PART II: Depositional Framework of Pleistocene Rocks in South Florida

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Abstract
Detailed stratigraphic analysis of the Pleistocene of south Florida, based on 56 measured sections, indicates that these deposits are divisible into five marine units separated by regional discontinuity surfaces. Marine units are correlated with eustatic high sea-level stands and discontinuity surfaces with subaerial exposure during low stands. Criteria for recognizing discontinuity surfaces include: (1) vadose sediment, (2) land-plant root structures, (3) laminated crusts, (4) diagenetic soilstones, (5) soils and soil breccias, (6) solution surfaces, (7) bored surfaces, and (8) freshwater limestones. Discontinuity surfaces are often found to be intraformational when related to formal stratigraphic designations presently in use. The Fort Thompson and Anastasia Formations contain four such surfaces, the Key Largo Limestone contains two, and the Miami Limestone contains one. When considered in detail, discontinuity surfaces in south Florida are found to vary in the amount of time they represent and the degree to which they are developed and preserved. The five marine units recognized in this study have been informally termed, from oldest to youngest, Q1 through Q5 (Q for Quaternary). Each stratigraphic unit is analyzed from the following viewpoints: (1) role of pre-unit topography, (2) isopach patterns, (3) lithofacies patterns, (4) ecologic facies patterns, and (5) interpretation of depositional environments. Pre-unit paleotopography strongly influenced isopach thicknesses and lithofacies patterns within individual Pleistocene units, although to a lesser degree upward in the section as paleotopography became more subdued. A marine embayment, occupied by the present position of Lake Okeechobee and the Florida Everglades, was bounded by topographically higher areas to the north and west. These topographic highs, mantled with older Pliocene and Miocene clastic sediments, served as sources for detritus throughout Pleistocene deposition. During Q1 deposition, quartz sandstones with admixtures of molluscan debris clearly reflect the proximity of these topographic highs. More open marine conditions persisted in the embayment between the highs where arenaceous, mollusk-fragment packstones are found interbedded with fossiliferous quartz sandstones. Important faunal elements within these embayment sediments include: Chione cancellata, miliolids, peneroplids, Manicina, Porites, and Schizoporella. In the northeastern portion of the study area, Q1 sediments are represented by arena-ceous, mollusk-fragment grainstones (deposited as barrier-beach and beach-dune sediments) and fine-grained quartz sands (lagoonal sediments). Lithofacies patterns within the Q2 unit are essentially the same as those found in the Q1 unit, except that molluscan packstone increases in the northern part of the embayment to the exclusion of quartz sandstone. Generally, the fauna of the Q2 unit is similar to that of the Q1 unit. However, more open shelf conditions are suggested by scattered occurrences of the coral, Montastrea, along the southern margin of the study area. The proportion of carbonates to terrigenous sediments increased markedly during deposition of the Q3 unit. Arenaceous, mollusk-fragment packstone deposited in the northern portion of the embayment grades southward into foraminiferal, mollusk-fragment packstone and grainstone that are relatively free of quartz. Fossiliferous quartz sandstones persisted in proximity to the topographic highs. A highly coralline facies composed of grainstone and packstone was deposited along the southern margin of the study area and marks the first appearance of large numbers of the hermatypic corals, Montastrea, Diploria, Porites astreoides, and Porites porites in association with encrusting red algae and Halimeda plates. Barrier-beach and lagoonal sedimentation persisted in the northeastern portion of the study area where mollusk-fragment grainstone, quartz sand, and sandstone were deposited. The upper contact of the Q3 unit represents one of the most pronounced of all the Pleistocene discontinuity surfaces. Pre-unit topography prior to Q4 deposition was greatly subdued as a result of Q3 sedimentation, especially along the southern margin of the study area where coral growth resulted in appreciable constructional topography. Maximum relief prior to Q3 deposition was greater than 120 ft (37 m), as contrasted with approximately 30 ft (9 m) prior to Q4 deposition. During Q4 deposition, detrital influx was greatly reduced in the western part of the study area, but continued to persist in the northeastern part in the form of quartz sand and sandstone deposited as a broad, arcuate shoal. To the east of this shoal, barrier-beach and lagoonal sedimentation prevailed. Within the embayment, mollusk-fragment packstone, wackestone, and quartz sandstone grade southward into highly burrowed peneroplid, miliolid, pellet packstone and grainstone, which locally contain abundant bryozoans (Schizoporella). This pelletal facies grades along the southern edge of the study area into a highly coralline, red algal packstone and grainstone facies that is characterized by Montastrea, Diploria, Porites, Halimeda, and encrusting red algae. Paleotopography developed on the upper surface of the Q4 unit gave rise to a relatively featureless, gently seaward dipping platform on which Q5 sediments were deposited. In the northeastern part of the study area, arenaceous, mollusk-fragment grainstone, fossiliferous sandstone, and fine-grained quartz sand were deposited as barrier-beach-lagoonal sediments that grade westward into quartz sandstones derived from terrigenous sediments shed off the nearby topographic high. During Q5 time, the area now occupied by Lake Okeechobee was the site of deposition of mollusk-fragment wackestone in what is believed to have been a restricted marine bay as suggested by local occurrences of Rangia cuniata and oysters. Southward along strike (along the present-day Atlantic coastal ridge), the barrier-beach-lagoonal complex gives way to an oolitic facies composed of arenaceous, oolith, pellet grainstone and packstone, believed to represent tidal-bar deposits. Another discrete oolitic tidal-bar belt was formed in the area now occupied by the lower Florida Keys. In contrast to the northern tidal-bar belt, these oolitic deposits are essentially quartz free and contain less mud (predominantly grainstone). In the area between the two tidal-bar belts of Q5 age, a highly coralline facies is found similar to those developed during Q3 and Q4 deposition. The environment of deposition favored for this coralline facies, as well as those of Q3 and Q4 age, is that of a migrating sand-shoal-patch-reef complex. This migrating shoal may have been initiated at a paleotopographic break in slope on the outer shelf margin and does not require a seaward barrier reef for its inception. Pelletal packstone and grainstone accumulated in sheltered water behind the sand-shoal-patch-reef complex and the oolite bars. These sediments are usually highly burrowed and contain miliolids, peneroplids and Schizoporella. Considered as a whole, the Pleistocene record of south Florida may be thought of as simple infilling of pre-Pleistocene paleotopography during repeated marine transgressions, modified by subaerial exposure and the production of discontinuity surfaces during low sea-level stands.

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