ABSTRACT

The first detailed history of relative sea-level changes known from lower Paleozoic strata in Antarctica is documented in this study of the Nelson Limestone. Basal beds of the formation constitute a nonmarine terrigenous valley-fill succession. Thickness of these strata varies from 0.1 m to 72 m, suggesting that significant topographic relief existed below the sub-Nelson unconformity. Locally, base level must have risen at least 72 m because, following fluvial infilling of the valley, a succession of carbonates over 400 m thick was deposited as seas inundated the Antarctic margin of Gondwana during late Middle Cambrian time. On a global scale, the broad relative sea-level rise recorded in the Nelson Limestone is synchronous with previously recognized eustatic events during Lejopyge laevigata time.

In the Nelson Limestone, smaller fluctuations of relative sea level are indicated by three sequences, each about 100 m thick. The lower sequence is bounded below by the sub-Nelson unconformity and includes the valley-fill succession, transgressive sandstone, and highstand carbonate deposits. The top of a subaerially exposed interval of carbonates in one section is interpreted as a sequence boundary with correlative conformity in the deeper-water areas. The subaerially exposed interval is roughly correlative with a succession of five or more slump deposits 1-2 m thick in another section. Subaerial exposure at nearly the same stratigraphic level as the slumps marked a fundamental shift in depositional style. Following marine flooding, an offshore carbonate shoal with an associated restr cted lagoon was established during deposition of the middle sequence. The upper sequence records marine flooding above the middle sequence and subsequent reestablishment of an offshore carbonate shoal prior to deformation and volcanism in the urea related to the Ross Orogeny.