Sand Ridge Field Research Articles

Stratigraphic framework of sediment-starved sand ridges on a mixed siliciclastic/carbonate inner shelf; west-central Florida

Seismic reflection profiles and vibracores have revealed that an inner shelf, sand-ridge field has developed over the past few thousand years situated on an elevated, broad bedrock terrace. This terrace extends seaward of a major headland associated with the modern barrier-island coastline of west-central Florida. The overall geologic setting is a low-energy, sediment-starved, mixed siliciclastic/carbonate inner continental shelf supporting a thin sedimentary veneer. This veneer is arranged in a series of subparallel, shore-oblique, and to a minor extent, shore-parallel sand ridges. Seven major facies are present beneath the ridges, including a basal Neogene limestone gravel facies and a blue-green clay facies indicative of dominantly authigenic sedimentation. A major sequence boundary separates these older units from Holocene age, organic-rich mud facies (marsh), which grades upward into a muddy sand facies (lagoon or shallow open shelf/seagrass meadows). Cores reveal that the muddy shelf facies is either in sharp contact or grades upward into a shelly sand facies (ravinement or sudden termination of seagrass meadows). The shelly sand facies grades upward to a mixed siliciclastic/carbonate facies, which forms the sand ridges themselves. This mixed siliciclastic/carbonate facies differs from the sediment on the beach and shoreface, suggesting insignificant sediment exchange between the offshore ridges and the modern coastline. Additionally, the lack of early Holocene, pre-ridge facies in the troughs between the ridges suggests that the ridges themselves do not migrate laterally extensively. Radiocarbon dating has indicated that these sand ridges can form relatively quickly (∼1.3 ka) on relatively low-energy inner shelves once open-marine conditions are available, and that frequent, high-energy, storm-dominated conditions are not necessarily required. We suggest that the two inner shelf depositional models presented (open-shelf vs. migrating barrier-island) may have co-existed spatially and/or temporally to explain the distribution of facies and vertical facies contacts.

Transgressive sand ridges on the mid-shelf of the southern sea of Korea (Korea Strait): formation and development in high-energy environments

The mid-shelf of the southern sea of Korea (Korea Strait) is covered by a series of shore-parallel sand ridges up to 63 km long, 3–9 km wide, and 22 m thick. The ridges are a moribund type but show well-organized, dipping strata, indicating appreciable progradation. The submerged paleo-channel system (the paleo-Seomjin River) associated with the sand ridge field suggests that the ridges primarily originated from river-mouth sandy shoals, which subsequently developed into shore-parallel, linear sand bodies in high-energy environments. The combined effect of coastal (tidal and longshore) and offshore ocean currents was possibly responsible for shore-parallel sediment transport and growth of the ridges. 14C dates, core lithology, and sea-level data suggest that the sand ridges mainly formed and developed during the middle period of the postglacial transgression. They are postulated to have developed sequentially from offshore to onshore in response to sea-level rise during that time. Very slow sea-level rise and high sediment supply through the paleo-channel system probably provided favorable conditions for forming the sand ridges. The ridges became inactive during the late transgression period as a result of a substantial reduction of wave and current strength and remained on the mid-shelf.

Evolution of radiative sand ridge field of the South Yellow Sea and its sedimentary characteristics

A sand ridge field of 22 470 km2 consists of fine sands and silts originally from the old Changjiang River sediment during the late Pleistocene period. Late Holocene sand stratum with its well-preserved larmnary bedding of more clay particles reflects the influence from the Yellow River. There are three genetic types of morphology of sand ridge field as follows: (i) reformed alluvial sandy bodies and old river valleys, located in the central and southern parts, formed from the end of Pleistocene to the present. (ii) Radiative current ridges and patrimonal valley type, located in the northeastern part, formed during the early or middle Holocene time. (iii) Eroded-depositional sandy bodies in the north and outer parts, and erosional trough in the north formed since the middle Holocene transgression. The sand ridge field has a periodic nature of developing processes: the period of sediment accumulation by rivers during cold epoch with low sea level and the period of erosional formation by tidal currents during warm epoch of transgression. The river-sea interactive process in the area is closely related to the climate change; the rising and falling of the sea level is the detonating agent of the coast zone land-sea dynamic interactive processes. They can be summarized as “transgression-dynamic-sedimentation” processes.

Active, moribund and buried tidal sand ridges in the East China Sea and the Southern Yellow Sea

Shallow seismic profiles and borehole data revealed active, moribund and buried tidal sand ridges in the East China Sea and the Southern Yellow Sea. The ridges consist of relatively clean, well-sorted sands with fairly uniform sequences. The microfossil assemblage exhibits a high diversity including both euryhaline and stenohaline species. The ridges are huge elongated sand bodies (several kilometres wide, 10–60 km long and up to 20 m thick) and occur in groups with a spacing of several to >10 km. Normally, they are parallel with each other, extending in the direction of the tidal currents. The field of tidal sand ridges on the East China Sea shelf covers an area of 57,000 km 2. Tidal sand ridges in the East China Sea and the Southern Yellow Sea were formed in estuarine and shallow shelf environments, where strong tidal currents reworked, transported and redeposited large amounts of relict sands from older deltaic and fluvial sediments. The development of tidal sand ridges in estuary-mouth areas is accompanied by a net sand transport from the shelf into the estuary. The evolution of tidal sand ridges is closely related to transgressions. Active tidal sand ridges are formed during sea-level rises. They show distinct morphology and active migrations in a lateral as well as in a landward direction. They are generally in equilibrium with present-day tidal processes and their distribution patterns are related to the flow field of the tidal currents. As the sea level rises further, these tidal sand ridges become moribund sand bodies with less distinct morphologies. Their upper surfaces are covered by a thin calcareous-rich layer with very high contents of foraminifera tests, reflecting a submarine hiatus condition on the shelf during the highstand period. During the subsequent steady fall in sea level, tidal sand ridges are often buried by fine-grained shelf and prodelta sediments. They have, therefore, a good chance of being preserved in stratigraphic records. The sequential buildup during a sea level rise (TR-tract) and a subsequent sea level fall (HS- and LS-tract) would consist of estuarine channel and tidal flat facies, tidal sand ridge facies, shelf mud facies, delta facies and fluvial facies in an ascending order. Tidally dominated estuary conditions may also occur when a major distributary is abandoned, such as at the Northern Branch of the modern Changjiang River mouth, where the accumulation of sand from an abandoned delta lobe and the development of tidal sand ridges have been observed. The observed features have important implications for the subsurface recognition of fossil tidal sand ridges, for the reconstruction of the regional palaeogeography and transgressional history and for the prediction of potential reservoir sandstones.

Side-scan sonar mosaic of a sand Ridge field: southern Mid-Atlantic Bight

Side-scan sonar coverage of a 1.5 km by 1.5 km area of the inner shelf depicts the morphology of part of a submarine ridge field. The presence of megaripples indicates that ridge sediments are presently reworked by currents. Megaripples occur in the coarser sands of the north-facing ridge flanks. Distribution of megaripples and the ridge asymmetry support the hypothesis that sand ridges respond as large-scale bedforms to south-setting flows. Megaripple crests were observed to be aligned shore-parallel which indicates a pre-survey episode of shore-normal bedload transport.

Seafloor response to flow in a southern hemisphere sand-ridge field: Argentine inner shelf

The inner continental shelf adjacent to Buenos Aires Province, Argentina, is characterized by a series of sand ridges, averaging 4.7 m in relief and spaced about 2.7 km apart. The ridges trend north-south, forming a 20 to 35° southward-opening angle with the northeast-southwest-trending coast. Although the adjacent coast is of depositional strand-plain origin, it is presently undergoing erosional retreat, and the formation of the ridges appears to be part of the retreat process. Seafloor grain-size distributions and bedform arrays are responses to the same velocity field that has formed the ridges. Along transects normal to the ridges, grain-size variation is 90° out of phase with the topography: the coarsest sediment is not at ridge crests or trough axes, but instead occurs on the landward slope; finest sands are on the seaward slope. Side-scan sonar reveals bedforms aligned obliquely to both ridges and the coast; these are inferred to be sand waves. The grain size and bedform patterns suggest that the formative flows are northward currents with an offshore flow component near the bottom, although flows observed during the study period were dominantly north to south. The grain-size gradients and bedform orientations are equivalent to those characteristic of the Atlantic shelf of North America with a north-south inversion. The comparison suggests that inner shelf transport directions in response to wind events can be predicted from the regional geometry and climate of the shelf.

Evolution of a classic sand ridge field: Maryland sector, North American inner shelf

ABSTRACTThe ridge and swale topography of the Middle Atlantic Bight is best developed on the Delaware‐Maryland inner shelf. Here sand ridges can be seen in all stages of formation. Several aspects of the ridge field are pertinent to the problem of ridge genesis. The first is ridge morphology. There is a systematic morphologic change from shoreface ridges through nearshore ridges to offshore ridges, which reflects the changing hydraulic regime. As successively more seaward ridges are examined, maximum side slope decreases, the ratio of maximum seaward slope to maximum landward slope decreases, and the cross‐sectional area increases. These changes in ridge morphology with depth and distance from shore appear to be equivalent to the morphologic changes experienced by a single ridge during the course of the Holocene transgression.A second aspect is the change in bottom sediment characteristics that accompanies these large‐scale morphologic changes. Megaripples, sand waves and mud lenses appear in the troughs between nearshore and offshore ridges. These changes indicate that the storm flows which maintain ridges are less frequently experienced in the deeper sector, and that the role of high‐frequency wave surge becomes less important relative to the role of the mean flow component in shaping the sea‐floor.A third aspect is the systematic relationship of grain size to topography. Grain size is 90° out of phase with topography, so that the coarsest sand lies between the axis of the landward trough and the ridge crest, while the finest sand lies between the ridge crest and the axis of the seaward trough. This relationship is characteristic of large‐scale bedforms.Finally, flow was measured and transport calculated on the same ridge during a one‐month period (November 1976). Threshold was exceeded only during storm events. Mean transport was southerly and a little seaward with respect to both the ridge crest and the shoreline. These flow measurements are in conformity with the pattern of smaller bedforms. A 43‐year time series of bathymetric change for this ridge reveals a systematic pattern of landward flank erosion, seaward flank deposition, and seaward crest migration.Sand ridges are considered the consequence of constructive feedback between an initial topography and the resulting distribution of bottom shear stress. The relationship between grain size and topography supports this model, but does not account directly for the oblique angle of the ridge with respect to the coastline. This feature may be due to a more rapid alongshore migration rate of the inshore edge of the ridge than the offshore edge, and the relationship between this migration rate, and the rate of shoreface retreat.