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often exhibits excellent porosity and permeability
and forms the major petroleum reservoir facies
throughout the Paradox Basin.
Grammer et al . (1996) described the algal
mounds as typically made up of individual plates
forming either a baffl estone fabric or packstone
fabric, and suggested that mound construction
may have been either by aggradational growth of
phylloid algae dominated mounds similar to mod-
ern Halimeda mounds (Hine et al ., 1988), and/or
as accumulations of individual plates by currents
and/or wave action.
From their combined subsurface and outcrop
study, Grammer et al . (2000) interpreted the algal
mounds as being deposited in moderate energy,
open-platform environments of varying depth,
but generally less than 20 m. The well-developed
algal mound geometries were thought to be the
result of an in situ accumulation of plates with
aggradational growth characterized by a baffl e-
stone fabric. Abundant fi brous and botryoidal
marine cements, common throughout the mounds
(Fig. 4c-e), were probably fast growing and synde-
positional based on modern examples (Grammer
et al ., 1993, 1999), and would have provided a
rigid framework for the mounds to continue their
aggradational growth (Roylance, 1990). These
mounds may have kept pace with sea-level rise
and grown into waters only 1-2 m deep, as indi-
cated by the presence of intertidal Chaetetes -
bearing facies or high-energy, well-washed shoals
that cap the mounds.
The biostromal algal-rich beds were interpreted
to have formed primarily through the hydrody-
namic accumulation of phylloid algal plates in
response to currents and wave action in water
depths of 5-10 m. These beds are characterized by
a wackestone to packstone fabric with more inter-
stitial mud and a lack of an interlocking fabric of
algal plates (i.e. baffl estone) characteristic of algal
mounds.
Algal mounds at 8-Foot Rapids
Well-developed mound-like algal buildups
are exposed over a distance of approximately
3.5 km near a location that is locally referred to as
8-Foot Rapids (see Fig. 3). As shown in Fig. 5, the
mounds exhibit a remarkable near-sinusoidal dis-
tribution in outcrop, with average mound heights
(a)
44 m
10.8 m
'Old Yeller'
54 m
SF
11.1 m
11.2 m
SC
AF
Algal mounds
Lower Ismay
IM
IF(D)
SF
5th
IF(D)
QSF
4th
Desert Creek
5th
NSC(O)
Ooid shoal
(b)
54 m
44 m
10.8 m
11.1 m
11.2 m
Algal mounds
Lower Ismay
Ooid shoal
Desert Creek
Fig. 5. (a) Photograph with line drawing showing distribution of algal mounds on the upstream, south side of the river at
8-Foot Rapids. (b) Amplitude of exposed algal mounds at this location ranges from 8 to 14 m with 'wavelengths' averaging
about 50-55 m.
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