Constance M. Soja

Significance of Silurian stromatolite-sphinctozoan reefs

Upper Silurian (Ludlovian) limestones from the Heceta Formation of southeastern Alaska (Alexander terrane) contain skeletal stromatolite reefs and stromatolite mud mounds that were colonized by sphinctozoan sponges. Internal growth cavities, synsedimentary marine cements, and stromatolite debris flows in slope deposits indicate that these reefs grew with relief at the seaward margin of the carbonate platform. The biotas under study have evolutionary significance because they contain the earliest widespread reef-building sphinctozoans and represent a previously unrecognized stage in the evolution of Phanerozoic microbial-metazoan (predominantly poriferan) reefs. On the basis of the presence of Silurian stromatolite-sphinctozoan deposits in southeastern Alaska, southwestern and west-central Alaska, and the Urals, this study also establishes a Late Silurian paleobiogeographic connection between the Alexander terrane, cratonic northwestern North America, and the Uralian region.

Coeval development of Silurian stromatolite reefs in Alaska and the Ural Mountains: Implications for paleogeography of the Alexander terrane

New insights into the paleogeography of the Alexander terrane (Alaska) have been obtained from a comparative study of limestones in southeastern Alaska and the Ural Mountains. Upper Silurian stromatolites preserved in the Heceta Formation of Alaska are remarkably similar in composition, biofabric, and environmental setting to Upper Silurian (Ludlovian) subtidal stromatolites in the Ural Mountains, particularly those discovered near the Ilych River, Northern Urals. The stromatolites were built by an unusual consortium of microbial taxa in association with distinctive sphinctozoan sponges (aphrosalpingids) and share a high degree of similarity with Upper Silurian subtidal stromatolites in the Nixon Fork terrane (southwestern Alaska). That the stromatolites in these three regions are not identical in taxonomic composition but share in common conspecific, reef-dwelling aphrosalpingids and congeneric microbiotas indicates that they evolved separately but in a geographically contiguous area. These fossils establish that the Alexander terrane as an island arc resided at a site that enabled migratory exchange of biotas with northwestern North America or Siberia (Nixon Fork terrane) and northern Baltica (Ural Mountains) in the Late Silurian, a paleogeographic setting that is compatible with Northern Hemisphere options derived from paleomagnetic, isotopic, detrital zircon, and other paleontologic evidence in southeastern Alaska.

Early Devonian paleomagnetic data from the Lower Devonian Karheen Formation suggest Laurentia-Baltica connection for the Alexander terrane

Paleomagnetic data from 29 sites within the Lower Devonian Karheen Formation pass fold and reversal tests, and define two polarity zones that are correlative between two separate stratigraphic sections. We interpret this magnetization to be Early Devonian and thus propose a paleopole for the Alexander terrane that places the terrane at about 14° (north or south). Due to uncertainties of the Early Devonian parts of the North American and Gondwana apparent polar wander paths, several interpretations of the Early Devonian position of the Alexander terrane relative to either North America (Laurentia) or Gondwana are permissible. We use a combined data set of paleomagnetic, detrital zircon, and paleontological observations to narrow the paleogeographic options. Eastern Australia and the Scandinavian part of Baltica lie within the paleolatitude bands, and both are possible sources of Precambrian detrital zircon grains found in the Karheen Formation; however, paleontological data from the underlying Heceta Formation are only consistent with a position near northern Laurentia-Baltica. Therefore, the combined data sets indicate an Early Devonian position near Baltica.

Island-arc carbonates: characterization and recognition in the ancient geologic record

Abstract Carbonates of island-arc origin that are preserved in Paleozoic-Mesozoic terranes of the North American Cordillera exhibit a distinctive suite of paleontologic and lithologic features and share a fundamental similarity with limestones forming in modern volcanic arcs. This study provides the first detailed synthesis of carbonate depositional systems in island arcs and documents primary sedimentary constituents based on facies relationships and faunal communities. Models are developed that show patterns in the long-term evolution of shallow marine organisms and the construction, evolution, and demise of carbonate platforms in island arcs. A suite of criteria is identified that may be used to differentiate island-arc carbonates from limestones that accumulated in other platform settings. Biogeographic isolation, prolonged subsidence, steep submarine slopes and tectonic instability of volcanic edifices contribute to the development of relatively high levels of species endemism, impoverished normal marine faunas, complex provincial affinities, and relict biotas in limestones that are characterized by exceptionally thick platform and periplatform sequences, fringing and barrier reefs at the shelf margin, extensive lagoonal deposits and rapid lateral and vertical facies changes. Although destructive tectonic and geologic processes in island arcs may hinder determining the original size and extent of the carbonate platform, and particular facies types may not be represented (e.g., fringing and barrier reefs may be replaced by sand shoals at the platform, margin), many characteristics have potential value for identifying carbonates of island-arc origin in the ancient rock record. Apart from being associated with calc-alkaline volcanic and volcaniclastic assemblages, the most valuable suite of features for recognizing island-arc carbonates is marine biotas that exhibit elevated levels of endemism and mixed paleobiogeographic affinities, extraordinary thicknesses of platform (shallow marine) and periplatform carbonates, and rapid facies changes between volcanic and carbonate rocks and between shallow and deep water limestones. Recognizing that an ensemble of features is distinctive within island-arc carbonates considerably enhances identification of volcanic arcs in the ancient geologic record and thus improves the likelihood of successfully unraveling the complex geologic history of ocean basins.

Silurian calcareous algae, cyanobacteria, and microproblematica from the Alexander Terrane, Alaska

Limestones of Silurian age (late Llandovery–Ludlow) from the Heceta Formation of southeastern Alaska (Alexander terrane) yield a diverse assemblage of calcified microbial organisms. Fourteen taxa are described, including the cyanobacteria Girvanella and ? Hedstroemia , possible cyanobacteria Epiphyton, Ludlovia , and Renalcis , three species of “ Solenopora ,” and microproblematica Rothpletzella and Wetheredella. Diagnoses and descriptions are also given of the new epiphytacean genus Hecetaphyton n. gen. and new microproblematica genera Tuxekanella n. gen. and Sphaerina n. gen.

Silurian microbial associations from the Alexander terrane, Alaska

Silurian calcareous algae, cyanobacteria, and microproblematica are abundantly preserved in the Alexander terrane of southeastern Alaska. They represent a diverse population of calcified microbes that contributed to the formation of a variety of shallow- and deep-water carbonate deposits. Five associations are recognized on the basis of recurring groups of microbial taxa. These include a Girvanella-Tuxekanella association that formed oncoids and thick encrustations on skeletal grains in shelf environments. A Renalcis association predominated in a stromatoporoid-coral reef that developed at the incipient shelf margin on a crinoid-solenoporid shoal (“ Solenopora ” association). Other organic buildups are characterized by a Ludlovia association, which constructed skeletal stromatolite reefs, and by an Epiphyton-Sphaerina association that contributed to the formation of a stromatolitic mud mound. A mixed microbial assemblage reflects transport and mixing of shallow-water microbial biotas that were deposited by turbidity currents, debris flows, and slumps in a slope environment.

Development and Decline of a Silurian Stromatolite Reef Complex, Glacier Bay National Park, Alaska

Abstract In Glacier Bay, Alaska, Silurian limestones record the development and demise of a stromatolite reef complex in the Alexander terrane. These microbial deposits are of regional and paleontological significance because they contain paleogeographically distinctive biotas and yield important insights into Phanerozoic stromatolites that inhabited normal-marine subtidal environments. Willoughby limestones exposed on Drake Island reveal that stromatolite growth at the platform margin influenced platform dynamics with the protection of peritidal and lagoonal habitats behind a reef-fringed rim, which experienced early lithification by the precipitation of synsedimentary marine cements. Relatively low-energy subtidal conditions in a restricted, shallow-marine lagoon are implied by the peloidal and mollusk-rich wackestones, packstones, and grainstones. At the platform margin, stromatolite boundstones and cementstones capped a reef-like mound by forming a thick microbial-cement crust on a core of outer lagoonal sediments. Well laminated, fenestral mudstones and peloidal wackestones-grainstones have an abrupt lateral contact with the microbial boundstones, indicating their formation in peritidal flats adjacent to the stromatolite reefs. Foreslope breccias consist of clasts derived from shallow subtidal environments on the shelf and at its rim. Cyclic repetition of these deposits indicate that sea level fluctuations and tectonic instability in the Alexander terrane contributed to periodic shedding of shallow-marine detritus to the upper slope, followed by the rebuilding of stromatolite reefs at the platform margin. Culminating stages in the Klakas orogeny induced large-scale, catastrophic collapse of the carbonate platform. Silurian stromatolites characterized by similar habitats and microbial-sponge biotas in southwestern Alaska and the Ural Mountains, Russia, together with paleomagnetic data, detrital zircon evidence, and the timing of late-stage Caledonide events, firmly establish genetic and temporal links of the Alexander terrane with other areas along the Uralian Seaway.

Paleoecology of Sponge-?Hydroid Associations in Silurian Microbial Reefs

Abstract Microbial boundstones from Alaska and Russia yield new insights into the paleoecology of Silurian biotas that inhabited stromatolite reefs. These high-energy reefs were built along the Uralian Seaway in the Late Silurian by a diverse suite of microorganisms in association with accessory metazoans, predominantly sphinctozoan sponges. Within the stromatolite framework, three species of small, solitary, sphinctozoans (aphrosalpingids) encrusted a variety of hard substrates, mostly skeletal remains but also microbial laminae and cavity surfaces. Fossils encrusted by the sponges include the problematic hydroid Fistulella, possible stromatoporoids (recrystallized), crinoids, the possible cyanobacterium Ludlovia, corals, and unidentifiable shelly debris. In addition to the ubiquitous microbial laminae, the sponges, Fistulella, and ?stromatoporoids were less commonly encrusted by Ludlovia, Renalcis, or crinoids. Well-developed attachment surfaces, including enlarged holdfasts, allowed the sponges to achieve stability on the seafloor after larvae settled randomly on available hard surfaces. A greater incidence of sponge encrustations on Fistulella than on other organisms indicates that some of the sponges may have enjoyed a commensalistic relationship while attached as juveniles to a living substrate. The sponges’ orientation on Fistulella in the sediment suggests that the relationship between the two taxa may have become parasitic, whereby the weight of the sponges caused Fistulella to collapse into the muddy substrate. Recognition of the intimate growth relationships shared by Silurian sphinctozoans, Fistulella, and other organisms expands the fossil record of encrusting sponges, identifies a novel sponge-?hydroid association, and reveals organismal responses to competition for space in mid-Paleozoic microbial reefs.

Late Silurian reconstruction indicated by migration of reef biota between Alaska, Baltica (Urals), and Siberia (Salair)

Abstract During the Late Silurian, reefs with similar frame-building biotas formed along the margins of the Uralian Seaway in areas that include present-day southeastern Alaska, the Ural Mountains, and the Salair Ridge in southwestern Siberia. The Uralian reefs grew at a passive platform margin, Alaskan and Salairian reefs originated in island arcs. The unique feature of these Ludlovian reefs is the framework structures that were built by microbial communities associated with atypical Silurian reef-related metazoans: sphinctozoan sponges (aphrosalpingids) and a problematic hydroid, Fistulella. The unusual but highly similar reef biotas imply that the paleo-Uralian sea during the Ludlow was partially enclosed, narrow, and subequatorial. In the Paleo-Asian Ocean, the Salair island arc may have been welded to the Siberian continental margin after the Early Caledonian orogeny, but other evidence suggests Salair persisted as an arc system until the Carboniferous. The microbial-sponge-hydroid reefs in Salair suggest a marine connection to similar reef communities of the Urals and Alaska evolving along the margins of the Uralian Seaway during the Late Silurian.

Potential Contributions of Ancient Oceanic Islands to Evolutionary Theory

Before much was known about the geology of continental margins and the occurrence of oceanic islands in suspect terranes, tests of theories related to island biogeography were undertaken mostly by ecologists and in paleontological investigations of non-insular biota. A significant new source of paleontological data with which to test models for species equilibrium over evolutionary versus ecological time scales and other important problems in evolutionary theory exists within circum-oceanic belts where ancient oceanic islands are preserved as allochthonous terranes. Research on faunas from these areas simplifies attempts to apply the theory of island biogeography to the fossil record and to trace the history of island populations over evolutionary time scales. Paleontologic data from a Paleozoic island arc (Alexander terrane) show significant fluctuations in the taxonomic diversity of brachiopods and thus question one of the main assertions of the dynamic equilibrium model. These data also support the concept of survival of relict taxa in ancient island refugia.