Phylloid algal mounds in the Paradox Basin, southwestern USA: an alternative to the in situ constructional growth model? - Perspectives in Carbonate Geology - page 234

Phylloid algal mounds in the Paradox Basin, southwestern USA:
an alternative to the in situ constructional growth model?

G. MICHAEL GRAMMER and AUDREY L. RITTER
Michigan Geological Repository for Research and Education, Department of Geosciences, Western Michigan
University, 1903 W. Michigan Avenue, Kalamazoo, MI 49008, USA (E-mail: [e-mail])

ABSTRACT

Phylloid algae are recognized as being common reef builders during the late Palaeozoic
and, in many parts of the world, are known to form important hydrocarbon reservoirs.
Most genetic models suggest that phylloid mounds develop primarily through in situ ,
constructional processes. The dominant processes in these models revolve around the
baffl ing of sediment by the algae themselves, most often in dense meadows, combined
with the binding of sediment by various organisms and pervasive early submarine
cementation. Most models suggest that phylloid mound deposition occurred in widely
distributed belts along open carbonate platforms or ramps, at or slightly above wave
base in water depths of 15-20 m. In the Paradox Basin, most of the individual algal
mounds and mound complexes seem to fi t previous interpretations, not only for depth,
but also for deposition in a relatively shallow open platform or ramp setting.
There are, however, local accumulations of individual phylloid algal mounds
exposed in outcrop whose distribution and geometry suggest deposition in an envir-
onment more closely associated with strong currents. These algal mounds, which are
exposed along the San Juan River in southwestern Utah, have 10-12 m amplitudes
and are distributed in a near sinusoidal pattern suggesting that they may have formed
under the infl uence of wave- or tide-generated ocean currents. In modern environ-
ments, tide-generated currents may pile similar-sized plates of the modern algae
Halimeda into dunes or sand waves along the sea fl oor with a similar near-sinusoi-
dal distribution. Ascertaining the conditions under which these mounds initiated
and grew may provide an alternative explanation for the genesis of phylloid algal
mounds at least locally, and may have important implications for exploration concepts
devised for algal mound reservoirs. High energy, current-emplaced mounds, for
instance, may be aerially limited in the subsurface and may trend preferentially along
strike (e.g. wave currents) or, conversely, along dip (e.g. tidal currents). In contrast,
constructional mounds may have formed in much broader areas across much of an
entire shelf or platform.

Keywords Phylloid algae, algal mounds, Halimeda.

INTRODUCTION 1989) and Russia (Antoshkina, 1998). The term

phylloid algae was fi rst used by Pray & Wray
(1963) to describe a growth form of leaf-like algae
that includes both calcareous red algae and codi-
acean green algae. The genus Ivanovia , a codi-
acean green algae, is the most abundant type of
phylloid algae in the Paradox Basin (Pray & Wray,
1963). Ivanovia is generally thought to have grown
in warm, shallow marine environments, at depths
of < 20 m (Wray, 1968; Wilson, 1975; Toomey,
1980). Deposition of phylloid-dominated mounds
has been attributed to a variety of mechanisms,

Late Palaeozoic phylloid algal mounds have
been described from many localities in Laurasia,
including the USA (Konishi & Wray, 1961; Pray
& Wray, 1963; Heckel & Cocke, 1969; Toomey
et al ., 1977; Mazzullo & Cys, 1979; Toomey, 1980;
Dawson & Carozzi, 1986; Roylance, 1990; Baars,
1992; Kirkland et al ., 1993; Grammer et al ., 1996,
2000; Soreghan & Giles, 1999; Forsythe, 2003;
Samankassou & West, 2002, 2003), Canada (Reid,
1986; Beauchamp et al ., 1989; Breuniger et al .,