Nonmarine sedimentation in an Early Cretaceous extensional continental-margin arc, Byers Peninsula, Livingston Island, South Shetland Islands
Upper Jurassic-Lower Cretaceous rocks of the Byers Group, exposed on Byers Peninsula, Livingston Island, record the expansion of Gondwana-margin continental-arc facies into a marine intra-arc basin. At least 1.3 km of marine clastic rocks are overlain by about 1.4 km of Lower Cretaceous nonmarine volcaniclastic strata assigned to the Cerro Negro Formation. The base of the nonmarine succession is marked by a low-angle unconformity. The lower 200-240 m is largely pale green- and gray-weathering, and consists mainly of welded and nonwelded silicic ignimbrites, with subordinate reworked silicic tuffs and ignimbritic conglomerates. A change in color to dark red-purple at the top of this interval broadly coincides with a change to a largely basaltic-intermediate provenance. The rest of the succession consists mainly of poorly sorted lithic lapilli-tuffs and tuffaceous breccias largely interpreted as debris-flow and flood-flow deposits. It also includes two welded silicic ignimbrite units rich in basaltic clasts, and is considered to represent a volcaniclastic apron flanking one or more basaltic andesite stratovolcanoes. Though it is dominated by syneruption deposits, this upper division also includes laterally impersistent, subsidence-driven inter-eruption facies, including basaltic conglomerates deposited in incised fluvial channels, minor lacustrine intervals, and rare paleosols. A thicker (100 m), peninsula-wide, mudstone/sandstone-dominated horizon represents a more extended period of inter-eruption deposition, during which the area was the site of a substantial lake. Throughout the Cerro Negro Formation, thickness and facies changes provide evidence of synsedimentary displacement across a series of ENE-trending normal faults, most with downthrow to the south. In the upper part of the formation, resulting differential subsidence led to southward thickening accompanied by increased preservation of inter-eruption facies, and on a smaller scale, trapping of a fluvial channel against the footwall of a synsedimentary fault. This tectonism appears to form part of an Early Cretaceous episode of arc-perpendicular extension well documented elsewhere in the Antarctic Peninsula region.