Paleobathymetric Interpretations Research Articles

Reversible subsidence on the North West Shelf of Australia

The Northwest Shelf (NWS) of Australia is characterized by offshore basins associated with Permian and Jurassic rifting and was only slowly subsiding by the Neogene. International Ocean Discovery Program (IODP) Expedition 356 targeted this region by coring four sites in the Northern Carnarvon and Roebuck Basins and two sites in the Perth Basin to the south on the Australian western margin. We use detailed lithological, physical property and age data with paleobathymetric interpretations, to infer tectonic subsidence apparently confined to the NWS that reverses (uplifts) with about the same amplitude and rate as an earlier subsidence event. About 300 m of tectonic subsidence occurred over one million years from 6 to 5 million years ago and then reverses when 300 m of tectonic uplift occurred from 2 to 1 Ma. The along strike extent of this subsidence pattern is ∼ 400 km. The similarity of magnitude and duration of the subsidence and uplift phases suggest that the subsidence is reversible. The results cannot be explained by glacial eustatic variability nor can the uplift event be attributed to sediments filling the accommodation space generated earlier. Reversible subsidence is a key fingerprint of dynamic topography. Although the rates of subsidence and uplift are roughly ∼ 300 m/Myr, a substantial portion of the changes occur over less than 1 Myr and the rates inferred from a detailed least squares analysis can reach up to about 500 m/Myr. These rates are incompatible with dynamic topography associated with motion of Australia over large-scale convection (10 to 40 m/Myr) or that associated with instability of the base of the lithosphere (<15 m/Myr). The vertical motions are too large to be associated with simple flexure of a plate and plate buckling in that the required amplitudes would lead to permanent deformation of the plate. A new geodynamic mechanism is required to fit the observations.

Mid- to late Pliocene (3.3–2.6 Ma) global sea-level fluctuations recorded on a continental shelf transect, Whanganui Basin, New Zealand

We present a ∼900 m-thick, mid- (3.3–3.0 Ma) to late Pliocene (3.0–2.6 Ma), shallow-marine, cyclical sedimentary succession from Whanganui Basin, New Zealand that identifies paleobathymetric changes, during a warmer-than-present interval of Earth history, relevant to future climate change. Our approach applies lithofacies, sequence stratigraphy and benthic foraminiferal analyses to two continuously-cored drillholes integrated with new and existing outcrop studies. We construct a depositional model of orbitally-paced, global sea-level changes on a wave-graded continental shelf. Unlike many previous studies, these shelf sediments were not eroded during sea-level lowstands and thus provide the potential to reconstruct the full amplitude of glacial-interglacial sea-level change. Paleobathymetric interpretations are underpinned by analysis of extant benthic foraminiferal census data and a statistical correlation with the distribution of modern taxa. In general, water depths derived from foraminiferal Modern Analogue Technique (MAT), are consistent with variability recorded by lithofacies.The inferred sea-level cycles co-vary with a qualitative climate record reconstructed from a census of extant pollen and spores, and a modern temperature relationship. A high-resolution age model is established using magnetostratigraphy constrained by biostratigraphy, and the dating and correlation of tephra. This integrated chronostratigraphy allows the recognition of 23 individual sedimentary cycles, that are correlated across the paleo-shelf and a possible “one-to-one” relationship is made to deep-ocean benthic oxygen isotope (δ18O) records. In general water depth changes were paced by ∼20 kyr duration between 3.3 and 3.0 Ma, after which cycle duration is ∼40 kyr during the late Pliocene (3.0–2.6 Ma). This record provides a future opportunity to evaluate the amplitude and frequency of global, Pliocene glacio-eustatic sea-level change, independent of the global benthic δ18O record.

Facies and depositional settings of the Middle Eocene-Oligocene carbonates in Kutch

Middle Eocene Fulra Limestone and Oligocene Maniyara Fort Formation represent platform carbonate deposits of Kutch at the north-western margin of India. These carbonates contain larger benthic foraminifera, including Alveolina, Assilina, Discocyclina, Lepidocyclina, Miogypsina, Nummulites and Spiroclypeus. This study presents paleodepositional and paleobathymetric interpretations for both formations using benthic foraminifera in combination with lithological association, sedimentary structures and early diagenetic features. The six carbonate facies comprising the Fulra Limestone indicate a depositional spectrum ranging from bar-lagoon to mid-ramp depositional conditions. It records several shallowing upward cycles, leading to emergence and formation of paleokarst. The four carbonate facies of the Maniyara Fort Formation represents deposition within the inner ramp setting in bar-lagoon and patch-reef environment, while intervening fine siliciclastics correspond to episodes of relative sea level fall. Nummulitic accumulations form low-relief bars within the fair weather wave base in both the formations. The depositional setting of the Paleogene carbonate in Kutch broadly resembles Eocene platformal deposits in the circum-Tethys belt.

Sequence stratigraphy of the Maastrichtian-Paleocene succession at the Dakhla Oasis, Western Desert, Egypt

The Maastrichtian-Paleocene succession at the Dakhla Oasis is marked by the presence of a typical Nile Valley Facies represented by the Dakhla and Tarawan formations in Edmonstone and Qur El Malik sections in the central and western parts of the oasis, while a mixed Nile Valley and Garra Al-Arbain facies represented by Dakhla, Kurkur and Tarawan formations in Teneida section in the eastern part of the oasis adjacent to the Abu Tartur Plateau. These sections were examined for their foraminiferal contents, lithologic characters and stratigraphic boundaries. The distribution of foraminifera in the studied sections is variable and inconstant, as the planktonics are concentrated only at certain levels, which may be considered as a time intervals of transgression and maximum flooding surfaces. Eight planktonic biozones are distinguished in this work; of theses two are of Maastrichtain age and six are of Paleocene age. Eight 3rd order depositional sequences are recognized in the studied Maastrichtian-Paleocene succession based on the time stratigraphic boundaries released from the planktonic foraminifera and sea level changes which are released from the paleoecologic interpretations. The distinguished sequences are subdivided into their systems tracts based on the paleobathymetric interpretations of P/B% and benthic biofacies analysis. These sequences are bounded by eight sequence boundaries (SB A - SB H) represented by unconformity surfaces and depositional hiatuses. The correlation of the sequence boundaries of the established depositional sequences with the eustatic sea level curve, suggesting that these depositional sequences were resulted from the interplay of eustatic sea-level changes and local tectonic activities.

Stratigraphy and paleobathymetric interpretation of the Cretaceous Karai shale formation of Uttatur Group, TamilNadu, India

The Karai shale Formation of the Uttatur Group is exposed in a bad land area at the western margin of the Cauvery Basin. This shale has been investigated based on foraminiferal fauna and clay minerals. The foraminiferal assemblages obtained contain predominantly calcareous benthic foraminifera, rare planktic and arenaceous foraminifera. The planktic foraminiferal index taxa Planomalina buxtorfi, Rotalipora reicheli, Praeglobotruncana stephani, and Hedbergella portsdownensis suggest the late Albian to middle Turonian age. The benthic assemblage dominated by Lenticulina, Gavelinella, Osangularia and Quadrimorphina, suggests an outer neritic (100–200 m) environment. The clay mineral content dominated by kaolinite-illite-montmorillonite indicates that the Karai shale was formed from weathering of igneous rocks.

Tectonic reconstructions for paleobathymetry in Drake Passage

A minimum-complexity tectonic reconstruction, based on published and new basin opening models, depicts how the Scotia Sea grew by Cenozoic plate divergence, dismembering a Jurassic sheared margin of Gondwana. Part of the Jurassic–early Cretaceous ocean that accreted to this margin forms the core of the Central Scotia Plate, the arc plate above a trench at the eastern end of the Scotia Sea, which migrated east away from the Antarctic and South American plates. A sequence of extensional basins opened on the western edge of the Central Scotia Plate at 50–30 Ma, decoupled from the South American Plate to the northwest by slow motion on a long transform fault. Succeeding the basins, seafloor spreading started around 30 Ma on the West Scotia Ridge, which propagated northwards in the 23–17 Ma period and ceased to operate at 6 Ma. The circuits of plate motions inside and outside the Scotia Arc are joined via rotations that describe Antarctic–Central Scotia plate motion in Powell Basin until 20 Ma, and along the South Scotia Ridge thereafter. The modelled relative motion at the northern edge of the Scotia Sea is thus constrained only by the plate circuit, but nonetheless resembles that known coarsely from the geological record of Tierra del Fuego. A paleobathymetric interpretation of nine time slices in the model shows Drake Passage developing as an intermediate-depth oceanographic gateway at 50–30 Ma, with deep flow possible afterwards. Initially, this deep flow would have been made tortuous by numerous intermediate and shallow barriers. A frontal pattern resembling that in the modern Scotia Sea would have awaited the clearance of significant barriers by continuing seafloor spreading in the Scotia Sea at ~ 18.5 Ma, at Shag Rocks Passage, and after 10 Ma southeast of South Georgia.

Sequence surfaces and paleobathymetric trends in Albian to Maastrichtian sediments of Ariyalur area, Cauvery Basin, India

Exposed Late Cretaceous (Albian–Maastrichtian) marine rocks of the Ariyalur area in the Cauvery Basin have been extensively studied based on biostratigraphy and paleobathymetry with paleobathymetric interpretation carried out using vertical and lateral relationships of rock facies, macro- and microfossil assemblages, textural characteristics and diagenetic changes of the lithologic units. The integration of these data reveals four Transgressive–Regressive (T–R) cycles, viz. Dalmiapuram, Garudamangalam, Sillakkudi and Kallankurichchi (in stratigraphic order). These T–R cycles have been compared with global published relative sea level curves of the study area. The major sea level changes during the Late Turonian and Late Maastrichtian in the study area correlate well with global sea level changes of Vail et al. (1977), Haq et al. (1987) and Miller et al. (2005). Based on biostratigraphy, stratal patterns and their relationship, the Late Cretaceous succession of the Ariyalur area is thus subdivided into four 2nd/3rd order sequences.

A New Dichotomous Microboring: Abeliella bellafurca isp. nov., Distribution, Variability and Biological Origin

Cluster-forming boring traces in carbonate substrates, characterized by regular dichotomous ramifications, are described as a new ichnospecies of Abeliella Mägdefrau 1937: A. bellafurca isp. nov. The new trace differs from A. riccioides Mägdefrau by a three- rather than two-dimensional display of tunnels and branchings and by penetrating carbonatic rather than phosphatic substrates. These boring traces occur in bivalve shells from the Oligocene to the Recent in shallow, illuminated marine settings. Two distinct morphotypes of A. bellafurca were observed, both originating from a single point of entry: one produces prostrate boring system in early developmental stages followed by deeper penetration, and the other, which forms only deep penetrating three-dimensional boring systems. Tunnels with dichotomous ramifications in modern marine environments are produced by the cyanobacterium Hyella stella Al-Thukair et Golubic. The organism penetrates the shells and shell fragments, whereby the apical cell divides alternately parallel and perpendicular to the direction of penetration. Bifurcation follows each longitudinal division of the apical cell. This type of cell division and bifurcations was described in Neoproterozoic endolithic cyanobacterium Eohyella dichotoma Green, Knoll et Swett. Both Proterozoic and modern organisms penetrated carbonatic ooid sand grains in shallow marine settings. Other species of Hyella and Solentia are known to form traces described as Fascichnus (Radtke). Dichotomously branched microborings in phosphatic substrates described as Abeliella riccioides Mägdefrau were attributed to boring fungi. Distinctions between borings produced by phototrophs vs. heterotrophs are discussed because they have bearing on paleobathymetric interpretations.

有孔虫化石からみた始新統幌内層および漸新統紅葉山層の地史（再検討）

The middle to upper Eocene Poronai and overlying Oligocene Momijiyama formations in Hokkaido are the major Paleogene strata studied by many workers in Japan. Recent progress in biostratigraphy and foraminiferal paleoecology requires a thorough revision of the regional geohistory interpretations.Depositional environments of the Poronai and Momijiyama formations are discussed based of the re-interpretation of the published foraminiferal data sets. Transgression, which is recognized in the lower sequences of the Poronai Formation in the Yubari, Ashibetsu and Ikushunbetsu areas, reached a maximum water depth in the upper bathyal facies. In addition, a temporal regression, which occurs in the middle part of the Poronai Formation in the Yubari area, is related to the eustatic sea level drop in the beginning of Late Eocene. Conversely, shallower water depths are inferred in the Yufutsu-Umaoi and Hobetsu districts. Based on these paleobathymetric interpretations and the distributions of the mega-fossil zones, the marine transgression during the deposition of the Poronai Formation was from northeast to southwest.Stratigraphic relationship between the Poronai Formation and the underlying Ishikari Group, which has been controversial, is demonstrated to be unconformable mainly due to the existence of transgressive phase at the basal part of the Poronai Formation.

Facies architecture and diagenesis of Belgian Late Frasnian carbonate mounds

Abstract Late Frasnian Petit-Mont Member carbonate mounds occur in the southern part of the Dinant Synclinorium and in the Philippeville Anticline (SW Belgium). These mounds are 30 to 80 m thick and 100 to 250 m in diameter. They are embedded in shale, nodular shale and argillaceous limestone. Based on facies mapping of 14 buildups and related off-mound sediments, these mounds typically started from below the photic and storm wave base zones and builtup into shallow water environments. Above an argillaceous limestone substrate, the first carbonate mound facies consists of spiculitic wackestone with stromatactis (PM1), which becomes progressively enriched in crinoids and corals (PM2), then in peloids, stromatoporoids and cyanobacteria (PM3). PM4 consists of algal–coral–peloid wackestone and packstone with green algae and thick algal coatings. A core of algal and microbial bindstone (PM5) sporadically occurs within large mounds. The uppermost part of these mounds may show a recurrence of facies PM2 and PM1. PM1 to PM3 are coloured red by hematite derived from microaerophilic iron bacteria; PM4 and PM5 are grey. The transition from the aphotic to the cyanobacterial photic zone is recorded in the succession PM2–PM3; the transition from the cyanobacterial to the green algal photic zone is recorded by PM3–PM5. Storm wave base was reached within PM3 and fair-weather wave base within PM5. This paleobathymetric interpretation suggests a depth of 100–150 m during initial establishment of PM1. Three types of mounds can be distinguished on the basis of geometry and facies architecture: (1) “Les Bulants”-type mounds display a continuous vertical facies succession (PM2–3–4–5) and low relief; (2) although exhibiting the same facies succession as “Les Bulants”, “Les Wayons”-type mounds show a distinct relief with steep flanks and bioclastic talus; (3) “St.-Remy” mounds consist exclusively of PM1 and PM2, bioclastic flank deposits are not observed. From (1) to (3), these mound types represent successive deepening down a ramp. Biostratigraphic correlation on a regional scale provides good evidence that relative sea-level changes largely controlled lateral and vertical transitions of facies. Beyond that, hypoxic conditions are indicated by the sponge and iron-bacteria consortium in lower parts of the mounds. This is in agreement with the general assumption of stratified water masses during Late Frasnian, preceding the prominent Lower Kellwasser crisis. Cementation began with a radiaxial synsedimentary cement. A fringe of meteoric phreatic cement, initially nonluminescent, then with a bright orange luminescence, occurs in all mounds. It is contemporaneous with a nonluminescent pervasive cement of grainstones deposited in littoral areas. Differentiation between the (reducing) mounds and the (oxidizing) littoral area resulted from better aquifer circulation in sedimentary bodies close to the recharge area. Late burial cements occlude all the remaining porosity and are contemporaneous with the opening of the Variscan fracture system.

BENTHIC FORAMINIFERA AND PALEOECOLOGY OF THE PLIOCENE YORKTOWN AND CHOWAN RIVER FORMATIONS, LEE CREEK MINE, NORTH CAROLINA, USA

Pliocene deposits assignable to the Yorktown Formation (Sunken Meadow, Rushmere, and Morgarts Beach Members) and Chowan River Formation (Edenhouse Member) yielded 129 species and subspecies of benthic foraminifera. Twenty-six taxa, selected on the basis of consistent occurrence (present in 50% or more of the samples) and relative abundance (3% or more of the benthic assemblage in at least one sample), account for 88% of the identifiable specimens. From among these 26, principal components analysis identified 14 species that account for nearly 90% of the faunal variation, and cluster analyses revealed distinct stratigraphic assemblages that in general conform to the lithologic subdivisions. The most dramatic faunal change corresponds to the boundary between the Sunken Meadow and Rushmere Members. Based upon the benthic foraminiferal faunas, the Sunken Meadow, Rushmere, and Morgarts Beach Members of the Yorktown Formation were deposited in middle to outer neritic, outer neritic, and middle neritic environments, respectively. The Edenhouse Member of the Chowan River Formation was deposited in an inner to middle neritic environment. Paleobathymetric interpretations based on benthic foraminifera are consistent with those based on other fossil groups. Paleotemperature implications of the benthic foraminiferal assemblages are not entirely consistent with previous studies. Whereas numerous studies have concluded that Pliocene temperatures at middle and high latitudes were significantly warmer than modern temperatures, the benthic foraminiferal faunas encountered in our study suggest paleotemperatures comparable to those of today. Evidence from benthic foraminifera at the Lee Creek Mine is not sufficient basis to question studies indicating warmer mid-latitude Pliocene seas. Rather, it is likely that localized conditions have overprinted the larger-scale climatic signal.

Three Component Sequence Stratigraphy

ABSTRACT Rapid developments in sequence stratigraphy have significantly changed the way of subdividing, correlating and mapping sedimentary rocks. Sequence stratigraphy has evolved from a theoretically controversial model into a practically accepted method. Currently there is a tremendous amount of interest in practicing sequence stratigraphy with different approaches being employed by various researchers. We believe that high resolution biostratigraphy, well-logs and seismic profiles are three fundamental components in the sequence stratigraphic analysis. They are interdependent and incomplete by themselves, and each of these components must be analyzed and integrated with each other using an iterative process in order to obtain accurate and reliable results. By using bioevents, fossil abundance and diversity histogram patterns, high resolution biostratigraphy provides a solid time framework that permits accurate correlations of sequences and systems tracts. Paleobathymetric interpretations facilitates lithofacies and seismic facies determinations. Analysis of well-log signatures within the aforesaid time framework permits delimitation of sequences and systems tracts. The geometry, configuration, continuity and amplitudes of seismic profiles offer another dimension of evidence in sequence stratigraphy because it visualizes the regional distributions and variations of sequences and systems tracts.

Effects of Water-Mass Changes on Quaternary Biostratigraphy, Gulf of Mexico

The quantitative record of benthic foraminifera in 61 cores from the northwestern Gulf of Mexico slope (283-1,341 m; 90-95{degree}W) indicates a strong relationship between the distribution of benthic foraminifera and that of water masses during the last 15,000 years, producing potentially useful biostratigraphic datums. In the extreme case, the effect of water-mass changes may be reflected in the local stratigraphic limits of a species. More commonly, such changes may significantly affect abundance variations of species in the stratigraphic column. During the last deglaciation (13-11 Ka), several common species of the Subantarctic Intermediate Water (e.g., Bulimina alazanesis, Osangularia culter) reappeared in the gulf with the reentry of this water mass, whereas Valvulineria 'A' disappeared because of a drastic reduction in the production of the Glacial North Atlantic Intermediate Water. Thus, within the depth range of these species coeval oceanographic events produced a biostratigraphic datum that is isochronous throughout the gulf. At the same time, at a water depth of about 800 m, the relative abundances of Bolivina albatrossi, Bolivina ordinaria, and Epistominella exigua increased, while those of Gavelinopsis translucens, Neocrosbyia minuta, and 'Valvulineria opima' decreased. The data also show that numerous species, including some recognized depth indicators, underwent changes in theirmore » depth adaptations during the deglaciation because of shifts of particular water-mass boundaries; these changes have to be taken into account in paleobathymetric interpretations of species occurrences or abundances.« less

Well Log-Seismic Sequence Stratigraphy Analysis: An Integrated Approach to Exploration and Development

Well log-seismic sequence stratigraphy analysis is a new technology that integrates high-resolution biostratigraphic and paleobathymetric data and the characteristics of the well log signatures with seismic reflection profiles. This methodology permits the biostratigrapher, geologist, and geophysicist to work together to subdivide a stratigraphic section into packages of sediments bounded by chronostratigraphically significant condensed sections and their associated maximum flooding surfaces and sequences boundaries using well logs and seismic profiles. Each sequence is subdivided into smaller lithogenetic (facies linked) units called systems tracts on the basis of characteristic well log patterns. Faunal and floral abundance and diversity data provide critical information to make reproducible chronostratigraphic correlations. The paleobathymetric interpretations permit the identification of rock types in relation to the depositional environment and systems tracts. The high resolution biostratigraphically is critical in integrated well log-seismic sequence stratigraphic analysis because it provides information for identifying fossil abundance and diversity peaks that are important for recognizing and correlating condensed sections. Added to the well log and seismic data set, it now provides a integrated data package that permits the inexperienced person to use the concepts of seismic well log-sequence stratigraphy to develop expertise and confidence in its practice. In order to obtain consistently reliablemore » results in well log-seismic sequence stratigraphy, a particular procedure must be followed and specific data sets must be used.« less

An Integrated Approach to Exploration and Development in the '90s: Well Log/Seismic Sequence Stratigraphy Analysis

ABSTRACT Well Log-Seismic Sequence Stratigraphy Analysis is a new technology that integrates high resolution biostratigraphic and paleobathymetric data and the characteristics of the well log signatures with seismic reflection profiles. This methodology permits the biostratigrapher, geologist and geophysicist to work together to subdivide a stratigraphic section into packages of sediments bounded by chronostratigraphically significant condensed sections and their associated maximum flooding surfaces and sequence boundaries using well logs and seismic profiles. Each sequence is subdivided into smaller lithogenetic (facies linked) units called systems tracts on the basis of characteristic well-log patterns. The systems tract boundaries, are identified on well logs, marked on two-way time logs or synthetic seismograms and correlated with corresponding systems tracts that have been independently identified on the seismic profiles using seismic-stratigraphic interpretation procedures (Vail and Wornardt, 1990). Faunal and floral abundance and diversity histograms provide critical information to make reproducible chronostratigraphic correlations (Huang and Wornardt, 1986; Shaffer, 1987; Wornardt, 1989; and Shaffer, 1990). The paleobathymetric interpretations permit the identification of rock types in relation to the depositional environment and systems tracts. The high resolution biostratigraphy is critical in integrated well log-seismic sequence stratigraphic analysis because it provides information for identifying fossil abundance and diversity peaks that are important for recognizing and correlating condensed sections and maximum flooding surfaces on well logs and seismic profiles. High resolution biostratigraphy and paleobathymetry data provides the additional data package that has heretofore been missing in seismic sequence stratigraphy. Added to the well log and seismic data set, they provide an integrated data package that permits the inexperienced person to use the concepts of seismic well log-sequence stratigraphy to develop expertise and confidence in its practice. In order to obtain consistently reliable results in well log-seismic sequence stratigraphy, a particular procedure must be fo lowed and specific data sets must be used.

Pitfalls of Paleobathymetric Interpretations Based on Trace Fossil Assemblages

Trace fossils have been used widely as indicators of original water depth in paleoenvironmental studies, but most paleobathymetric interpretations in ichnology have been based on only a small number of standard ichnofacies. In addition to bathymetry, it is important to recognize that salinity, oxygen concentration, and substrate character also play a major role in controlling the distribution of trace fossils. Several pitfalls of paleobathymetric interpretation based on trace fossils can lead to erroneous paleoenvironmental reconstructions. The most common of these pitfalls, of which historical geologists should be wary, include the following: 1) occurrence of a particular ichnogenus does not necessarily indicate the presence of the ichnofacies of the same name, and a standard ichnofacies may be identified without the presence of the namesake trace fossil; 2) environmental shifts of certain trace fossils have occurred through time, so the paleoenvironmental significance of a particular ichnotaxon may have changed during its histoty; 3) the nine standard ichnofacies that have enjoyed wide use by ichnologists have broad environmental significance that may include, but certainly extends beyond, bathymetry; 4) not all environmental situations are represented in the nine standard ichnofacies, so many trace fossil associations in the real world cannot be easily fit into those categories.

Quantitative Paleobathymetry Using Oxygen Isotopes and Shape Changes in Benthic Foraminifera: ABSTRACT

Accurate estimates of paleodepth are of critical importance to oil exploration in determining environment of deposition and geologic history. Models based on the test shape and the /sup 18/O//sup 16/O ratio in benthic foraminifera from the northwestern Gulf of Mexico indicate that a resolution of +/- 75 ft can be achieved in paleobathymetric reconstructions. The proportion of /sup 18/O and /sup 16/O incorporated into the tests of benthic foraminifera varies with bottom water temperature in a predictable manner. This depth/temperature relationship is the result of the temperature dependence of oxygen isotopic fractionation between sea water and calcium carbonate, and it allows the tests of benthic foraminifera to be used as indicators of paleotemperature. Since subbottom water temperatures on the outer shelf and slope decrease systematically with increasing water depth, these paleotemperatures can be used to reconstruct paleobathymetric trends. Paleobathymetric interpretations can also be independently inferred from Fourier shape analysis of benthic foraminiferal species. Combining the oxygen isotope and shape relationships relative to water depth increases the resolution of paleobathymetric reconstructions and provides an independent check on interpretations based on faunal assemblages and sedimentological data. These paleodepth models should allow extinct taxa to be used for paleobathymetric reconstructions as well.

Upper Cretaceous foraminiferal assemblages of the external units of the Rif (Northern Morocco) - a paleobathymetric model of the Late Mesozoic North African continental margin

An analysis of foraminiferal assemblages, combined with a sedi-mentologic study has been utilized to propose a paleobathymetric zonation for the upper cretaceous sediments of the External Units of the Moroccan Rif. General faunal aspects -such as diversity, ratio of planktonic to benthic foraminifers , ratio of agglutinated to hyaline calcareous tests, abundance of primitive "flysh-type" agglutinants, dissolution-effects on the faunal composition -and the distribution of characteristic benthic foraminifers have been compared to coeval assemblages from other parts of the world. The paleobathymetric interpretation of these assemblages, when considered together with sedimentological features, fits into a model of a continental-margin succession with a general trend of deepening from South to North, resp. from neritic series to abyssal sub-CCD sediments. This general pattern is complicated locally in some elevated areas (generated by compressional movements or uplifted parts of tilted blocks). These paleobathymetric features and the palinspastic reconstruction confirm the model for the external Rif representing a continuous succession of slightly overthrusted segments of the Mesozoic North African continental margin.

Benthic Foraminiferal Morphology: New Approach to Paleodepth Interpretations in Northern Gulf of Mexico: ABSTRACT

Variation in benthic foraminiferal morphology is a potentially powerful tool in paleoenvironmental and paleobathymetric interpretations. Recognition of distribution patterns of particular characters in modern Gulf of Mexico taxa will enable these distributions to be applied to bathymetric interpretations of Tertiary core samples. The morphology of the 295 most commonly recorded benthic foraminiferal species in the northern Gulf of Mexico was scored into 68 categories describing test shape, chamber shape, chamber arrangement, apertural characteristics, and surface sculpture. Cluster analyses of these data at 300 localities resulted in maps showing the distribution of particular sets of features. Many of these morphologic biofacies are depth relatable. Canonical variate analysis was used to determine which variables were most important in distinguishing the various biofacies. This approach to paleobathymetric interpretations should be applicable throughout the Cenozoic and perhaps into the Mesozoic, as morphology can be considered as an adaptive response to environmental factors. Even though different taxa are involved throughout the Cenozoic, similar characters can indicate adaptations to similar environments. Hence, this modern data set can be applied to determine paleobathymetry in older samples. End_of_Article - Last_Page 247------------

Observations Concerning Benthic Foraminiferal Genus Melonis: ABSTRACT

For many years, the benthic foraminifer Melonis pompilioides has been used as an important deep-water index species indicating deposition in abyssal environments. However, uncertainties about the character and importance of its morphological features have caused problems in species identification, which in turn have produced paleobathymetric interpretations that differ from those based on seismic, sedimentary, and stratigraphic data. Incorrect identifications of Melonis pompilioides and the probably automatic assignment of an abyssal environment to sections containing this species have led to controversy and, at times, to serious questions regarding the reliability of this genus for paleoenvironmental interpretations. An attempt has been made to rectify the problems of identification of Melonis pompilioides and related forms so that micropaleontologists can accurately and consistently identify specimens from the Melonis species complex and can recognize misidentifications by others. Characteristics such as pores, umbilical size, sutures, apertures, and height/width ratios were studied and evaluated. Other studies have shown that depth of water may not be the single or dominant factor controlling the distribution of this species and that discretion should be used in establishing a paleoenvironmental determination. End_of_Article - Last_Page 533------------