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Richly fossiliferous Upper Cretaceous-Paleocene beds exposed on Seymour Island provide an excellent opportunity for a combined sedimentological-paleoecological analysis of a high-latitude shallow marine sequence. The depositional paleoenvironments of this sequence were interpreted on the basis of the stratigraphy, sedimentological analysis (including grain-size analysis) of the sediments, and auto- and synecological evaluation of the fauna present.

The sequence is composed of two formations. The underlying López de Bertodano Formation (upper Campanian to Paleocene) consists of 1,190 m of gray to tan, friable, sandy, muddy siltstone, and is subdivided into 10 informal units. The lower six units (informally named the Rotularia Units) contain a depauperate macrofauna and are dominated by the annelid Rotularia. They were deposited in a shallow marine environment, near a delta or estuary. Units 7 through 10 include an abundant macrofauna (Molluscan Units). Units 7 through 9 are interpreted as progressively deeper water deposits, with Units 7 and 8 representing middle shelf facies, and Unit 9, the outer shelf facies. Macrofauna in the most offshore portion of Unit 9 is characterized by an epifaunal suspension-feeding bivalve asssemblage dominated by Pycnodonte cf. P. vesiculosa, and an increased percentage of cosmopolitan ammonites. Regressive facies appear 30 m below the inferred Cretaceous/Tertiary boundary (contact between Units 9 and 10). Unit 10 was probably deposited in a middle shelf to inner shelf environment.

The overlying Sobral Formation (Paleocene) follows unconformably, and is composed of as much as 255 m of maroon, well-laminated silts at the base, followed by cleaner sandstones that become more glauconitic and crossbedded toward the top. The Sobral is not very fossiliferous and represents the filling of the basin by the progradation of a deltaic system. Units 1 and 2 are interpreted as pro-delta facies followed by clean sands of a coastal barrier (Unit 3). The uppermost Sobral (Units 4 and 5) contains the delta top facies, mostly representing the lateral accretion of distributary channels.

Three unconformity-bounded depositional sequences are recognized in the studied section. The lower sequence extends from Units 1 to 9 of the López de Bertodano Formation (upper Campanian to upper Maastrichtian). The intermediate sequence is restricted to Units 9 and 10 of the same formation (upper Maastrichtian to lower Paleocene); the upper sequence is represented by the Sobral Formation (Paleocene). Uncertainties in the dating of these sequences make their comparison with worldwide sea-level fluctuations premature.

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