SILURIAN STROMATOLITE REEFS AS INDICATORS OF ENVIRONMENTAL CHANGE ALONG THE URALIAN SEAWAY

Strikingly similar reefs dominated by microbial-sponge communities flourished in the Late Silurian along the Uralian Seaway. They occur in outcrop in Alaska's Alexander terrane (AT) and in Russia (Pay-Khoy, Ural Mountains, and Salair Ridge of sw Siberia). Silurian reefal limestones from the AT originated as an offshore barrier in an island arc during waning volcanism. Pay-Khoy and Uralian reefs were rigid organic structures that grew at a passive platform margin on eastern Baltica. The Salair Ridge is an accreted fragment of mosaic structure within the Altaj-Sayan foldbelt. After early phases in the Caledonian Orogeny, the western part of the Altaj-Sayan was a shelf marginal to the Siberian craton.

The carbonate platforms in each region were rimmed by skeletal stromatolite reefs, which were constructed by a distinct consortium of microbial taxa (such as Girvanella, Renalcis, Wetheredella, Rothpletzella, Ludlovia, Sphaerina, Hecetaphyton, and “Solenopora”), sphinctozoan sponges (Aphrosalpinx, Nematosalpinx, and Palaeoschada) and possible hydroids (Fistulella) in association with depauperate normal-marine Silurian metazoans (rugose corals, bryozoans, stromatoporoids, brachiopods, crinoids, etc.). By occurring only in Alaska, Pay-Khoy, the Urals, and Salair, these distinctive microbial-sponge deposits confirm that the Uralian Seaway was an important marine corridor allowing transmigration of biotas between northern Laurentia, eastern Baltica, and Siberia in the Late Silurian. Interbedded units imply that the Uralian Seaway was a partially enclosed, narrow, subequatorial sea affected by fluctuating environmental conditions perhaps associated with late-stage Caledonide activity. Silurian reefs in Salair (and Farewell terrane, sw Alaska) are incompletely known but support the Uralian Seaway model, which circumscribes the location of the AT close to Baltica and to the northern margin of Laurentia and/or Siberia in the Late Silurian.