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pleistocene barrier bar seaward of ooid shoal complex near miami, florida
This article was originally published in the AAPG Bulletin, Vol. 61/4, April, 1977.
An ooid sand barrier bar of Pleistocene age was deposited along the seaward side of an ooid shoal complex southwest of Miami, Florida. The bar is 35 km long, about 0.8 km wide, elongate parallel with the trend of the ooid shoal complex and perpendicular to channels between individual shoals. A depression 1.6 km wide, interpreted as a back-barrier channel, isolates the bar from the ooid shoals. During sea-level fall and subaerial exposure of the bar, the ooid sand was cemented in place, preventing migration of the barrier. No Holocene analogue of this sand body is recognized, perhaps because of the relative youthfulness of Holocene ooid shoals. This Pleistocene ooid shoal complex, with its reservoir-size barrier bar, may serve as a refined model for exploration in ancient ooid sand belts.
Most of south Florida's population lives on a topographic feature known as the Atlantic Coastal Ridge (Davis, 1943). South of Miami this southwest-trending ridge consists of the oolitic facies of the Pleistocene Miami Limestone of Hoffmeister et al (1967). The feature has been recognized as an ooid shoal retaining much of its original morphology (Hoffmeister et al, 1967). General features of the ooid shoal are evident on standard topographic maps with a 5-ft (1.5 m) contour interval. The ridge consists of a series of slight topographic highs interpreted as shoal areas, separated by sinuous low areas or glades which are interpreted as relict tidal channels.
During the 1930s, surveyors were hired by the Civilian Conservation Corps (CCC) to plane-table map Dade County. They constructed a detailed topographic map (Friedman, 1939) for the more populated parts of the county with a contour interval of 1 ft (0.3 m). This long overlooked map provides added detail revealing a topographic feature interpreted as a barrier bar, seaward of the ooid shoal. In addition, the detail of this map reveals a channel or narrow lagoon between the ooid shoal and the barrier bar. Although the barrier is composed of the same material as the ooid shoal and has identical porosity and permeability characteristics, it is separated from the shoal to form a separate sand body of reservoir dimensions.
The 64-km coastal ridge between Miami and Florida City consists of broad areas roughly 4.8 km across, which are more than 2.7 m above sea level and are separated by narrow sinuous depressions. The depressions are approximately 0.8 km wide, several kilometers long, and generally less than 2.1 m above sea level. The 7-ft and 9-ft (2.1 and 2.7 m) contours, which emphasize these features, are reproduced in Figure 1. The entire ridge system, which is 4.6 m above sea level near Miami, slopes southward to 3.0 m above sea level near Florida City. Westward the ridge slopes gently toward the Everglades at an average incline of 0.02 m/km. Eastward the ridge drops abruptly with short slopes ranging from 0.3 to 3.0 m/km. Near Miami this slope forms a 3.0-m precipice called the Silver Bluff, interpreted as a wave-cut notch by Parker et al (1955, p. 130). South of Black Creek (Fig. 1), the eastern slope is more gentle but still is more than 0.3 m/km.
|Figure 1. Significant morphologic features southwest of Miami are illustrated by the 7-ft and 9-ft (2.1 and 2.7 m) contour intervals. Profiles along A, B, and C are illustrated in Figure 4. [Larger image]||Figure 2. Fingerlike extensions (arrows) of northwest-trending high ground are interpreted as spit accretion features building southward at end of barrier bar. [Larger image]|
Black Creek is a natural boundary between two areas of differing topography. These areas differ in three aspects. First, between Black Creek and the Miami River, a distance of about 35 km, only one of the sinuous depressions breaks entirely through the east ridge (Fig. 1). In contrast, from Black Creek southwest to beyond Florida City, a distance of about 24 km, there are six depressions cutting entirely through the ridge. Second, south of Black Creek, areas above 2.7 m and the intervening sinuous depressions are elongate east-west or southeast-northwest. This alignment is preserved throughout area 1 of Figure 1. In contrast, in area 2 of Figure 1 the sinuous depressions bend to a north-south or northeast-southwest orientation as do the areas above 2.7 m elevation. Third, and as a partial function of the first observation, the 7-ft (2.1 m) contour line is broken and irregular southwest of Black Creek. In contrast, the contour line northeast of Black Creek is very straight and unbroken.
Along the eastern side of the ridge, these two areas of contrasting morphology intersect in an area of complex topography illustrated in detail in Figure 2. The northeast-trending topographic high has a narrow extension 0.4 km wide and 1.2 km long at its southern terminus. On the southwest side of this high, similar projections extend westward from the main body (arrows, Fig. 2).
We interpret the topography of area 1, as did Hoffmeister et al (1967), as a series of ooid shoal areas separated by relict tidal channels. In the terminology of Ball (1967), this area would be a tidal bar belt similar to several Holocene ooid sand bodies in the Bahamas, such as Schooner Cays, Joulters Cays, and the ooid shoals at the south end of the Tongue of the Ocean. The ooid shoals are preserved as low rises of ooid grainstone. Individual ooids are partly leached aragonite or are replaced by blocky calcite. Skeletal grains, pelloids, and quartz grains are minor grain types. The common cement is a mosaic of low magnesium calcite (Fig. 3a).
|Figure 3||Figure 4|
|Figure 3. (above left) Photomicrographs of Miami Limestone lithologies. a, Ooid grainstone of tidal bars; b, c, ooid grainstones of barrier bar; d, molds of former "bryozoan facies" of Hoffmeiser et al (1967) west of ooid shoal complex; e, f, skeletal and pelloid grainstones east of barrier bar. Samples are located on Figure 5 by lower case letters. Bar scale is 1 mm. [Larger image]
Figure 4 . (above right) Three profiles across ooid shoal complex illustrate topographic and geographic distinction of barrier bar and its absence in southern half of the study area. [Larger image]
|Figure 5. Paleogeographic reconstruction of Pleistocene ooid shoal complexes based primarily on present topography. Lower case letters indicate position of samples illustrated in Figure 3. [Larger image]|
Three sets of observations suggest that the barrier bar originated near Miami and built southwestward toward Black Creek. First, the presence of the barrier northeast of Black Creek and its absence southwest of Black Creek, together with the southward curvature of inter-shoal channels as they join the back-barrier channel, suggest southwestward growth of the barrier bar. This channel curvature probably resulted as the barrier restricted free flow from the channels, forcing the channels to exit farther south. A corollary of this interpretation is that the bar formed after significant shoal development. Second, southward growth of the bar is supported by topography at the southern end (Fig. 2). The finger-like projections extending southward and southwestward from the bar are reminiscent of spits building at the end of a bar. Accretion ridges are visible on aerial photographs which predate the South Florida building boom. Figure 6 is a reproduction from a 25 year-old aerial photograph in which accretion ridges, shoals, channels, and the barrier bar can be identified. Third, present longshore processes bring quartz sand to the Miami area from far in the north, and similar processes may have operated in the Pleistocene to bring quartz sands into the ooid shoal complex (the oolite interfingers with quartz sands north of Miami), where they served as the nuclei for ooids. These same longshore and/or swash zone processes probably account for the southerly extensions of the Pleistocene barrier bar.
|Figure 6a. Aerial photograph (1940) of part of study area. [Larger image]||Figure 6b. Interpretation illustrating ooid shoals, inter-shoal channels, back-barrier channel, and barrier bar. [Larger image]|
This barrier bar seaward of the oolite shoal is a heretofore unrecognized sand body of large dimensions. It is 1.6 km or more wide, slightly over 32 km long, and about 9.1 m thick. No similar feature is known to be associated with Holocene ooid deposits, although one area in the Bahamas, Joulters Cays, appears to be developing features that may coalesce to form a significant bar (Harris, in prep.). Bahamian Holocene ooid shoals are very young, less than 3,000 years old (Newell and Rigby, 1957; Martin and Ginsburg, 1965); perhaps barrier bar building is a later phase of ooid shoal development, so that, given the right physiographic conditions, a maturing ooid shoal complex will develop a significant seaward barrier.
The barrier is one of the highest features in Miami and underlies many of the areas over 6.1 m in elevation. Depositional slopes dip in all directions off the bar and, if it were covered with an impermeable seal, the trap would have about 4.6 m of closure and a very high permeability and porosity, making it an excellent hydrocarbon reservoir prospect in a producing area. Where limestone is quarried actively, this bar is a carbonate sand accumulation that would be of considerable economic value. It is suggested that workers drilling or quarrying ancient oolites consider the possibility that a barrier bar may be present in their particular area and use the Miami Oolite as a possible model for exploration.
Although this barrier bar shares morphologic similarities with many quartz sand barrier bars, caution must be taken when pushing the analogy to include origin and evolution of the bar. The barrier bar under discussion consists almost entirely of oolite material that probably formed on nearby ooid shoals or on the bar itself. This is in contrast to quartz sand barrier bars, which commonly are composed of sand derived from tens or hundreds of kilometers away. Another contrast results from differences in geographic setting. Holocene quartz sand bars commonly are a few kilometers from the shoreline, whereas this carbonate barrier bar developed with a broad, shallow-carbonate platform separating it from the Gulf of Mexico on the west and a rather narrow shelf between it and the Straits of Florida on the east. This apparently occurred during a high sea-level stand about 120,000 years B.P. (Osmond et al, 1965). Finally, many quartz sand barrier bars have migrated landward to their present position with Holocene sea-level rise and retain internal sedimentary structures of migration rather than formation (Swift, 1975). We believe this bar is in or very near its original position. This is supported by a facies change from primarily oolite to skeletal calcarenites in the Pleistocene rocks a few hundred meters seaward of the bar. Also, during sea-level fall from the 120,000 years B.P. high stand, the bar was rapidly cemented in place subsequent to subaerial exposure and freshwater replacement of interstitial sea water. This cementation and possibly submarine cementation prevented migration of the bar during sea-level fall. (No petrographic evidence for submarine cement has been found, although it is common in some Holocene ooid shoals.) Probably the barrier bar retains a suite of sedimentary structures representative of early processes of formation rather than seaward or bankward migration.
Elucidation of these formative processes awaits meticulous study of the internal structure of the barrier bar. At this time, the internal structure is poorly known. Exposures are rare but, from those that are present, the following may be pointed out. Nowhere in the barrier bar have proven eolian-dune features been observed. The most common sedimentary structure is medium-scale crossbedding, dipping eastward and southeastward at 10 to 30°. Southwestward dips are present at the back (northeastern) side of the barrier bar. At one locality, beachlike deposits occur (low angle cross-bedding, penecontemporaneous cementation and erosion; Parker et al, 1955, p. 103), and at several localities thin (1.27 to 5.08 cm thick) lenses of lime mudstone are at the top of the bar. From extremely limited information, it seems that most of the bar is a subtidal sand accumulation. How much of the sand body was reworked during sea-level fall is not known. Clearly, the bar was an emergent feature for an unknown period of time during which it was analogous to offshore quartz sand barrier islands, such as those north of Miami today.
A barrier bar was present in the Pleistocene ooid shoal complex southeast of Miami, Florida, and can be recognized readily in detailed topographic maps of Dade County. It is a feature that developed late in the history of this particular shoal, after the establishment of a tidal bar belt and, as yet, has no analogue in the comparatively young Holocene ooid-forming areas. The size and geometry of the barrier bar make it an excellent model for exploration in areas where oolites are of economic importance, either as hydrocarbon reservoirs or as deposits of high-grade calcium carbonate. Although the barrier bar was analogous morphologically and physiographically to quartz sand barrier islands, it probably formed essentially in place and should retain sedimentary structures indicative of bar formation rather than of bar migration. Future studies of the internal structure of this barrier bar are needed to test this hypothesis.
U.S. Department of the Interior, U.S. Geological Survey
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