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Abstract
Journal of Sedimentary Research, Section
B: Stratigraphy and Global Studies
Vol. 67 (1997)No.
4. (July), Pages 686-697
Nonmarine Sedimentation in an Early Cretaceous Extensional Continental-Margin
Arc, Byers Peninsula, Livingston Island, South Shetland Islands
B. Hathway
ABSTRACT
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 uccession 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 syn-eruption 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.
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