The Amery Group
Exposures of the Amery Group sedimentary sequence in the northern Prince Charles Mountains represent the only known substantial in situ occurrence of Paleozoic or Mesozoic strata east of the Transantarctic Mountains. Although exposure within the Amery Oasis is limited to an area of <300 km2, possible Amery Group correlatives have also been recognised within ODP (Ocean Drilling Program) drill cores obtained from Prydz Bay, close to the northern end of the Lambert Graben system. Permian plant macrofossils (Glossopteris and Vertebraria), of equivalent age to some Amery Group fossils, have also been identified within glacial erratics at Mount Rymill, 250 km south of the Amery Oasis in the southern Prince Charles Mountains. This discovery suggests that unexposed Amery Group sediments may also be present throughout much of the Lambert Graben.
The Amery Group, comprising a >3000m thick sequence of fluviatile sediments, is dominated by sandstone packages interbedded with lesser shales, coals and conglomerates. The entire sequence has been stratigraphically subdivided into three main units. In ascending order, these are the Radok Conglomerate, the Bainmedart Coal Measures and the Flagstone Bench Formation. Within the Amery Oasis area these strata are gently folded, with dips of less than 10 degrees. Faults cross-cutting Amery Group exposures have vertical displacements of up to 2 km, and in some cases display evidence of dextral strike-slip faulting. Middle to Late Permian spore and pollen assemblages within the Radok Conglomerate and Bainmedart Coal Measures, as well as Triassic megafloras and palynofloras within the Flagstone Bench Formation, imply a Middle Permian to Late Triassic (260 to 210ma) age for the Amery Group sediments.
Environment of Deposition:
The Amery Group sedimentary sequence was deposited in the Lambert Graben at a time, prior to Gondwanan break-up, when it was continuous with the Son-Mahanadi Graben of southeastern peninsular India. Consistent palaeocurrent directions and stratigraphic sequences in both areas indicate that a major braided river system flowed axially northwards down the graben from Antarctica into India for ~1000 km. In Antarctica, the deposition of the Bainmedart Coal Measures and the Flagstone Bench Formation by this river system can be correlated with the deposition of the Raniganj Coal Measures and the Mahadeva Sequence, respectively, in India.
The transition within the Amery Group from the glossopterid-dominated vegetation of the Bainmedart Coal Measures to the peltasperm-, lycophyte- and corytosperm-dominated vegetation of the Flagstone Bench Formation is considered to reflect a change from a cool, humid climate during the Permian to a warmer and drier one during the Triassic. The shift from humid to semi-arid conditions is also recognisable in the sediments themselves. Coals disappear at the top of the Bainmedart Coal Measures (latest Permian) and are replaced by redbeds and localised calcrete development in the lower and middle Flagstone Bench Formation (Early to Middle Triassic). In the upper Flagstone Bench Formation (Late Triassic), the resumption of more humid conditions is indicated by a return to northerly directed palaeocurrents, an abundance of thick channel sandstone packages, and a well-preserved fossil flora.