Geology of the Porcupine Cave Area

The Manitou Dolomite accumulated on a broad, stable, westward-facing cratonic shelf bathed in shallow, well-oxygenated marine waters teeming with life (Foster, 1972; Gerhard, 1972; Stewart and Poole, 1974). After deposition as fossiliferous limestone, the strata were largely transformed into dolomite in the Porcupine Cave area, a process of recrystallization that obliterated much of the Ordovician fossil record.

At the cave site, the Manitou Dolomite is a 55-m-thick, well-bedded, gray, resistant unit (figure 5.3), which forms substantial cliff faces topped by the extremely resistant Harding Quartzite. The dolomitization process is incomplete, and close examination of the Manitou reveals layers in which crinoid debris occurs. A locally mappable series of algal nodule beds is found 12 m above the base. These nodules are each about 1 cm in diameter and occur in a series of 20- to 80-cm-thick layers over a 2- to 5-m zone that has been traced along the entire ridge face. The same horizon has been found on adjacent ridges in the area, and similar algal nodules have been reported from the Manitou by Chronic (1964:106) and Gerhard (1972:9). The Manitou is rich in chert beds, the chert occurring as isolated concretions, concretion-rich beds, and thin laceworks of chert interwoven with dolomite. Chert colors range from cream to brown and black. In plan view some of the chert concretions appear to be filling in regular networks of cracks or burrow systems.

In the cave area, the Manitou Dolomite was deposited directly on a low-relief, lightly weathered Precambrian granite surface. Basal Cambrian quartzites, which are common elsewhere in Colorado, were either eroded or not deposited in this region. Fluctuations in sea level resulted in the accumulation of a succession of transgressive and regressive deposits (Gerhard, 1972) as well as in episodic subaerial exposure and erosion. Prior to the deposition of the overlying Ordovician Harding Quartzite, the surface of the Manitou Formation was exposed and subjected to intense weathering. A karst surface with orange-red soils formed on the unconformity. In places, cavities and fissures were etched tens of meters down into the carbonate rock (figure 5.4). Burial by the Harding Quartzite and younger strata entombed the weathered surface. By establishing deep zones of weakness and pathways for fluid flow, this Paleozoic weathering set the stage for later cave formation and established a Paleozoic ancestry for Porcupine Cave.

Uplift of the Ancestral Rocky Mountains in the Pennsyl-vanian caused the Paleozoic shelf sediments to be eroded from the crest of a series of basement-cored uplands (Mallory, 1958, 1972; De Voto, 1972; Kluth and Coney, 1981), the closest being the Hartsel Uplift (the western flank of the ancient Frontrangia uplift), which is exposed 30 km north-northeast of Porcupine Cave (De Voto, 1971). In the cave area, the landscape changed dramatically from a low-relief passive margin to an intermontane basin termed the Eagle Basin. This basin was invaded by an arm of the western sea and hosted accumulations of dark, organic-rich shale. The Early to Mid-Pennsyl-vanian (Morrowan and Atokan) Belden Shale characterizes the deep and possibly anoxic marine embayment. In places, evaporites also occur (Shawe et al., 1995). The margins of the narrow basin are characterized by thousands of meters of arkosic

Location Porcupine Cave Colorado

P/Pm

Porcupine Cave

Penn/Perm Minturn Fm.

pm I Pennsylvanian Molas Fm.

Mississippian Leadville Fm.

ßc i Devonian Chaffee Fm.

Ordovician Fremont Fm.

Oh I Ordovician Harding Fm.

Karst Alteration

Ordovician Manitou Fm.

Algal Oncolites

Precambrian Granite

FIGURE 5.1 Geological sketch of the vicinity of Porcupine Cave. The asterisk marks the entrance to Porcupine Cave. (Modified from Quinlivan, 1959.)

30 meters

FIGURE 5.2 Cross section showing inclined strata and Porcupine Cave. The resistant Harding Quartzite forms the ridge crest.

30 meters

FIGURE 5.2 Cross section showing inclined strata and Porcupine Cave. The resistant Harding Quartzite forms the ridge crest.

synorogenic debris belonging to the Mid-Pennsylvanian through Early Permian (Atokan through Wolfcampian) Min-turn and Maroon formations. These formations are represented by well-exposed redbeds south-southeast of the cave and by dark shales to the west of the cave site. The site was probably buried by more than 2 km of this strata, requiring subsidence of a comparable amount (c in figure 5.5).

During a period spanning more than 100 million years in the late Paleozoic and early Mesozoic, the Ancestral Rocky Mountains were worn down, and a regional unconformity was mantled by the Jurassic Morrison Formation. The South Park area was for a second time reduced to a low-relief landscape. This gentle Jurassic alluvial plain was inundated by the Cretaceous Interior Seaway about 100 Ma ago. Although Cretaceous strata have since been entirely eroded from the vicinity of the cave, one can develop an estimate of what was once there by examination of regional facies distribution patterns and subsurface drilling data (e.g., Ettinger, 1964). The Manitou Dolomite was buried by an additional cover of more than 1000 m (e in figure 5.5). After regression of the Interior Seaway to the southeast about 70 Ma ago, the area was again a gently undulating coastal plain. The Laramie Formation with its characteristic coal deposits (Washburne, 1910) was deposited across South Park.

Approximately 67 Ma ago the area was uplifted by the Lara-mide Orogeny, and the Front Range east of the cave site was borne high along the east-dipping Elkhorn thrust (Sawatsky, 1964). At the cave site, strata dip about 35° to the east. The tilting is associated with a series of east-dipping thrusts and gentle north-south basement-involved folds. It is likely that the deformation is Laramide in age, as it reflects the style of widespread Laramide compressive deformation documented in South Park by Ettinger (1964) and Sawatsky (1964). An-desitic volcanoes mantled the deforming land in lava flows (Raynolds, 1997). Large volumes of synorogenic debris making up the South Park Formation were shed to the west of the rising Front Range. It is probable that the compressional deformation resulting in the tightly faulted anticline at the cave site took place during this time and that the cave site was uplifted as overlying strata were breached by erosion (f in fig-

Fremont Dolomite

Ridge Crest

Harding Quartzite

, O D5q Q Terra rosa and karst

Cave Entrance ■

Cave Entrance ■

, O D5q Q Terra rosa and karst

Manitou Dolomite

Manitou Dolomite chert nodules

10 meters

SSil algal oncolltes

^gfpit

Precambrian Granite

FIGURE 5.3 Stratigraphic section at Porcupine Cave. Black blebs are chert zones; note the layer of algal nodules about 12 m above the base of the Manitou Dolomite.

Precambrian Granite

FIGURE 5.3 Stratigraphic section at Porcupine Cave. Black blebs are chert zones; note the layer of algal nodules about 12 m above the base of the Manitou Dolomite.

ure 5.5). After termination of the Laramide Orogeny the landscape around the cave site was subjected to a long period of weathering, which resulted in the development of yet a third low-relief, gently rolling surface, with the resistant Harding Quartzite holding up ridges (De Voto, 1971).

At the end of the Eocene a series of lava flows and ignim-brites crossed this landscape, flowing along easterly trending valleys located both north and immediately south of the cave site (Epis and Chapin, 1975). The principal ignimbrite, the Wall Mountain Tuff (36.7 Ma), swept as far east as the Denver Basin. The cave ridge stood above these flows, and the ignim-brites are preserved as relict patches, some of which, with their characteristic obsidian cooling rinds, are still present in the paleovalleys surrounding the cave. The erosional surface defined at this time has been termed the Rocky Mountain Surface, and it is a very widespread, low-relief surface covering many thousands of square kilometers (Epis and Chapin, 1975; Epis et al., 1980; Bradley, 1987; Chapin and Kelley, 1997). It is probable that, as the land around the cave was slowly etched

FIGURE 5.4 Panel diagram of the ridge containing Porcupine Cave. Note that, besides Porcupine Cave, there are numerous other karst fissures extending well down into the Manitou Dolomite. There are probably additional undiscovered caves along this ridge.

1000

J. 2000

3000

5000

40 30 20 10 0 Million Years BP

Cretaceous Interior Seaway Subsidence

FIGURE 5.5 Subsidence profile at Porcupine Cave. Asterisks indicate times when the cave area was prone to dissolution and weathering. Note that, after initial weathering in the Paleozoic, the cave strata were deeply buried until uplifted during the Laramide Orogeny. In the late Tertiary, as the region was weathered and underwent at least one episode of aggradation, it is likely that there were multiple times of cave formation (see text for discussion).

40 30 20 10 0 Million Years BP

Cretaceous Interior Seaway Subsidence

FIGURE 5.5 Subsidence profile at Porcupine Cave. Asterisks indicate times when the cave area was prone to dissolution and weathering. Note that, after initial weathering in the Paleozoic, the cave strata were deeply buried until uplifted during the Laramide Orogeny. In the late Tertiary, as the region was weathered and underwent at least one episode of aggradation, it is likely that there were multiple times of cave formation (see text for discussion).

Million Years BP

by this phase of erosion, the groundwater table was fluctuating near the elevation of the cave (g in figure 5.5). Dissolution may have taken place along the old fissures and joints defined by the Paleozoic karsting. This gives the cave the potential for a Mid-Tertiary phase of development.

After the ignimbrites flowed past, the area around the cave ridge aggraded several hundred feet (h in figure 5.5) until about 8.57 ± 0.16 Ma ago (Chapin and McIntosh, pers. comm., 1997), when a small basaltic volcano erupted just 2.5 km southwest of the cave. A hornblende/olivine-rich tongue of scoriaceous mafic lava flowed east from the vent. The morphologies of both the vent and the flow are still evident south of the cave. The lava is resistant to erosion and forms inverted topography where the flow, which once covered the valley floor, now forms a resistant ridge at about the same elevation as the cave (2915 m). Once again the groundwater table may have been at the elevation of the cave site (i in figure 5.5), and the cave may have been further enlarged. This gives the cave a potential Late Tertiary phase of development.

To the east of the cave site, a thick package of Middle to Late Miocene lacustrine strata accumulated on the gently subsiding floor of the South Park basin. The lacustrine facies is termed the Wagon Tongue Formation, and the proximal gravely facies are termed the Trump Formation (De Voto, 1964). Relicts of both facies occur in the Herring Park area south and west of the cave. Wagon Tongue facies sediments can also be found beneath the Late Miocene lava flow south of the cave.

Over the past eight million years the valley has again been down-cut, exhuming the remnants of the Wall Mountain ignimbrite and the Eocene paleovalley, leaving the cave high above the modern water table.

During the Pleistocene, the cave area saw a series of ponded bogs, and fens formed on the valley floor in South Park. Some of these are still present in the area, and some have been dissected by modern erosion. One or more sinkhole-type openings formed over the now dry cave, allowing access for Pleistocene fauna. It is likely that much of the fauna was carried in by wood rats and carnivores, although the presence of disarticulated bones from a large camel suggests that some animals may have fallen into an open chasm. Small rodents and carnivores used the cave for many thousands of years until the openings became choked with sediment, sealing the cave. The cave and its contents awaited discovery by nineteenth-century miners prospecting along the weathered unconformity at the top of the Manitou Dolomite. Ore bodies are concentrated at a similar unconformity on the top of the Mis-sissippian Leadville Limestone.

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