POST-DEPOSITIONAL INFLUENCES ON STROMATOLITE MORPHOLOGY, PALEOPROTEROZOIC NASH FORMATION, MEDICINE BOW MOUNTAINS, WYOMING

Wyoming's Paleoproterozoic Nash Formation hosts some of the most spectacular Precambrian stromatolites in the world. This formation, composed of tan, silicified, stromatolitic metadolomite with interbedded phyllite and quartzite, is near the top of a sedimentary assemblage deposited along the passive, southern margin of the Wyoming Province, circa 2000 Ma. Although sporadic research has been conducted on Nash Fork stromatolites from 1926 (E. Blackwelder) to recent years, the seminal work is that of S.H. Knight (1968). Our current investigation seeks to determine to what extent the present morphology of Nash Fork stromatolites is a product of post-depositional processes. Although the Nash Fork contains many examples of classic stromatolite domes and hemispheroids, there is wide variability in size and shape from wavy, thinly laminated beds to giant oblate spheroids (>5m diameter). Many small domes include as nucleus a tabular fragment of underlying dolomite. Laminae commonly completely envelop the nucleus, transitioning upward to laminae laterally linked to adjacent domes. Such features suggest soft-sediment displacement during which a fragment of a partially lithified bed was incorporated into less rigid laminated material as the latter wrapped around it. This interpretation contrasts with the oncolite analogy in which frequent overturning of the nuclear object permitted microbial growth on all surfaces. We also question the traditional assumption by previous workers that height, slope and axis orientation of individual domes can be taken as representing biogenically constructed sea-floor relief. Many outcrops exhibit over-close packing of hemispheroids, continuity of laminae across interdome troughs, and lateral passage of hemispheroidal forms into distorted, asymmetric lamination. These features suggest that the original laminated deposit could have been a sea-floor microbial carpet with minor relief before it wrinkled and deformed during lateral movement by down-slope sliding or pre-lithification compaction. We conclude that interpretations of Nash Formation depositional environments should not include an unquestioned assumption that present stromatolite morphology represents original sea-floor relief and unaltered evidence of microbial mat development.