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Garden Island; the earliest (and first-illustrated) bryozoan reef in North America (basal Chazyan, early Middle Ordovician; Lake Champlain, New York-Vermont)



Garden Island; the earliest (and first-illustrated) bryozoan reef in North America (basal Chazyan, early Middle Ordovician; Lake Champlain, New York-Vermont)



Abstracts with Programs - Geological Society of America 34(1): 72



The oldest bryozoan reef in North America is in the sea-cliff at the southern end of tiny Garden Island in Lake Champlain, 1.4 mi (2.2 km) E7 degrees S from the dock at Valcour (Keeseville 7.5' quad.; visible in a 1776 British naval battle painting). This bryoherm lies at the base of the Scott Limestone Member low in the Day Point Formation. Our purpose is to utilize taxonomic updatings and new collections to quantify the species composition and contributions here. Only 10 bryozoan species were identified from 714 zoaria seen in 75 peel-sections made from the Garden Island bryozoan reef and its immediate surroundings. Batostoma chazyensis (incl. campensis), encrusting to massive to locally stubby-branching trepostome, overwhelmingly dominates the reef core, and thin-branching trepostome Champlainopora (Atactotoechus) chazyensis the reef flank. Each comprises 50-90% of its ecozone's rock volume and 60% and 90% (respectively) of its zone's zoaria. The former was the principal frame-builder in the core, now layered cruststone and bindstone with micritic matrix containing crinoid and brachiopod debris; the latter grew prolifically on the flank, their broken fallen branch fragments now forming the skeletal sediment preserved as rudstone with partly sparry and partly micritic matrix. A third species, ceramoporoid Ceramoporella (Cheiloporella) adamarhombica, is uncommon in the core, where its thin crusts locally constitute 10-15% of rock volume and zoarial count. Two species, bifoliate Chazydictya chazyensis and fenestrate Phylloporina reticulata, are rare throughout, comprising only 2-3% of the zoaria and no more than traces volumetrically. Five more are even rarer, 0.1-0.5% of the zoarial counts: small irregular trepostomes Nicholsonella pulchra and Jordanopora heroensis, tiny arthrostylid Helopora mucronata, and bifoliates Eopachydictya gregaria and Stictopora (Rhinidictya) fenestrata.

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