Geology Reference
In-Depth Information
the various buildups into three end-members.
In the Paradox Basin, the 8-Foot Rapids bioherms
fi t most closely into their Type 1 and Type 3
models. Type 1 mounds are described as 'con-
structed of cup-shaped algal thalli' and consist
of low-diversity buildups dominated by phyl-
loid green algae. The authors indicated that the
mounds formed by the growth of phylloid algae
that were 'closely packed and juxtaposed next to
and above one another', forming a constructional
framework characterized by 'intercup voids (that)
contain various fi llings, such as irregular grains,
uniform peloidal sediment, early marine cements,
and homogeneous mud'. The internal fabric, pelo-
idal sediment/cement, abundance of syndeposi-
tional marine cements, and low biotic diversity of
Type 1 mounds are consistent with the observa-
tions from the Paradox Basin mounds described
by Grammer
et al
. (1996, 2000).
Type 3 mounds were described by Samankassou
& West (2003) as mounds that formed from the
mechanical accumulation of algal plates in a
muddy environment. These mounds have some-
what higher biotic diversity, abundant broken
thalli, and variable internal void spaces mostly
fi lled with cements, and are also often capped by
shallow-water facies. Samankassou & West (2003)
proposed that these types of mounds probably
formed primarily through transport of algal frag-
ments by bottom feeders and scavengers, although
they suggested that metre-scale megaripples might
also be formed by 'currents' (Hamblin, 1969). The
possibility of large dunes or sand waves forming
'mounds' is especially intriguing for the phyl-
loid algal mounds observed in outcrop at 8-Foot
Rapids.
found in the subsurface typically develop on a
regional thick unit of underlying 'shale' (Peterson
& Hite, 1969; Wilson, 1975). Third, there is a gen-
eral thinning of regional evaporites over the crests
of the mound fi elds, also observed in the subsur-
face. Fourth, there is a strong component of lateral
variability within mound fi elds as indicated from
fi eld development histories (Petroleum Frontiers,
1984; Herrod & Gardner, 1988).
These trends indicate that there was some
kind of depositional or structural control on the
regional alignment of many of the mound fi elds in
the subsurface of the Paradox Basin. In addition,
the pattern of individual mound fi elds developing
on local areas of anomalously thick shales, also
aligned in a strike-parallel direction, suggests a
concomitant control mechanism. It is also clear
that, by the end of algal mound growth, there were
areas of positive topographic relief as documented
by the thinning of overlying evaporites, suggest-
ing that the mounds accreted vertically.
Previous studies by Grammer
et al
. (1996, 2000)
concluded, as did those of many previous work-
ers, that the major control on the regional distribu-
tion of mound fi elds was the change in relative sea
level through time and that the primary mechanism
for relative sea-level change across the basin was
most probably glacio-eustatic in origin (Crowell,
1978; Goldhammer
et al
., 1991). They suggested,
however, that the general distribution of individ-
ual mound fi elds may be the result of a combina-
tion of both structural and depositional controls.
Baars & Stevenson (1982) and Stevenson & Baars
(1986, 1988) argued persuasively for a series of
deep-seated NW-SE trending faults that remained
active through Pennsylvanian time in this region.
The strong correlation between algal mound fi eld
orientation and these regional faults is strong cir-
cumstantial evidence that the two are interrelated.
One possibility is that the algal mound fi elds
developed on a series of slightly down-thrown
blocks associated with these regional faults. This
would explain both the regional trend of the
mound fi elds as well as the underlying expanded
'shale' successions. Differential and increased
subsidence in the area of mound development
would create increased accommodation space for
the mounds to grow vertically and would explain
the propensity for the mounds to stack in some
fi elds. Another possibility is that the 'shale' thicks
formed in response to current and/or wave action
that resulted in strike-parallel mud banks, simi-
lar to those found today off the coast of South
America (Rine & Ginsburg, 1985).
Paradox Basin mounds - overview
Although the processes and controls of algal
mound fi eld development are still poorly under-
stood and widely debated in the Paradox Basin,
Grammer
et al
. (1996, 2000) identifi ed a number
of generally recognized and regionally pervasive
trends that encompass most of the algal mound
accumulations studied in the basin. First, most
of the individual mound fi elds are oriented with
their long dimensions in a strike-parallel (i.e.
NW-SE) direction, and with the exception of
the giant Aneth Field (Weber
et al
., 1995), a
majority of the fi elds are 1.5-3 km wide in dip
direction by 1.5-11 km long in strike direction,
and consist of a number of coalesced mound units
(Fig. 3). Second, the individual mound fi elds
