| Summary: | Includes bibliographical references. Includes illustrations and maps. Foraminiferal Biostratigraphy, Paleoecology, and Systematics of DSDP Site 273, Ross Sea, Antarctica. Phase Three of DSDP Leg 28 explored one site in the western Ross Sea basin. At Site 273, 346.5m of sediment were cored with a recovery of 83.4m or 25%. Two lithologic units were recognized along with two biostratigraphic zones. Further examination of detailed sedimentary analyses from the initial reports of Leg 28 and the study of two foraminiferal assemblages has resulted in the relocation of the lithostratigraphic and biostratigraphic boundaries. The lithologic boundary has been raised 2.5m to a distinct contact. This change is supported by paleontologic evidence. Two foraminiferal assemblages have been recognized. The upper occurs in Zone II, above Core 5 section 1, 76cm. The lower assemblage of Zone I, occurs below Core 5 section 1, 76cm. Both are dominantly calcareous benthic assemblages. The fauna of Zone II contains one arenaceous species, four planktonic species and 44 calcareous benthic species. The total of 49 can be subdivided into 21 (42.8%) exclusive species, 16 (32.6%) long-ranging species, and 12 (24.6%) species considered to be reworked. The fauna of Zone I contains 77 calcareous benthic species and two planktonic species. One new species, Trochoetphidie lla uni foramina, has been recognized. Twenty-six species (33%) are common to both zones. Sixteen are considered to be long-ranging while ten species found in Zone I occur in Zone II by reworking. The taxa of Zone II are relatively large, thick-walled, and white in color. This contrasts with the taxa of Zone I which are characteristically smaller, thin-walled, and generally depauperate. Zone II has been assigned a Pleistocene/Recent age (0-1.8 m.y.b.p.) while Zone I has been assigned a Middle Miocene age (11.0-16.9 m.y.b.p.). A disconformity which represents approximately 9.2 m.y. of lost geologic record separates the two units and zones. Lithologic Unit 1 and Zone II may represent as little as 0.7 m.y. as based on diatom biostratigraphy. Lithologic Unit 2 and Zone I represent latest Early Miocene, Middle Miocene and very earliest Late Miocene deposition. Paleoecological interpretations indicate that Middle Miocene analogs of present day antarctic surface and Ross Sea shelf waters existed during the Middle Miocene. It is also hypothesized that a Middle Miocene analog of the saline, dense, corrosive form of Ross Sea shelf water did not exist at this time. This is indicated by the absence of arenaceous foraminifera in Zone I. Interpretation of related paleoecologic factors provides evidence for 200-300m of downward movement of the Ross Embayment after the Middle Miocene. This complements similar tectonic up lift of the nearby Transantarctic Mountains. M.S. (Master of Science)
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