Ancient Reef Research
Connie Soja

Connie Soja

Professor of Geology
Geology, 343 Ho Science Center
p 315-228-7200

Research on Ancient Reefs in Alaska, Russia, Australia, and Mongolia


Portrait of Professor SojaOverview

My research focuses primarily on fossil-bearing rocks of Paleozoic age (~350-440 million years old) that are exposed in the Alexander terrane (AT) of Alaska; the Ural Mountains of western Russia and Salair region of Siberia; north Queensland, Australia; and in the Gobi Desert of Mongolia. Collaborative work with professional colleagues and Colgate students has focused on elucidating species evolution, diversification, and extinction processes in ancient reefs, particularly those that developed in geologically active areas. Our research has also helped develop paleontologic tools to determine where crustal pieces have traveled through space and time (terrane analysis). This knowledge will enable geologists to predict how our planet's physical and biological realms will be reshaped by similar processes in the future.

Research begins in the field by collecting data that will document the ancient ecology and geographic distribution of marine communities as well as the sedimentologic context in which these organisms lived, died, and were buried. This information allows us to examine specific questions related to patterns of reef community development and dispersal mechanisms (particularly in areas of active tectonism); speciation or extinction rates and processes; and paleobiogeographic origins assessed through comparative analysis. My research approach is "holistic" so that our analyses are as comprehensive (i.e., community based) as possible. This strategy fosters collaboration and is flexible enough to be accessible to undergraduate students with diverse interests.

Map showing distribution of suspect terranes, western NAA little background information may be helpful. In the 1980s, geologists proposed that many parts of Alaska and much of the west coast of North America were not originally part of the continent but had formed separately elsewhere before undergoing geologic adoption by the mainland. These "orphaned" crustal fragments are called suspect terranes to emphasize their exotic origins, as reflected in geologic deposits that are suspiciously unlike neighboring rock of North America's craton. Geologists now generally agree that plate tectonic motion rafted these terranes from their natal sites in ancient oceans to North America, where they became wedded to the growing continental margin. Geologically, western North America represents a mammoth jigsaw puzzle, configured of pieces of old seafloor crust, volcanic island chains, atolls, and microcontinents. The poor geologic match between rocks underlying the Alexander archipelago, where I do my research in Alaska, and those in adjacent western North America led to southeastern Alaska's designation as the Alexander terrane (AT), one of nearly a hundred crustal segments of unknown origin now identified in North America.

Reconstruction of chambered spongeMy paleontological evidence confirms that marine organisms once flourished in warm, subtropical seas surrounding the volcanic islands of the AT. The unclear affinities of many of these fossils, however, provide inconclusive information about which islands or continental margins were the source of some of the species that eventually colonized the AT.

Despite these challenges, our recent detailed examinations of the limestone bedrock in Alaska, the Ural Mountains and Salair, and Australia have yielded distinctive fossils that match the geology of southeastern Alaska with that of Russia (but not Australia, as proposed by another research team) for the Late Silurian. For the first time, the location of the Alexander terrane can be circumscribed to the Northern Hemisphere for a particular interval of time. Thus by placing the origin and distribution of Alaskan fossils in an evolutionary and tectonic context, this research has contributed new insights into the geologic history of the AT, portions of west-central Russia, and, to some extent, other areas.

Summary

Joint investigations with other geologists and Colgate students on rocks from three continents were crucial for solving one of the enigmas surrounding the geographic placement of southeastern Alaska in the Late Silurian-Early Devonian. In particular, the provincial (regional) affinities of the microbial-sponge biotas that we discovered in Alaska and Russia, together with geologic data (paleomagnetic, detrital zircon, and isotopic) compiled by other geologists in the AT, confirm that in the mid-Paleozoic, the AT was located in the Northern Hemisphere along the Uralian Seaway, which served as an important migratory route for marine organisms. Future research in Mongolia and elsewhere will help to reveal if closure of the Uralian Seaway was an underlying cause of the Late Silurian mass extinction.

Map of Alexander TerraneJoint research with Colgate students also yields new insights into the evolutionary paleoecology of a wide range of organisms, including brachiopods, nautiloids, trace-making organisms, sponges, hydroids, and a diverse suite of microorganisms that were reef inhabitants.

These studies emphasize the importance of integrating paleontological evidence with other geologic data to deconstruct terrane histories.

Furthermore, our investigations promote a "paradigm shift" in our understanding of how marine communities of the past functioned by acknowledging that: (1) post-Precambrian stromatolites (microbial mats and mounds) had a persistent presence in many reef (and level-bottom) habitats; (2) Phanerozoic microbial communities were not restricted to the intertidal zone nor did they function primarily as "disaster" taxa following times of global ecologic crisis; instead they were important ecologic and evolutionary entities in their own right; and (3) Silurian reefs in Alaska, Russia, and Mongolia contribute to a growing paleobiogeographic database for elucidating terrane placement and the conditions that were conducive (or detrimental) to Phanerozoic stromatolite growth and preservation.

Thanks to the following students for their contributions to this research:

Leah Kittredge '93; Katrina Gobetz '94; Nikki Bazie '94; Jen Thibeau '95; Erika Zavala '95; Colleen Brogenski '97; Lena Krutikov '97; Brian Flynn '98; Allison Gleason '98; Stacey Joyce '00; Lisa Mayhew '00; Jann Vendetti '01; Luke Dwyer '02; Megan Mitchell '02; Alicia Newton '02; and Christy Visaggi '02.

References

  • Coney, P.J., Jones, D.L., and Monger, J.W.H., 1980, Cordilleran suspect terranes: Nature, v. 288, p. 329-333.
  • Gehrels, G. and Saleeby, J., 1987a, Geologic framework, tectonic evolution, and displacement history of the Alexander terrane: Tectonics, v. 6, p. 151-173.
  • Rigby, J.K., Nitecki, M., Soja, C.M., and Blodgett, R.B., 1994, Silurian aphrosalpingid sphinctozoans from Alaska and Russia: Acta Palaeontologica Polonica, v. 39, p. 341-391.
  • Soja, C.M., and Antoshkina, A.I., 1997, Coeval development of Silurian stromatolite reefs in Alaska and the Ural Mountains: Implications for paleogeography of the Alexander terrane: Geology, v. 25, p. 539-542.