Early Paleozoic sedimentation, magmatism, and deformation in the Pensacola Mountains, Antarctica: The significance of the Ross orogeny

Combined sedimentological, structural, and geochemical studies of a lower Paleozoic succession within the Pensacola Mountains, Antarctica, suggest that it probably formed in a foreland basin setting during the Ross-Delamerian orogen, a complex early Paleozoic convergent margin of Antarctica and Australia. The lower Paleozoic succession lies unconformably on a deformed(?) Neoproterozoic sequence (referred to here as Sequence 1) and is divided into three unconformity-bounded sequences (Sequences 2–4). The oldest sequence, Sequence 2, comprises Middle–Upper Cambrian platformal limestone (Nelson Limestone) and overlying Lower Ordovician silicic volcanic rocks of the Gambacorta Formation (U-Pb zircon age of 501 ± 3 Ma). The volcanic rocks crystallized from a high-temperature anhydrous magma derived from a lower crustal igneous source and may represent magmatism on the inboard side of a magmatic arc now largely absent from this part of the margin. Sequence 3 (Wiens Formation), in part conformable with Sequence 2, represents deposition by unconfined ephemeral streams followed by a marine transgressive unit. The base of Sequence 4 (Neptune Group) is a major erosion surface marked by karstification of the exposed Nelson Limestone and by calcrete pedogenesis. The Neptune Group is an alluvial fan complex typical of many syn- and post-orogenic red beds. The predominance of nonmarine and shallow marine sequences, and the facies and paleocurrent directions within the basin, suggest that it may be more typical of a “piggyback” basin than of a foredeep basin, with the alluvial fan complexes derived from advancing thrust sheets. Growth folds, progressive unconformities, and deformed clasts of underlying strata within basal conglomerates are consistent with active deformation during sedimentation and the proposed tectonic setting. The presence of variably plunging folds, some of which are transected by a slaty cleavage, suggests that deformation was in an oblique-slip setting perhaps due to oblique convergence along this part of the Antarctic margin during the Ross-Delamerian orogeny.

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