Glacio-eustatic Sea-level Fluctuations Research Articles

Quaternary sedimentary history of Adana, Cilicia and Iskenderun basins: northeast Mediterranean Sea

Detailed single-channel airgun and 3.5 kHz profiles from the northeastern Mediterranean Sea show that the continental shelf is formed by superimposed deltaic successions (depositional sequences), separated by major erosional unconformities. Each depositional sequence is composed of a sigmoid prograding package overlain by an oblique prograding package, representing respectively delta progradation during high and low sea levels of interglacial and subsequent glacial stages. During the glacio-eustatic lowstands of sea level, deltas prograded seaward. The present-day shelf break denotes the topset to forest transition at maximum progradation during the last glacial period. During post-glacial transgressions, the deltas initially lost their dynamic equilibrium with the environment and rapidly retreated landward, leading to the deposition of a thin veneer of sediments originating from reworking of formerly coastal sediments. At the highstand position the deltas were re-established in the ancestral Adana Bay and foreset progradation started. Chronology suggests that the Cilicia and Iskenderum Basins are subsiding at rates of 0.38 m 1000 yr −1 and 0.33 m 1000 yr −1, respectively. Seismic reflection profiling shows that delta architecture in the Adana, Cilicia and Iskenderun Basins is mainly controlled by glacio-eustatic sea-level fluctuations and continuous basin subsidence.

Interbasinal cyclostratigraphic correlation of Milankovitch band transgressive-regressive cycles: Correlation of Desmoinesian-Missourian strata between southeastern Arizona and the midcontinent of North America

The cyclic pattern of deposition in the Horquilla Limestone of southeastern Arizona resulted from glacio-eustatic sea-level fluctuations with periodicities within the Milankovitch band. The depositional surface fluctuated from above sea level to below the depth of autochthonous carbonate production. By means of characteristics of the cycles, correlations can be established not only between sections in the Pedregosa basin of southeastern Arizona, but also to the midcontinent of the United States. These correlations exceed the resolution of biostratigraphy and distinguish the eustatic component of relative sea-level fluctuations in widely spaced basins with different depositional settings.

Clastic sequences developed during late Quaternary glacio-eustatic sea-level fluctuations on a passive margin: Example from the inner continental shelf near Barnegat Inlet, New Jersey

Analysis of 100 km of seismic reflection (GEOPULSE) profiles from a 47-km 2 grid on the New Jersey low-mesotidal inner continental shelf near Barnegat Inlet reveals that the upper 30 m is composed of 3 unconformity-bounded units. Vibracores (1-6 m long) recovered from 12 sites contain several lithofacies, reflecting a variety of depositional environments that existed during late Quaternary glacioeustatic sea-level fluctuations on this slowly subsiding passive margin with low sediment supply. Environments include rivers during glacial and stadial lowstands of the sea, and a barrier island-lagoon complex, as well as pro-barrier ebb-tidal delta, shore-attached and -detached ridges, and below storm-wave-base shelf during interglacial or inter-stadial highstands. Following the last major interglacial (∼125 ka), sea level fell and rivers eroded a planar, seaward-dipping surface (R1) by early Wisconsinan time (∼70 ka). As sea level rose during the mid-Wisconsinan (∼55 ka), a barrier island system migrated to within 0.2-1.7 km of the modern barrier shoreline, leaving a 4- to 6-m-thick record (depositional sequence I). Maximum highstand of the mid- Wisconsinan sea level was 20 m below present. Sea level fell again during the late Wisconsinan (∼20 ka), and rivers again flowed across the exposed shelf, creating an unconformity (R2). A barrier/lagoonal system developed under rising sea level by at least early middle Holocene time, based on peats (8810 ± 170 yr B.P.) cored from -12 m. The mid-Wisconsinan (∼55 ka) barrier system was preserved under these early Holocene transgressive sediments (lower unit of depositional sequence II, lower transgressive tract). The modern barrier and inner continental shelf deposits (upper unit of depositional sequence II, upper transgressive tract) are relatively thin (average 3-4 m) pebbly sands overlying a prominent unconformity (a transgressive surface, R3) formed by marine erosion during the Holocene sea-level rise. Both the ebb-tidal delta and shore-attached linear sand ridges act as partial shields to wave and tidal- current erosion of the muddy sub-strate below R3. In the nearshore area, this shielding is creating a transgressive surface (marine unconformity) of relatively high relief (3-7 m).

Late Neogene sequence stratigraphy of a carbonate-siliciclastic transition: Southwest Florida

A network of high-resolution seismic profiles and 23 lithologically and/or geophysically logged boreholes have been analyzed in order to evaluate the effects of sea-level cyclicity, karstification, and an intercalated, carbonate to siliciclastic transition on the late Miocene to Quaternary stratigraphy of the Charlotte Harbor area in southwest Florida. Six late Neogene disconformity-bounded depositional sequences are identified between two regional unconformities. The lower regional unconformity (R-3) varies from about 20 m to more than 115 m below sea level. The lows are present as northwest-southeast- and northeast-southwest-trending troughs, and the high areas as sharp, irregular reflections and isolated sinkholes indicative of subaerial exposure. The R-2 unconformity, which overlies and truncates the late Neogene sequences, occurs as a strong, flat-lying reflector seaward of the present coastline and a series of downcutting channel complexes in back-barrier areas. The six depositional sequences prograde to the south-southeast with a proximal-distal transition from short, steeply dipping oblique clinoforms to long, low-angle clinoforms. Three lithofacies, which generally occur in a vertical sequence, are (from the bottom up) (1) quartz-dominated sand with phosphate sand and gravel; (2) calcareous clays; and (3) poorly indurated to well-indurated, sandy, molluscan calcirudite. The pebbley sand facies are transgressive lags gradationally overlain by prograding, laminated clays deposited in an estuarine to prodelta shoreface environment. The calcirudite transitionally overlies the clay facies and was deposited in a shallow subtidal to subaerial chenier-like setting which may be indurated and/or indicate subaerial exposure and may form the base of another sequence. The karstified antecedent topography has controlled the distribution of the depositional sequences in two ways. First, fluvial systems are localized in the antecedent lows, which restricts the distribution of fluvially transported sands. Second, the relatively impermeable clays, which occur as thin beds outside the lows, thicken to 20-40 m within the karstic depressions. The clay deposits limit vertical drainage and initiate the development of fluvial systems on the carbonate terrain. The sequences were deposited during the late Miocene Tortonian Stage, which is increasingly viewed as a time of high-frequency, glacio-eustatic sea-level fluctuations. Although high-resolution chronostratigraphic data are not available, the sequences contain thin and/or poorly preserved transgressive sands, overlain by prograding regressive sediments, and they represent discrete sea-level cycles. A regional pattern of karstic dissolution has controlled the transition by localizing deposition of impermeable prodelta clays and the developing paleo-fluvial system. The introduction of quartz sand into the carbonate terrain was, in turn, controlled by the karstic relief and a localized sediment supply derived from the paleo-fluvial systems.

Some pitfalls in paleosol interpretation in carbonate sequences

In Quaternary carbonate units composed mostly of eolianites, paleosols are important stratigraphic markers that help differentiate episodes of carbonate deposition tied to glacio-eustatic sea level fluctuations. Paleosols used in this manner can be misinterpreted, and thus lead to errors in interpretation of the geologic record. Some possible pitfalls include: failure to differentiate between terra-rossa paleosols and calcarenite protosols; failure to recognize that separate paleosols may merge laterally into composite paleosols; failure to recognize that single paleosols may bifurcate in highly weathered bedrock; and failure to recognize soil-derived material that infills karst features. The Quaternary carbonates of the Bahamas are used to illustrate these pitfalls, which may occur in carbonates of any age.

Secular changes in the amount and texture of dolomite

Research Article| February 01, 1991 Secular changes in the amount and texture of dolomite Duncan F. Sibley Duncan F. Sibley 1Department of Geological Sciences, Michigan State University, East Lansing, Michigan 48824 Search for other works by this author on: GSW Google Scholar Author and Article Information Duncan F. Sibley 1Department of Geological Sciences, Michigan State University, East Lansing, Michigan 48824 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1991) 19 (2): 151–154. https://doi.org/10.1130/0091-7613(1991)019<0151:SCITAA>2.3.CO;2 Article history First Online: 02 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 Duncan F. Sibley; Secular changes in the amount and texture of dolomite. Geology 1991;; 19 (2): 151–154. doi: https://doi.org/10.1130/0091-7613(1991)019<0151:SCITAA>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 SocietyGeology Search Advanced Search Abstract The amount of dolomite in Cenozoic carbonates is lower than throughout most of the Paleozoic, a difference that could be due to changes in either chemical conditions or physical conditions that lead to extensive dolomitization. The abundance of mimetic (texturally retentive) dolomites in the Cenozoic suggests that chemical conditions were unusually favorable for dolomite formation, and, therefore, it must be physical conditions that explain the paucity of Cenozoic dolomites. One reasonable explanation for the paucity of Cenozoic dolomites is that glacio-eustatic sea-level fluctuations reduced the amount of time carbonate sediments were in contact with dolomitizing solutions. 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.

A simulation model of island reef morphology: the effects of sea level fluctuations, growth, subsidence and erosion

Glacioeustatic sea level fluctuations continually cover and expose reefs, alternately allowing growth or erosion to operate. In a simulation model we examine the simultaneous effects of sea level change, island subsidence, reef growth, subaerial erosion, marine backwearing, and fluvial erosion (from central highlands) on reef development. Using values obtained from the literature, we vary the rates of these processes and compare the reefs produced. Our results indicate that subaerial erosion, subsidence and growth are of comparable importance in determining reef morphology. Fore reef terraces, as developed by the model, are primarily drowned growth features; marine backwearing is of little importance in their development. Reef terraces form readily at depths that never had a stable sea stand, their depth is influenced by growth, subaerial erosion, and subsidence rates. Thus reef terraces often do not indicate former sea stands. We examine the causes of reef drowning and attribute it primarily to rapid subsidence and subaerial erosion, not to truncation through marine backwearing. We propose that reefs deeply submerged today are not necessarily “drowned out”, but may be vertically stable through many sea level cycles. Fluvial erosion is likely an important agent of lagoon formation on high islands in areas with high erosion rates.

Early Pleistocene glacio-eustatic sea-level fluctuations as deduced from periodic changes in cold- and warm-water molluscan associations in the Shimokita Peninsula, Northeast Japan

Sea-level oscillations of apparently glacio-eustatic origin are recognized in the Lower Pleistocene Hamada Formation, based on changes of molluscan associations. There are two cold-water molluscan associations, the Turritella saishuensis-Macoma-Clinocardmium and the Acila-Macoma Associations, and one warm-water Limopsis crenata-Limatula-Nuculana Association. Their distribution in time and space indicates that the study area was dominated by cold water, and was invaded periodically by warm water at least three times in the Early Pleistocene between 1.10 Ma and 0.89 Ma. Judging from their species composition, the paleodepth was greater during the times of warm-water intrusion than during the times dominated by cold water. The phenomena are reasonably explained by a combination of climatic and sea-level changes.

Upper Ordovician – lower Silurian sequences of the Bohemian Massif, central Europe

AbstractUpper Ordovician – lower Silurian sequences of the Bohemian Massif (in the Moldanubian Zone and Barrandian preserved in the Prague Basin; Rožmitál area; metamorphic ‘islets’ in the mantle of Central Bohemian Pluton; and in the Železné hory area), as well as along the northern margin of the Bohemian Massif in the Saxothuringian and Lugian (= West Sudetian) zones, show evidence of regressive–transgressive facies changes which can be related to glacio-eustatic sea-level fluctuations recognized elsewhere. Glacio-marine diamictites of late Ordovician (Hirnantian) age occur in the Prague Basin and Rožmitál area, and in north Bavaria and Thuringia. They provide evidence of temporary cooling of the region. Facies changes caused by synchronous sea-level changes allow stratigraphic correlation of even the faulted, weakly metamorphosed and biostratigraphically poorly dated sections in the Bohemian Massif. Correlation is based on sections in the Prague Basin in which the changes are best recorded and dated.

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

Mixed Carbonate and Volcaniclastic Slope Facies in a Middle to Upper Miocene Archipelago, Las Negras and Rodalquilar Areas, Southeastern Spain: ABSTRACT

The Las Negras and Rodalquilar areas contain excellent exposures of middle to upper Miocene carbonate and volcaniclastic deposits that fringe earlier formed Neogene volcanic substrates in an archipelago setting. The Miocene sedimentary sequence consists of three depositional sequences composed of reworked volcanic rocks and reef crest to distal slope strata that display 100-200 m of relief over 0.5-2.0 km. The entire sequence is truncated by a regional subaerial unconformity. The lower two depositional sequences are composed predominantly of shallow-dipping, normal marine, distal to proximal slope wackestones and packstones. The two sequences are separated by a megabreccia composed of Tarbellastreaea and Porites blocks and volcaniclastic sandstones and conglomerates. The reef blocks likely developed as upslope patch reefs, and, with the volcaniclastics, were eroded and transported via mass movements and tractive currents to a distal slope position. The upper (third) depositional sequence consists predominantly of foreslope strata of a Porites-dominated fringing reef complex. The base of this sequence is characterized by a megabreccia of Porites reef blocks, shallow-marine packstones, or volcaniclastic conglomerates. Five volcaniclastic wedges, interpreted as fan delta deposits, alternate with prograding carbonate foreslope deposits likely as a response to high frequency relative sea level fluctuations during a new sea levelmore » fall. The apparent correlatability of similar deposition sequences throughout the region indicates the importance of sequence development from allogenic processes, including glacio-eustatic sea level fluctuations and tectonism associated with the isolation of the Mediterranean Sea during the latest Miocene.« less

Orbital Forcing of Glacioeustasy: Evaluation of the Alluvial Clastic Source/Sink Term: ABSTRACT

Orbital forcing of climate change at key latitudes has long been recognized as a probable cause of variation in continental ice budget and thereby sea level. The precession cycle, variation in tilt of the earth's axis relative to the sun, and variation in the eccentricity of the earth's orbit about the sun all affect seasonal solar insolation. The seasonal solar insolation signal at any particular latitude is complex, with periods ranging upward from 20,000 yr to hundreds of thousands of years, Further, nonlinear responses of the earth system (e.g., slow buildup and rapid destruction of continental ice sheets) can convert modulation of short-period signal into long-period sea level variation. The alluvial valley/floodplain can be an important factor in sand supply to the marine environment. The alluvial system is constantly linked to the marine depositional system by sea level. With rising sea level, the alluvial system may store sand, thus depriving the marine system. With falling sea level, incisement may release sand previously sequestered in the alluvial system. To investigate these relationships in the context of numerous, rapid, and complex glacioeustatic sea level fluctuations, the authors have incorporated into their two-dimensional forward model an interactive, model-driven alluvial source/sink sediment supply termmore » which includes numerous user-specified parameters. Valley/floodplain geomorphic evolution includes incisement, terrace erosion, and floodplain sedimentation. Sensitivity tests suggest the system seldom reaches equilibrium for geologically reasonable parameterizations. Source/sink relations, which might be ignored if one assumed sea level to be moving slowly in one direction for long periods, can drastically modify availability of sand to the marine environment.« less

Forward Modeling of High-Frequency, Glacio-Eustatic, Depositional Sequences: An Example from Middle Pennsylvanian Shelf Carbonates of the SW Paradox Basin, Honaker Trail, Utah: ABSTRACT

Middle Pennsylvanian (Desmoinesian) shelf carbonates in the southwest Paradox basin display three superimposed orders of stratigraphic cyclicity with a systematic, vertical succession of facies, parasequence, and sequence stacking patterns. Fifth-order parasequences (34 parasequences in 645 ft section; averaging 20 ft thick, 40 k.y. duration) are grouped into fourth-order sequences (averaging 100 ft thick approximately 400 ky. duration), which in turn vertically stack to define a third-order accommodation cycle ({>=} 650 ft thick 2.4 m.y. duration). Fifth-order parasequences are comprised of shallowing-upward packages of dominantly subtidal shelf carbonates with sharp parasequence boundaries (exposure or flooding surfaces). Fifth-order parasequences are packaged into fourth-order sequences (type 1) bounded by regionally correlative subaerial exposure surfaces. These type 1 sequences contain a downdip, basinally restricted lowstand wedge of evaporites and quartz clastics, a regionally correlative transgressive shaly mudstone (condensed section), and a highstand systems tract composed of thinning-upward, aggradational, fifth-order parasequences. Systematic variation in the thickness of fourth-order sequences (thinning followed by upward thickening), as well as the number of fifth-order parasequences/sequences (decreasing followed by increasing number), define a third-order accommodation trend, which is also regionally correlative. High-frequency parasequences and sequences are interpreted as aggradational allocycles generated in response to glacio-eustatic sea level fluctuations drivenmore » by Milankovitch climatic forcing. The fifth-order parasequences record the earth's orbital eccentricity cycle (mean period of 41 ky.), and the fourth-order sequences result from the long eccentricity cycle (mean period of 413 k.y.).« less

Postglacial transgressions in late paleozoic basins of Western Argentina: A record of glacioeustatic sea level rise

The areas marginal to main glaciated centers are appropriate sites to distinguish the record of glacioeustatic sea level fluctuations. The Late Paleozoic basins of western Argentina show a marginal position with respect to the glaciated areas of eastern South America (Brazil). In three of these basins (Tarija, Calingasta-Uspallata and Central Patagonian) widespread glacial-marine diamictites and their gravity-reworked counterparts pass upward to open-marine fine clastics (mudstones, shales and scarce fine sandstones) with fossils of the Levipustula zone of Middle Carboniferous (Late Namurian-Westphalian) age. The presence of fine-grained marine sequences capping the diamictitic deposits is interpreted as the sedimentary response to a glacioeustatic sea level rise, expressed as a transgression in stratigraphic terms. The glacioeustatic nature of this sea level rise is confirmed by the interbasinal, facies-based correlation of the transgressive episode and the paleolatitudinal positioning of this portion of Gondwanaland during Carboniferous times.

Glacio-Eustasy, Depositional Environments, Diagenesis, and Reservoir Character of Goen Limestone Cyclothem (Desmoinesian), Concho Platform, Central Texas

The Goen Limestone (Desmoinesian) of Concho and Runnels Counties, central Texas, consists of three stacked transgressive-regressive sequences. Each cycle consists of a thin basal transgressive unit (clayey facies) deposited in a deeper water lower ramp setting, and thick regressive limestones (algal, foraminiferal, and bryozoan facies) representing middle to upper ramp-mound, mound-associated, and back-mound environments. Glacio-eustatic sea level fluctuations were the dominant control on the observed cyclicity and facies distribution. The major porosity forming process in the Goen Limestone was early leaching by freshwater lenses that migrated in response to glacio-eustatic sea level changes. The dominant porosity reducing processes included early cementation in a meteoric-phreatic environment and late-stage saddle dolomitization in a deeper burial connate environment. The hydrocarbon-producing regressive middle and upper ramp units (foraminiferal and bryozoan facies) underwent the most complex diagenetic histories and contain extensive secondary pores. The presence or absence of penetrating freshwater lenses and the distribution of green phylloid algae and secondary pore-filling cements were the major factors controlling Goen reservoirs. Numerous cyclic carbonate reservoirs with depositional and diagenetic histories similar to the Goen are present in the Desmoinesian section of central Texas (e.g., Caddo, Odom, Gardner, Capps, and Dog Bend limestones). Late Paleozoic mound-associated and back-mound carbonate reservoirs may be of greater importance in petroleum exploration than algal mounds, one of the most frequently exploited late Paleozoic carbonates of North America.

Field and Modelling Studies of Cambrian Carbonate Cycles, Virginia Appalachians

ABSTRACT Field studies coupled with computer modelling of carbonate cycles in the Elbrook-Conococheague Formations (Middle to Upper Cambrian) in the Appalachians have helped define the mechanisms of formation. The sequence is up to 1.6 km thick and composed of cyclic, peritidal carbonates. These formed on an aggraded, rimmed shelf on a mature, passive continental margin. The cycles (1-7 m thick) have lower parts of subtidal/intertidal carbonate and caps of dolomitic laminite containing minor quartz arenite, shale and breccia. Sedimentation rates for these peritidal carbonate environments exceeded long-term subsidence of the platform (0.01 to 0.1 m/1,000 yr), thus, the shelf stayed near to sea level (i.e., was aggraded). Milankovitch (20,000 to 100,000 yr), low-amplitude sea-level fluctuations (few meters) superimposed on 3rd-order sea-level fluctuation of 1 to 3 m.y. duration and less than 20 m amplitude caused periodic submergence of the gently sloping platform (less than 0.02 m/km slopes), resulting in deposition of over 400 peritidal cycles in about 26 m.y. Cycles that formed during 3rd-order sea-level rise are thick and limestone-rich, whereas those formed during 3rd-order fall are thin, quartzose, and highly dolomitized. The modelling suggests that fewer and thinner cycles formed during the long-term falls compared to the rises, reflecting decreased accommodation. Cycle-frequency decreases into areas of lower subsidence rate. Open-marine cycles typical of the outer shelf and cycles formed during 3rd-order sea-level rise contain digitate bioherms, grainstone, and ribbon carbonates in lower parts. They reflect high subsidence rate and an open-marine setting. Inner shelf cycles and cycles that formed during 3rd-order sea-level fall commonly are thrombolitic, or have thin subtidal parts (grainstone, conglomerate or stromatolite) and reflec the low subsidence rates. Interior areas of Quaternary carbonate shelves are characterized by incipiently drowned facies and pinnacle reefs and are punctuated by soil/caliche horizons and karst surface that reflect 100 m glacio-eustatic sea-level fluctuations. In contrast, many ancient shelves, like the Cambro-Ordovician shelf of the Appalachians, were flat-topped and dominated by cyclic peritidal sequences lacking evidence of high amplitude sea-level fluctuations. Such aggraded shelves may represent the typical state of mature carbonate shelves that formed in response to low-amplitude Milankovitch sea-level fluctuations as opposed to the large-scale fluctuations that typified the Quaternary.

Rhizoliths in ancient eolianites

Rhizoliths are abundant within Permo-Pennsylvanian eolian rocks of southeastern Utah. They are also present at the tops of several eolian sand bodies within the Pennsylvanian Fountain Formation of southeastern Wyoming and at a single stratigraphic level within the Jurassic Navajo Sandstone of southeastern Utah. Ranging in diameter from less than 1 mm to 15 cm, these cylindrical structures are composed of siliciclastic-free micrite and calcite spar. Cylinders ramify throughout the uppermost 2–3 m of thin, flat-topped sand bodies. Large cylinders are compound rhizoliths and display alveolar fabric, a feature resulting from rootlet calcification that is characteristic of modern and ancient calcretes. Development of rhizoliths required flat, geomorphically stable substrates for plant growth, highly permeable surficial sediments, and sources of calcium ions. The restriction of rhizoliths to planar deflation surfaces suggests that plants were unable to colonize or stabilize eolian bedforms. Paleozoic rhizoliths appear to be restricted to coastal areas with onshore winds. Associated evaporites suggest an arid to semi-arid paleoclimate. Rhizoliths formed when carbonate allochems derived from an upwind epeiric sea were dissolved and calcite reprecipitated around plant roots. Quaternary rhizoliths are best developed in similar sedimentologic and geographic settings. The abundance of rhizoliths and carbonate allochems in Quaternary and late Paleozoic low-latitude coastal dunes suggests that they are genetically related to glacio-eustatic sea-level fluctuations.

Delineation of Late Quaternary Depositional Sequences by High-Resolution Seismic Stratigraphy, Louisiana Continental Shelf: ABSTRACT

Interpretations of over 20,000 line km of single-channel, high-resolution seismic reflection profiles, coupled with nearshore vibracores and logs of industrial platform borings, provide the data base for determining the history and stratigraphy of late Quaternary sea level fluctuations on the Louisiana continental shelf. Regional unconformities, formed by subaerial exposure of the shelf during glacio-eustatic sea level withdrawals and modified by shoreface erosion during ensuing transgression, serve as markers to identify the boundaries of depositional sequences. Unconformities are recognizable on seismic profiles by high-amplitude reflectors as well as discordant relationships between reflectors. Within the upper Quaternary section, six depositional sequences have been recognized. Five of these are related to glacio-eustatic sea level fluctuations, involving sea level fall close to, or beyond, the margin of the continental shelf. Three of these fluctuations culminated in the deposition of shelf margin delta sequences. Extensive fluvial channeling characterizes the regressive phase of these sequences. Transgressive phases are marked by infilling of fluvial channels, flood-plain aggradation, truncation, or deposition of sand sheets, depending upon sediment supply and rate of sea level rise. Sequences 4 and 5 are correlated with the late Wisconsinan glacial stage and Holocene transgression. The upper portion of sequence 5 consists of anmore » early Holocene Mississippi delta complex. Abandonment and transgression of this delta are responsible for the formation of sequence 6. Although these deposits cover a smaller area, this demonstrates that deltaic processes can produce sequences similar to those driven by glacially controlled sea level changes.« less

Forward modeling of bank-margin carbonate diagenesis

We have applied forward modeling to investigate deposition and early diagenesis resulting from the interaction of high-frequency glacioeustatic sea-level fluctuation with bank-margin carbonates in a subsiding basin. The model output yields lithologies predicted by the static model, but the relation of these lithologies to any particular subaerial exposure surface is obscured by the complexity of glacioeustatic sea-level history. The geologic history of our synthetic stratigraphic sequences could not be unraveled by traditional stratigraphic and sedimentologic study methods. Forward modeling is the preferred approach to analysis of carbonate sequences.

Lithostratigraphy of Pliocene strata, Central and Southern Hawke's Bay, New Zealand

Abstract Te Aute Formation is here elevated to Te Aute Group and defined as all marine strata, in Central and Southern Hawke's Bay, that unconformably overlie Miocene and older rocks and, at most places, unconformably underlie Pleistocene strata. The group is characterised by shallow marine siliciclastic mudstone and sandstone, calcareous sandstone, and bioclastic limestone. The strata crop out discontinuously in a structurally complex region characterised by active compressional tectonics, with a northeast-southwest structural grain. Te Aute Group is subdivided into six formations which are, in order of decreasing age: Mangatoro Formation, Kairakau Limestone, Mokopeka Sandstone, Awapapa Limestone, Raukawa Mudstone (Argyll Sandstone Member), and Te Onepu Limestone. It is characterised by rapid vertical and lateral facies changes reflecting tectonic deformation and glacio-eustatic sea-level fluctuations during deposition.