Geometry and stratification of uppermost Cretaceous and Paleogene units on Seymour Island, northern Antarctic Peninsula

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Abstract
The Cretaceous-Paleogene sequence on Seymour Island has been mapped in terms of four previously named units: the López de Bertodano Formation (Cretaceous-Paleocene), the Sobral Formation (Paleocene), the Cross Valley Formation (late Paleocene), and the La Meseta Formation (Eocene). The basal surfaces of the Sobral and Cross Valley Formations include portions with clear evidence of erosional unconformity. The base of the La Meseta Formation laps onto a steep buttress unconformity at both of its exposed margins. The scale of lenticularity of the units increases upward throughout the sequence. The López de Bertodano Formation and most of the Sobral Formation are built of essentially tabular units, and have been effectively characterized by isolated measured sections. The Cross Valley Formation is a channel-filling lens. The La Meseta Formation has a complex, asymmetric, trough-like form. It is built of large-scale lenses characterized by different shelly facies that do not all extend across the full width of the trough. Consequently, mapping between measured sections is essential to establish the lateral relationships. Even then, the geometric relationships do not identify time lines; establishing the real age relationships will require very carefully located faunas. The observed relationships between the length and height of lenticular units can be used to estimate the expected vertical error when faunas are projected into a composite section. The whole sequence is essentially a gently dipping homocline. The youngest units have slightly gentler dips, indicating progressive or repeated tilting. But a larger component of the variance in bedding attitude is contributed by steeper dips in the sedimentary filling of channels and slump scars. The regional tilt, the development of growth faults, facies changes, and the orientation of erosional surfaces have a symmetrical arrangement from which a common cause is inferred. The best estimates of the timing of the major unconformities and erosional episodes are only partially compatible with current models of global sea-level change.

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