Slow Sea-level Rise Research Articles

Late Quaternary Stratigraphy and Development of Tidal Sand Ridges in the Eastern Yellow Sea

Abstract The eastern Yellow Sea is characterized by a number of tidal sand ridges which occur as a series of linear sediment bodies in the shelf (shelf sand ridges) and as a group of individual sand bodies in the nearshore (nearshore sand ridges). The shelf sand ridges are present in water depths of 50–90 m and show large, elongate shapes with a length up to 200 km. In contrast, the nearshore sand ridges are much smaller in size (up to 34 km length) and occur in water depths shallower than about 30 m. Detailed interpretation of seismic and lithologic data, using radiocarbon dating to constrain the ages of the ridges, has shown that the regional sea-level changes played a major role in the existence and development of different morphologic features of these sand ridges. The shelf sand ridges developed mainly during the postglacial transgression (ca. 14,000–9,500 yr B.P.), possibly during episodes of stillstand or very slow rise of sea level. The shape of the shelf sand ridges more or less results from an erosional process dominantly acting during the postglacial transgression. The substratum of the shelf sand ridges consists of the regressive or lowstand deltaic mud deposits, probably formed prior to the last glacial maximum (> ca. 17,000 yr B.P.). In contrast, the nearshore sand ridges have undergone tidal action during the recent highstand of sea level (ca. < 7,000 yr B.P.) and show a typical modern geomorphology of erosional sand ridges. The substratum of the nearshore sand ridges consists of remnants of the last interglacial tidal deposits. Large dunes indicate a strong hydrodynamic influence on the entire ridge surface at present.

Modeling downstream fining in sand-bed rivers. I: formulation

In this paper a numerical modeling formulation is presented for simulation of the development of the longitudinal profile and bed sediment distribution in sand-bed rivers. The objective of the model application, which is presented in the companion paper (Wright and Parker, 2005), is to study the development of two characteristics of large, low-slope, sand-bed rivers: (1) a downstream decrease in bed slope (i.e. concave upward longitudinal profile) and (2) a downstream decrease in characteristic bed sediment diameter (e.g. the median bed surface size D 50). Three mechanisms that lead to an upward concave profile and downstream fining are included in the modeling formulation: (1) a delta prograding into standing water at the downstream boundary, (2) sea-level rise, and (3) tectonic subsidence. In the companion paper (Wright and Parker, 2005) the model is applied to simulate the development of the longitudinal profile and downstream fining in sand-bed rivers flowing into the ocean during the past 5000 years of relatively slow sea-level rise.

Snowfall-Driven Growth in East Antarctic Ice Sheet Mitigates Recent Sea-Level Rise

Satellite radar altimetry measurements indicate that the East Antarctic ice-sheet interior north of 81.6 degrees S increased in mass by 45 +/- 7 billion metric tons per year from 1992 to 2003. Comparisons with contemporaneous meteorological model snowfall estimates suggest that the gain in mass was associated with increased precipitation. A gain of this magnitude is enough to slow sea-level rise by 0.12 +/- 0.02 millimeters per year.

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 the Coastal Depositional Systems of the Changjiang (Yangtze) River in Response to Late Pleistocene-Holocene Sea-Level Changes

ABSTRACT The paleo-Changjiang (Yangtze) incised-valley fills, approximately 80-90 m thick, provide an opportunity to document the evolution of coastal depositional systems with large sediment supply in response to late Pleistocene-Holocene sea-level fluctuations on time scales of 103 to 104 years. The sedimentary facies of the incised-valley fills record three main depositional systems: fluvial, tide-dominated estuary, and tide-dominated delta. Radiocarbon ages for the incised-valley fills suggest that these depositional systems developed before about 11 ka, between 11 and 8 ka, and after approximately 8 ka, respectively. By applying sequence-stratigraphic concepts, the evolution of the depositional systems can be divided into three systems tracts--a lowstand systems tract (LST), a transgressive systems tract (TST), and a highstand systems tract (HST). Sea-level changes on a 104-year time scale controlled the basic architecture of the sequence of the incised-valley fill. On the other hand, sea-level changes on a millennial time scale affected the stacking pattern of the systems tracts. In particular, the continuous sea-level rise with episodic rapid rises during the last deglaciation affected the stacking pattern of the TST, which is characterized by a combination of aggradation and backstepping. The aggradation of fluvial and estuarine systems was dominant and the shoreline migrated only gradually landward under the relatively slow rise in sea level, and a very rapid sea-level rise around 12 and 10 ka caused the system to migrate abruptly landward. Unlike the transgressive estuarine phase, the stacking pattern of the regressive tide-dominated delta (HST), which developed within the almost filled incised valley and on the surrounding interfluve zones, was characterized by seaward progradation with clinoform architecture. It was initiated with aggradational and progradational stacking about 8 ka during the last phase of decelerated sea-level rise, and was followed by a progradational phase after the highest sea level about 6 ka.

Holocene coastal dune development and environmental changes in Apulia (southern Italy)

Abstract The coastal area of southern Apulia is characterised by marine deposits of Pleistocene and Holocene ages. It shows the effects of different phases of coastal morphology evolution. During the Middle and Upper Pleistocene, a number of marine terraces were shaped. In some places, they are characterised by thin beach deposits along with a dune belt and relative back dune deposits. Two important morphogenetic phases occurred during the Holocene. The former was characterised by deposition of beach deposits placed slightly above present mean sea level, and by formation of dune belts which were radiometrically aged about 6000 years BP. This event marks the end of the rapid postglacial transgression that occurred during the Holocene Climatic Optimum. During the younger phase, dated at about 2500 years BP, deposition of aeolian sands occurred. Archaeological and morphological evidences suggest that this phase should be related to a low sea-level stand followed by a slow sea-level rise up to the present position and by humid-temperate climatic conditions. The collected data about the Holocene dune belts occurring along the Apulian coastline suggest that main phases of dune development could be related to the effects of sea-level changes, climatic conditions, and in a subordinate way, to human activity.

Architecture and evolution of the tide-dominated Changjiang (Yangtze) River delta, China

The Changjiang (Yangtze) River, China, has formed a large tide-dominated delta on the western coast of the East China Sea. This paper presents the architecture and Holocene evolution of the delta, based on analyses of three borehole samples taken from the present delta plain. The deltaic sediments are 25–30 m thick and composed largely of subaqueous deposits, which can be divided into three facies: prodelta, delta front and delta plain, in ascending order. Tide-influenced sedimentary structures such as sand–mud couplets and bi-directional crosslaminations characterize the delta plain and delta front deposits. An upward-coarsening facies succession from prodelta to delta front is overlain by an upward-fining facies succession from the uppermost delta front to the delta plain. The deposits at the boundary between delta plain and delta front are composed of well-sorted, coarse deposits because it is in the zone of highest energy. The depth from the top of the intertidal facies of the delta plain to this boundary is almost the same, 9–11 m, among the three cores. Sediment accumulation rates are highest in the delta front facies (ca. 10 m/kyr maximum) and decrease toward both delta plain and prodelta facies (ca. 1 m/kyr). The evolution of the Changjiang delta, which began in a bay approximately 8 kyr bp, can be divided into two stages by its stacking pattern: aggradational and progradational. The aggradational system developed during the slow rise of sea level from 8 to 6 kyr bp, and the progradational system developed after sea level reached or approached its present position. The evolution of distributary channels and related river-mouth sand bars was an important process of delta migration or progradation.

Nearshore morphology and late Quaternary geologic framework of the northern Monterey Bay Marine Sanctuary, California

A combination of side-scanning sonar and high-resolution seismic reflection data image seafloor bedrock exposures and erosional features across the nearshore shelf. Sediment-filled troughs incise the inner shelf rock exposures and tie directly to modern coastal streams. The resulting bedrock geometry can be related to its resistance to erosion. Comparison of the depth of the transgressive erosional surface to recently developed sea level curves suggests a period of slow sea level rise during the early stages of post-interglacial marine transgression. The slow rise of sea level suggests an erosional episode that limited the preservation of buried paleo-channels beyond 70 m water depth. Seafloor features suggest that localized faulting in the area may have influenced the morphology of bedrock exposures and the coastline.

Proximal–distal facies relationships and sedimentary processes in a storm dominated carbonate ramp (Kimmeridgian, northwest of the Iberian Ranges, Spain)

Abstract Facies analysis of the upper Kimmeridgian rocks in the outcrops located near Ricla (Zaragoza province, northeast Spain) and the integration of the resultant data in a broader context (the northern part of the Iberian Basin), has produced two general models showing the facies distribution and the processes that controlled the sedimentation in the Kimmeridgian carbonate ramp. Using these two models the transition from shallow to relatively deep environments of the carbonate ramp is examined in detail. Model 1 corresponds to the development of a mixed carbonate-siliciclastic ramp during a slow rise and stillstand of sea level (Sequence 1-HST), whereas Model 2 represents the growth of a pure carbonate ramp during a rapid rise of sea level (Sequence 2-TST). Carbonate production was higher in the shallow ramp domains (coral reefs and oolitic shoals in Model 1 and reefs in Model 2) than in deeper domains, where there is no indication of significant pelagic or benthic production. The activity of unidirectional return flows induced by winter storms and hurricanes, played an important role in the redistribution of the sediment across the ramp, generating different coarse-grained deposits. In the inner and mid-ramp settings dunes, lower scale bedforms and tempestites occur in Model 1, and storm lobes, bars and tempestites in Model 2. Moreover, a significant bulk of the carbonate mud produced in shallow areas would eventually be resedimented in the outer ramp as suspended load in the density currents. Stillstand of sea level in Model 1 involved a rapid progradation of the inner and proximal mid-ramp carbonate and siliciclastic facies. The rapid relative sea level rise of Model 2 is determined by the dominance of the carbonate facies and by the presence of aggradational geometries in the transitional area between shallow and deep-ramp domains. The presence of relatively thick sections in the outer-ramp settings (instead of condensed sections, as observed in Model 1) during times of sea level rise (Model 2) can mainly be explained by the increase of the shallow production in the reef dominated areas.

Recent geological evolution and human impact: Fraser Delta, Canada

Abstract Throughout the Holocene, the river dominated Fraser Delta on the Pacific coast of Canada has prograded by continuous channel switching and avulsion into a deep (&gt;300 m) basin. However, at the beginning of the 20th century the delta was modified to provide a navigable channel and port facilities for the city of Vancouver. Now most of the sand brought down by the river (35% of the sediment load) is removed from the system by dredging. The remaining fine-grained sediment is transported in a plume past the intertidal estuary within the distributary channels then deflected northwards by the dominant flood tidal flow into the basin. Two causeways to the south of the main channel and one to the north that cross the intertidal zone to the delta foreslope act as barriers to the dominant northward sediment transport causing estuarine and localized seabed erosion. An eroded distributary channel failure complex has been exposed on the delta foreslope, off the southern causeways, by flood tidal flows that scour the seabed and form northward migrating subaqueous dunes, further increasing the delta slope. This, combined with slow sea-level rise and seismicity, intensifies the risk of further erosion and instability of the delta, particularly along the subaqueous delta front and the intertidal estuaries.

Sea-level change and storm-surge deposition in a late Holocene Florida salt marsh

ABSTRACT A 300-km length of west Florida's coastline is dominated by an open-marine coastal marsh system. Located along the central part of this sediment-poor region, Waccasassa Bay is presently a broad, shallow embayment rimmed by an expansive Juncus roemerianus salt marsh. In this system, sediments were first deposited in a forested swamp from ~ 4400 to 1800 cal yr BP. Contemporaneously, large oyster reefs grew in the embayment near a paleo-river mouth. These deposits indicate a period of slow sea-level rise during the Late Holocene that correlates with the establishment or regression of other west Florida coastal systems. Circa 1800 cal yr BP, a rapid transgression of 2-4 km led to salt marsh growth over the former swamp, and a brackish marsh developed over a previously unflooded upland surface. Calculated rates of shoreline retreat are 10 to > 20 m/yr at this time and compare to < 3.0 m/yr during most of the Late Holocene. Timing of the event in Waccasassa Bay corresponds with transgressive sequences in other Atlantic and Gulf coast systems and supports a relative sea-level fluctuation ca. 1800 cal yr BP. Following this event, sediments overstepped during the transgression were reworked onto the low-lying tidal marsh. Rapid accretion along the shore edge outpaced sea-level rise and led to the local development of high intertidal and supratidal levees. Evidence indicates that storm-driven surge was the principal mechanism of sediment transport for these deposits. In Waccasassa Bay, storm-surge deposition has played a significant role in marsh surface accretion, distinguishing the system from more typically tide-dominated wetlands. Within the past ~ 150 years, most of the supratidal environment became tidally inundated and there was a widespread transition into the modern salt marsh. This change and evidence from adjacent portions of the Florida coast implicate a rise in sea level within the past several hundred years.

Late highstand patterns of shifting and stepping coastal barriers and washover-fans (late Messinian, Sorbas Basin, SE Spain)

The late Messinian Sorbas Member, up to 75 m thick, consists in its type area of a parasequence set of three prograding coastal barriers (sequences I–III), associated with lagoon and washover sediments. Around the town of Sorbas these strata can be studied exceptionally well due to absence of burrowing by raised Messinian salinities and exposure along a network of up to 30 m deep canyons. Fifteen vertical sections were logged and carefully correlated. This permits to reconstruct and discuss patterns of relative sea-level movements between decimetres, up to 15 m within a parasequence. Excellent examples of non-tidal transgressive facies are characterized by lagoon and washover sediments instead of the usual combination of washover and tidal deposits (channel and flood-tidal delta). Implications for the sandstone connectivity are given. The lower two sequences are deposited in a relatively large, tectonically enhanced wedge-shaped accommodation space. They show both fining-up, deepening sequences, followed by prograding coarsening-up shoaling sequences and can be compared to the classical parasequences of the Western Interior Basin (USA). Progradation of sequence II was interrupted by a major slide event (most likely triggered by an earthquake), which caused more than 400 m seaward slumping of a stretch of 1 km of coastal sands. The architecture of sequence III is more complex due to limited accommodation space characteristic for the late highstand, so that this setting was very sensitive to sea-level fluctuations. This resulted in an intricate pattern of juxtaposed and superposed lagoonal muds, washover fans and swash zones. So-called ‘stranded’ coastal barriers occur, which were left behind after seaward jump of the coastline over more than 1.5 km during forced regression. The pattern of reconstructed sea-level positions is well comparable to the sequential pattern shown by the correlated equivalents along the northern basin margin, which belong to the so-called Terminal Carbonate Complex. The problem of ranking the complex sequence III as one or more parasequences and its consequences for cyclostratigraphy are shortly discussed. Two models of washover formation are given, respectively during more rapid and more slow sea-level rise. The influence of synsedimentary folding on the location of barriers is discussed and also the source area of extra- and intraclasts supplied to barriers and washovers. After deposition of the Sorbas Member the sea withdrew from the Sorbas Basin, probably as a result of the major downdrop in the Mediterranean at the maximum isolation during the Salinity Crisis. It is suggested that the semi-enclosed setting of the basin resulted only in limited, localized erosion, in contrast to the deeper adjacent Vera Basin, which was more open to the Mediterranean.

The Rise and Fall of Last Interglacial Sea-Levels

The Rise and Fall of Last Interglacial Sea-Levels

Stratigraphic evolution of the inner continental shelf in response to late Quaternary relative sea-level change, northwestern Gulf of Maine

Accumulations of deltaic and littoral sediments on the inner continental shelf of Maine, Gulf of Maine, preserve a record of postglacial sea-level changes and shoreline migrations. The depositional response of coastal environments to a cycle of regression, lowstand, and transgression was examined with seismic-reflection profiles, vibracores, and radiocarbon dates collected from sediments at the mouths of the Kennebec and Penobscot Rivers. Sequence-stratigraphic analysis of these data reveals two distinctly different successions of late Quaternary deposits that represent end members in an evolutionary model for this glaciated coast. Seaward of the Kennebec River, coarse-grained shorelines with foreset beds occur at depths of 20–60 m and outline the lobate margin of the Kennebec River paleodelta, a complex, rock-framed accumulation of glaciomarine and deltaic sediments capped by estuarine and marine deposits. Sand derived from this system today supports large barrier spits and extensive salt marshes. In contrast, the mouth of the Penobscot River is characterized by thick deposits of glaciomarine mud overlain by marine mud of Holocene age, including gas-charged zones that have locally evolved into fields of pockmarks. The distinct lack of sand and gravel seaward of the Penobscot River and its abundance seaward of the Kennebec River probably reflect differences in sediment sources and the physiography of the two watersheds. The contrasting stratigraphic framework of these systems demonstrates the importance of understanding local and regional differences in sediment supply, sea-level change, bedrock structure, and exposure to waves and tides in order to model river-mouth deposition on glaciated coasts. Evolution of shelf deposits was largely controlled by relative sea level, which locally fell from a highstand (+60 to +70 m at 14 ka) contemporary with deglaciation to a lowstand (−55 m at 10.8 ka). The sea-level lowering was accompanied by fluvial incision of older deposits, producing a regressive, basal unconformity. Major rivers deposited abundant sediment over this surface. Sea level then rose at varying rates, extensively reworking formerly emergent parts of the shelf and producing a shoreface ravinement surface in areas exposed to waves. A tidal ravinement surface has developed in sheltered embayments where erosion is due mainly to tidal currents. Incised valleys in both settings preserve transgressive estuarine deposits that contain lagoonal bivalves and salt-marsh foraminifera at depths of 15–30 m. These deposits accumulated ca. 9.2–7.3 ka, locally a period of relatively slow sea-level rise.

Submerged sand ridges on the western continental shelf off Bombay, India: evidence for Late Pleistocene-Holocene sea-level changes

The morphology, distribution and probable origin of submerged sand ridges on the western continental shelf off Bombay, India have been studied using echosounding, side-scan sonar and high-resolution seismic reflection techniques supplemented by seabed sampling. 7–8 rows of well defined, broad, symmetrical submerged sand ridges were found on the middle and the outer shelf 100–200 km offshore and between 75 and 100 m water depth, the ridges being more prominent between 85 and 90 m. These ridges are 0.5 to 10 km wide, 1.5 to 18 m high, several tens of km long, with a few kilometers spacing and a trend of NNW-SSE parallel to the shelf edge. The inter-ridge areas are characterised by long, narrow palaeo-lagoonal-like depressions. Some rounded/smooth ridges appear to be moribund while others have undergone reworking as indicated by superimposed bedforms/sand waves and reefs. Ridges also occurred on the inner shelf where they were either eroded during the Holocene transgression or covered by Recent sediments. The sand ridges originated as dunes which were submerged and reformed into very large sand bars. Further drowning and lithification favoured the development of organic growth and sand wave formation which must have taken place during a period of stillstand or slow sea level rise in the early to mid-Holocene. The superimposed bedforms reflect the prevailing flow regime on the shelf.

Depositional patterns of sand ridges in tide-dominated shallow water environments: Yellow Sea Coast and South Sea of Korea

The continental shelf of the Yellow Sea and the Korean South Sea is a tide-dominated shelf. A number of sand ridges occur in this area which form bathymetric highs. Surveyed were one large sand ridge (Jangan Satoe) along the western (Yellow Sea) coast and a group of mid-shelf sand ridges in the South Sea, to delineate their depositional pattern and probable origin in tide-dominated shallow water environments. Analyses of sediment samples, side-scan sonographs, and seismic profiles reveal that these ridges show regionally different characteristics in morphology, bedforms, and sediment. The sand ridge in the Yellow Sea shows a composite form of various bedforms including sand waves and megaripples. This ridge is interpreted to be in an accretional active stage, maintained by strong tidal currents. In contrast, the sand ridges on the mid-shelf of the South Sea show rounded, single forms without bedforms on them. They are interpreted to represent moribund-type sand ridges in the shelf environment, which were formed near the shoreline during the Holocene sea-level transgression. These ridges developed off the paleo-Seomjin River during a period of a stillstand or slow sea-level rise approximately between 10,000 and 7000 yrs B.P.

Mangrove Retreat with Rising Sea-level, Bermuda

Low island mangroves keep up with slow sea-level rise by peat accumulation. Holocene stratigraphic records show that they maintain the same pace as sea-level rise at rates up to 9 cm/100 years. Tide gauge records from Bermuda since 1932 show sea-level rise at a rate of 28 cm/100 years. The largest mangrove area (6·26 acres) at Hungry Bay has for the last 2000 years been building peat at a rate of 8·5 to 10·6 cm/100 years. Retreat of the seaward edge has caused loss of 2·24 acres of mangroves, commencing in the last few hundred years, with a second dieback between 1900 and 1947, and a third dieback in the last decade. The substrate elevation of the seaward margin of mangroves is below mean sea-level, the normal lower limit for mangroves. Present dieback shows problems of erosion indicating that the Bruun Rule of beach erosion with sea-level rise is also appropriate for mangrove swamps. Stratigraphy shows that before 4000 BP sea-level rose at a rate of 25 cm/100 years, from 4000 to 1000 years BP the rate of sea-level rise declined to 6 cm/100 years during which time mangroves established, and in the last 1000 years there was an increase to 14·3 cm/100 years, during which time the mangroves died back. This study indicates that low island mangroves will experience problems with the rates of sea-level rise predicted for the next 50 years.

Relative sea-level changes and the development of a Cambrian transgression

AbstractThe Lower Cambrian in northwest Scotland is one example of a Lower Palaeozoic ‘orthoquartzite-carbonate’ succession deposited on a slowly subsiding, peneplained Precambrian basement during a period of relative sea-level rise. This particular setting led to the development of a very wide, low gradient shelf which was extremely sensitive to minor sea-level changes. The basal quartz arenite section (Lower Member-Pipe Rock) is a transgressive, tide-dominated systems tract, but lacks a systematic parasequence architecture because of three factors: a fluvial sediment flux was insufficient to induce shoreline progradation, accommodation space was limited during sea-level falls (which are commonly expressed by widespread erosional surfaces), and sediment yield to the shelf by transgressive reworking was a major contributor towards the preserved stratigraphy. The storm-dominated Fucoid Beds represent a condensed section and also show the effects of rapid and widespread facies belt oscillations because of the low shelf gradient. An overlying highstand systems tract is also lacking, partly due to the absence of a large fluvial sediment yield and also due to lowstand and transgressive reworking. An erosively based tidal sandsheet at the top of the Fucoid Beds, interpreted to be a lowstand systems tract, therefore rests directly on the condensed section of the underlying sequence. This was in turn reworked into linear tidal sandbanks (Salterella Grit) during slow sea-level rise, prior to the next major transgression. The limited accommodation space therefore introduced a preservational bias towards deepening-upward trends on a parasequence and sequence scale. The oscillations in facies belts, episodic subareal exposure and the potential to remove substantial portions of systems tracts suggests that Lower Palaeozoic ‘orthoquartzite’ successions may exhibit regular and abrupt vertical shifts in depositional environment which, given their subtle lithological character, may require detailed analysis to identify. Such successions may also display incomplete development of several components of transgressive-regressive sequence architecture.

Episodic sea level changes in the western Indian Ocean: A review

Abstract In the past, sea level changes in the Indian Ocean received much less attention than those in the Atlantic and Pacific oceans. Here the sea level variation in the western part of the Indian Ocean has been considered. Rather than concentrating on the yet unproven, slow, and gradual sea level rise due to the so‐called greenhouse warming, attention is paid to the much more dramatic sea level variations that can occur within a few hours due to storm surges and tsunamis. In that part of the western Indian Ocean that is considered here, the storm surges are generated from the sea level atmospheric pressure fields and tangential surface wind stresses associated with travelling weather systems such as tropical cyclones. One can adapt the results from tsunami runup studies to storm surges since both phenomena fall into the classification of long gravity waves.

Mississippi-Alabama outer continental shelf topographic features formed during the late Pleistocene-Holocene transgression

A geophysical survey of the outer Missisippi-Alabama continental shelf revealed three types of topographic features: 1) reef-like mounds (RLM), 2) isobath-parallel ridges, and 3) shallow depressions. RLM appear to be bioherms. Their bases cluster in two isobath bands, 105 to 120 m and 74 to 82 m, implying the late Pleistocene-Holocene transgression was nonuniform. The deeper RLM probably formed during a slow sea level rise at the beginning of the deglaciation. An 11 ka BP C14 age was obtained from the summit of one, at a depth of 90 m. Depth relationships suggest the shallower RLM and most ridges formed about the time of the mid-deglacial Younger Dryas cooling episode.