Up to 50 m thick archaeocyath-microbial reefs.
(Coalesced small reefs with 2 m size).
Locally with large growth-framework cavities filled with submarine cements and internal sediments. 
2 groups:
- Renalcis dominated (rare archaeocyaths).
- variable composition with Epiphyton.
Maximum abundance of archaecyaths is 30 %.

Abstract of ref. 1063
The Shackleton Limestone formed a carbonate platform that bordered part of the Greater Antarctic craton in middle and late Early Cambrian time. In the Holyoake Range of the central Transantarctic Mountains, this unit records deposition on a stable shelf on which flourished ecological reefs composed of microorganisms and archaeocyathans. Burrow-mottled lime mudstone, wackestone and packstone with patch reefs represent accumulation in shelf areas of relatively low to moderate energy. Thick ooidal grainstone units reflect deposition in higher energy shoals and as sand sheets that were associated with extensive reef complexes. The framework of these reefs was principally the product of micro-organisims, by inference mostly cyanobacteria. Archaeocyathans constitute as much as 30% of some reefs, but commonly they forms less than 10% and are absent from some. On the basis of microbial composition, three reef types and are recognized. The first type is a Renalcis boundstone that lacks archaeocyathans. Within these, abundant upward-directed thalli of Renalcis formed a framework that trapped fine-grained sediment. The second type, which forms the core of some larger reefs, is composed of stroamtoctis-bearing, microbial boundstone. The third, yet most common, reef type is variable in composition. It is characterized by the presence of abundant Epiphyton, but may include archaeocyathans, and the microbial microfossils Girvanella and Renalcis as well as cryptomicrobial clotted micrite. In this type of reef, frame-building organisms typically constructed highly porous structures that had small interparticle and fenestral pores and large growth-framework cavities, as well as rare metre-sized caverns. Within these spaces, Epiphyton and, less commonly Ranalcis, encrusted framework elements, fine-grained sediments accumulated, and pervasive sea-floor cements were precipitated. Boundstone fabrics in the Shackleton Limestone are highly complex, with fabrics analogous to younger, more metazoan-rich reefs, as well as deep-water stromatactis-bearing mud-mounds. The Epiphyton-Girvanella-arachaeocyathan frameworks and stromatactis-bearing boundstone, both of which seemingly first appeared in the middle Early Cambrian, are regarded as the precursors, in structure, composition, and preferred hydrologic setting, of the more extensive reefs and complex framework styles of later Phanerozoic time. 

