Late Neogene sequence stratigraphy of a carbonate-siliciclastic transition: Southwest Florida

Abstract

A network of high-resolution seismic profiles and 23 lithologically and/or geophysically logged boreholes have been analyzed in order to evaluate the effects of sea-level cyclicity, karstification, and an intercalated, carbonate to siliciclastic transition on the late Miocene to Quaternary stratigraphy of the Charlotte Harbor area in southwest Florida. Six late Neogene disconformity-bounded depositional sequences are identified between two regional unconformities. The lower regional unconformity (R-3) varies from about 20 m to more than 115 m below sea level. The lows are present as northwest-southeast- and northeast-southwest-trending troughs, and the high areas as sharp, irregular reflections and isolated sinkholes indicative of subaerial exposure. The R-2 unconformity, which overlies and truncates the late Neogene sequences, occurs as a strong, flat-lying reflector seaward of the present coastline and a series of downcutting channel complexes in back-barrier areas. The six depositional sequences prograde to the south-southeast with a proximal-distal transition from short, steeply dipping oblique clinoforms to long, low-angle clinoforms. Three lithofacies, which generally occur in a vertical sequence, are (from the bottom up) (1) quartz-dominated sand with phosphate sand and gravel; (2) calcareous clays; and (3) poorly indurated to well-indurated, sandy, molluscan calcirudite. The pebbley sand facies are transgressive lags gradationally overlain by prograding, laminated clays deposited in an estuarine to prodelta shoreface environment. The calcirudite transitionally overlies the clay facies and was deposited in a shallow subtidal to subaerial chenier-like setting which may be indurated and/or indicate subaerial exposure and may form the base of another sequence. The karstified antecedent topography has controlled the distribution of the depositional sequences in two ways. First, fluvial systems are localized in the antecedent lows, which restricts the distribution of fluvially transported sands. Second, the relatively impermeable clays, which occur as thin beds outside the lows, thicken to 20-40 m within the karstic depressions. The clay deposits limit vertical drainage and initiate the development of fluvial systems on the carbonate terrain. The sequences were deposited during the late Miocene Tortonian Stage, which is increasingly viewed as a time of high-frequency, glacio-eustatic sea-level fluctuations. Although high-resolution chronostratigraphic data are not available, the sequences contain thin and/or poorly preserved transgressive sands, overlain by prograding regressive sediments, and they represent discrete sea-level cycles. A regional pattern of karstic dissolution has controlled the transition by localizing deposition of impermeable prodelta clays and the developing paleo-fluvial system. The introduction of quartz sand into the carbonate terrain was, in turn, controlled by the karstic relief and a localized sediment supply derived from the paleo-fluvial systems.