Alexander Island - Geology

Geology

Except for a 10 to 30 km (6 to 19 mi) strip along its eastern edge, the bedrock beneath the ice sheet, which covers Alexander Island, consists of a complex assemblage of metasedimentary and metavolcanic rocks. These rocks, which are locally overlain and intruded by igneous rocks, are known as the "Lemay Group." It consists of turbiditic sandstone (subgraywacke and arkose) interbedded with secondary siltstone, mudstone, tuff, volcaniclastic sedimentary rock, agglomerate, and conglomerate, which have all been metamorphosed to prehnite–pumpellyite to blueschist facies. These metasedimentary rocks contain poorly preserved ammonites, pelecypods, gastropods, radiolarians, and fragmentary plant fossils. The LeMay Group also contains north-south, fault-bound slices of accreted ocean floor, ocean islands and seamounts, which includes pillow basalts, volcaniclastic sedimentary rocks, and chert. The rocks, which comprise the LeMay Group are asymmetrically folded and thrust faulted to the west, contain chaotic zones, and exhibit abundant coeval soft-sediment deformation. Based upon biostratigraphy, the oldest known rocks found within the LeMay Group are Lower Jurassic and range up to late Lower Cretaceous. The oldest rocks within this group may date back to Permo-Carboniferous times (350–260 million years ago). The LeMay Group is interpreted to be an accretionary prism that was built out from the west (fore-arc) side of a long-lived magmatic arc, the roots of which are now exposed on the Antarctic Peninsula, between Early Jurassic and Paleocene times, when subduction ceased.

The easternmost 10 to 30 km (6 to 19 mi) wide strip of Alexander Island is underlain by a 7 km (4 mi) thick sequence of highly fossiliferous, shallow marine to subaerial fluvial sandstone, mudstone, and conglomerate of the Fossil Bluff Group. The marine strata within the Fossil Bluff Group contain numerous ammonites, belemnites, gastropods, pelecypods, and other marine fossils. The upper fluvial sedimentary rocks of the Fossil Bluff Group contain fossil forests with standing trunks as tall as 5.5 m (18 ft), other plant fossils, and associated paleosols as found at Citadel Bastion. These sedimentary rocks are essentially unmetamorphosed and gently folded. They have been affected by strike-slip, normal, and thrust faulting at different times. As determined form the fossils found in it, the Fossil Bluff Group accumulated over a period of time ranging form Early Jurassic to mid-Cretaceous. The sedimentary rocks of the Fossil Bluff Group unconformably overlie and are faulted against the LeMay Group. Earth scientists argue that the Fossil Bluff Group accumulated within a fore-arc basin on the western side of a long-lived magmatic arc, the roots of which are now exposed along the Antarctic Peninsula.

Two different groups of igneous rock have been found within Alexander Island. One group of igneous rocks consist of volcanic and plutonic rocks of Late Cretaceous to Paleocene age. They unconformably overlie and intrude metasedimentary and other rocks of the LeMay Group and the sedimentary rocks of the Fossil Bluff Group. Some of these volcanic rocks consist of distinctive magnesium-rich andesites, which were created by the subduction of a segmented ancient mid-ocean ridge about 80 million years ago. Finally, there are several small outcrops of Neogene alkaline volcanic rocks, which formed after the cessation of subduction.