Early Cambrian reefs, reef complexes, and associated lithofacies of the Shackleton Limestone, Transantarctic Mountains

Abstract

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‐organisms, by inference mostly cyanobacteria. Archaeocyathans constitute as much as 30% of some reefs, but commonly they form less than 10% and are absent from some. On the basis of microbial composition, three reef types are recognized. The first type is a Renalcis boundstone that lacks archaeocyathans. Within these, abundant upward‐directed thalii of Renalcis formed a framework that trapped fine‐grained sediment. The second type, which forms the core of some larger reefs, is composed of stromatactis‐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 Renalcis, 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‐archaeocyathan frameworks and stromatactis‐bearing boundstones, 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.

Correlation of Byrd and Selborne Groups, with implications for the Byrd Glacier discontinuity, central Transantarctic Mountains, Antarctica

“…Synthesis of trilobite collections from the Shackleton by Palmer & Rowell (1995) has shown that they range from basal Atabanian through Botomian, and questionally into the Toyonian. It is generally agreed that the Shackleton Limestone was deposited on a shallow-water carbonate shelf or platform with minor terrigenous components, emergent in part, and populated by archaeocyathan bioherms (Laird 1963;Laird et al 1971;Burgess & Lammerink 1979;Rees et al 1989;Myrow et al 2002;Stump et al 2002a). However, before this study, a stratigraphy for the Shackleton Limestone had not been recognised.…”

Treatise Online, no. 38: Part E, Revised, Volume 4, Chapter 18: General features of the Archaeocyatha

“…The bioconstr uctional capacity of archaeocyaths has been documented in many recent works, representing all regions and spanning their appearance in the Tommotian to their virtual demise in the Toyonian (ZhurAvlevA, 1960b(ZhurAvlevA, , p. 59, 1966(ZhurAvlevA, , 1972aJAmes & KobluK, 1978;JAmes & KlAPPA, 1983;gAnDin & Debrenne, 1984roWlAnD, 1984;roWlAnD & gAngloFF, 1988;Debrenne, gAnDin, & roWlAnD, 1989;JAmes, KobluK, & KlAPPA, 1989;rees, PrATT, & roWell, 1989;JAmes & grAvesTocK, 1990;Debrenne, gAnDin, & ZhurAvlev, 1991;KennArD, 1991;Kruse, 1991b;WooD, ZhurAvlev, & chimeD Tseren, 1993;Kruse, ZhurAvlev, & JAmes, 1995;riDing & ZhurAvlev, 1995;Kruse & others, 1996;ZhurAvlev, , 2001cWooD, 1999;coPPer, 2001;PrATT & others, 2001;roWlAnD & shAPiro, 2002;Kiessling, Flügel, & golonKA, 2003). The resultant bioconstructions (meterscale mounds) all had the same basic plan and can be categorized into component domains occupied by associations of lime mud, archaeocyaths, renalcid calcimicrobes, and/or cement.…”

“…Reefs were either dominantly renalcid calcimicrobial or (rarely) dominated by metazoans (ZAmArreño, 1977;gAnDin & Debrenne, 1984;selg, 1986;sTePAnovA, 1986;Debrenne, gAnDin, & roWlAnD, 1989;rees, PrATT, & roWell, 1989;Debrenne & grAvesTocK, 1990;JAmes & grAvesTocK, 1990;WooD, ZhurAvlev, & chimeD Tseren, 1993;moreno-eiris, 1994). Indeed, historically, Tommotian archaeocyaths initially invaded purely calcimicrobial reefs, which had first proliferated in the preceding basal Cambrian NemakitDaldynian stage.…”

Wave-Modified Turbidites: Combined-Flow Shoreline and Shelf Deposits, Cambrian, Antarctica

“…The overlying Byrd Group is of lowermost Paleozoic age and is generally thought to unconformably overlie the Goldie Formation (Laird et al 1971;Stump 1995). The carbonate-ramp deposits of the Lower Cambrian Shackleton Limestone (Rees et al 1989) is the oldest unit, and its age is constrained by trilobite and archeocyathan fauna (Debrenne and Kruse 1986;Palmer and Rowell 1995). A number of siliciclastic units overlie the Shack-FIG.…”