Professor Dave Lageson (Montana State University) - "Variability in stromatolite morphology, Paleoproterozoic Nash Formation, Medicine Bow Mountains, Wyoming: A case study in biogenicity"


Wednesday, March 13, 2019, 4:00pm to 5:30pm


Haller Hall, Geological Museum, 24 Oxford Street



The Paleoproterozoic Nash Formation in the Medicine Bow Mountains, SE Wyoming hosts some of the most spectacularly preserved Precambrian stromatolites in the world.  The Nash Formation is composed of tan, silicified, stromatolitic metadolomite with interbedded black phyllite and quartzite, occurring near the top of a sedimentary protolith assemblage that was deposited along the passive, southern margin of the Wyoming Province, ca. 2000 Ma.  Although sporadic research has been conducted on Nash Fork stromatolites since 1926 (E. Blackwelder) up to the modern day (Bekker and Eriksson, 2002), the classic, seminal work was that of S.H. Knight (1968).  Our current investigation seeks to understand variations in shape and apparent growth patterns of structures that have been traditionally interpreted as biogenically-formed stromatolites, but have been altered by a spectrum of syn- and post-depositional processes including tectonic deformation and greenschist facies metamorphism during the Medicine Bow orogeny (1.78-1.74 Ga; Chamberlain, 1998).  Although the Nash Fork contains many fine examples of “classic” stromatolite domes and hemispheroids, there is wide variability in size and shape from wavy, thinly laminated beds (interpreted to have been microbial mats) to giant oblate spheroids (>5 m diameter).  Many domes have a nucleus formed from a tabular fragment of underlying dolomite (or an exotic block of phyllite) with laminations that completely wrap-around the nucleus, transitioning upward to laminations that are laterally-linked with adjacent domes.  Almost all outcrops display, to varying degree, evidence of over-close packing of adjacent domes, dome flattening (oblate) parallel to stratification, and highly disrupted zones that laterally wedge into adjacent nondisrupted beds.  Many of these features suggest a passive margin with mixed siliciclastic and carbonate sediment with vertically extensive  accumulations of microbial growth structures (and in some cases truly massive stromatolites) that experienced pervasive soft-sediment deformation prior to lithification, and which may have been periodically swept by high-energy storm events or very vigorous tides.  In addition, expulsion of formation water during burial could also account for the chaotic, brecciated appearance of some beds. The great diversity in stromatolite (and pseudo-stromatolite) size and shape in the Nash Formation, coupled with mixed depositional environments and diagenetic/tectonic overprints, provides an ideal laboratory for the discrimination of biogenic and nonbiogenic signatures in rocks from earliest Earth.


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