Cucharas Pass and
Trinchera Peak

James S. Aber
Emporia State University

Table of Contents
Cucharas Pass Raton Basin
Culebra Range Trinchera Peak

Cucharas Pass

Cucharas Pass, at 9941 feet (3030 m), represents the divide between the Cuchara and Purgatoire basins, which drain respectively the northern and southern sides of Spanish Peaks. The pass is the less-eroded "connection" between Spanish Peaks and the Culebra Range to the west. Within the vicinity of the pass, a nearly complete sequence of strata is preserved and exposed, from upper Paleozoic through Tertiary.

Colorado highway 12 follows the Cuchara River valley from the north and crosses the pass into the Purgatoire drainage basin to the south. Designated as the Highway of Legends, this route is said to involve hidden cities and buried gold. Cuchara is the Spanish word for spoon. According to legend, the valley was formed when a giant laid down his spoon in a rain, which left a lasting impression. Culebra is the Spanish word for snake. Regardless of the legendary aspects, the highway does provide excellent access to higher terrain in vicinity of Spanish Peaks and the Culebra Range.

East and north of Cucharas Pass
Raton Basin

The region east of Cucharas Pass is part of the Raton Basin. Eastward from the pass, upper Cretaceous and lower Tertiary strata are exposed along the county road to Cordova Pass. Beginning at Cucharas Pass, upper Cretaceous strata are present, and progressively younger strata are encountered along the county road to the east. These strata are described below in ascending order (from oldest to youngest).

Upper Cretaceous (lower portion, undivided) -- In ascending order, Graneros Shale, Greenhorn Limestone, Carlile Shale, Niobrara Formation, and Pierre Shale. Interbedded shale, chalk, and limestone beds. Total interval is 1100 m thick, of which the Pierre Shale is 700 m (Lindsey 1996). These sedimentary strata dip steeply toward the east on the western flank of the Raton Basin.

Trinidad Sandstone and Vermejo Formation (upper Cretaceous, upper portion, undivided) -- Arkosic sandstone, siltstone, shale, and coal. Total thickness is about 150 m (Lindsey 1996). These sedimentary strata dip toward the east on the western flank of the Raton Basin.

Poison Canyon and Raton Formations (uppermost Cretaceous and Paleocene, undivided) -- Arkosic sandstone, siltstone, and conglomerate. Total thickness estimated 600-700 m (Lindsey 1996). The eastward dip of these formations is less steep than underlying formations.

Cuchara Formation (Eocene) -- Poorly consolidated, arkosic sandstone and conglomerate. Locally metamorphosed to hornfels, slate, and quartzite in contact with Spanish Peaks intrusions. In vicinity of Cucharas Pass, the formation is informally subdivided in two members (Lindsey 1995, 1996).

  1. Conglomerate of Cordova Pass. Coarse conglomerate interbedded with sandstone and mudstone. Conglomerate is composed of rounded cobbles and boulders of gneiss, red and gray sandstone, limestone, and vein quartz. Boulders may exceed 1 m in diameter. These materials were derived from the Proterozoic and Paleozoic rocks of the Culebra Range and deposited as alluvial fans along the mountain front. The conglomerate member interfingers with the sandstone member. Estimated thickness near Cucharas Pass is 800 m.

    Conglomerate of Cordova Pass is metamorphosed adjacent to a dike in the left foreground. Trinchera Peak is visible in the right background. Cordova Pass, June 2003 J.S. Aber.

  2. Sandstone member. Interbedded sandstone, mudstone and conglomerate. Poorly consolidated, cross bedding prominent, weathers to rounded spheroidal shapes. Locally includes logs, stumps, and wood fragments. Estimated thickness near Cucharas Pass is 300-550 m; as much as 1000 m thick to the north toward La Veta.

    Sandstone facies of the Cuchara Formation seen at the golf course in La Veta. Poorly consolidated arkosic sandstone and pebbly sandstone outcrop in rounded blocks that weather rapidly. Nov. 2008 J.S. Aber.

The Cuchara Formation represents sediment eroded from the Culebra Range during the Laramide orogeny. This sediment accumulated as alluvial fans and river deposits in the Raton Basin adjacent to the mountain range. Total preserved thickness of the Cuchara Formation is 1800 m (Lindsey 1995). Most of the Cuchara Formation is gently dipping to near horizontal in position, which suggests that only slight deformation of the Raton Basin has taken place since the Eocene. Around Spanish Peaks, the Cuchara Formation was altered by contact metamorphism to create strata that are more resistant to erosion. However, the sedimentary origin of these rocks is still clearly evident in the layered appearance of Spanish Peaks.

Sill of White Peaks (Miocene) -- A series of three, ridge-forming peaks make up the White Peaks, which are held up by light-gray porphyritic granite. The granite is a sill contained within the Pierre Shale north of Cucharas Pass. The sill has a total length of 5 km and is 700 m thick. Intrusion of the sill is thought to postdate intrusion of East and West Spanish Peaks, because radial dikes of Spanish Peaks do not cut across the sill (Penn and Lindsey 1996).

View northeastward across the Cuchara Valley with West Spanish Peak in the right distance. The White Peaks are a thick sill injected into upper Cretaceous Pierre Shale that stands vertically on the western flank of the Raton Basin. In front of White Peaks and lower is the Dakota hogback, comprised of lower Cretaceous sandstones, also standing in vertical position. Image date 6/03; © J.S. Aber.
Late autumn view from Spring Creek Trail above Cuchara, looking eastward. WSP = West Spanish Peak. NWP = North White Peak, MWP = Middle White Peak, SWP = South White Peak. For description of the Dakota, see Monument Lake. Image date 11/03; © J.S. Aber.

West of Cucharas Pass
Culebra Range

The foothills of Culebra Range begin immediately west of Cucharas Pass. The foothills ascend steeply westward to the crest of the range, in which the peaks exceed 4000 m (13,000 feet) elevation. The entire sequence, from the foothills to range crest, is contained in the Sangre de Cristo Formation. As the name suggests, most of these rocks have blood-red color.

Sangre de Cristo Formation (Pennsylvanian and lower Permian) -- Red, arkosic sandstone, siltstone, shale, and conglomerate. Cross-bedding, ripple marks, channels, mud cracks, and other sedimentary structures are common. Lowermost 200 m of formation is gray sandstone, which supports the ridge crest of the Culebra Range in this vicinity. Conformable on the underlying Whiskey Creek Limestone Member of the Madera Formation (middle Pennsylvanian), which outcrops on the western side of the mountain crest. Total thickness of the Sangre de Cristo Formation exceeds 4000 m (Wallace and Lindsey 1996).

Within the foothills, the Sangre de Cristo Formation is disrupted by numerous folds and faults. The strata define two broad folds that parallel the range crest--an anticline in the eastern foothills and a syncline in western foothills (Lindsey 1996), and thrust faults offset strata on the eastern side of the range crest.

View westward from near Cucharas Pass toward the Culebra Range on the horizon. Trinchera Peak is visible to left side of scene. It reaches 4120 m (13,517 feet) elevation, the highest peak in the northern part of the Culebra Range. In the middle distance is the valley of Cucharas Creek, which drains from Trinchera Peak. Image date 6/03; © J.S. Aber
Roadcut exposure of the Sangre de Cristo Formation on Colorado highway 12. Redbeds dip steeply to the east (left) at this location. Image date 6/99; © J.S. Aber.

The Sangre de Cristo Formation represents the sedimentary deposits of the Ancestral Rocky Mountains. Uplifted in the same location as the modern Culebra Range, these older mountains underwent erosion, and the resulting sediment accumulated as alluvial fans in an adjacent basin. Conglomerate beds suggest that the mountain source was not far from the depositional site of the Sangre de Cristo Formation. The bright red color is common for Permian strata of the southern plains region, known as the "Permian basin." Similar rocks can be seen in New Mexico, Texas, Oklahoma, and Kansas. Uplift of the Ancestral Rockies took place at the same time as the Ouachita orogeny in Arkansas, Oklahoma and Texas.

Trinchera Peak

Trinchera Peak marks the crest of the northern portion of the Culebra Range. It is composed of vertical to slightly overturned, gray, quartzose sandstone of the basal Sangre de Cristo Formation. The erosional resistance of the quartzose sandstone accounts for its topographic prominence at the ridge crest. The western flank of the Culebra Range continues with Madera Formation overthrust by crystalline basement rocks, which can be seen clearly from the crest of the range.

Madera Formation (middle Pennsylvanian) -- Gray limestone and cherty limestone, interbedded with gray and red, arkosic sandstone, siltstone and shale. Thickness exceeds 900 m (Wallace and Lindsey 1996).

Crystalline (early Proterozoic) -- Various mafic to felsic gneisses and augen gneisses, ~1.6 billion years old (Wallace and Lindsey 1996). Felsic pegamtites and mafic dikes, some of which may be considerably younger.

Gray sandstone of the basal Sangre de Cristo Formation stand in near-vertial position beneath Trinchera Peak. Note the red rocks on east (left) flank of peak; this is a fault zone and gray sandstone is repeated farther to left. Image date 8/03; © J.S. Aber.
View northward from Trinchera Peak along the western side of the Culebra Range. The light gray strata in scene center comprise the Whiskey Creek Limestone Member of the Madera Formation. Image date 8/01; © J.S. Aber.
View westward from the crest of the Culebra Range. The deformed gray strata are part of the Madera Formation, and the reddish zone at top of section is crystalline rock (see next image). Image date 8/03; © J.S. Aber.
Tilted and deformed strata of the Madera Formation (gray) are capped by reddish crystalline rocks. The contact between gray and red rocks marks the Culebra Thrust Fault (Wallace and Lindsey 1996). Image date 8/03; © J.S. Aber.
View from the west with the Culebra Range and Trinchera Peak (center) on the eastern horizon. Notice how the clouds are a subtle reflection of ridge crest topography. Image date 3/02; © J.S. Aber.

Glaciation of the Culebra Range.

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GO 547/ES 747 © J.S. Aber (2008).