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4. Characteristics of the Shelf Sedimentary Cover
Overview
The west Florida continental shelf consists of a thin veneer of unconsolidated sediments overlying a phosphatic, dolomitic limestone supporting a karstic surface. On the inner shelf off west-central Florida, these unconsolidated surface sediments are generally < 3 m thick, exhibit a patchy distribution, and consist of a mixture of siliciclastics and carbonates. The underlying limestone surface regularly crops out to form ledges and hard bottoms.
The inner continental shelf can be zoned into three main areas, or provinces: (1) a "shallow bedrock" sand-ridge system north of Tampa Bay, (2) a middle zone dominated by the Tampa Bay ebb-tidal delta , and (3) a southern "deep bedrock" sand-ridge system south of Tampa Bay.
The Tampa Bay ebb-tidal delta is the primary modern source of sediment to the inner continental shelf. Continuous sediment cover attributed to this supply extends approximately 10 km offshore and, combined with some smaller inlets linked to Tampa Bay, has resulted in late-Holocene sediment accumulation of 5 m or greater in a nearshore zone 20-25 km alongshore centered on the mouth of Tampa Bay. Other small ebb-tidal deltas a few km's in dimensions are maintained at the mouths of inlets throughout this coastal system, but are not significant modern contributors of sediment to the continental shelf. Should an inlet close, the ebb tidal-delta deposits are soon dispersed and accreted to the barrier island shoreface by waves and alongshore currents.
Sand-ridge morphologies predominate on the continental shelf north and south of the Tampa Bay ebb-tidal delta and beginning just seaward of the barrier islands shoreface. The northern and southern sand-ridge systems (provinces 1 and 3 mentioned above) display distinctive differences in orientations and sedimentary facies associations. These differences are also correlated with different bedrock elevation as well as a change in orientation of the coastline.
The northern sand ridge system (province 1) exhibits a consistent NW-SE ridge system trending at oblique angles to the shoreline that vary from 30-65 degrees. The ridge trends are most variable as they sweep around the Indian Rocks Beach headland. Seaward of 10 m water depth the ridge trends are more consistently oriented at approximately 50 degree from the shoreline, the oblique angle opening to the north. Sand ridge widths are on the order of 1 km, with a 2 to 2.5 km ridge spacing.
The southern sand ridge systems (province 3) are also orientated at oblique angles with the coastline but display mixed NW-SE and NE-SW trends. NE-SW trends at 45-55 degrees relative to the coastline are found south of Tampa Bay in the nearshore (out to ~10 m water depth). This switches to NW-SE trends at lower angles of 24 degrees south of the Venice headland. From 10 Ð15 m water depth NW-SE and NE-SW trends with 30-50 degrees are mixed. In greater than 15 m depth NE-SW trends of 50-60 degrees appear most common.
Two important contrasts between the northern and southern sand ridge systems are: (1) sediment textural trends in relation to ridge topography, and (2) smaller-scale sand wave morphologic variability. In the northern area, low side-scan backscatter associates with the full bathymetric expression of the sand ridges and sand waves, indicating more uniform textural patterns for these sand bodies at the level of 100 kHz acoustic imaging. The ridges in the north also are commonly mantled with smaller sand waves or form as a sand ridge through the coalescence of several large sand waves (Harrison et al., 2000). In contrast, the southern sand ridges may exhibit significant textural changes across a ridge (Twichell et al., 2000). Smaller scale sand waves are not a significant feature within these sand bodies.
The combination of approximately 300 vibracores, 500 surface sediment samples, and numerous hardbottom and ledge samples were used to define nine lithofacies. Lithofacies include Miocene carbonate platform, Pleistocene restricted marine, and Holocene back barrier/estuarine to shallow open-marine sediments deposited during the most recent sea-level rise. Facies associations and 14C dates indicate that in the northern portion of the study area, barrier islands may have developed at least 8 ka and up to 20 km from the present shoreline. In the southern portion of the study area, barrier island facies associations are not found offshore. This differential development was likely due to differences in the availability of sand, coupled with a slowing in the rate of sea-level rise overprinted on different shelf elevations from north to south. The southern shelf elevations are up to 5 m deeper at equivalent distances offshore and would have flooded earlier. The present coastal barrier-island system formed over a wide range of time scales from decades to millennia. The oldest of the barriers have been dated at 3,000 years (Stapor et al., 1988) and others have formed during the past two decades.
Surface sediments on the inner shelf consist of fine quartz sand, and coarse sand and gravel size marine biogenic carbonates. Black, phosphatic-rich sediments are locally abundant, but are always subordinate to the quartz- and carbonate-rich sediment types. The distribution is patchy and the transition between sediment types is generally abrupt. Sediment is probably reworked by waves and tides, characteristic of siliciclastic shelf systems, but the patchy distribution and generally abrupt transition between sediment types, indicates that these processes do not completely redistribute the sediment. Instead, the sediment distribution is more a reflection of the source of input, which is more characteristic of carbonate shelf systems.
There are multiple sources of sediment input, but all come from within, or immediately adjacent to, the basin. Quartz sand is most likely a product of reworking of coastal deposits from earlier sea-level highstands as previously mentioned. The carbonate component is produced and deposited in situ by marine carbonate production. The black, phosphatic-rich sand is likely from the bioerosion and reworking of the numerous limestone outcrops as well as from non-lithified units of the Hawthorn Group.
The source for sand to initiate early Holocene barrier development in the north, may have been ancient coastal deposits that were stranded on the platform surface during a previous sea-level highstand. Quartz sand terraces, representing ancient coastlines, commonly veneer peninsular Florida. The intersection of one of these quartz sand deposits by the coastline approximately 8 ka, may have provided the sediment required for barrier-island development. The lack of evidence for early Holocene barrier-island development in the south may simply reflect the absence of available sand.
Based upon benthic foraminifera associations and test types, it is suggested that the climatic conditions during this early Holocene barrier island development were much more arid than they are today, similar to modern Laguna Madre of the Texas Gulf coast (Brooks et al., in review a, b). This coincides with a world-wide period of much drier and windier conditions. Evidence suggests that these climatic conditions may have been accompanied by a slowing in the rate of sea-level rise, which, coupled with the intersection of the coast at that time with the above mentioned sediment source, may have initiated barrier-island development.
Overall, the shelf off Tampa Bay (province 2) seems to have the thickest sediment cover associated with the enormous ebb-tidal delta. The shelf in the southern part (province 3) of the study area has the broadest areas of exposed bedrock. The inner shelf seaward of the Indian Rocks headland (province 1) seems to be the most active in terms of sediment transport.
Detailed Studies: Sand Ridges off Indian Rocks Beach Headland (Province 1)
Morphology and Processes
The west-central Florida sand ridges off the Indian Rocks Beach headland are far more complex morphologically than they appear on standard coastal bathymetric charts (Harrison, 1996). The mixed carbonate-siliciclastic sedimentary cover produces a high acoustic contrast such that the ridges, superimposed bedforms, and significant morphologic details are readily visible in side-scan data, perhaps more so than in areas where such acoustic contrast does not exist (see Harrison et al., 2000). These ridges are covered with a bedform hierarchy consisting of large sand waves whose crests trend parallel to the sand ridges, small sand waves, sediment-starved sand waves, and small ripples all having multiple orientations. These large sand waves become aligned to form single sand ridges, which may extend for many km's. The sand-ridge field extends from within 2 km of the beach to over 25 km offshore, where they become larger, more widely spaced. They seem to lack the bedform hierarchy and are not as complex morphologically. These smaller, complex sand ridges are laterally restricted to an area off a major coastal headland.
The west-central Florida ridges contrast with the US Atlantic margin ridges in that they:
- have their large sand-wave crests oriented parallel to ridge-axis trend,
- are smaller in all dimensions,
- have a significantly higher carbonate content,
- have limestone bedrock hardbottoms exposed in topographically low areas that support mature benthic communities,
- are not shoreface-connected, and
- have the most proximal sand ridges at the headland apex, which are more shore-parallel in orientation leaving a ~2 km gap between the ridges and the shoreface suggesting little sediment exchange between these two environments.
The smaller size of west-central Florida ridges is a function of the sediment-starved, low-energy nature of this inner shelf and controls sand-ridge and sand-wave morphology.
The seafloor is active and not relict as indicated by:
- relatively young AMS 14C dates (~1,200 to 1,600 yrs BP) at 1 to 1.6 m subsurface depths,
- possible shifts in sharp grayscale boundaries and patterns seen in time-series side-scan mosaics,
- maintenance of sharp textural and acoustic boundaries and small bedforms in area where bioturbation is constantly active,
- sediment textural asymmetry indicating selective transport across large sand waves and sand-ridge crests,
- morphological asymmetry of large sand waves and sand ridges with steeper, southwest-facing flanks, and
- current-meter data showing that the critical threshold velocity for sediment transport is frequently exceeded.
Radiocarbon dating has indicated that these sand ridges can form relatively quickly (within past ~1,300 yrs) on relatively low-energy inner shelves once open marine conditions are available and that high energy, storm-dominated conditions are not necessarily required.
Two lines of evidence suggest that the offshore sand ridges and the shoreface do not exchange sediment to any significant degree. First, the ridge topography does not extend to the shoreface from the inner shelf leaving a 2-3 km gap of flat and relatively featureless topography between the two areas. Secondly, the carbonate content of the offshore sand ridges is higher than that found on the adjacent beaches indicating that two different facies (mixed siliciclastic/carbonate sand facies forming the ridges and the fine quartz sand facies forming the shoreface and beaches) are spatially partitioned and temporally sequestered.
Facies and Stratigraphy
The ridges are composed mostly of a mixed siliciclastic/carbonate sand facies with interlayered, coarser shelly sand facies suggesting migrating bedforms. The shelly sand facies also separates these higher energy facies from the stratigraphically lower, reduced-energy, muddy sand and organic-rich mud facies. Two models of shelf evolution are presented: model #1 indicates that the muddy sand facies is a low energy, open-shelf lagoon backed by open-marine marshes (organic-rich mud facies; Edwards, 1998). The shelly sand facies marks a period of water deepening with sea-level rise and concomitant sand-ridge formation as energy increased with greater water depth. The barrier-island chain formed landward during the deepening phase. Model #2 indicates that a landward transgressing barrier island passed through the area of the modern inner shelf leaving the base of the shelly sand facies as the ravinement surface cut into the muddy sand facies, which formed in the back-barrier lagoon. The organic-rich mud facies in this scenario were a fringing marsh formed along the mainland. Sedimentologic data within the cores do not allow a conclusive choice between these two models. However, it is most likely that both models are valid with the coastal system changing from one to another in time and space.
Detailed Studies: Late Holocene Estuarine-Inner Shelf Interactions; Evidence of Estuarine Retreat Path? Tampa Bay, Florida (Province 2)
An important component in the development of continental shelves during the sea-level rise since the last glacial maximum, particularly along passive margins, has been the landward retreat of fluvial-deltaic-estuarine systems. Although laterally restricted in places the surficial features generated by the landward tracking of such large paleo-drainage systems such as the Hudson, Delaware, Savannah, Altamaha, and Albemarle Rivers and the suite of rivers associated with the Chesapeake Bay estuary are impressive in bathymetric complexity and cross-shelf continuity. Indeed, features such as estuarine retreat shoals/blankets, shelf valleys and levees inherited from earlier estuary mouth environments then mantled by a hierarchy of sand ridges and bedforms may dominate certain shelf sectors such as the Georgia Embayment (South Atlantic Bight of Swift, 1976).
In the Tampa Bay area there was not a large paleo-fluvial system, nor a high-relief valley system that extended across the exposed west Florida shelf during that last sea-level lowstand (Donahue, 1999). Instead, a broad mid-platform depression existed in the proto Tampa Bay area with its western boundary located probably within 40 km west of the present day estuary.
As sea level started to rise since the Last Glacial Maximum the shoreline, starting at the Ð120 m isobath, transgressed rapidly across the shelf but did not track up a shelf-valley system until it reached the western, most-seaward limit of the mid-platform depression. Hence, there is no estuarine retreat path on the west Florida shelf well seaward of Tampa Bay similar to those described on the shelf off the US East Coast.
Instead, the inner shelf seaward of Tampa Bay is dominated by two sand bodies, the present-day modern ebb-tidal delta and a more distal and deeper sand plain, which we interpret to be an older, but a short-lived ebb-tidal delta (Donahue et al., 2000). This sand body may have formed during a sea-level stillstand or slowdown and then was abandoned during an ensuing accelerated sea-level rise. The modern ebb-tidal delta has prograded out onto the inner shelf during the past few thousand years as the rate of sea-level rise decelerated, the modern barrier-island system formed and narrowed the mouth of Tampa Bay. This, in turn, focussed the ebb-jet, which exported sands seaward to form the ebb-tidal delta. The most seaward part of the study area consists of sediment-starved sand waves surrounded by coarse, molluscan shelly material and/or exposed hardbottom. These sandwaves respond to modern shelf hydraulics and do not appear to be influenced by the modern estuarine system.
Detailed Studies: The Inner Shelf off Sarasota, Florida: A Complex Facies Boundary in a Low-Energy Environment (Province 3)
The west-central Florida inner continental shelf is the transition between a beach system composed of nearly pure siliciclastic sand and a shelf of nearly pure carbonate sediment. The geometry of the transition between the beach and shelf facies on this low-energy, sediment- starved inner shelf was poorly understood, and for this reason the innermost shelf off Sarasota, Florida was mapped in detail using sidescan-sonar imagery, seismic-reflection profiles, surface sediment samples, and short cores (Twichell et al., 1999, 2000). Seaward of a smooth lower beach face that is covered with siliciclastic sand, the inner shelf is shaped by a series of low-relief, nearly shore-perpendicular ridges that are 3-11 km in length, 1-3 km in width, 1-4 m in height, and are asymmetrical with the steeper side facing northwest. These ridges rest on a flat erosional surface and contain the majority of the Holocene sediment in the area. The ridges are separated by flat-floored troughs where there are exposures of older strata and hardgrounds, which are partially covered by a veneer of shelly gravel. Despite the low relief of these Holocene deposits, the distribution of different surface sediment types is closely tied to this subtle morphology. The steeper northwestern sides of the ridges are covered with a coarse shell hash while the gentler southeastern sides are covered by fine siliciclastic sand. The transition between these facies is abrupt and occurs at the crest of the ridge. This facies distribution suggests that fine siliciclastic sediment is winnowed from the northwestern, updrift, sides of the ridges, presumably by south-flowing, storm-generated currents. A coarse shell lag is left armoring the updrift sides of the ridges. The fine sediment, in turn, is deposited on the gentler, downdrift, sides of the ridges. This pronounced partitioning of the surficial sediment has not been observed on other sand ridges. The difference in sediment distribution appears to be the result of the siliciclastic sand being easily transported by these currents while the carbonate shell hash falls below the threshold of sediment movement. The orientation of the ridges is nearly perpendicular rather than oblique to the strongest currents and may further enhance the sorting of these two facies. The resulting facies boundaries on this low-energy, sediment-starved inner continental shelf are complex and suggest that the remarkably subtle ridge morphology has had a strong control on sediment redistribution by modern processes.
continue to Hardbottom Development and Significance
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