Scott W. Snyder

Eocene-Oligocene boundary: Biostratigraphic recognition and gradual paleoceanographic change at DSDP Site 549

Drilling at Site 549, located on the Goban Spur in the northern Bay of Biscay (Irish continental margin), penetrated an apparently continuous section of upper Eocene–lower Oligocene nannofossil chalks. The Eocene-Oligocene boundary as identified by nannofossils does not coincide with that recognized using planktonic foraminifers. Regardless of which biostratigraphic criteria are used, no dramatic nannofloral or planktonic foraminiferal change occurred at the boundary. There was a gradual transition from warm-water to cool-water assemblages through the late Eocene, with more intense cooling indicated by low-diversity assemblages in lower Oligocene sediments. A bottom-water temperature drop occurred in the late Eocene through early Oligocene as indicated by an 18 O enrichment in benthic foraminifers, although some of this enrichment may have been caused by increased ice volume. A major benthic foraminiferal faunal change, from a Nuttallides truempyi –dominated assemblage to one dominated by wide- and long-ranging taxa, occurred in the early late Eocene. No major benthic assemblage changes occurred at the Eocene-Oligocene boundary.

The Goban Spur transect: Geologic evolution of a sediment-starved passive continental margin

Leg 80 of the DSDP-IPOD program drilled a transect of four core holes (548–551) across the continent-ocean boundary at Goban Spur, a prominent southwest-trending structural high on the Irish continental slope. Multichannel seismic-reflection profiles show that, during rifting, continental basement rocks of Goban Spur were broken up by northwest-trending listric normal faults to form a series of half-graben basins. Two of these half-grabens were sampled during Leg 80 (Sites 548 and 549). Site 550 was located on the adjacent oceanic crust of Porcupine Abyssal Plain. Site 551 was located on transitional crust at the foot of Goban Spur. The objectives were lo analyze the structural, the depositional, and the paleoenvironmental development of this sediment-starved passive continental margin. At Sites 548 and 549, basement comprises continental Hercynian metasediments of Devonian age; at Sites 550 and 551, the basement is tholeiitic basalt. The oldest syn-rift sediments (Barremian age, or perhaps late Hauterivian) were penetrated at Site 549, lying uuconformably below Aptian? strata. Seismic sequence analysis reveals that Aptian? strata also overlie this unconformity farther northeastward in the basin. An unconformity above the Aptian? section marks the end of rifting and the beginning of sea-floor spreading. An Albian age for the initiation of sea-floor spreading was corroborated at Site 550 where abyssal late Albian chalks rest upon and are interbedded with oceanic basalts, indicating an initial water depth of ∼2,000 m. As sea-floor spreading progressed, Goban Spur subsided rapidly, so that by Cenomanian time, bathyal to abyssal chalks were accumulating at deeper sites. After two periods of partial stagnation in the Aptian-Albian and in the Turonian, chalk deposition in well-oxygenated environments took place at all sites, modified chiefly by shifts in deep-circulation patterns and the calcite compensation depth (CCD), by periodic influx of terrigenous detritus during low stands of sea level (especially in the Cenozoic), and by frequent displacement of older carbonates from the slope to abyssal sites. A number of major unconformities correspond to those most often reported from other widespread locations in the North Atlantic Basin and on surrounding continental shelves and coastal plains. Several unconformities are preserved undisturbed in our cores and can be correlated with sea-level fluctuations, with paleoceanographic events, and with tectonic movements. A thick Quaternary section at Site 548 records prominent fluctuations of glacial-inter-glacial paleoclimates. An even thicker Paleogene section at Site 549 provides unusually well-preserved and uninterrupted sequences suitable for detailed sedimentological and stratigraphic studies.

A flume study of the movement of foraminifera tests

Abstract Assemblages of benthic foraminifera are used as indicators of paleo-environmental conditions, but if an assemblage represents post-mortem transport the reliability is limited. A repeatable method of determining such reliability was investigated by measuring the traction velocities of foraminifera in a flume with a moveable sediment substrate. Twenty-five specimens of each of twelve species were weighed and measured for various dimensions (intercepts). Volume, nominal diameter, operational sphericity, maximum projection sphericity and effective density were calculated. Each specimen was placed in a flume and the traction velocity was determined; the mean velocity was calculated for each species. Mean velocities varied from 5.1 to 18.7 cm/sec. The values for each species were compared and it was determined which were moved by velocities from the same statistical populations and which were not. If applied to assemblages, the tested specimens that were transported (sorted) would have similar velocities, but an assemblage composed of specimens representing different velocities would have remained in place, being unsorted. The method is repeatable and practical in that the specimens can be manipulated and observed easily. Only operational sphericity and weight have significant correlations with the traction velocity of foraminifera. However, with multiple regression techniques, the traction velocity for each species could be estimated with the following regression equation: X 1 = 18.4 − 11.4X 2 − 38.9X 3 where X1 is the mean traction velocity (cm/sec); X2 is the mean value of the maximum projection sphericity; X3 is the mean weight (mg) and 18.4 is the intercept of the regression plane. Thus, it may be possible to determine with flume runs if specimens in an assemblage have been transported or have remained in place. Also, estimates of traction velocities are possible without flume runs if sufficient data are obtained regarding the weight and dimensions of the specimens. The latter method may eliminate the need for flume runs.

Assessment of postmortem transportation of modern benthic foraminifera of the Washington continental shelf

A linear regression equation was used to estimate the traction velocity for each of the 31 most common benthic foraminiferal species in surface sediments of the Washington continental shelf. Using Scheffes method of multiple comparisons, the traction velocities of these species were compared individually and by groups in order to detect species that would be transported in the same statistical population. Three traction-velocity groups were recognized: 1) four species with roughly equidimensional tests and low traction velocities; 2) 19 species with elongate, inflated or coiled, moderately inflated tests and intermediate traction velocities; and 3) eight species with elongate, highly compressed or coiled, discoidal tests and high traction velocities. Depending upon the bottomcurrent velocity, one of these three traction-velocity groups might be expected to numerically dominate a current-sorted assemblage. However, each major faunal association, defined by cluster analysis in the preceding paper, is composed of mixtures from all three groups. Direct observations of bottom currents suggest that velocities at the sediment-water interface are generally too low, with directions too erratic to effectively sort species identified in this study. Hence, faunal assemblages appear to be dominated by indigenous species whose distributions reflect the oceanographic and sedimentologic factors that limited living representatives. INTRODUCTION The accuracy and reliability of foraminifer-based paleoenvironmental reconstructions depend, in part, upon the ability to distinguish in situ from allochthanous assemblages. Because microfossil assemblages are susceptible to displacement by bottom currents, recognizing the disruptive effects of postmortem transportation is especially important. Experiments that predict the behavior of foraminiferal tests as sedimentary particles can help to assess whether or not sorting by currents has modified an assemblage (Kontrovitz et al. 1978; Kontrovitz and Snyder 1981). The Washington continental shelf is an appropriate area to study the effect of bottom currents on the transportation of sedimentary particles because extensive data on the physical oceanography and geology of the region, and an adequate sample base are available. Low-level radioactive waste introduced into the Columbia River by the Hanford reactors from 1943 to 1971 stimulated interest in potential environmental effects (Pruter and Alverson 1972; Foster 1972). The study of radionuclides traced through the river and onto the shelf has contributed to the large volume of data on current patterns and sedimentary processes across this margin. The preceding paper compares distributional patterns of benthic foraminifera with large-scale patterns of sediment distribution. The objective of this paper is to assess the extent to which bottom currents have sorted assemblages so that the influence of postmortem transportation can be evaluated. METHODOLOGY Traction velocities were estimated for 31 of the 106 benthic foraminiferal species identified in this study (Appendix I). These 31 were selected because they occur in 15 or more samples or in two or more samples with a relative abundance greater than 5 percent (Appendix II). Their cumulative abundance ranges from 90.2 to 100 percent of the assemblage, with a mean of 98.2 percent (S.D. = 1.9 percent). Approximately twenty specimens of each species were individually measured to determine their long, intermediate and short dimensions. Maximum projection sphericity (MPS) was then calculated for each specimen. MPS is defined as the 3 () where S is the short dimension of the particle, and L and I are the long and intermediate dimensions, respectively. The traction velocity for each specimen was computed using the following regression equation: Y = 22.3 19.8(MPS), where Y is estimated traction velocity and 22.3 is the intercept of the y-axis (see Kontrovitz et al. 1978; Kontrovitz and Snyder 1981). Mean traction velocity was then interpreted for each species. Within-species versus between-species variation was evaluated using Scheffes method of multiple comparisons in order to group species having similar estimated traction velocities. Scheffes method results in fewer significant differences than most other methods of multiple comparison, and it is not sensitive to unequal numbers of observations among groups. Because the method is so rigorous, Scheffe (1959) recommended using the 0.10 level of significance. F-distribution tables at this level are provided by Winer (1971). The species within each group delineated by this method should be transported together (Kontrovitz et al. 1978; Kontrovitz and Snyder 1981). micropaleontology, vol. 36, no. 3, pp. 259-282, pls. 1-8, 1990 259 This content downloaded from 157.55.39.112 on Wed, 07 Sep 2016 05:24:39 UTC All use subject to http://about.jstor.org/terms S. W. Snyder, W. R. Hale, M. Kontrovitz: Postmortem transportation of modern foraminifera of the Washington continental shelf MEAN TRACTION VELOCITY (CM/SEC) RELATIONSHIPS AMONG MEAN TRACTION VELOCITIES (EACH SPECIES COMPARED TO ALL OTHER SPECIES) ADERCOTRYMA GLOMERATA A-4.56 NONION LABRADORICUS 6.01 LAGENAMMINA ATLANTICA 6.15 RECURVOIDES TURBINATUS ---6.-45GLOBOBULIMINA AURICULATA --6.48 ANGULOGERINA ANGULOSA 6.81 EGGERELLA ADVENA (AVE.) --7.09 GAUDRYINA ARENARIA 7.11 BULIMINELLA ELEGANTISSIMA 7.49 EGGERELLA ADVENA (LARGE) ---7 .58--UVIGERINA JUNCEA 7.64 NONIONELLA STELLA 7.77 PULLENIA SALISBURYI 7.89 HAPLOPHRAGMOIDES BRADYI 8 EPONIDES LEVICULUS 808 REOPHAX ARCTICA --4 BUCCELLA ANGULATA 1 TROCHAMMINA PACIFICA 8.6 EPISTOMINELLA VITREA --8.74 NONION AURICULUS ELPHIDIUM EXCAVATUM 9.02 CLAVATA FORM REOPHAX SCORPIURUS 9-37 ALVEOLOPHRAGMIUM JEFFREYSI 9.40 TEXTULARIA EARLANDI 10.47 SPIROPLECTAMMINA BIFORMIS -10.48 TROCHAMMINA SQUAMATA ---1084 BOLIVINA PACIFICA 10.94 TROCHAMMINA NITIDA 1 Q TROCHAMMINA ROTALIFORMIS 1 1. ALVEOLOPHRAGMIUM 1 1.75 COLUMBIENSE TROCHAMMINA KELLETTAE 13.27 -5

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.

Upper Eocene ejecta of the New Jersey continental margin reveal dynamics of Chesapeake Bay impact

Evidence for the Chesapeake Bay Crater as the source for New Jersey continental margin ejecta is provided by fine-grained tektites and coarse-grained unmelted ejecta. The Upper Eocene ejecta deposit, now demonstrated to be part of the North American strewn field, occurs on the New Jersey continental margin at Ocean Drilling Program (ODP) Sites 904 and 903. The mineralogy, major oxide composition of the ejecta materials, and biostratigraphic age of the enclosing sediments link the origin of these ejecta to the recently recognized Chesapeake Bay impact crater, located only 330 km away. Sediments associated with the ejecta provide information about the dynamics of impact events. The 35-cm-thick ejecta-bearing layer can be subdivided into three subunits that indicate a sequence of events. Bottom subunit III documents sediment failure and deposition of gravel-sized fragments, middle subunit II records deposition of abundant sand-sized ejecta by gravity settling, and upper subunit I contains a 12-cm-thick sedimentary deposit containing rare silt-sized tektites and evidence of waning currents. These events are interpreted by linking sediment deposition to seismic ground motion and subsequent tsunami waves triggered by both the Chesapeake Bay impact and slope failures.

Evidence for changes in Mesozoic and Cenozoic oceanic circulation on the south-western continental margin of Ireland: DSDP/IPOD Leg 80

Summary A transect of four coreholes, drilled by the Glomar Challenger across the Irish continental margin at the Goban Spur, evidences a dynamic palaeoceanographic regime during the late Mesozoic and Cenozoic. Shallow marine waters invaded the rift-stage grabens of the Goban Spur in the early Barremian. Thereafter, the margin subsided rapidly, producing a pelagic depositional regime by late Barremian time. Deep marine conditions were maintained as sea-floor spreading began in the early Albian, and chiefly pelagic deposition continued to the present. Among a series of significant post-rift oceanographic changes, one of the most notable is the familiar fluctuation of oxic and anoxic sea-floor environments during the Cenomanian and Turonian. Another marked change took place during the late Palaeocene, when cooler, oxygen-rich, northern bottom waters reached the Goban Spur as a consequence of rifting and sea-floor spreading between Greenland, Rockall Plateau, and Norway. Later during the Cenozoic, the initial production of Antarctic bottom water, several accelerations of polar icecap growth, and fluctuating eustatic sea-level produced a variety of circulatory shifts on the Goban Spur. A particularly significant sedimentological consequence of these interacting processes was the widespread creation of numerous erosional and non-depositional unconformities.

Ferruginization and Phosphatization of Foraminifera in Pleistocene/Holocene Sands on Mid-Atlantic Continental Shelf: ABSTRACT

Pleistocene/Holocene sands up to several meters thick, which contain 5% to 40% phosphate grains, occur on the continental shelf of Onslow Bay, North Carolina. Altered foraminiferal specimens, 98% of which belong to the genus Quinqueloculina, exhibit gradational surface discoloration (white to dark yellow-brown) that progresses from late to early-formed chambers. The percentage of extensively altered specimens varies directly with phosphate concentration in the sand fraction. Microprobe analyses of polished sections from completely discolored specimens indicate that alteration involves a decrease in %CaO and concomitant enrichment in %FeO and %P2O5. Degree of alteration diminishes from the outside to the inside of exterior-facing chamber walls (mean v lues are: 70 to 78 to 82% CaO; 18 to 11 to 7% FeO; 0.8 to 0.5 to 0.4% P2O5). Interior chamber walls are less altered (mean values are: 84% CaO, 6% FeO, 0.3% P2O5). On a CaO-FeO-P2O5 diagram the compositional changes through successive chambers of a single specimen parallel those from unaltered through altered specimens. The chemical compositions of completely discolored specimens fall on a proposed alteration trend between unaltered calcareous specimens and chamber fillings. Chamber fillings contain 0.9% CaO, 49% FeO, 12% MgO, and 1.6% P2O5; they are generally black. Relative concentrations of CaO-FeO-MgO plot within the compositional range of siderite and magnesite. Constant MgO values (7.5%) in alte ed foraminiferal tests demonstrate that initial diagenesis involves conversion to high-magnesium calcite. Subsequent alteration is largely ferruginization and minor phophatization of the test and the diagenetic materials forming within the chambers. End_of_Article - Last_Page 521------------

Relationship of Benthic Foraminiferal Biofacies to Lithofacies in Phosphatic Miocene Sediments, Mid-Atlantic Continental Shelf: ABSTRACT

Changes in benthic foraminiferal assemblages accompany changes in total sediment texture and mineralogy (primarily percent phosphatic grains) throughout the Pungo River Formation in Onslow Bay, North Carolina. Only Burdigalian (late-early Miocene) deposits have been cored in southern Onslow Bay. Basal phosphorite sands (30% phosphate) are overlain by phosphatic (8%) muds and slightly phosphatic (4%) quartz sands. Elongate buliminaceans (Bolivina, Bulimina, Buliminella, Uvigerina) comprise over 50% of the benthic assemblage in phosphorites. They also predominate (43%) in phosphatic muds where Siphogenerina and Florilus become conspicuous faunal elements. Diverse trochospirally coiled forms (mainly Hanzawaia, also Valvulineria and Cibicides) become predominant in quartz san s; buliminaceans decline to 30% of the fauna. Pungo River deposits in northern Onslow Bay are Burdigalian, Langhian (early-middle Miocene), and Serravallian (middle Miocene) in age. Burdigalian deposits are nonphosphatic, muddy quartz sands in which Hanzawaia predominates and buliminaceans comprise only 22% of the fauna; Florilus accounts for 5%. Hanzawaia remains the dominant genus in the slightly phosphatic (4%) quartz sands of the Langhian and the phosphatic (10%) sands of the Serravallian; buliminaceans increase to 29% of the fauna, but Florilus nearly disappears. Both vertically and laterally through the Miocene of Onslow Bay, nutrient-loving buliminaceans thrive where phosphate content increases. Florilus and Siphogenerina are associated with the influx of fine-grained terrigenous ediments. The Hanzawaia-dominated assemblage thrives in clean, End_Page 529------------------------------ coarser-grained substrates. Regional trends in the distributional patterns of these taxa may aid in locating additional phosphate deposits. End_of_Article - Last_Page 530------------

Reliability of Microfossil Assemblages as Paleoenvironmental Indicators: ABSTRACT

Ostracoda and Foraminifera have been used throughout the petroleum industry as paleoenvironmental indicators even when the reliability of the assemblages cannot be demonstrated. Such reliability can be evaluated only by knowing if the fossils have been subject to transport by water currents. In this study, threshold velocities of shells were measured in a flume with a moveable sediment substrate. The velocities were different for the several genera and some species thereby providing a means of evaluating fossil assemblages. An assemblage composed of specimens with the same threshold velocities would have been transported (sorted) or represent the residual after other shells had been moved away. An assemblage with shells having different velocities would be undisturbed and would be reliable as an environmental indicator. The best predictors of threshold velocity for the small calcareous shells are several measures of shape including the maximum projection sphericity and operational sphericity. They can be used to predict (.01 level) threshold velocities and provide a reliable objective means of testing the usefulness of fossil assemblages. End_of_Article - Last_Page 1686------------