Glacial Lowstand Research Articles

Late quaternary transgressive/regressive sequences from Taranaki continental shelf, western New Zealand

Interpretation of seismic reflection profiles (3.5 kHz, airgun), and piston core and borehole stratigraphic logs, collected from mid-outer shelf depths (50–130 m) on the Taranaki continental shelf, western New Zealand, have delineated at least 4 alternating late Quaternary seismic and lithologic couplets. Each couplet comprises an acoustically transparent, upper silty unit that overlies a highly reflective sandy unit, characterised by a channelised erosive base. Radiocarbon and biostratigraphic nannofloral dating of selected samples provides chronostratigraphic control for the succession, indicating that the entire sequence is probably late Quaternary in age (< 600 kyr B.P.). Based mainly on nannofossil paleoenvironmental interpretations and seismic reflection characteristics, it is proposed that the silty units represent interglacial highstand deposits and that the intervening sandy units were deposited either as glacial lowstand or transgressive nearshore deposits that developed under rising sea-level conditions. Erosive, irregular basal unconformities were probably developed during relative sea-level fall and represent Type 1 sequence boundaries; there may also have been a component of wave planation during the subsequent transgression. High-amplitude channel-fill seismic units are possibly fluvial and/or paralic sediments, deposited during the sea-level lowstand and/or the ensuing transgression. Tentative correlation of late Quaternary units on the Taranaki shelf is made to the deep-sea oxygen isotope stage curve to provide temporal resolution of the stratigraphic section. Four possible sedimentation models are considered, based on different temporal interpretations of the observed litho- and seismic stratigraphy. Estimated sedimentation rates for each model are compared to existing data-sets as a validation tool. The data are inconclusive, but appear to support scenario 1 which is characterised by moderate average sedimentation rates over the past 300,000 years, typically 0.57 m/kyr, with an anomalous phase of slow deposition (or non-deposition and/or erosion) during stage 6.

Stacking Pattern and Stratigraphic Model of Reservoirs from a Modern Analogue to Ancient Deltaic and Siliciclastic Deposits: Golfe du Lion, SE France: ABSTRACT

The {open_quotes}Golfe du Lion{close_quotes} Pleistocene shelf deposits are used as modern analogues of ancient deltaic and shallow water siliciclastic deposits. At least 6 cyclic superimposed sedimentary packages constituting a seaward thickening wedge are documented, including major progradational clinoforms units bounded by regional unconformities, with their updip terminations. Seismic correlations and geometry models, together with age dating and facies analysis of the uppermost units have been used at regional scale. They allowed to differentiate allocyclic from autocyclic events, to improve our knowledge of the nature of tectonic control on architecture, and to rely the different seismic units, inside the packages, to the environmental changes from interglacial highstand to glacial lowstand in 4 to 5th order cycles. An alternative model of {open_quotes}forced regression{close_quotes} during sea level lowstand was documented and validated. The different illustrated sedimentary units, related to specific parts of the relative sea level curve along a short duration/high amplitude cycle, are: (i) the {open_quotes}forced regression{close_quotes} deposits (ii) the late lowstand massive sands onto the outer shelf (iii) the early transgressive backstepping (?) beach sands, (iii) the late transgressive backslapping parasequences. The sand content inferred from the seismic facies, comforted with the uppermost units facies from data cores, is in agreementmore » with this model.« less

Palaeoecology of enigmatic tube microfossils forming “cryptalgal” fabrics (Late Quaternary, Red Sea)

Tubular microfossils are important sediment producers in the aphotic zone of the Red Sea. They occur on forereef slopes and as incrustations on hardgrounds of basinal carbonates as well as encrusting drowned reefs. The tubular microfossils (mean inner diameter 10 μm, outer diameter 50 μm) are straight to slightly curved and commonly branched. Because of their size and mode of calcification, they may represent remnants of fungi or cyanobacteria. Combined growth of the microfossils on protected upper surfaces of substrates and little sedimentation produce strictly geofugal cryptalgal fabrics. Although this fabric suggests phototactic growth, the tubular microfossils are eurytopic. In the Red Sea they occur between 63 and 765 m below present sea level. Within the basin the cryptalgal fabrics formed during last glacial lowstand of sea level and along the slope during Holocene rise of sea level. Limiting factors recognized so far seem to be space competition with faster growing encrusters, mainly the corallinaceans, and burial by sediments. During glacial lowstand of sea level salinity exceeded the tolerance levels of most planktonic taxa resulting in low rates of sedimentation in the basin. Along the cliffed slope sedimentation remained low during Holocene transgression. Before the mid-Mesozoic calcified cyanophyceans were very common. It is proposed that, as a consequence of the radiation of calcareous plankton during that time, suitable conditions of low sedimentation in deep water became less common.

Late Quaternary sedimentation, lower Colorado River, Gulf Coastal Plain of Texas

Research Article| August 01, 1994 Late Quaternary sedimentation, lower Colorado River, Gulf Coastal Plain of Texas MICHAEL D. BLUM; MICHAEL D. BLUM 1Department of Geology, Southern Illinois University, Carbondale, Illinois 62901 Search for other works by this author on: GSW Google Scholar SALVATORE VALASTRO, JR. SALVATORE VALASTRO, JR. 2Radiocarbon Laboratory, Balcones Research Center, University of Texas, Austin, Texas 78758 Search for other works by this author on: GSW Google Scholar Author and Article Information MICHAEL D. BLUM 1Department of Geology, Southern Illinois University, Carbondale, Illinois 62901 SALVATORE VALASTRO, JR. 2Radiocarbon Laboratory, Balcones Research Center, University of Texas, Austin, Texas 78758 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1994) 106 (8): 1002–1016. https://doi.org/10.1130/0016-7606(1994)106<1002:LQSLCR>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation MICHAEL D. BLUM, SALVATORE VALASTRO; Late Quaternary sedimentation, lower Colorado River, Gulf Coastal Plain of Texas. GSA Bulletin 1994;; 106 (8): 1002–1016. doi: https://doi.org/10.1130/0016-7606(1994)106<1002:LQSLCR>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Investigations in the lower Colorado River Valley, Gulf Coastal Plain of Texas, have resulted in the development of a spatially and temporally controlled history of changes in channel and flood-plain erosional and depositional processes. When combined with paleoclimatic and stratigraphic data from the upper Colorado River drainage and the record of glacio-eustasy in the Gulf of Mexico, this study permits evaluation of the relative influence of different external controls on channel and flood-plain behavior, the development of alluvial landforms, and the development of alluvial stratigraphic sequences.Late Pleistocene and Holocene alluvial deposits of the lower Colorado River have been subdivided into allostratigraphic units, with chronological control afforded by radiocarbon ages. In the bedrock-confined valley, up to 10 m of late Pleistocene (∼20,000-14,000 yr B.P.) sediments referred to as the Eagle Lake Alloformation (ELA) underlie a terrace at 17-20 m above the present-day channel. Deposition of the ELA was followed by bedrock valley incision, then deposition of a complex Holocene valley fill referred to as the Columbus Bend Alloformation (CBA). Columbus Bend Allomembers 1 and 2 (CBA-1 and CBA-2) underlie a terrace at 12-14 m above the present-day channel. CBA-1 was deposited ∼12,000-5,000 yr B.P., whereas CBA-2 was deposited ∼5,000-1,000 yr B.P. Columbus Bend Allomember 3 (CBA-3) consists of channel and flood-plain deposits that represent the past 600 yr of activity.Allostratigraphic units within the lower Colorado valley correlate with allostratigraphic units in major valley axes of the upper Colorado drainage and with records of climatic and environmental change, suggesting that alluvial deposits record basinwide responses to climatically controlled changes in discharge regimes and sediment supply. Basal unconformities for Holocene valley fills, however, appear to be 1,000-2,000 yr younger in the upper Colorado drainage than they are in the lower Colorado valley. This time-transgressioe episode of bedrock valley cutting was initiated by climatically controlled reductions in sediment supply, but conditioned by limits on rates of up-stream propagation of incision through a large drainage basin. By contrast, unconformities within Holocene valley fills document time-parallel episodes of flood-plain abandonment and soil formation, but little additional bedrock valley cutting, and indicate decreased flood magnitudes following shifts to drier climatic conditions.Flood-plain morphology and sedimentary facies changed through time in response to changes in climate coupled with a protracted degradation of upland soil mantles, which altered the rate at which precipitation was transferred to stream channels as runoff. During the late Pleistocene through middle Holocene, runoff was filtered through deep upland soils, floods were for the most part less flashy and contained within channel perimeters, and flood plains were constructed by lateral migration without significant vertical accretion; hence, the ELA and CBA-1 contain few vertical accretion facies. Exposure of bedrock surfaces during the late Holocene resulted in increased flood stages, deep overbank flooding, and construction of flood plains by vertical accretion; hence, CBA-2 and CBA-3 contain thick vertical accretion facies.Allostratigraphic units and bounding unconformities persist through the bedrock-confined valley to the Quaternary alluvial plain, but stratigraphic architecture changes substantially in the downstream direction as a result of the last glacio-eustatic cycle. On the alluvial plain, late Holocene CBA-2 and modern CBA-3, deposited contemporaneously with the present interglacial highstand, onlap and bury the ELA and CBA-1, which were emplaced during the last full glacial lowstand and the transgression that followed. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Glacial‐Holocene Biogenic Sedimentation Patterns in the South China Sea: Productivity Variations and Surface Water pCO2

A bathymetric transect of cores in the South China Sea extending from 4200‐m to less than 1000‐m water depth has been examined for glacial‐interglacial changes in carbonate and organic carbon sedimentation. Typical “Pacific carbonate cycles” (high carbonate content during glacials and low carbonate content during interglacials) characterize cores from water depths deeper than 3500 m. In contrast, “Atlantic carbonate cycles” (low carbonate during glacials and high carbonate during interglacials) are observed in cores from depths shallower than 3000 m as a result of increased dilution of carbonate by terrigenous material during glacial low stands of sea level. Glacial‐interglacial changes in the carbonate chemistry of South China Sea intermediate and deep waters resulted in significant changes in the positions of the carbonate compensation depth (CCD) and the aragonite compensation depth (ACD). During the last glacial the CCD and ACD were at least 400 and 1200 m deeper, respectively, than at present. Organic carbon accumulation rates in the South China Sea were approximately 2 times higher during the last glacial than the Holocene. Carbon isotopic analyses and C/N ratios of the organic matter indicate that only a small fraction of the increase in glacial organic carbon accumulation can be attributed to input of terrestrial carbon. On the basis of this we conclude that surface water productivity in the South China Sea was approximately 2 times higher during the last glacial maximum. This is consistent with previous studies which have demonstrated that glacial productivity was higher in low‐ to mid‐latitude regions of the Atlantic and eastern Pacific. The deglacial decrease in organic carbon accumulation is accompanied by a decrease in δ13Corg. Using the relationship between δ13Corg and [CO2](aq) developed by Popp et al. [1989], we estimate that surface water pCO2 values in the South China Sea during the last 25,000 years were very similar to atmospheric CO2 concentrations.

A high‐resolution Late Quaternary upwelling record from the anoxic Cariaco Basin, Venezuela

Results are presented of a high‐resolution study of the planktonic foraminiferal faunas from two piston cores recovered from the Cariaco Basin in the southern Caribbean Sea. The Cariaco Basin is a small anoxic marine basin on the northern continental margin of Venezuela in an area today characterized by both seasonal trade wind‐induced upwelling and pronounced dry and wet seasons. Our data indicate that large changes in the intensity of upwelling, and hence trade wind strength, occurred in this region during the last glacial‐interglacial transition and throughout the Holocene. During the last glacial lowstand of sea level, the Cariaco Basin was effectively isolated from the open Caribbean along its northern margin by the then largely emergent Tortuga Bank. Oxic conditions existed in the deep Cariaco Basin at this time, and surface productivity was low. About 12,600 years ago, the abrupt initiation of strong upwelling over the basin and the onset of permanent anoxia in the deep waters are coincident with the rapid rise of sea level that accompanied the peak interval of meltwater discharge from the Laurentide Ice Sheet into the Gulf of Mexico. Strong upwelling between 12,600 and about 10,000 years ago may be related to intensified trade winds resulting, in part, from cooler sea surface temperatures in the Caribbean and Gulf of Mexico. After about 10,000 years ago, upwelling intensity was reduced, though highly variable. A preliminary frequency domain analysis of the Holocene portion of the Cariaco Basin time series suggests that solar forcing may explain a significant component of the century‐scale variability observed in the record of upwelling and trade wind strength.

High Production and Highstand Shedding from Deeply Submerged Carbonate Banks, Northern Nicaragua Rise

ABSTRACT Because the shallow isolated carbonate banks on the northern Nicaragua Rise, the Nicaragua/Honduras and southern Jamaica carbonate shelves, and many other modern carbonate banks worldwide, are covered by an average of 20 to 30 m of water and by a thin blanket of coarse carbonate sediments, other carbonate sedimentologists have considered these banks good examples of incipiently drowned, or even drowned carbonate banks. However, based on recent research on the northern Nicaragua Rise, we can demonstrate that these banks currently are healthy producers of large volumes of periplatform sediments (fine aragonite and magnesian calcite), which are exported almost entirely to the deep surrounding slopes. These sediments, which were deposited during the past 9000-10,000 years, form peripl tform wedges on the middle and upper slopes, which are seen clearly on 3.5 kHz seismic profiles. Radiocarbon ages of the wedge surface sediment range between 230 and 610 YBP and provide clear evidence for the contemporaneous production of sediments on the shallow bank and shelf and their instantaneous export to the upper slopes. For the last 5000 years, sedimentation rates ranged from 2000 mm/ky off Pedro Bank to 1300 mm/ky off Jamaica. These rates are somewhat lower than, but of the same order of, magnitude as sedimentation rates on the western (leeward) slopes of Great Bahama Bank. Deposition of the metastable carbonate sediments discussed in this paper occurred as soon as sea level rose to flood the bank and shelf which flank Walton Basin, following the last glacial lowstand. They com rise a wedge-shaped package of transgressive and highstand sediments on the upper and middle slopes of Pedro Bank and the southern Jamaica shelf.

High‐resolution planktic stable isotope record and spectral analysis for the last 5.35 M.Y.: Ocean Drilling Program Site 625 northeast Gulf of Mexico

Planktic δ18O, δ13C, and biostratigraphy are presented from a nearly complete Pliocene and Quaternary sequence from Ocean Drilling Program hole 625B, northeast Gulf of Mexico. A high‐resolution chronology is derived for the last 5.35 m.y. by tuning the δ18O record to variations in orbital obliquity. Most ages derived for biostratigraphic datums in hole 625B are similar to the same events based on magnetobiochronologies from outside the basin. The δ18O record indicates that Pliocene and early Pleistocene temperature decrease/ice buildup in the northern hemisphere was nonlinear, occurring via a series of steps and gradual shifts beginning about 3.8 Ma. Late Pliocene (2.30–1.66 Ma) and early Pleistocene (1.66–0.73 Ma) δ18O anomalies with magnitudes of −1.4 to −2.0‰ indicate glacial meltwater discharge from the Mississippi River implying that an ice sheet has been present at mid‐latitudes in North America periodically since 2.3 Ma. Spectral analysis of the δ18O signal indicates that the 100‐kyr period dominated during approximately the last 0.5 m.y. and the 41‐kyr period was dominant during the early Pleistocene and entire Pliocene. A contour plot of the δ18O spectrum indicates that the early to mid‐Pleistocene climate transition commenced about 1.0 Ma with the first occurrence of a strong low‐frequency component to the climate system. This low‐frequency component gradually became the 100‐kyr frequency by about 0.6 Ma. Between 0.7 and 0.6 Ma, the 41‐kyr period decayed and via intermediate frequencies gave way to near‐100‐kyr periodicities. The magnitude of glacial‐interglacial δ13C variation during the middle to late Pleistocene at site 625 is greater than observed in open‐ocean records and may have resulted from increased flux of isotopically light, terrestrial organic carbon into the basin during glacial lowstands of sea level.

Clastic Sequences Developed During Glacio-Eustatic Sea Level Fluctuations on a Sandy Barrier Coastline: ABSTRACT

Seismic reflection (Geopulse) profiles from a 125 km{sup 2} grid on the New Jersey low-mesotidal sandy barrier coastline reveal a 30-m sediment thickness composed of three unconformity bounded parasequences. Vibracores from 12 sites reveal a variety of depositional environments that existed during the most recent glacio-eustatic sea level fluctuations on this stable passive margin. Environments include fluvial, during glacial lowstands; barrier islands with back-barrier lagoons and pro-barrier ebb tidal deltas and shore-attached ridges; and below storm-wave base shelves during interglacial highstands. Following the last major interglacial (Sangamon, {approx} 125KA), sea level fell and rivers eroded a planar seaward dipping surface by the early Wisconsinan time ({approx}70KA). As sea level rose during the mid-Wisconsinan ({approx} 50KA), a barrier island shoreline migrated to within 0.2-1.7 km of the modern shoreline. Maximum highstand of mid Wisconsinan sea level was 20 m below present. Sea level fell again during the late Wisconsinan ({approx}20KA). A barrier island/lagoonal system developed under rising sea level at least by the early middle Holocene based on peats (8,800 BP) in lagoonal muds cored from {minus}12 m. The root of the mid-Wisconsinan barrier system was preserved under early Holocene transgressive sediments. The modern coastal and inner continental shelf deposits aremore » relatively thin (3-4 m) pebbly sands overlying a prominent ravinement surface unconformity formed during the Holocene sea level rise. Isopach maps constructed for this surface indicate up to 7 m of relief eroded into muds beneath the ebb tidal delta and the shore-attached ridges. The ridges and delta shielded the underlying fine sediments during sea level rise. Based on a reasonable Atlantic shoreline transgression rate of 2 m/yr, the modern Barnegat Inlet ebb tidal delta has existed in its general location for at least 800 years.« less

Seismic Depth Conversion Problems Associated with the Mississippi Canyon in the Vicinity of Ewing Bank Block 305 Field, Offshore Louisiana: ABSTRACT

Ewing Bank Block 305 field is located on the outermost edge of the continental shelf, approximately 145 km south of New Orleans, Louisiana. The geology of the field is fairly typical of other fields found offshore of Louisiana, except there is a huge, partly filled, Pleistocene submarine canyon that overlies the field. This canyon, which is commonly referred to as the Mississippi Canyon, causes a large seismic depth conversion problem that makes it virtually impossible to correctly map the structure of the field using normally processed marine seismic data The Mississippi Canyon is a large erosional feature that was formed at the mouth of the ancestral Mississippi River during the last glacial sea level lowstand. In the vicinity of the field, the canyon is about 10 km wide and 1 km deep, which is approximately two-thirds the size of the Grand Canyon of the Colorado River, and is filled mostly with Holocene sediments. Because the sediments that fill the canyon are very young, their interval velocities are much lower than the interval velocities of the sediments that form the canyon walls. This creates abrupt near surface lateral velocity variations that distort the geometry of seismic reflectors such that the timemore » geometries seen on seismic lines are not at all indicative of the structure of the subsurface. Using a model-based processing technique and available well control, the authors have been able to solve this difficult depth conversion problem and have gained a better understanding of the geology of this field.« less

Quaternary Sediments and Facies Relationships in the Sound of Iona, Western Scotland: ABSTRACT

This study presents the sedimentological relationships between interbedded carbonate marine sediments and siliciclastic glacial deposit in the Sound of Iona. In this area there is an abrupt vertical and lateral facies transition between siliciclastic sediments deposited in turbid, cold water and biogenic limestones deposited in clear, warm water. The reason for this abrupt transition is that highly productive, carbonate-secreting organisms were rapidly deposited during the temperate-climate marine transgression. The carbonates abruptly cover preexisting glacial lowstand deposits. Terrigenous influx is low during the transgression because glacial erosion has removed siliciclastic sediment from coastal source areas. The Sound of Iona is carpeted by a Holocene deposit of carbonate sand, estimated from seismic data to be 3-5 m thick. Point-count data show that the sand averages 75% carbonate, with comminuted barnacle shells comprising over 50% of the sand. Four facies are recognized: (1) rippled carbonate sands lying on the margins of the sound, which grade laterally into (2) centrally located carbonate sand waves and sand ribbons, which lie downstream to (3) in-situ coralline algae deposits (maerl) which in turn overlie (4) relict glacial drift deposits. The sand-wave facies forms the thickest deposit ({approximately}5 m). The sand is localized over a bathymetric high formedmore » from an older deposit of glacial drift. Carbonate-sand deposits are thin or absent distal to the sand-wave facies; relict glacial deltaic deposits lie exposed in these outlying localities. The barnacle-rich, rippled carbonate-sand facies is deposited over preexisting glacial deposits, producing an abrupt vertical facies transition. These carbonate sands are transported by wind-generated wave and low-velocity tidal currents.« less

Geohydrology of the Laura fresh-water lens, Majuro atoll: A hydrogeochemical approach

In small limestone islands, the depositional history and subsequent chemical interactions between ground water and the aquifer host rock play critical roles in the occurrence, movement, and chemical quality of ground water. The hydrogeochemistry of the Laura fresh-water lens, Majuro atoll, Marshall Islands, is an example of these relations. Laura is underlain by two principal hydrologic units. The upper unit is a back-reef-marginal-lagoonal deposit which formed during the Holocene interglacial stage. It is composed of moderately permeable carbonate sediments. The lower hydrologic unit consists of highly permeable limestone that was subaerially exposed, most likely during a Pleistocene glacial lowstand. Similar stratification is found at Bikini and Enewetak atolls. The upper hydrologic unit contains a calcium bicarbonate-rich fresh-water lens, in which a potable fresh-water nucleus as much as 14 m thick occurs on the lagoon side of the island. Storage in the fresh-water nucleus ranged from 1.70 x 10 6 to 2.08 x 10 6 m 3 during 1984-1985. Ground-water occurrence and flow are governed by an asymmetric distribution of lithofacies about the longitudinal axis of the island and an abrupt increase in permeability at the contact between the upper and lower hydrologic units. The highly permeable lower hydrologic unit contains sea water and truncates the fresh-water-sea-water mixing zone. The fresh-water lens and associated fresh-water-sea-water mixing zone are the site of continuously occurring diagenetic reactions that significantly affect the porosity and permeability of the aquifer. Non-equilibrium dissolution-precipitation reactions, coupled with variations in CO 2 input, control the chemical evolution of Laura ground water. At the present rate of chemical weathering, 465 m 3 of sediment are being dissolved and transported to the sea by ground water each year. This dissolution results in an annual increase in porosity of 0.01%. The primary factors controlling the occurrence and flow of ground water in the leeward reef islet of Laura are (1) the depositional history of the upper hydrologic unit, which has resulted in a greater accumulation of low-permeability (fine-grained) sediments beneath the lagoon side of the island and a high- to low-permeability (coarse-to fine-grained sediment) gradation between the ocean and lagoon; and (2) the diagenetic history of the lower hydrologic unit, which has resulted in a highly permeable basement.

Carbonate Platforms, Sequence Stratigraphy, and Sea Level: ABSTRACT

Besides tracing reflectors, the mapping of stratigraphic sequences marks a major advance in seismic interpretation. However, construction of sea level curves from sequence stratigraphy is complicated by other factors besides sea level influencing sequence geometry. One such factor is lithology. This point is examined by comparing siliciclastic systems and carbonate platforms. During the Pleistocene, the siliciclastic sediment supply to the deep sea was at its maximum during glacial lowstands of sea level. Pleistocene carbonate platforms were exactly in antiphase to this rhythm. They produced and exported most sediment during interglacial highstands when the platforms were flooded (highstand shedding). In the Bahamas and other platforms, accumulation of both bank-derived fines and sandy turbidites is higher by a factor of 2 to 9 during the interglacials, and turbidites tend to cluster in these highstand intervals. Highstand turbidites also differ in composition from their lowstand counterparts. Turbidite abundance and composition together provide a faithful record of the Pleistocene sea level cycles not easily erased by diagenesis. Geometrically, platform respond to sea level by forming highstand turbidite wedges and lowstand drapes of pelagic sediment--the opposite of siliciclastic systems. Drowning unconformities are another example for the significance of lithologic change in sequence stratigraphy. Flanks of carbonate platforms are generally steeper than siliciclastic slopes. When carbonate platforms are drowned and buried by siliciclastics, an unconformity ensues because the clastics are unable to assume the steep carbonate slope angle or because they are shed from other directions than the carbonates. Examples of drowning unconformities include the Lower Cretaceous platforms off West Africa and off eastern North America as well as the middle Cretaceous unconformity in the Gulf of Mexico. Highstand shedding and drowning unconformities of platforms illustrate that not all depositional systems respond alike to changes in sea level and that sequence boundaries may be caused by lithologic change. These lithologic turning points need not be related to sea level. In a very general way, sequence boundaries can be viewed as changes in the pattern of sediment input and dispersal in a basin. Sea level fluctuations are one way to induce such changes, but tectonic movements and environmental change represent important alternatives, demonstrated by the seismic stratigraphy of the deep Gulf of Mexico. End_of_Article - Last_Page 1522------------

Mass movement of fine-grained sediment to the basin floor, bering sea, Alaska

Large volumes of fine-grained sediment have been transported to the deepbasin floor in the Bering Sea, especially during glacial low stands of sea level. Turbidity currents and several types of mass movement have been the chief transporting agents.

Rhone Deep-Sea Fan: A review

The Rhone Fan is a large Plio-Pleistocene turbidite deposit in the western Mediterranean Sea. The fan is fed from the broad Rhone River delta, but only one canyon, the Petit-Rhone, has fed most of the major turbidite depositional sequences that have been mapped. Slumping of sediment from intercanyon areas on the delta slope also has provided much sediment for the fan. The lack of Recent turbidite deposition on the fan suggests that turbidite sedimentation dominates during glacial low stands of sea level, building major leveed valley sequences, while surficial slumping of the valley levee deposits and pelagic sedimentation seem to mark high stands of sea level during interglacial periods.

Origin and Significance of Grapestone, Bahama Islands

ABSTRACT Bahaman grapestone is produced by cementation of recrystallized ooids in the marine environment. Initial binding of grains is accomplished by growth of algae and encrusting forams in the substrate. Later cementation and infilling of the composite may be due to continued growth of these organisms in the interior of the aggregate and to chemical or biochemical precipitation of cement. Data indicate the recrystallized ooids making up grapestone were formed at an earlier stage in sedimentation as the sea transgressed onto the Bahaman Platform at the end of the last glacial lowstand. Thus, grapestone is produced through a sequence of depositional conditions rather than a single set of conditions. Given a supply of firm grains, uneven water turbulence, high water circulation rates, and very low sedimentation rates favor the formation of grapestone.

Phreatic vs. Vadose Diagenesis: Stratigraphy and Mineralogy of a Cored Borehole on Barbados, W. I.

ABSTRACT Meteoric water diagenesis may take place in the vadose-subaerial environment or in the fresh water phreatic environment. This paper presents data to suggest meteoric water diagenesis may occur more rapidly in the phreatic diagenetic environment. Continuous drill core was recovered from a borehole in the radiometrically dated 105,000 years Before Present (B.P.) reef tract on the south coast of Barbados, W. I. Diagenetic environments have passed through the sediment of this borehole in response to Pleistocene glacio-eustatic fluctuations of sea level and gradual tectonic uplift of the island. Carbonate sediments from the upper 1.5 m of the borehole have been in the vadose diagenetic zone since initial emergence from the depositional environment. These carbonate sediments are for the most part mineralogically unstable; they have not been affected by extensive dissolution. Fresh water phreatic lenses associated with high stands of sea level have occupied the pore systems of the sediments between 1.5 m and 15.3 m depth at least twice and possibly three times. Those zones occupied at least once by a fresh phreatic lens show complete mineral stabilization of the inter-coral matrix to low-Mg calcite, and extensive cementation and dissolution. Sediment from the lowest part of the core still retains most of its depositional mineralogy and shows only minor dissolution or cementation. This portion of the borehole has alternately been subjected to the vadose zone (during glacial low stands of the sea) and the marine phreatic zone (during interglacial high stands). Although interpretations of sea level fluctuations indicates the sediment from this borehole spent more time in the vadose diagenetic environment than all other environments combined, it is significant that only those portions of the borehole section once occupied by fresh phreatic pore fluids show extensive diagenetic modification. Indeed, the lower portion of the borehole section, despite prolonged exposure to the vadose diagenetic environment, shows only minor diagenetic modification. Thus on the south coast of Barbados, early carbonate diagenesis takes place more rapidly in the fresh water phreatic zone than in the other diagenetic environments combined.