ABSTRACT Evidence for subaerial exposure of an early Famennian platform is recorded within the fore-reef slope succession in the Napier Range reef complex of the northern Canning Basin. This observation supports recent sequence-stratigraphic interpretations of fluctuations in relative sea level throughout reef evolution in this basin, and contrasts with previous outcrop-based interpretations that there were no sea-level falls except at the Frasnian-Famennian boundary. Remnants of the paleokarst surface are preserved in allochthonous reef-flat blocks that are temporally and spatially related to lenticular siliciclastic units. Subaerial exposure of the platform margin during a third-order sea-level fall in the early Famennian led to the formation of karst cavities on the reef flat and collapse of parts of the reef margin. During this time, siliciclastic sands were fed to the slope, where they were deposited as a series of aprons along the margin. In the early stages of the ensuing transgression, renewed marine conditions enabled cyanobacteria to colonize the cavities. As sea level continued to rise, siliciclastic sands were reworked across the platform, filling the encrusted cavities. The reef-flat blocks were dislodged during the late transgression n response to instability that may have been caused by triggers such as rising pore-fluid pressures and/or tectonic activity. They were emplaced on the siliciclastic slope aprons towards the end of sand deposition as sea level on the platform continued to rise and carbonate production resumed.
Abstract The Itremo region of central Madagascar has an importance in the evolution of the East African Orogen (EAO) that belies its size. Unusually for the southern EAO (Mozambique Belt), it is made up of low-grade metasedimentary rocks and therefore preserves an almost unique window into upper crustal deformation during this key period of the Gondwana supercontinent cycle. In this paper new field mapping of three linked regions in the Itremo Sheet and in the upper part of the underlying mid-crustal Antananarivo Block are presented. From these a complete structural section through the eastern Itremo Sheet is produced and the complex deformation record preserved there is then discussed. An early deformation (D1) consists of 10 km scale recumbent isoclinal folds that predate intrusion of a c. 780–800 ma igneous suite. Metamorphic aureoles around these plutonic bodies overprint D1-related fabrics. Local deformation accompanies intrusion of the c. 780–800 Ma, suite (D2). Extensive E-W contractional deformation occurs between 780 and c. 570 Ma, that is here amalgamated as a composite D3 event, which includes thrusts and at least two phases of upright folds. Post-551 Ma, normal shearing (D4) marks the boundary between the Itremo Sheet and the underlying Antananarivo Block (the Betsileo Shear Zone), and may have also been responsible for formation of the Saronara Shear Zone. Finally, E-W open folding and dextral shear zone development marks a late N-S contractional event that is interpreted as a far-field response to collision between the northern Bemarivo Belt and central Madagascar.
The Cambrian succession in the Arrowie Basin, Flinders Ranges, South Australia comprises the carbonate-dominated Hawker Group overlain by the sandstones and siltstones of the Billy Creek Formation, Wirrealpa Limestone and the clastic-dominated Lake Frome Group. Cross-bedding and other current data indicate a change in provenance for the clastic sediments at about the level of the Wirrealpa Limestone. It is suggested that the clastics of the Hawker Group and the Billy Creek Formation were derived from the Curnamona Craton to the east and southeast whereas those of the Lake Frome Group were derived from the Gawler Craton. This change occurred at approximately the same time as a change in provenance within the upper Backstairs Passage Formation of the Kanmantoo Group on Fleurieu Peninsula and may indicate that the top part of the Backstairs Passage Formation would correlate approximately with the Billy Creek Formation. The provenance change suggests that the Delamerian Orogeny caused uplift of the Curnamona Craton prior to that of the Gawler Craton.
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