar to that of the Merritt Island well. This similarity gives some indication of the tidal flats' potential areal extent, which, at a minimum, would range from the Merritt Island well south to the South Beaches well.

 Core at 2,138 feet BLS in the South Beaches well (Appendix A7) has a brecciated texture with angular clasts of grayish-brown dolostone floating in a matrix of yellowish-brown dolostone. This zone may represent a caliche similar to that found at 1,821 feet BLS in the Merritt Island well or perhaps the angular nature of the fragments may be more indicative of a collapse breccia related to karstification.

 The uppermost Oldsmar (upper 40 feet of the formation) lacks key sedimentary structures that might be indicative of a specific depositional environment. However, the observed mineralogical suite of this interval that includes glauconite, pyrite, collophane and clay is associated with unique chemical and depositional environmental conditions. Glauconite occurs in the form of well rounded, dark green, sand-sized peloids with concentrations ranging from one to as much as ten percent of the total rock (South Beaches and Port Malabar wells). Glauconitization occurs at the sediment seawater interface at depths of 195 feet down to 3,250 feet in open marine waters with temperatures of 59 degrees F (15 degrees C) or less (Odin and Fullagar, 1988). Low sedimentation rates and bottom turbulence are also necessary for glauconitization and as a result glauconitic sediments represent depositional hiatuses in the sedimentary record (Odin and Fullagar, 1988). Assuming glauconitization proceeds at a minimum water depth of 195 feet, then the uppermost Oldsmar glauconitic carbonates may record a significant, and possibly rapid, sea level rise since tidal flat deposits con-