Showing papers on "Foraminifera published in 1974"

Pacific Pleistocene Sediments: Planktonic Foraminifera Dissolution Cycles and Geochronology

Abstract: Late Pleistocene equatorial Pacific sediments can be correlated by the ratios of certain solution-susceptible versus solution-resistant planktonic foraminifera. These ratios fluctuate in a cyclic manner whose timing is in phase with glacial-interglacial fluctuations established by oxygen isotope geo-chronology. Glacial stages are characterized by less solution of foraminifera than interglacial stages.

Late Miocene—Early Pliocene Paleomagnetic Stratigraphy, Paleoclimatology, and Biostratigraphy in New Zealand

Abstract: The paleomagnetic stratigraphy, biostratigraphy, and paleoclimatology have been studied in two marine sections of late Miocene to early Pliocene age in New Zealand. A total of over 850 separately oriented cores were collected from 270 sites. The Blind River section (41°43′ S.) is now adjacent to the southernmost subtropical (temperate) water mass, but planktonic foraminifera indicate that the area was covered by subantarctic water during much of late Miocene and early Pliocene time. The Mangapoike River section (38°55′ S.) records temperature oscillations mainly within the subtropical water mass during late Miocene–early Pliocene age, with perhaps one subantarctic interval during latest Miocene time. The Miocene-Pliocene boundary in New Zealand has consistently been placed at the first evolutionary appearance of Globorotalia puncticulata at the boundary between the late Miocene Kapitean Stage and the early Pliocene Opoitian Stage. This boundary lies within sediments deposited during the Gilbert Reversed Epoch between the Nunivak Event (base at 4.14 m.y. B.P.) and the Gilbert C Event (top at 4.33 m.y. B.P.) in both sections. Thus, the Miocene-Pliocene boundary, as recognized in New Zealand, is dated as 4.3 ± 0.1 m.y., which appears to be slightly younger than the type (International) Miocene-Pliocene boundary in Italy (4.9 to 5.1 m.y.). Biostratigraphic ranges of planktonic foraminifera between New Zealand and the Mediterranean differ in detail, perhaps due to different paleo-oceanographic histories. A major cooling episode during the early Gilbert Reversed Epoch is recorded at Blind River and Mangapoike River. This cooling is more pronounced in the southern section examined, where it is represented by the occurrence of a central subantarctic planktonic foraminiferal assemblage. In the northern section, cooling was also pronounced, although of shorter duration, represented by a probably northern subantarctic assemblage. The Miocene-Pliocene boundary in Europe has still only been dated indirectly by means of non-Mediterranean sections. Interpretation of late Cenozoic paleomagnetic data from Mediterranean deep-sea cores collected from Glomar Challenger is rejected.

Oxygen and carbon isotope measurements of Late Cretaceous and Early Tertiary foraminifera

Abstract: Oxygen and carbon isotope measurements on paleontologically inferred "planktonic" and "benthonic" foraminifera of Late Cretaceous and Early Tertiary age have shown that the "planktonic" foraminifera indeed inhabited surface waters of the oceans, while the "benthonic" foraminifera inhabited deep waters. Over a five-million-year period near the end of the Cretaceous, various planktonic foraminiferal species retained the same order of depth stratification beneath the surface of the ocean. The surface water in the Cretaceous had temperatures and 6C13 values for dissolved bicarbonate similar to those existing today in South Atlantic surface waters. The bottom-water temperature, however, was perhaps about 10? C warmer than today. Oxygen isotope studies of the Cretaceous pelecypod Inoceramus reveal that it had a benthonic habitat and that it shows consistently heavier oxygen isotope values than the benthonic foraminifera values. Isotope results from the foraminifera clearly indicate that there was a sharp climatic deterioration from middle to late Maastrichtian (68 to 66 million years B.P.). Whether or not this climatic change was the primary cause of the massive extinction of taxa near the end of the Cretaceous can not be proven, but the timing of these two events is sufficiently coincidental to suggest causal effect. Oxygen and carbon isotope measurements of Late Cretaceous and Early Tertiary foraminifera

Cenozoic Planktonic Foraminifera and Biostratigraphy, Arabian Sea Deep Sea Drilling Project, Leg 23A

Abstract: Recent reconstructions of the Mesozoic and Cenozoic spreading history of the Indian Ocean (McKenzie and Sclater, 1971) have provided a hypothetical framework within which geostructural and faunal events may be examined. Drilling during DSDP Leg 23A (March-April, 1972) was conducted with the aim, not only of determining the sedimentary and biostratigraphic history of selected regions in the Arabian Sea, but of comparing the ages determined for igneous basement with the predictions of McKenzie and Sclater's model. The degree of corroboration and the implications of observed sea-floor ages for the geotectonic interpretation of Indian Ocean history are discussed elsewhere in this volume. Sediments were recovered from seven holes at six sites (219 through 224) in the Arabian Sea. Location and recovery data are presented below (Figure 1, Table 1). In terms of their objectives, and to a lesser extent their geographic positions, the sites may be divided into three groups. Sites 219 and 220 are associated with the ChagosLaccadive Ridge, a north-south-trending structure extending southward from the eastern margin of the Arabian Sea. The former is located near the crest of the ridge, in a relatively deep gap between shoal areas represented geographically by the Laccadive and Maldive Island groups. Prior to the drilling of Site 219, coral rock and related debris represented the only sedimentary rock recovered from this ridge. As a result, its Tertiary sedimentary history was unknown, as well as the nature of the underlying basement. Site 220 is situated slightly to the southwest, in deeper water along the western flank of the ChagosLaccadive Ridge within a quiet magnetic zone (McKenzie and Sclater, 1971). It was anticipated that igneous basement at this location would be no older than 38 m.y. (Late Eocene). The primary objectives of drilling at Site 220 included the sampling of this basement and the age determination of a shallow unconformity observed in seismic records. The extensive sedimentary sequences at these sites contained the only abundant and well-preserved planktonic foraminiferal faunas recovered during Leg 23A.

Paleocene Benthonic Foraminiferal Biostratigraphy, Biogeography and Paleoecology of Libya and Mali

Abstract: The Paleocene of Mali is developed along the inner shelf margin of a broad, shallow seaway which extended from Algeria to northern Nigeria. The Paleocene of Libya is developed in a structurally controlled embayment of the Tethys Sea which extended southwards through the Sirte Basin as far as the Chad border (Tibesti Mountains). The foraminiferal assemblages in Mali are dominated by shallow-water cibicidids, rotaliids, Rosalina, nonionids, Elphidiella, and various larger foraminifera, while planktonic foraminifera are rare or absent. The foraminiferal assemblages in Libya are developed in two lithotopes, one similar to that in Mali and containing numerous elements in common, the other, developed in a deeper-water basinal shale environment, containing a "Midway" type of fauna, together with planktonic foraminifera. Vertical and horizontal relationships between benthonic assemblages in the Sirte Basin are shown and paleoenvironmental reconstructions are made at approximately 2-million-year intervals. The Paleocene of Mali was deposited in a broad, shallow shelf sea at water depths probably less than 50 m. The Paleocene of the Sirte Basin of Libya was deposited in a transgressive sequence characterized by basin infilling at maximum depths somewhat in excess of 200 meters. Paleocene benthonic foraminiferal biostratigraphy, biogeography and paleoecology of Libya and Mali

Benthic Ecology of the High Arctic Deep-Sea.

Abstract: An analysis is made of 75 quantitative benthic samples collected by Mini-LUBS, and 28 qualitative benthic samples collected with the “small biological trawl”, from Fletcher's Ice Island, T-3, while it was drifting over the Alpha Cordillera region of the High Arctic Ocean during October, 1969 through February, 1970 and in March, 1972. The depth range was 1000 to 2500 m. Benthic foraminiferans account for about 53%, bivalves for 27%, sponges for 7%, and polychaetes for 5% of the total biomass. Other groups make up the remaining 8%. The weight ratio of macro- to meiofauna is 1:1. Numerically, excluding Foraminifera, polychaetes comprise 42%, nematodes 16%, sponges 11%, and bivalves 8% of the total fauna. The remaining 23% is composed of 13 other taxa. Biomass in the Amerasian Basin at depths of 1000 to 2000 m is extremely low (0.04 g/m); it is comparable to depths of 5000 to 6000 m in the oligotrophic red-clay area of the mid-Pacific Ocean, and is 40 times less than biomass at comparable depths from Antarctica and off Peru. Diversity, as calculated by the Shannon-Weaver method, is low, suggesting that the Arctic ecosystem is young, as reported in earlier studies (Dunbar, 1968; Menzies et al., 1973). Although the H' values are low, no biocoenoses of oligomixity in the deep Arctic are revealed, contrary to previous statements and beliefs. There may be fewer major benthic groups in the Arctic Ocean than in other parts of the world oceans. Following the conventional terminology of Petersen (1913) and Thorson (1957), we have called the High Arctic biocoenoses of the Alpha Cordillera region a Thenea abyssorum-Spirorbis granulatus community.

Arctic Ocean Sediments, Microfauna, and the Climatic Record in Late Cenozoic Time

Abstract: Deep-sea cores from the Central Arctic Basin yield significant faunal and lithologic evidence of normal and low salinity cycles superimposed upon temperature fluctuations in late Cenozoic time. Lowest temperatures correspond to the upper Pleistocene (the Brunnes normal polarity epoch), whereas higher temperatures and lower salinities were recorded by planktonic foraminifera during the Matuyama reversed polarity epoch.

Planktonic foraminifera associated with Antarctic sea ice

Abstract: Juvenile specimens associated with diatom community, probably Globigerina pachyderma or G. quinqueloba

Cretaceous Planktonic Foraminifera from the Eastern Indian Ocean

Abstract: Cretaceous planktonic foraminifera occur at four sites drilled on Leg 26: on the Broken Ridge (Site 255), in the Wharton Basin (Sites 256 and 257), and on the Naturaliste Plateau (Site 258). In the Wharton Basin only assemblages of Albian age were found. These differ from each other in preservation and in their specific composition. The sequence on the Naturaliste Plateau ranges from upper Albian to Santonian. Rich and well-preserved faunas were found in the Albian, Cenomanian, Coniacian, and Santonian. The main features of all Cretaceous assemblages from this part of the Indian Ocean are the low diversity and the predominance of nonkeeled forms of the genera Hedbergella, Globigerinelloides, Whiteinella, and Archaeoglobigerina. This indicates a cool paleoenvironment for this area during the Cretaceous. Range charts for Sites 257 and 258 are given, and most of the species are discussed in the systematic part of this report. INTRODUCTION During Leg 26 Cretaceous planktonic foraminifera were recorded at four different sites in the eastern part of the Indian Ocean: at Sites 255 (Broken Ridge), 256 and 257 (Wharton Basin), and 258 (Naturaliste Plateau). The lithology of the Cretaceous sequences in these three areas differs considerably from one area to another. (See Figure 1.) At the Broken Ridge site indurated and tilted limestone with intercalating chert was penetrated. Foraminifera could not be isolated from these hard rocks and were studied in thin section. The assemblages in the Wharton Basin occur in brown detrital clay which was deposited near the lysocline. Many tests show dissolution effects. Planktonic foraminifera occur only in the Albian part of these two sites. Site 258 on the Naturaliste Plateau is the only one where, over a relatively long stratigraphic interval, ranging from the Albian to the Santonian, good planktonic assemblages were found. The present description and discussion is therefore mainly based on the study of material from this site. The assemblages found are of particular interest since, so far, relatively few data have been available from the southern hemisphere. However, a number of important papers have been published on Cretaceous planktonic foraminifera from Australia (e.g., Edgell, 1957; Belford, 1960; and Belford and Scheibnerova, 1971); they allow a correlation of our sites with the onshore Cretaceous of Western Australia. A Turonian faunule from the Naturaliste Plateau near Site 258 has been described by Burckle, Saito, and Ewing (1967). PLANKTONIC FORAMINIFERAL ASSEMBLAGES FROM SITES 255-258 Site 255 Sixteen thin sections of limestone and chert from the following samples were studied: Sample 8, CC; Sample 9-1, 116-118 cm; Sample 9, CC; Sample 10-1, 72-74 cm; Sample 10-1, 74-76 cm; Sample 10, CC; Sample 11, CC. Most of the limestone, and in part also the chert, contains planktonic foraminifera in great abundance. Practically all of them are small globigerine-shaped forms and planispiral tests belonging to the genus Globigerinelloides, which cannot be specifically identified. Rare oblique sections of double-keeled globotruncanid forms may possibly be attributed to Globotruncana area or related forms, which suggests a Santonian to Campanian age of this limestone. This corresponds with the Santonian age determined by nannoplankton. Apart from planktonic foraminifera, most of the samples contain numerous benthonic foraminifera, fragments of Inoceramus, including abundant isolated calcite prisms of the ostrakum, as well as fragments of other pelecypods and crinoids. Some of the samples contain glauconite, which frequently fills the tests of radiolarians. Much of the limestone is partly silicified and fossil remains, particularly the tests of foraminifera and the prisms of Inoceramus may be partially or totally silicified. A section through the boundary between the siliceous limestone and chert shows that this boundary may cut across foraminiferal tests, which clearly demonstrates that the silicification is postdepositional. Site 256 At this site Cretaceous planktonic foraminifera occur only within the 2 meters of sediment immediately above the basalt. They were recorded in Sample 8-6, 115-119 cm; Sample 8, CC; and Sample 9-1,120-122 cm. The main feature of these assemblages is the very small size of the specimens, the rather poor preservation (the tests being etched and recrystallized), and their virtually monospecific composition (Plate 1, Figures 1-6). Most of the specimens can be assigned to Hedbergella planispira, but some show transitional features to H. 745

Secondary Calcification of the Planktonic Foraminifer Neogloboquadrina pachyderma as a Climatic Index.

Abstract: Oscillations in the ratio of two principal types of secondary calcification of the test surface in Neogloboquadrina pachyderma populations closely parallel paleoclimatic oscillations over the last 6 million years in a deep-sea core drilled in the temperate South Pacific. The nature of secondary calcification in fossil planktonic foraminifera represents a useful index in interpreting Cenozoic climates.

Upper Cretaceous Benthonic Agglutinated Foraminifera, Leg 27 of the Deep Sea Drilling Project

Abstract: Upper Cretaceous brown, zeolitic clays were penetrated in the northeast Indian Ocean at Site 260 in the Gascoyne Abyssal Plain and at Site 261 in the Argo Abyssal Plain. The sediments which are assumed to have been deposited below the lysocline, contain an assumedly autochthonous fauna of characteristic agglutinated foraminifera. The 21 genera distinguished are: Haplophragmoides, Labrospira, Recurvoides. Paratrochamminoides, Haplophragmium, Pseudobolivina, Bolivinopsis, Plectorecurvoides, Trochammina, Praecystammina, Uvigerinammina, Verneuilina, Dorothia, Plectina, Glomospira, Glomospirella, Ammodiscus, Saccammina, Pilulina, Hormosina, and Hyperammina. Sixteen of the 44 recognized species are new, 12 were described for the first time from the Upper Cretaceous zeolitic clays of the northwestern part of the Pacific Ocean (Leg 20, Sites 196 and 198A), and 16 are known from earlier micropaleontological publications. Based on the systematic composition, two assemblages of agglutinated foraminifera are distinguished: an upper assemblage with Praecystammina globigerinaeformis and a lower assemblage with Haplophragmium lueckei. The brown clays at Site 260 with agglutinated foraminifera of the upper assemblage also contain reworked planktonic foraminifera of Albian (?), Cenomanian, and upper Turonian-Coniacian age. Thus, the age of sediments containing the upper assemblage of foraminifera is not older (within the Upper Cretaceous) than upper Turonian-Coniacian. The upper and lower assemblages of agglutinated foraminifera have some species in common. Therefore, the age of the sediments with the lower assemblage which do not contain planktonic forms is also considered Upper Cretaceous, but slightly older than the upper assemblage. The agglutinated foraminifera are characterized by having small dimensions (0.15-0.40 mm) and a fine-grained or homogeneous thin wall with a smooth surface, features that reflect deep-water conditions. The upper assemblage of agglutinated foraminifera with Praecystammina globigerinaeformis has been recognized in the Upper Cretaceous of the Indian (Leg 27) and Pacific (Leg 20) oceans. Thus, this type of microfauna can be used for correlating noncalcareous deep-water brown clays, which are usually extremely poor in organic remains. The assemblages of agglutinated foraminifera described herein differ strongly from those in contemporaneous Cretaceous marine sediments on continents, where deep-water brown clays of this age are unknown.

Pliocene biostratigraphy of the Labrador Sea: calcareous plankton

Abstract: Planktonic foraminifera abundant, calcareous nannoplankton rare, correlated with Pacific Ocean, Mediterranean Sea and New Zealand

Distribution of Benthonic Foraminifera in Upper Jurassic and Lower Cretaceous Deposits at Site 261, DSDP Leg 27, in the Eastern Indian Ocean

Abstract: Benthonic foraminifera from 18 samples selected from Site 261 (Figure 1) were studied in order to determine their age and stratigraphic range. The samples, which were selected by V. A. Krasheninnikov, were washed and treated during the leg; the microfauna was selected at the Geological Institute of the Academy of Sciences of the USSR in Moscow. The following samples have been studied at Site 261: 29, CC; 30-2, 7-9 cm; 30-4,44-46 cm;30-3, 63-64 cm; 30, CC; 31-3, 10-12 cm; 31-2, 22-23 cm;31-4,32-34 cm; 31-5, 60-62 cm; 31, CC; 32-4, 40-42 cm; 32-2, 59-60 cm; 32-3, 83-85 cm; 32, CC; 33, CC; 35-5, 120-122 cm; 35-5, 128130 cm; and 35, CC. All of the samples studied contain foraminifera, together with radiolarians, ostracods, sponge spicules, and, in one sample (Sample 30, CC), fish scales. The foraminiferal assemblages are characterized by the following features: 1) All the foraminifera are benthonic. Agglutinated forms are predominant and some assemblages consist entirely of these forms (Samples 31-2, 22-23 cm; 30-2, 79 cm). 2) Some assemblages contain many species and numerous specimens (Samples 32, CC; 32-3, 83-85 cm; 32-4, 40-42 cm; etc.). Others contain few species represented by only sporadic specimens (Samples 35-5, 120122 cm; 32-2,59-60 cm; 31-4, 32-34 cm; 29, CC). 3) Most of the benthonic foraminifera are well preserved. Agglutinated foraminifera as a rule are better preserved than calcareous ones. 4) The foraminifera from Site 261 exhibit considerable diversity with over 90 species present. These have been attributed to 44 genera from 12 families. The systematic composition of the foraminiferal fauna is as follows: Family Astrorhizidae (Rhizammina, Hyperammina, Hippocrepina, Saccorhiza) Family Saccamminidae (Pelosina, Sorosphaerà) Family Ammodiscidae {Ammovertella, Ammolagena, Lituotuba, Glomospira, Glomospirella) Family Hormosinidae (Reophax) Family Lituolidae {Haplophragmoides, Ammobaculites, Haplophragmium, Placopsiliná) Family Textulariidae {Textularia, Bigenerinà) Family Trochamminidae (Trochammina) Family Ataxophragmiidae {Verneuilinoides, Dorothia) Family Nodosariidae {Lenticulina, Planularia, Marginulina, Vaginulina, Frondicularia, Falsopalmula, Dentalina, Nodosaria, Bojarkaella, Lingulina, Lagena, Pseudonodosaria) Family Polymorphinidae (Pyrulina, Ramulina) Family Ceratobuliminidae (Epistomina?) Family Spirillinidae {Spirilliná) Figure 1. Location of Leg 27, Site 261.

Paleocene and Early Eocene Microfacies, Benthonic Foraminifera, and Paleobathymetry of Deep Sea Drilling Project, Sites 236 and 237, Western Indian Ocean

Abstract: ABSTRACT A significant and abrupt change in the sedimentary regime at Site 237 provides evidence for an early Tertiary subsidence of the Mascarene Plateau area in the western Indian Ocean. Between approximately 62 and 57 m.y.B.P. pelagic sediments were deposited at Site 237 in upper bathyal (200-600 m) depths with admixtures of slumped material from nearby shoals at a rate of about 68 m/m.y. The sea floor began to sink in the late Paleocene (57-58 m.y.B.P.). Introduction of shallow-water material ceased and normal pelagic sedimentation proceeded, at a rate about six times slower, at lower bathyal (600-2500 m) depths. Deepening at Site 237 at this time permitted development of deep-water benthonic foraminiferal faunas similar to those at Site 236 to the north. A further decrease in the rate of sedimentation occurred near the end of the Paleocene and in early Eocene, from approximately 54 to 50 m.y.B.P. During that period of time an interval of nondeposition spanned the Paleocene/Eocene boundary at Site 236. The area at Site 237 has subsided approximately 2000 meters over the past 52 m.y. This sequence of events on the Mascarene Plateau is similar to, and synchronous with, the subsidence history of the ChagosLaccadive Ridge to the east.

Neogene Planktonic Foraminifera of the Indian Ocean (DSDP, Leg 26)

Abstract: This report is the result of a study of the Neogene deposits cored in the Indian Ocean during Leg 26 of the Deep Sea Drilling Project. The main purposes of the study were: (a) qualitative, and in some degree quantitative, study of the planktonic foraminiferal assemblages found in the holes drilled; (b) .determination of the stratigraphic ranges of the main species; (c) establishment of biostratigraphical zonations in the area under study; (d) correlation of the sites drilled; and (e) where possible, the drawing of ecological paleoclimatological as well as taxonomical conclusions. Time restrictions in the publication schedule did not permit several aspects of the conclusions drawn to be investigated in detail. For the same reason, unfortunately, no study of benthonic fauna was completed. Hopefully, it will be carried out in the neai future. In spite of these shortcomings, I hope that the main results of this study are adequately based and correct. To save space, the following abbreviations are used for the samples cited in text and in the plates explanations. Each sample is characterized by numbers which signify core number, section number of that core, and, set off by commas, depth from the top of the section in centimeters. Thus 3-2, 40-42 cm signifies that this sample was taken from Core 3, Section 2, at a depth of 40-42 cm from the top of the section. Samples from the core catchers are designated by the number of the core and the letters "CC." Thus, a sample taken from the core catcher material of Core 5 will be marked as 5, CC.

Diversity of Benthic Foraminifera on the Georgia Continental Shelf

Abstract: Two indices of species diversity, the species number s and the Shannon-Wiener function H, bring out the difference between the benthic foraminiferal assemblages on the inner part of the Georgia continental shelf and those on the middle and outer parts. The diversity values rise from the shoreline to water depths of about 15 m. From 15 m to the shelf edge (at about 50 m) and beyond, the values remain steady or increase very slightly, with s nearly 50 and H nearly 3. This may be explained by the fact that in terms of variations in water temperature, salinity, and substrate conditions, the nearshore habitat on the Georgia shelf is much more “unstable” or “unpredictable” than the offshore habitat. Such differences in diversity gradients could be expected in comparable late Tertiary foraminiferal assemblages. Foraminiferal equitability values on the different parts of the Georgia shelf do not show any distinct trends.

Paleogene planktonic foraminifera of New Zealand and the Austral region

Abstract: The 103 documented Paleogene planktonic foraminifera from 21 New Zealand sections which were previously divided into 14 zones have been further subdivided into 26 subzones and a correlation is attempted with Trinidad. A correlation of the following Austral regions is also made: 9 southern Australian sequences, a composite New Zealand sequence based on 21 sections, 4 deep-sea cores from the South Pacific, and 2 sequences from Argentina and 1 from Chile. A detailed examination is undertaken of the Upper Eocene-Lower Oligocene faunas in order to determine the accurate placement of the Eocene-Oligocene boundary. Although there are a number of taxa common to the Boreal, Tropical-subtropical, and Austral belts, some of their extinction levels appear to be diachronous. A record drop in sea-water temperature of 9 degrees C from oxygen isotope analyses in the Upper Eocene of New Zealand effectively reduced species diversity and was the probable cause of the diachronous nature of the initial appearances and extinctions. It is postulated that the cooling probably began in Antarctica in the Upper Eocene, spreading northwards to the New Zealand region, then the cooling affected the already potentially cooler Boreal region in the lowermost Oligocene and finally lowered the temperature of the warmer Tropical-subtropical Belt later in the Lower Oligocene. A time sequential paleo-biogeographic model of the Upper Eocene-Lower Oligocene planktonic foraminifera is presented.

Late Pleistocene Benthic Foraminifera of the Southern Champlain Sea: Paleotemperature and Paleosalinity Indications

Abstract: Lake Champlain (Fig. 1) is a long, narrow lake lying between New York and Vermont. It trends north south and has a maximum depth of about 244 metres. The Champlain Valley was glaciated during the Woodfordian stade and between 14,500 and 11,500 years BP, the lake basin was gradually freed of ice. During this interval, pro-glacial lakes at several levels occupied the Champlain Valley (Chapman, 1937 ; Schafer and Hartshorn, 1965; Thomas, 1964; Connally and Sirkin, 1970). With the departure of the ice blocking the St. Lawrence lowland, which was then below sea level, the Champlain Sea penetrated into the Champlain Valley (Chapman, 1937; Gadd, 1964). This southern arm of the Champlain Sea gradually shallowed as glacio-isostatic uplift proceeded unti l the sill at its northern end reached sea level be­ tween about 8500 and 10,000 years BP (Fillon, 1970; and Chase and Hunt, 1972), and the present fresh­ water lake was created.