SEA-LEVEL CHANGES AND STRATIGRAPHY OF THE NELSON LIMESTONE (MIDDLE CAMBRIAN), NEPTUNE RANGE, ANTARCTICA

The first detailed history of relative sea-level changes known from lo wer Paleozoic strata in Antarctica is documented in this study of the Nelson Limestone. Basal beds bf the formation constitute a nonmarine t errigenous valley-fill succession. Thickness of these strata varies fr om 0.1 m to 72 m, suggesting that significant topographic relief exist ed below the sub-Nelson unconformity. Locally, base level must have ri sen at least 72 m because, following fluvial infilling of the valley, a succession of carbonates over 400 m thick was deposited as seas inun dated the Antarctic margin of Gondwana during late Middle Cambrian tim e. On a global scale, the broad relative sea-level rise recorded in th e Nelson Limestone is synchronous with previously recognized eustatic events during Lejopyge laevigata time. In the Nelson Limestone, smalle r fluctuations of relative sea level are indicated by three sequences, each about 100 m thick. The lower sequence is bounded below by the su b-Nelson unconformity and includes the valley-fill succession, transgr essive sandstone, and highstand carbonate deposits. The top of a subae rially exposed interval of carbonates in one section is interpreted as a sequence boundary with correlative conformity in the deeper-water a reas. The subaerially exposed interval is roughly correlative with a s uccession of five or more slump deposits 1-2 m thick in another sectio n. Subaerial exposure at nearly the same stratigraphic level as the sl umps marked a fundamental shift in depositional style. Following marin e flooding, an offshore carbonate shoal with an associated restricted lagoon was established during deposition of the middle sequence. The u pper sequence records marine flooding above the middle sequence and su bsequent reestablishment of an offshore carbonate shoal prior to defor mation and volcanism in the area related to the Ross Orogeny.