Distinctive Middle Cambrian sponge-calcimicrobe reefs in Iran

1 Pages·1999·0.24 MB·

by  P D Kruse

MEMOIRS OF THE QUEENSLAND MUSEUM
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DISTINCTIVE MIDDLE CAMBRIAN decimetre-thick limestone bed ofabundant eocrinoid
SPONGE-CALCIMICROBE REEFS IN IRAN. ossicles. Scattered, widely conical Rankenella are
Memoirs ofthe QueenslandMuseum 44: 298. 1999:- preserved upright in life position, suggesting
Following the virtual demise ofarchaeocyaths in the attachment to a hardground. Sponges, ossicles,
Toyonian stage and consequent collapse ofthe Early trilobites and hyoliths are encrusted by Girvanella,
Cambrianarchaeocyath-calcimicrobereefconsortium. whichalsoformsraftsandonkoids.Texturewithinthis
Middle and Late Cambrian reefs remained generally biostromal bed ranges from floatstone-rudstone to
devoid of metazoan input, being almost entirely Girvanellaboundstone,withevidenceofmicrobialand
microbial.Thefewexceptionsinthisintervalgenerally oxea-bearing sponge-body automicrites.The lower,
includesomeminorcontributionbyspiculatesponges. reef-bearing portion of overlying unit 3 is massive,
Onesuchspiculatesponge-calcimicrobereefsystemin comprising pale grey stacked bioherms of similar
the Middle Cambrian ofnorthern Iran is distinctive in textureandcompositiontotheunit2Rankenellabed.In
that the spiculate sponges constitute a major this interval, Rankenella adopts the entire range of
componentofthe reefframework. co-occurring cup shapes from narrowly conical
Thereefsare in units2 and3 oftheMilaFormation throughto explanate. Clotted-peloidal biohermal mud
in the eastern Elburz (Alborz) Mountains. The Mila is interpreted as automicrite. Substrate, peribiohermal
Formation consists of five units, together ranging in and overlying sediment is commonly a bioclast
age from Middle Cambrian to Early Ordovician. rudstonerichinorthidebrachiopodvalves.Spongesare
Trilobites permit correlation of unit 2 and the contributorstobioconstructioninareeftracttowardthe
reef-bearing lower unit 3 with the late Middle top oflower unit 3. Component bioherms ofthis reef
Cambrian, and upper unit 3 with the Chinese tract are constructed by ramose Rankenella encrusted
Kushanian(terminalMiddletoearliestLateCambrian) by thick coatings of Girvanella to form a
and Changshanian (Late Cambrian) stages. The reefs Rankenella-Girvanella framestone with only minor
are well exposed in a road section 3 km north of limemudpockets.Intersticesarerimmedbyonetotwo
Shahmirzad.Thereefsareconstructedbyaconsortium generations of columnar cement and occluded by
of the anthaspidellid sponge Rankenella and a coarse equant cement. By comparison with Early
presumed variety of microbes including the Cambrianreefs,RankenellaandGirvanellaplayedthe
calcimicrobe Girvanella. Rankenella is otherwise roles of archaeocyaths and calcimicrobes:
knownonlyfromtheOrdian-eaiiyTempletonianstage framework/substrate and encrusting/binding
of the Northern Territory, Australia.That stage is respectively. In many Early Cambrianreefs, however,
equivalent respectively to the late Toyonian-early lime mudrepresentsamuchgreatercomponent, while
Amgan and Longwangmiaoan-Maozhuangian stages calcimicrobes were capable of building massive
G
of Siberia and China.Unit 2 comprises fossiliferous frameworkunaidedby metazoans. Porifera, Middle
interbeds of grey limestone/dolostone and Cambrian, Iran, calcimicrobe, Rankenella,
yellow-brown marly shale, with desiccation cracks, Girvanella, reef, automicrite.
bidirectional ripples and probable tempestites and
hardgrounds. Peter D. Kruse (email: [email protected]),
The stratigraphically lowest known appearance of Northern Territory Geological Survey, PO Box 2901,
Rankenella is in upper unit 2, in a single DarwinNT0801, Australia: 1June 1998,