William G. Siesser

Late Miocene Origin of the Benguela Upswelling System off Northern Namibia

Deep Sea Drilling Project cores collected at site 362/362A suggest the time of initiation of the Benguela upwelling system off northern Namibia. Studies of sediment accumulation rates, diatom abundances, microfossil temperature preferences (for planktic foraminifers, calcareous nannoplankton, and silicoflagellates), productivity (expressed as the organic carbon content), and phosphorus incorporation in calcareous skeletons all suggest that major, sustained upwelling began in the early late Miocene. Upwelling brought cold, nutrient-rich waters to the surface, and the modern Benguela system dates from this time (about 10 million years before the present).

Tertiary Sea-Level Movements around Southern Africa

Sedimentological, micropaleontological and seismic-profiling data elucidate the history of Tertiary sea-level movements around southern Africa. These new data show that landward movement of the sea began in early Paleocene time and continued into the early Eocene. The sea probably reached its maximum Paleocene height during the early Eocene, and is today represented by outcrops up to at least 204 m, and probably as high as 360 m, above modern sea level. A brief regressive pulse occurred during the middle Eocene, with renewed transgression in the late Eocene. A major regression followed, probably spanning all of Oligocene and early Miocene times. This regression exposed much of the continental shelf and is clearly represented on seismic-reflection profiles by a widespread unconformity. The major Neogene transgression began in the middle Miocene and probably reached its greatest extent in the late Miocene or early Pliocene. The overall middle Miocene to early Pliocene transgression was interrupted by a brief regressive pulse near the Miocene-Pliocene boundary. Seas withdrew again in the late Pliocene. Rocks deposited during the Miocene-Pliocene transgression are today found up to 330 m above sea level. This scheme should be viewed as showing only the gross movements of the seas around southern Africa during the Tertiary. Nevertheless, the timing of these southern African transgressions/regressions is closely parallel to the timing recently established for sea-level movements in other parts of the world.

Paleogene Calcareous Nannoplankton Biostratigraphy: Mississippi, Alabama, and Tennessee: ABSTRACT

Distribution and abundance of calcareous nannoplankton in the Paleogene formations of Mississippi (MS), Alabama (AL), and Tennessee End_Page 1471------------------------------ (TN) are reported in this study. These data have been used to assign each of the following nannoplankton-bearing formations and members to Martinis 1971 internationally recognized calcareous nannoplankton zones. Table End_of_Article - Last_Page 1472------------

Petrology and Origin of Some Phosphorites from the South African Continental Margin

ABSTRACT Phosphorites (phosphatized limestones) are areally widespread on the South African continental margin. These rocks have packstone or wackestone textures and are typically composed of microfossils (chiefly planktonic foraminifers), with lesser amounts of macrofossil fragments and terrigenous grains set in a collophane/micrite matrix. The average P2O5 content of these rocks is about fifteen percent. Some varieties are exceedingly ferruginous, owing to the intimately mixed goethite in their matrices. The faunal and textural characteristics of these phosphorites are strikingly similar to those of certain unphosphatized Miocene limestones dredged from the same area. Mineralogical and geochemical evidence indicates that the lime mud matrix of some of these limestones was replaced by francolite. Phosphatization was accomplished under submarine conditions by permeating, phosphate-rich solutions.

Relict and Recent Beachrock from Southern Africa

Beachrock forms a broken pavement along part of the southwestern coast of South Africa. The rocks are found in the intertidal zone as well as slightly above high-tide and below low-tide level. The beachrock is composed chiefly of quartz grains, with lesser amounts of skeletal fragments, cemented by micrite. Carbonate mineralogy of the rocks consists of low-Mg calcite and aragonite, with low-Mg calcite predominating. Laminated calcrete (wholly low-Mg calcite) covers much of the beachrock exposures. The presence above spring-tide level, poor correspondence to the composition of the surrounding beach, and presence of abundant low-Mg calcite suggest that the beachrock is relict. A C 14 date on the beachrock of 25,860 −1,190 +1,040 yr B.P. confirms this; the laminated calcrete layer overlying the beachrock formed 25,430 −1,210 +1,050 yr B.P. It is believed that this beachrock formed in an intertidal environment during withdrawal of the sea from its Wurm I/II interstadial high. Subsequent subaerial exposure has resulted in diagenetic alteration of some of the original metastable carbonate minerals to the stable variety, low-Mg calcite. Recent beachrock occurs in the intertidal zone at Vilanculos, Mozambique. The rocks consist of layers of coarse-grained skeletal fragments cemented by fibrous aragonite, alternating with layers of quartz grains cemented by micrite. The carbonate mineralogy is almost wholly aragonite. The components and mineralogy of the surrounding beach correspond closely to that of the beachrock. C 14 dates of 910 −140 +120 and 920 −150 +140 yr B.P. were obtained for these rocks.

Paleoproductivity of the Indian Ocean during the Tertiary Period

Abstract Studies of oceanic paleoproductivity have lagged in part because of the absence of a suitable proxy for measuring paleoproductivity. A transfer function has recently been proposed which directly translates carbonate mass accumulation rates into a quantitative measure of productivity. This transfer function ha been applied at six ODP sites in the Indian Ocean in order to investigate temporal and spatial changes in Indian Ocean productivity during the Tertiary Period. Early Paleogene productivity in the Indian Ocean was considerably higher than in the late Paleogene and Neogene. A warmer, more confined sea, with large riverine nutrient input from close lying land masses may account for the generally higher early Paleogene productivity. Productivity increased dramatically at two times during the Paleogene: during early Paleocene calcareous nannofossil Zone NP5 and during early Eocene Zone NP11. These productivity increases may be related to abrupt climatic changes which reorganized oceanic circulation and stimulated major changes in productivity. Productivity began to fall in the Oligocene as a result of global cooling and the initiation of the modern Indian Ocean circulation system. This trend continued into the Neogene, which was characterized by generally low productivity, interspersed with occasionally higher levels, especially at the more northern sites in the Indian Ocean.

An investigation of the suitability of four methods used in routine carbonate analysis of marine sediments

Abstract Atomic absorption spectrophotometry (AAS), EDTA titration, gasometric measurement of carbon dioxide, and acid soluble weight-loss are all methods that may be used for the analysis of carbonate in marine sediments. Forty-two samples were analysed by each of these four methods in order to evaluate their respective advantages and disadvantages. Statistical evaluation of the carbonate values obtained shows that there is no significant difference between the mean carbonate values obtained by AAS and gasometry and EDTA. There is a significant difference between the carbonate values obtained by AAS and EDTA, and between the weight-loss method and the other three methods. The data show that major errors are inherent in the weight-loss determination. In terms of accuracy and precision, expense, analytical skill, and time available for analysis, gasometry is judged to be the most suitable method for routine carbonate analysis of large numbers of marine sediments.

Significance of Upper Triassic nannofossils from the Southern Hemisphere (ODP Leg 122, Wombat Plateau, N.W. Australia)

A taxonomic and biostratigraphic investigation has been carried out on Upper Triassic (Carnian-Rhaetian) nannofosils from Sites 759, 760, 761 and 764 drilled on the Wombat Plateau during ODP Leg 122. The recovery of continuous sequences containing well preserved nannofossils has enabled us to refine the previous taxonomy and biostratigraphy of this interval. Fossil assemblages are of two major types: (1) previously described calcareous taxa were recovered at Sites 761 and 764; and (2) sideritic forms, which may represent diagentic replacement of calcareous nannofossils, were observed in material from Sites 759 and 760. The sideritic forms proved difficult to study taxonomically due to inadequate optical properties. Calcareous nannofossil assemblages in the Upper Triassic are dominated by Prinsiosphaera trisassica. We show that the multitude of identities of this species in the light microscope are the result of selective etching on a layered structure. We propose an evolutionary lineage for the earliest known coccoliths, with Crucirhabdus primulus as the ancestor. This species gave rise to C. minutus and Archaeozygodiscus koessenensis. The Upper Triassic can be subdivided based on the sequential first occurrences of C. primulus and Eoconusphaera zlambachensis in the upper Norian. The late Norian and Rhaetian were times of slow evolution of calcareous nannofossils. However, we noted three morphmetric changes in this time-interval which possess biostratigraphic utility: (1) P. triassica increases in diameter from an average of 6-mu-m to over 9-mu-m; (2) E. zlambachensis evolves from a stubby to an elongated shape; and (3) C. primulus increases in size. Upper Triassic assemblages from the Wombat Plateau are similar in composition and diversity to those which have been described in detail from the Alps. In both areas, nannofossiliferous sediments interfinger with massive limestones deposited in reef and peri-platform environments. Stable isotopic analyses of Wombat Plateau nannofossil assemblages indicate that they thrived in open ocean conditions. Biostratigraphy allows sequence chronostratigraphic interpretation of ODP Site 761 and supports the chronostratigraphic cycle charts of Haq et al. (1987).

Mid-Tertiary Braarudosphaera-Rich Sediments on the Exmouth Plateau

Nannofossil assemblages enriched in Braarudosphaera occur in lower Oligocene to lower Miocene sediments at Ocean Drilling Program Sites 762 and 763 on the central Exmouth Plateau. Braarudosphaerids appear here rather abruptly in the lower Oligocene (in Zone NP21). They reach their greatest numbers in the lower Oligocene (in Zones NP22 and NP23), where they comprise up to 10% of some samples. Braarudosphaera bigelowii is the overwhelmingly dominant species, occurring together with rare specimens of B. discula and Micrantholithus pinguis. The holococcoliths Peritrachelina joidesa and Lantemithus minutus are also associated with the Braarudosphaera enrichment. There are two populations of B. bigelowii: one of normal size (10-14 μm) and one of large specimens (20-22 μm). The larger braarudosphaerids are more common than the smaller forms. Braarudosphaera-nch sediments are absent at Wombat Plateau sites during the same time interval. We attribute this to latitudinal control, because the Wombat sites are about 4° north of the central Exmouth Plateau sites. We believe that the occurrence of braarudosphaerids is related to an Oligocene to early Miocene oceanographic event on the Exmouth Plateau. We suspect that mid-ocean up welling of cool, low-salinity, nutrient-rich water along a divergent zone created the Braarudosphaera-nch sediments in the South Atlantic and Indian oceans.

Calcareous Nannoplankton in Continental-Shelf Sediments, East China Sea

Calcareous nannoplankton have been investigated in 22 samples collected from the East China Sea continental shelf and 36 indigenous taxa identified. Nannoplankton abundance and diversity increase progressively in a seaward direction, reflecting changes in water temperature, salinity and terrigenous sediment input to the shelf. The chief environmental factors controlling the distribution of modem nannoplankton assemblages are the cold inshore Coastal Current flowing southward and the warm Kuroshio Current flowing northeastward over the southern middle and outer shelf. The continental shelf can be divided into three zones based on the nannoplankton assemblage: 1) an inner-shelf zone (Gephyrocapsa oceanica Zone) which is low in nannoplankton abundance and diversity and in which the species present are overwhelmingly dominated by G. oceanica; 2) a zone approximating the middle shelf (Gephyrocapsa oceanica-Emiliania huxleyi Zone) in which abundance and diversity are moderate-with G. oceanica and E. huxleyi the dominant species and G. oceanica predominating; and 3) an outer-shelf zone (Emiliania huxleyi-Gephyrocapsa oceanica Zone) of much abundance and diversity, with E. huxleyi predominant over G. oceanica.