Rocky Mountain Geology 36.2
Middle Cambrian offshore microbialites and shoaling successions, western Wyoming: Implications for regional paleogeography
Larry T. Middleton
Middle and Upper Cambrian strata in western Wyoming and throughout the Rocky Mountain region comprise relatively thick successions of nearshore and shelf siliciclastics and carbonates deposited during numerous marine transgressions and regressions. The overall pattern is one of an offshore (westward) transition from coarse-grained facies of the Flathead Sandstone into fine-grained siliciclastic and carbonate strata of the Gros Ventre and Gallatin Groups. Within each formation there is clear evidence of upsection shallowing conditions. This was due to shoreline progradation and/or development of offshore peritidal carbonate buildups
The Middle Cambrian Death Canyon Limestone of the Gros Ventre Group contains both small-scale (typically less than a few meters thick) and larger scale (up to 10 m) cycles of shoaling-upward carbonates. The formation can be divided into lower and upper cliff-forming units. The lower cliff-forming unit is comprised of laminated dolomitic mudstones and packestones near the base that pass upward into fossiliferous and peloidal wackestones and packstones. Near the top of the lower unit intraclastic packstones occur. The overall trend reflects a transition from quiet water, subtidal settings to shallower subtidal and possibly intertidal conditions.
The upper cliff-forming part of the formation contains the most striking evidence of shallow, peritidal deposition. This includes an overall increase upsection in the percentage of allochems such as peloids, ooids, intraclasts, and oncoids. Near the contact with the overlying Park Shale, the Death Canyon Limestone contains large microbial buildups in the forms of thrombolites and stromatolites. These domical mounds are up to 1 m in height and 1.5 m in width at their base. Thrombolites exhibit a coarse, clotted fabric and laminae, if present, are indistinct. The stromatolites are well laminated with the outer surface displaying small, discrete knobs. Associated with these buildups are inter-mound packstones containing ooids, large intraclasts, and skeletal debris. Flat-pebble conglomerates also occur locally. These algal mounds and the associated facies are similar to those found in modern shallow subtidal areas and lower intertidal flats. A depositional model of peritidal sedimentation around offshore, low-relief islands and within tidal channels is proposed. Thus, the upper Death Canyon Limestone comprises shoaling-upward successions that developed far offshore of the Middle Cambrian strand-line.
Keywords: microbialites, shoaling successions, paleogeography, Cambrian, Wyoming
Geochemistry and tectonic setting of Paleoproterozoic metavolcanic rocks of the southern Front Range, lower Arkansas River Canyon and northern Wet Mountains, central Colorado
R. A. WOBUS, M. J. FOLLEY, K. M. WEARN, AND J. B. NOBLETT
Across a 5000-km2 area of central Colorado, previously unstudied Paleoproterozoic metabasalts (amphibolites) and metarhyolites (felsic gneisses) comprise a bimodal metavolcanic association within a dominantly metasedimentary terrain. Extending southward from about 39° N latitude in the Southern Front Range to about 38° 15' N latitude in the Wet Mountains, and from the mountain front westward to the Wet Mountain Valley and Pleasant Valley fault system, this area includes the exceptional exposures within the lower Arkansas River Canyon from Howard downstream to Canon City. Regional metamorphism from garnet to sillimanite grade, pervasive deformation, and intrusion by three generations of Proterozoic plutons have largely obscured original stratigraphic relationships and primary structures within these metamorphic basement rocks, although a few pyroclastic features persist locally within the felsic members. These metavolcanic rocks are compositionally similar to the much better preserved bimodal section in the Salida area, dated at 1728 ± 6 Ma by Bickford (1986), which emerges from beneath Paleozoic cover rocks about 15 km beyond the western edge of the area of this report.
Geochemical studies of 45 samples (30 amphibolites and 15 felsic
gneisses) delineate two groups of metavolcanic rocks ranging
in silica content from 45–55% in one group and from 65
to almost 80% in the other. Along a 100-km transect from north
to south, metavolcanic rocks of the lower-silica group
(amphibolites) show an increase in total alkalies (from 2% t
o 5–6%) and large ion lithophile trace elements as well as an
increase in degree of enrichment in light rare earth elements
(from LaN/LuN < 2 to LaN/LuN
5). Rocks with higher
silica content (felsic metavolcanic rocks) occur mostly in
the Arkansas Canyon area and contain 6–8% total alkalies;
they show strong fractionation between light and heavy rare
earth elements with moderate to pronounced negative europium
anomalies.
Tectonic discriminant diagrams using relatively immobile high-field-strength elements indicate volcanic arc settings for both mafic and felsic populations. Metavolcanic rocks from the northern Wet Mountains and Arkansas Canyon suggest a mature arc environment, possibly on an expanding continental margin. The isolated metabasalts to the north in the southern Front Range, where no felsic metavolcanic rocks have been identified, are more primitive island-arc tholeiites; they may represent pyroclastic rocks with a source beyond the study area.
These new data from a wide area of central Colorado reinforce results from the well-studied Paleoproterozoic bimodal are assemblages to the west near Salida and Gunnison. They also allow the extension across a wider geographic area of previous tectonic models for the Paleoproterozoic evolution of the Colorado province (as defined by Bickford et al., 1986). These models (Condie, 1986; Reed et al., 1987; Karlstrom et al., 1987) portray the rapid addition of juvenile crust to the southern margin of the Wyoming province by accretion of individual volcanic arcs or larger, previously amalgamated are terranes, resulting in the southward expansion of the craton by about 1300 km from 1800–1650 Ma.
Key Words: Paleoproterozoic • central Colorado • bimodal metavolcanic rocks • tectonic setting • volcanic arcs
Origin and emplacement of igneous rocks in the central Wasatch Mountains, Utah
T. A. VOGEL, F. W. CAMBRAY AND K. N. CONSTENIUM
The calc-alkaline igneous rocks in the central Wasatch Mountains
were emplaced between 36–30 Ma. They form a belt comprised
of eleven stocks and the Keetley volcanic field aligned along
the crustal suture between the Archean Wyoming province and
accreted Paleoproterozoic terranes. Magmatism associated with
this belt and its westward continuation into the Bingham mining
district has been related to mid-Cenozoic extension. These rocks
consist of two types of stocks based on texture: a western type,
which is coarse grained, and mostly equigranular, and an eastern
type (including the Keetley volcanic rocks), which is fine grained
and porphyritic. The compositional variation in the western
stocks (Little Cottonwood, Alta, and Clayton Peak stocks) forms
three distinct compositional groups. The compositional variation
in the eastern stocks is similar to the compositional variation
in the Alta stock. Major and trace element variations in these
rocks resemble those of subduction-related magmas. However,
the high K2O contents and low
sr
values are not consistent with this origin. These magmas
formed from melting of mafic igneous rocks. We propose that
magmas were generated by decompression melting due to
gravitational collapse of the crust that had been thickened
during Cretaceous to early Cenozoic deformation. Magmas rose
to varying levels in the crust along an east–west lineament.
The igneous rocks of the central Wasatch Mountains have
Nd(t)
similar to most of the Phanerozoic igneous rocks in the
miogeocline (MG), but have significantly lower
Sr(t).
That anomaly has been explained as due to melting of a basement
long depleted in Rb (Farmer and DePaolo, 1983, 1984). However,
the Wasatch igneous belt rocks are high-potassium, calc-alkaline
rocks and all have very similar incompatible trace element
patterns, whereas only a few MG rocks are calc-alkaline or
high potassium. Furthermore, in the MG rocks incompatible
trace element patterns are variable. One possible explanation
for the dilemma of long-time depletion of Rb in these
high-potassium, calc-alkaline rocks is that the crust may
have been recently charged with Rb and K during the Sevier-Laramide
event (100–40 Ma) by dehydration of the subducting slab. This
event was followed by melting during mid-Cenozoic collapse of
the orogen (ca. 40–20 Ma). The source of the magmas was
melting of mafic rocks in the lower crust. Some of these
magmas ponded, formed magma chambers, and differentiated.
Some involved little ponding and erupted directly on the
surface in the form of the Keetley volcanic field. Continued
melting and extension produced new magmas from a similar crustal
source. These magmas were emplaced below a series of pull-apart
structures associated with strike-slip displacement along an
east–west suture. This suture may have been controlled
by the Archean-Proterozoic boundary. Some magma bodies were
emplaced quickly to the surface without significant fractionation.
Others coalesced and fractionated over a protracted period of
time. These magma bodies interacted with crustal rocks, and
differentiated to relatively evolved compositions.
Key Words: Calc-alkaline magmas • magma evolution • magma generation • magma emplacement • magma ascent • crustal melts • pull-apart structures • extensional duplexes • strike-slip faults • Wasatch Mountains • Sevier orogenic belt