Chapter 11
Stromatolites: Yel lowstone
Analogues
Stromatolites are important types of geological structures, found primarily
in the Precambrian Era, but present to lesser extent throughout the rest
of
earth history up until the present. Stromatolites are important geologically
because they provide a useful fossil record of the Precambrian, and because
many mineralized deposits are found within or associated with them. A
number of definitions of "stromatolite" have been used, but a recent
definition (Walter, 1976) attributable originally to Stanley Awramik and
Lynn Margulis (Stromatolite Newsletter, February 1974, p. 5) follows:
"Stromatolites are megascopic organosedimentary structures produced
by
sediment trapping, binding, and/or precipitation as a result of growth and
metabolic activity of organisms, primarily blue-green algae."
  The paper by Barghoom and Tyler (1965) can well be said to have issued
in the modem era of paleomicrobiology. This seminal paper presented
evidence from thin sections that filamentous microorganisms, perhaps blue-
green algae, were well preserved in 2-billion-year-old rocks of the Gunflint
formation. Following this paper, there has been an explosion of papers
describing microfossils from a wide variety of Precambrian sedimentary
rocks, some as old as 3 million years. I do not propose to review all of
this
work here, most of which is not relevant to the Yellowstone project.
However, of considerable importance for my own thinking along these
lines was the statement in the original Barghoorn and Tyler paper: "The
black chert is characterized by discontinuous anastomosing pillars oriented
roughly perpendicular to the gross structure of the algal dome. . . . The
general appearance is that of a nest of thimbles, strikingly similar in
morphology to structures associated with certain modem algal growths
(Figure 2, Parts 1 and 2)." The figures referred to were thin sections
of
siliceous sinter from currently forming geyserite deposits in Yellowstone