This trip will cross the northern Sangre de Cristo Range, from Westcliffe to Crestone, Colorado, by way of the Hermit Pass Road and the Rito Alto pack trail (Fig. 1 below; road and trail shown on Fig. 2). The traverse is designed to give the geologist a sample of the structure and stratigraphy of this part of the range. Emphasis will be on the relationship between the horst of the Sangre de Cristo Range and adjacent down-dropped valleys, on the Laramide thrusted structure of the range, and on the stratigraphy and depositional environments of Pennsylvanian and Permian sedimentary rocks in the range.

The northern Sangre de Cristo Range is composed mostly of Early and Middle Proterozoic crystalline rocks and Paleozoic clastic sedimentary rocks (see geologic map, Fig. 2). Proterozoic rocks, mostly gneiss and quartz monzonite, are overlain on the west side of the range by about 100 m of early Paleozoic quartzite, dolomite, limestone, and shale. Early Paleozoic rocks are in turn unconformably overlain by Pennsylvanian and Permian clastic rocks. Southeast of the range, in Huerfano Park, Paleozoic rocks are overlain by Jurassic and Cretaceous rocks of the Raton basin.

The Middle Pennsylvanian Minturn Formation and the Pennsylvanian and Permian Sangre de Cristo Formation (see restored sections, Fig. 3) were deposited in the central Colorado trough between the Uncompahgre and Front Range highlands of the Ancestral Rocky Mountains (Fig. 4). Large alluvial fans shed detritus from the Uncompahgre highland northeasterly into the seaway of the central Colorado trough (Lindsey, Clark, and Soulliere, 1986). Alluvial fans and fan deltas prograded over the sea bottom, depositing 4,000 m of sandstone, conglomerate, siltstone, and shale.

During Late Cretaceous to Eocene time, rocks of the Sangre de Cristo Range were folded and thrusted during the Laramide orogeny (Lindsey and others, 1983) (see structure section, Fig. 5). The Laramide Uncompahgre highland was thrust against the basin fill of the central Colorado trough, forming a fold-thrust belt that extends most of the length of the Sangre de Cristo Range. From east to west, the northern part of the range contains 1) the Alvarado thrust plate of gneiss, representing the western edge of the Laramide Wet Mountain highland; 2) an autochthonous folded terrane of Minturn and Sangre de Cristo Formation; 3) a zone of thrusts in Paleozoic rocks, typified by the Spread Eagle Peak and Crestone thrusts along the route of the field trip; and 4) a large upper plate, consisting of a simple section of Crestone Conglomerate Member of the Sangre de Cristo Formation on Proterozoic rocks. The upper plate is located south of the area of the field trip and extends across the entire range east of the Great Sand Dunes National Monument. This plate represents the eastern edge of the Uncompahgre highland, thrust over the fill of the central Colorado trough.

From late Oligocene time to the present, the Sangre de Cristo Range was uplifted and the adjoining San Luis and Wet Mountain Valleys were down-dropped by Rio Grande rifting (Lindsey, Andriessen, and Wardlaw, 1986). Rifting accompanied and followed intrusion of stocks, sills, and dikes of felsic and mafic igneous rock. The horst of the northern Sangre de Cristo Range rose rapidly in early Miocene time. The Sangre de Cristo fault, which bounds the west side of the range, has been active in Holocene time. The Alvarado normal fault, on the east side of the range, has not been active since late Pleistocene time.

To follow the route of the field trip, consult the geologic maps of the Horn Peak (MF-1623) and of Rito Alto Peak and the northeast part of Mirage (MF-1787) quadrangles. Figure 6 shows the geology around stops 1 through 4; figure 19 shows the geology at stop 5; and figure 25 shows the geology at stop 6.

0 mi: Leave Westcliffe; go west on Main Street to the intersection with the mural of the Sangre de Cristo Range on the store front (southwest corner); turn left and go a few blocks to Hermit Road; turn west on Hermit Road and drive across the Wet Mountain Valley toward the Sangre de Cristo Range. From the point where it enters the foot of the range, the Hermit Road follows the valley of Middle Taylor Creek to Hermit Pass.

The Wet Mountain Valley is a graben formed during Rio Grande rifting from latest Oligocene to present time. The graben is bounded on the east by the Westcliffe fault and on the west by the Alvarado fault (concealed beneath glacial till and outwash). The valley fill is exposed in road cuts north and south of town but not along the Hermit Road. The fill has been assigned to the Pliocene and Miocene Santa Fe Formation (Scott and Taylor, 1975). At the south end of the valley, the Santa Fe Formation is beveled by a conspicuous pediment surface that extends into the foothills of the Sangre de Cristo Range.

6 mi: Cross cattle guard on Hermit Road; wooded hills ahead are glacial till of Pinedale (about 12,000 years) and Bull Lake age. Road cuts and canyon of Middle Taylor Creek are in Pinedale till.

The various tills (two of Bull Lake, one of Pinedale age) in the Sangre de Cristo Range are distinguished by thickness of weathered rinds on sandstone clasts, degree of weathering and smoothing of moraine surfaces, and extent to which large erratics are exposed above the surface of the moraine. Individual tills are also mapped from distinctive textures and outlines of moraines on aerial photos. Valleys on the east side of the range were occupied repeatedly by glaciers that extended out to the Wet Mountain Valley. Those on the west side were also glaciated, but in many valleys, the glaciers stopped before reaching the San Luis Valley.

7.2 mi: Enter the area of the geologic map of the Horn Peak quadrangle (MF-1623, by Lindsey, Scott, and others, 1984).

8.5 mi: Rest stop at campground. The campground (not shown on the quadrangle map) is located immediately above the beaver ponds along Hermit Road, about 2 mi into the northwest corner of the quadrangle from the point where the road enters the map.

Immediately before turning into the campground, we passed an outcrop of gneiss (Xgn on map) on the right. This is one of only two outcrops of gneiss known in the valley of Middle Taylor Creek. The next outcrop up the valley, located above the campground, is of red sandstone of the Permian and Pennsylvanian Sangre de Cristo Formation. The two outcrops help fix the approximate position of the concealed Alvarado fault, to be discussed at stop 2.

10.0 mi: Steeply dipping beds of red conglomerate, sandstone, and siltstone of the Sangre de Cristo Formation crop out along the road; beds face southwest. Fining-upward alluvial cycles characterize this part of the Sangre de Cristo Formation. Conglomerate and sandstone beds have channeled bases, trough crossbedding, and grade upward into dark red siltstone and shale. A few siltstone beds have nodular calcareous zones that probably represent paleosols.

10.2 mi: Road crosses the Spread Eagle Peak thrust fault (concealed here). The fault is composed of two to three strands; the eastern strand separates red sandstone and siltstone of the Sangre de Cristo Formation from gray sandstone and shale of the Middle Pennsylvanian Minturn Formation. West of the Spread Eagle Peak thrust fault, follow the geology along the Hermit Road on the geologic map of the Rito Alto Peak and northeastern Mirage quadrangles (MF-1787, Lindsey and others, 1985). We will be on this map for the remainder of the field trip.

Stops 1-4, first day, are located on Figure 6.

10.7 mi:--Looking southeast, one can see the SPREAD EAGLE PEAK THRUST (Fig. 7, below). The main strand of the thrust passes through the grassy saddle east of peak 12193 and thence down the ravine to Middle Taylor Creek. Steeply dipping redbeds of the Sangre de Cristo Formation crop out in the cliffs east of the thrust. West-dipping gray sandstone beds of the Minturn Formation comprise a faulted slice on peak 12193. A major strand of the thrust fault cuts the Minturn Formation west of the peak. West of the fault strand, the Minturn extends in an essentially unbroken section to the top of the range.

11.6 mi:--Overview of the ridge south of Middle Taylor Creek (Fig. 8, below) and the Wet Mountain Valley. Features to observe are:

1) Spread Eagle Peak thrust.

2) Wet Mountain Valley and Alvarado fault.

3) Section of Minturn Formation.

The SPREAD EAGLE PEAK THRUST is exposed on peak 12,193, on the ridge south of Middle Taylor Creek (see attached map). The thrust dips steeply west here (more than 80 degrees), but is interpreted to flatten at depth. Throughout much of its extent, the thrust displays a bifurcating pattern. The thrust extends north and south for a total distance of more than 20 km, outlining a large thrust plate in folded Minturn and Sangre de Cristo Formations (structure section, Fig. 5). The front of the thrust plate is composed of an anticlinorium of Minturn Formation. The rear part of the plate, located west of the crest of the range, consists of a large syncline (Gibson Peak syncline) of Sangre de Cristo Formation. Rocks of the Spread Eagle Peak plate override tightly folded red sandstones of the Sangre de Cristo Formation, exposed east of peak 12193. Zircons from a light-colored felsic dike in the thrust south of peak 12193 have been dated as 26.5 +/- 1.1 Ma by the fission-track method by C. W. Naeser. The dike is unsheared and is interpreted to have been emplaced during Rio Grande rifting that formed the Wet Mountain Valley graben.

The poorly exposed ALVARADO FAULT (located to the left, outside figure 8) lies along the west side of the Wet Mountain Valley (structure section, fig. 5). Rocks east of the Alvarado fault show net movement upward and over those to the west even though today's topography suggests the opposite. Where the fault is exposed in the range north of Middle Taylor Creek, Early Proterozoic gneiss is juxtaposed against red sandstone of the Sangre de Cristo Formation. In the footwall (west side) of the Alvarado fault, Sangre de Cristo sandstones are juxtaposed westward over one another. Mapping farther north shows that the Alvarado is a complex zone of faults that has downdropped the Wet Mountain Valley on the east during Miocene time. Thus, the history of the Alvarado fault is one of west-directed Laramide thrusting, which brought Precambrian rocks against late Paleozoic sedimentary rocks on the west, followed by late Tertiary normal faulting, which downdropped the Wet Mountain Valley on the east.

A SECTION OF MINTURN FORMATION (Lindsey, Clark, and Soulliere, 1985) was measured and described along the ridge from peak 12,193 to Eureka Mountain, at the crest of the range. At nearly 2,000 m thick, this section is the most complete section of Minturn in the northern Sangre de Cristo Range. The entire formation is organized into coarsening-upward prograding deltaic cycles of mostly gray and brown sandstone, siltstone and shale. Many cycles in the lower 1,500 m contain thin intervals of graded and massive sandstone beds interpreted as prodelta or delta-front turbidites; hence, this part of the section is called the "turbidite-bearing facies." The cycles are evident in alternating cliffs of thick sandstone and slopes of shale on the ridge south of Middle Taylor Creek. Cycles in the upper 500 m of Minturn contain thin fossiliferous marine limestone; this part of the section is called the "limestone-bearing facies"; these cycles can be viewed at stop 4 on Hermit Peak.

11.8 mi: Light-colored talus and outcrop along road are felsic sills and dikes of probable late Oligocene age. Felsic dikes and sills are common throughout much of the range.

12.0 mi:--At 12,000 ft on Hermit Road, overlooking Middle Fork of Taylor Creek and Hermit Lake (southeast). Consult figure 6 for local geology. Features to see include:

1) Prograding deltaic cycles in the Minturn Formation (ridge to south).

2) Turbidites in Minturn deltaic cycles (this outcrop).

PROGRADING DELTAIC CYCLES (Fig. 9) in the Minturn Formation are defined by coarsening-upward sequences. Grain size, bed thickness, and amplitude of crossbedding tend to increase upward in the prograding deltaic cycle. In the lower 1,500 m of Minturn Formation in Middle Taylor Creek, deltaic cycles generally consist of: 1) prodelta shale, siltstone, and sandstone; 2) prodelta or delta-front sandstones deposited by turbidity flows; 3) delta-front deposits of crossbedded conglomeratic sandstone; and 4) delta-plain and alluvial-plain channel sandstone and overbank siltstone and shale, arranged in fining-upward subcycles. The lower Minturn cycles are interpreted as deposits of a fan delta prograding onto a sea bottom below wave base. Diagnostic marine fossils in facies (1) are rare; channel-filling sandstones are common in delta-plain to alluvial facies (4) and fossil plants in place are found in some overbank deposits. In the upper 500 m of Minturn Formation in Middle Taylor Creek, deltaic cycles consist of: 1) prodelta and interdeltaic shallow-marine limestone, shale, siltstone, and sandstone; 2) delta-front conglomerate and conglomeratic sandstone, locally containing large foresets interpreted as Gilbert-type deltas formed in shallow water; 3) fining-upward subcycles of sandstone, siltstone, and shale deposited by braided or meandering streams. Some cycles appear to lack recognizable delta-front deposits. The upper Minturn cycles are interpreted as deposits of fan deltas prograding on a shallow shelf above wave base. Models showing depositional environments for the two types of deltaic cycles are summarized in Figure 10.

TURBIDITES are deposits of sediment density flows that are, in the geologic record, identified by the characteristic Bouma (1962) sequence of sedimentary structures (Fig. 11). Turbidites indicate a mechanism of deposition, not a depositional environment. Although geologists commonly associate them with deep-marine sediments, turbidites have been described from a wide range of depositional environments, ranging from glacial lake sediments to the prodelta sandstones of the Cretaceous epicontinental seaway of western North America. The Minturn turbidites of the Sangre de Cristo Range are prodelta or delta-front deposits. Most features of Minturn turbidites can be viewed in the excellent exposures at stop 3 (Fig. 12, below).

The turbidites of the Minturn Formation (Fig. 13) in the Sangre de Cristo Range are proximal in the classification of Walker (1967), averaging about 1 m in thickness and commonly containing granules and small pebbles (Soulliere and others, 1984). Most contain incomplete Bouma cycles of massive or graded intervals overlain by a thin silty or shaly parting; however, more complete Bouma cycles are common. Examine the outcrops at stop 3 for examples of these. Other features include flame structures, shale clasts floating in massive and sandy intervals, amalgamation of beds, rare flute casts, Zoophycus trace fossils (best found in float), and intervals of ripple cross-laminated sandstone. Turbidites in deltaic cycles indicate rapid deposition, perhaps by slumping or by rapid surges of flood water coming from an alluvial fan that fed the delta. Gradient and proximity of the onshore fan, position and shape of the shoreline, depth and slope of the sea bottom, and coastal wave energy are some of the factors that could affect deposition and preservation of prodelta turbidites.

The turbidites viewed at stop 3 form a lens 124 m thick that extends along strike (approximately along paleostrike, also) more than 13 km. The turbidite lens was deposited as a broad fan at the front of a fan delta as it prograded onto the sea floor of the central Colorado trough. Crossbedding from delta-front and deltaic-alluvial sandstones above the turbidites indicate transport toward the northeast, approximately the same as for all of Minturn-Sangre de Cristo time.

13.2 mi:--Hermit Pass and Hermit Peak (13,350 ft). Walk up the road to the pass. Refer to figure 6 for local geology. At Hermit Pass, features to see include:

The ANTICLINORIUM of the Spread Eagle Peak thrust plate is well-displayed on the ridge to the northeast of Hermit Pass, across the valley of North Taylor Creek (Fig. 14, below). The Cotton Lake anticline crosses the saddle; the Rito Alto syncline is barely visible west of Spread Eagle Peak; and the thrust crosses the ridge immediately east of the peak.

 

Figure 14.--Photograph and sketch of ridge from peak 13,524 to Spread Eagle Peak (SEP), showing anticlinorium of Minturn Formation and Spread Eagle Peak thrust. MT, main turbidite member. View north from Hermit Pass; field of view in sketch is slightly wider than photo. In sketch: PPsu, Sangre de Cristo Formation, undivided; Pmu, Minturn Formation, upper part; Pmt, Minturn Formation, turbidite member; Pml, Minturn Formation, lower part.

Large DELTAIC FORESETS (Fig. 15) are exposed on the east side of Rito Alto Peak. The foresets are essentially the same as the deltaic sedimentation units of Collinson (1968) and the lacustrine deltas described by Gilbert (1883). In the Minturn, deltaic foresets range from 1 to 5 m thick; they commonly overlie marine limestone beds or other prodelta deposits (Fig. 9B). Because the foresets formed in a body of standing water, their thickness is a measure of water depth.

The MINTURN-SANGRE DE CRISTO FORMATION CONTACT (Fig. 16, below) is well-exposed on Hermit and Rito Alto Peaks. Although the contact is commonly evident by the change from predominantly gray (Minturn) to red (Sangre de Cristo) color, thin marine limestone marker beds are more reliable. In this part of the range, a marker limestone and associated marine beds below the contact have been mapped for 13 km along strike.

After viewing these features, hike to the summit of Hermit Peak (Fig. 16, south of Hermit Pass). On the way up, pause to examine various beds showing evidence of alternating marine shelf and continental environments. On the summit, view key beds that were used to map contact between the Minturn and Sangre de Cristo Formations; these beds contain evidence for an abrupt change from marine to continental environments (section and photos, Fig. 17). Things to see on Hermit Peak:

Prograding deltaic cycles containing shallow marine limestone are exposed in the Minturn Formation on the east side of Hermit Peak. The typical limestone-bearing cycle consists of: 1) prodelta and interdeltaic shallow-marine limestone, shale, siltstone, and sandstone; 2) delta-front conglomerate and sandstone, locally containing large deltaic foresets deposited by distributaries discharging into shallow water; and 3) fining-upward subcycles of crossbedded sandstone, siltstone, and shale deposited by braided streams. Rocks of fining-upward alluvial cycles locally contain remains of plants such as Calamites in place. The cycles on Hermit Peak deviate from the idealized progression outline above.

FOSSILIFEROUS MARINE LIMESTONES in the Minturn Formation represent a variety of shallow-marine environments. Such limestone typically contains brachiopods, crinoid columnals, corals, and bryozoans; some contain oolites, fusulinids, and conodonts. Fusulinids and conodonts, although rare in the upper part of the Minturn and lower part of the Sangre de Cristo Formation, indicate a Middle Pennsylvanian age; fusulinids, identified by R. C. Douglass of the U. S. Geological Survey, are Desmoinesian.

The MARKER LIMESTONE (1-2 m thick) is a local stratigraphic marker bed characterized by dark radioactive siltstone and shale in the lower part and fossiliferous limestone in the upper part. The dark siltstone contains plant trash, pyrite, and anomalous traces of uranium, copper, lead, and zinc. The marker bed extends north and south from Hermit Peak a total distance of about 13 km. It is overlain everywhere by an interval of distinctive crossbedded sandstone and siltstone about 60 m thick. On Hermit Peak, the unit contains sandstone and siltstone interpreted by Clark and Walz (1985) as sand shoal, delta front, and braided stream deposits. South of Hermit Peak, the sandstone and siltstone unit contains deltaic foresets as well as other delta-front and braided-stream deposits. Lenses of dark siltstone in the upper part of the sandstone contain marine fossils, plant trash, pyrite, and anomalous traces of uranium, copper, lead, and zinc.

The MINTURN-SANGRE DE CRISTO CONTACT on Hermit Peak is defined by the first thick succession of fining-upward braided stream deposits above the marker limestone. At other localities, other limestone beds occupy the position of the marker limestone. Although the top of the Minturn is commonly marked by abundant marine limestone beds, no correlation of the beds with the marker limestone can be proven because they are separated by faults of large displacement. Some workers have attempted to define the contact by the appearance of redbeds, a definition that also works well at Hermit Peak. This definition is discouraged because the redbeds of the Sangre de Cristo Formation are of diagenetic origin and the red-gray contact probably crosses stratigraphic boundaries on a regional scale.

FINING-UPWARD CYCLES of crossbedded conglomeratic sandstone, siltstone, and shale characterize the lower member of the Sangre de Cristo Formation (Lindsey and Schaefer, 1984). Such cycles are comparable to those described by Miall (1978) for braided alluvial deposits. The basal sandstone of each cycle has a sharp, scoured base, small channels filled with conglomerate, and abundant trough crossbedding. Finer-grained rocks higher in the cycle contain ripple cross-lamination and, locally, mudcracks. Shale at the tops of the cycles is deep red and in places contains calcareous nodules interpreted as paleosols. Cyclic sequences of the lower member are interpreted as deposits of braided streams laid down on the lower reaches of alluvial fans.

South and southwest of Hermit Peak, a SECTION OF THE SANGRE DE CRISTO FORMATION (Lindsey and Schaefer, 1984) was measured from the summit of Eureka Mountain westward through Groundhog basin (Fig. 18, below). The section contains a lower member (red sandstone, siltstone, and shale, about 600 m thick) and the overlying Crestone Conglomerate Member, about 1,100 m thick (top eroded).

Day 2: Hermit Pass to Crestone Campground 

Drive from Westcliffe to Hermit Pass via Hermit Road; the drive is over 13 mi of rough road and takes about 1.5 hrs. The remainder of the day will be spent on foot on and off the Rito Alto trail. The hike to stop 5 takes about 4 hrs; the hike from stop 5 to stop 6 east of Crestone campground takes another 3 hrs. Total hiking distance is about 9 miles. The end of the second day is at the trailhead parking lot east of the Crestone campground. Our route will be on the geologic map of the Rito Alto Peak and northeast part of Mirage quadrangles all day (MF-1787, by Lindsey and others).

0 mi: Begin hike at Hermit Pass. The hike to stop 5 will pass through redbeds of the lower member of the Sangre de Cristo Formation. To reach the Rito Alto trail, descend the mine road and pack trail leading west from Hermit Pass about 1.7 mi to the trail junction.

1 mi: Mercury mine site (concealed by talus) at end of road; trail leading southwest begins. The mine was a prospect for uranium in the 1950's. About 100 m before the end of the mine road, good examples of fining-upward alluvial cycles occur in the Sangre de Cristo Formation beside a section of steel pipe. Red siltstone in one cycle contains calcareous nodules representing possible paleosols.

1.7 mi: Junction with Rito Alto trail; follow the south (left) fork of the trail.

2 mi: Trail crosses ridge between Rito Alto and San Isabel Creeks. A 1-m-thick red debris-flow conglomerate crosses the trail. The base of the Crestone Conglomerate Member crosses the ridge about 200 m west of the trail.

2.3 mi: Trail to San Isabel Lake. Take the upper trail that follows the contour around the cirque at the head of San Isabel Creek.

2.5 mi: On the north wall of San Isabel canyon, coarse streamflow conglomerates of the lower member of the Sangre de Cristo Formation crop out along the trail.

3.5 mi: Trail crosses ridge southwest of Eureka Mountain; leave the trail and climb peak 12,569 to the west.

3.6 mi: Peak 12,569. Groundhog basin is to the west. The cirque walls of both halves of the basin are composed of Crestone Conglomerate Member of the Sangre de Cristo Formation. A section of Sangre de Cristo Formation (Lindsey and Schaefer, 1984) was measured from the summit of Eureka Mountain, southwest across point where you left the Rito Alto trail, offset to the mouth of Groundhog Basin, and thence up the ridge south of the basin. Descend to the north cirque of Groundhog Basin by following the ridge northwest from peak 12,549. You will cross the base of the Crestone Conglomerate Member of the Sangre de Cristo Formation. Proceed west to conglomerate outcrops (stop 5) at about 12,000 ft.

4.0 mi:--North cirque of Groundhog Basin. Consult Figure19 for local geology. Things to see (Sangre de Cristo Formation photographs, Fig. 20) are:

LOW-ANGLE CROSSBEDDING (less than 5 degrees) and horizontal lamination are common in sandstones near the basal contact and within the Crestone Conglomerate Member. These structures are closely associated with ripple cross lamination. The crossbedding is commonly defined by concentrations of heavy minerals. Such low-angle crossbedding has been observed in braided alluvial deposits (Miall, 1978), but has not been described much in the literature. Sandstones with low-angle crossbedding and horizontal lamination are interpreted here as sheetflow deposits.

DEBRIS FLOW DEPOSITS are characteristic and abundant in the Crestone Conglomerate Member. They range in thickness from a few cm to tens of meters, and in grain size from sand to boulders several meters across. Thin (about 1 m) beds of matrix-rich debris flow conglomerate (diamictite), interbedded with thicker intervals of streamflow conglomerate and sandstone, can be observed along the north wall of the cirque. Thick (15-20 m) debris flow conglomerates strike across the cirque 200 m west of the lake.

Debris-flow conglomerates in the Crestone Conglomerate Member are classified as clast-rich or clast-poor (mudflows). Both types are massive but may contain intervals of current-reworked sand and gravel; such compound units were formed by more than one flow. Commonly, the lowermost parts of clast-rich flows exhibit reverse grading that may represent a sieve effect whereby small particles tend to settle into interstices between larger ones during emplacement of the flow. Also, the tops of flows may contain large boulders that were buoyed up by the dense matrix. The mechanisms of flow movement and emplacement in the Crestone Conglomerate are probably diverse and complex.

The Crestone Conglomerate Member is interpreted as the deposit of torrential streams and debris flows on the proximal reaches of alluvial fans, upstream from braided alluvial plains of the lower fan (represented by sandstone cycles of the lower member). Fan deltas represented by the Minturn Formation lay to the northeast in an inland sea of the central Colorado trough.

DISTINCTIVE CLASTS in conglomerates include red syenite and pink quartz monzonite. The source of the red syenite clasts is unknown, but their distribution defines a northerly facies of the Crestone Conglomerate Member. Clasts of pink quartz monzonite containing large orthoclase phenocrysts were eroded from the Early Proterozoic rocks correlated with the Boulder Creek Granodiorite. Rocks correlated with the Boulder Creek are exposed in the Mirage quadrangle (MF-1787) at the mouth of Rito Alto Creek, as well as farther south in the range. In the absence of red syenite clasts, the clasts of pink quartz monzonite define a facies of Crestone Conglomerate south of Groundhog Basin. Farther south, at Medano Pass, a third conglomerate facies is defined by the predominance of gneiss clasts (derived from unit Xgn, Early Proterozoic gneiss) and the virtual absence of pink quartz monzonite clasts. Together with northeasterly paleocurrents determined by crossbedding, the clast facies define the easterly dispersion of rock types from the source area of the Uncompahgre highland into the central Colorado trough.

From stop 5 walk to the mouth of Groundhog Basin; keep to the east side of the basin to avoid brush and marshy ground; cross and recross the small stream that flows from the basin. At the basin mouth, you can examine good outcrops showing sedimentary features of the lower member of the Sangre de Cristo Formation (Fig. 21). Sedimentary features of the Crestone Conglomerate Member (Fig. 21) can be observed on the ridge south of the basin. All of these features are located along the principal reference section (Fig. 19) (Lindsey and Schaefer, 1984). OPTION: if you wish to examine the Crestone Conglomerate Member of the Sangre de Cristo Formation again, ascend the south ridge of Groundhog basin (this option will add several hours to the trip).

To return to the Rito Alto trail from the mouth of Groundhog basin, proceed east.

5.2 mi: Rejoin the Rito Alto trail at a point 1.2 mi southeast of the point where you left it. Walk down the trail, which follows North Fork Crestone Creek.

5.5 mi: Trail crosses contact between lower member and Crestone Conglomerate Member. Trail ahead passes through Crestone Conglomerate in axis of Gibson Peak syncline.

6.0 mi: Axial trace of Gibson Peak syncline (Fig. 22, below). Axis is best observed to northwest. To southeast, note steep west limb of syncline.

6.2 mi: Pinedale till.

6.6 mi: Trail junction; follow west fork down North Crestone Creek.

6.8 mi: Outcrops of Crestone Conglomerate Member.

7.2 mi: Trail descends to North Crestone Creek; cross approximate location of contact between Crestone Conglomerate and lower member.

7.8 mi: Cross from lower member, Sangre de Cristo Formation, into Minturn Formation.

8.0 mi: Bed of algal limestone in Minturn.

8.3 -8.5 mi:--Trail ends; jeep road begins. Stop at the anticline along the trail (Fig. 23, below); note strong cleavage in redbeds of Minturn Formation. The Crestone thrust is exposed on valley wall to the northwest (Fig. 24, below). The thrust brings Precambrian and lower Paleozoic rocks over an overturned section of Minturn Formation. Consult Fig. 25 for a map of the thrust and local geology at stop 5 and Fig. 26 for a structural cross section through the thrust.

Features to examine here include:

CLEAVAGE AND DRAG FOLDS characterize the footwall rocks of the Crestone thrust. The anticline exposed along the trail extends 2 km north and south within a block bounded by strands of the thrust (Fig. 25); south of North Crestone Creek, the anticline is overturned to the east. Cleavage in footwall rocks is evident as much as 1 km east of the main thrust. Except in drag folds, cleavage typically dips 35-40 degrees west and appears to be oriented subparallel to the thrust, probably intersecting the thrust at a low angle. The cleavage is slaty in siltstone and shale of the Minturn Formation, where oriented metamorphic muscovite and chlorite is visible under the microscope. The Minturn Formation at stop 6 is composed of quartzose redbeds, a local facies referred to the Pennsylvanian Kerber and Sharpsdale Formations by previous workers. In brittle rocks such as the Ordovician Harding Sandstone, fracture cleavage is abundant in both the footwall and hanging wall of the Crestone thrust.

The VARIABLE DIP of the main thrust is evident from the mapped relations. Near the top of the ridge north of North Crestone Creek, the thrust dips about 35 degrees west; there, all lower Paleozoic formations have been removed by thrusting. As the thrust descends into the valley of North Crestone Creek, it steepens to 70-80 degrees west and remains at that attitude as it crosses the ridge south of the creek. Small felsic dikes (evident from light colored float) occupy the thrust at the top of both ridges and on the north side of the valley above a prospect that is visible from the trail. Strands of the thrust cut footwall rocks and isolate drag folds, completing the picture of a complex thrust zone composed of numerous faulted slices deformed by cleavage and folding.

Small FELSIC DIKES (at 8.4 mi) are common in the thrust fault; the dikes are unsheared but contain metamorphic muscovite and chlorite. Similar dikes elsewhere in the range have been dated as late Oligocene by the fission-track method on zircon (see discussion of Spread Eagle Peak thrust at stop 1). The dikes are interpreted as having been emplaced during opening of the thrust fault by Rio Grande rifting. Mafic dikes of lamprophyre, such as those north of Rito Alto Creek, were probably intruded into open fractures at the same time.

CHLORITOID porphyroblasts (Karig, 1964) occur in siltstone of the Minturn Formation north of the jeep road (at 8.4 mi). Chloritoid is significant because it indicates greenschist metamorphism (about 300 degrees C or above). Color alteration index of conodonts, determined by B. R. Wardlaw (USGS) from the ridge north of North Crestone Creek, yielded a comparable temperature of metamorphism (Lindsey, Andreissen, and Wardlaw, 1986). Chloritoid crystals have grown across the slaty cleavage of muscovite and chlorite, indicating that metamorphism continued after thrusting. Metamorphic muscovite and chlorite in felsic dikes in the thrust suggest continued or recurrent heating as late as late Oligocene time.

A VERTICAL DEPOSITIONAL CONTACT (at 8.5 mi) between Ordovician Harding Sandstone (quartzite here) and Middle Proterozoic quartz monzonite (correlated with the Silver Plume Quartz Monzonite of central Colorado) is exposed in hanging-wall rocks north of the trail. This contact is interpreted as the east flank of a large anticline in the frontal part of the Crestone thrust plate. Evidently, the Crestone thrust began to form as a large recumbent fold. The thrust fault developed by attenuation of the overturned limb of the fold. Further compression thrust the upper limb of quartz monzonite and lower Paleozoic rocks over the overturned limb of Minturn Formation. The main zone of thrusting developed in shaly beds in Mississippian to Devonian Chaffee Formation and the lower part of the Minturn Formation. This level of detachment has been observed in many parts of the northern Sangre de Cristo Range.

8.4 mi: Float of felsite dike; cleavage and chloritoid in phyllite outcrop at road level, north side of road. Bedding is steeply overturned; cleavage dips about 35 degrees west.

8.5 mi: Vertical depositional contact between Harding Sandstone and Middle Proterozoic quartz monzonite, north side of jeep road.

8.8 mi: Trailhead parking lot, Crestone campground. The trail and road below the parking lot pass through Middle Proterozoic quartz monzonite correlated with the Silver Plume Quartz Monzonite in the Colorado Front Range.

9.5 mi: Leave mouth of North Crestone Creek and enter the San Luis Valley. The Sangre de Cristo fault cuts alluvial fan deposits of Pinedale and Bull Lake age northwest of the canyon mouth (see map, fig. 12). The scarp is marked by a north-trending line that separates trees at two levels. The scarp disappears southward where it is projected through the village of Crestone.

The Sangre de Cristo fault (Fig. 27) is a major normal fault that downdrops the alluvial fill of the San Luis Valley against the Precambrian and Paleozoic rocks of the Sangre de Cristo Range. The fault has imposed a steep face to the west side of the Sangre de Cristo Range. Numerous scarps offset Holocene alluvium and alluvial fan deposits of Pleistocene Pinedale and Bull Lake age along the west side of the range. The range itself forms a large horst that stands between the Rio Grande rift grabens of the San Luis and Wet Mountain Valleys.

10.7 mi: Enter village of Crestone. Drive through the village of Crestone and west to Colorado route 17.

22 mi: Junction with Colorado route 17; turn north to Salida; pass the Moffat post office and store.

34 mi: Junction of Colorado route 17 and U. S. route 285; tufa mound of Mineral Hot Springs lies east of highway. Gravel road east of the highway goes to Valley View Hot Springs. The hot springs are low-temperature (less than 100 degrees C) geothermal waters that ascend the range-bounding Sangre de Cristo fault. A scarp of the fault cuts a fan of Pinedale age south of the hot springs; the scarp is plainly visible by looking due east from the highway.

39 mi: Village of Villa Grove. Scarps of Villa Grove fault zone are visible as linear shadows and dark vegetation in valley to east. The scarps were described first by Scott (1970) and later in more detail by McCalpin (1982). Scarps of the Sangre de Cristo fault are visible at the foot of the range.

54 mi: Poncha Pass.

59 mi: Village of Poncha Springs; route 50 east to Salida; route 285 north to Denver.

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